/**
 * finder.js
 */

/* import */
import {
  matchAttributeSelector,
  matchDirectionPseudoClass,
  matchDisabledPseudoClass,
  matchLanguagePseudoClass,
  matchPseudoElementSelector,
  matchReadOnlyPseudoClass,
  matchTypeSelector
} from './matcher.js';
import {
  findAST,
  generateCSS,
  parseSelector,
  sortAST,
  unescapeSelector,
  walkAST
} from './parser.js';
import {
  filterNodesByAnB,
  findLogicalWithNestedHas,
  generateException,
  isCustomElement,
  isFocusVisible,
  isFocusableArea,
  isVisible,
  resolveContent,
  sortNodes,
  traverseNode
} from './utility.js';

/* constants */
import {
  ATTR_SELECTOR,
  CLASS_SELECTOR,
  COMBINATOR,
  DOCUMENT_FRAGMENT_NODE,
  ELEMENT_NODE,
  FORM_PARTS,
  ID_SELECTOR,
  INPUT_CHECK,
  INPUT_DATE,
  INPUT_EDIT,
  INPUT_TEXT,
  KEYS_LOGICAL,
  NOT_SUPPORTED_ERR,
  PS_CLASS_SELECTOR,
  PS_ELEMENT_SELECTOR,
  SHOW_ALL,
  SHOW_CONTAINER,
  SYNTAX_ERR,
  TARGET_ALL,
  TARGET_FIRST,
  TARGET_LINEAL,
  TARGET_SELF,
  TEXT_NODE,
  TYPE_SELECTOR
} from './constant.js';
const DIR_NEXT = 'next';
const DIR_PREV = 'prev';
const KEYS_FORM = new Set([...FORM_PARTS, 'fieldset', 'form']);
const KEYS_FORM_PS_VALID = new Set([...FORM_PARTS, 'form']);
const KEYS_INPUT_CHECK = new Set(INPUT_CHECK);
const KEYS_INPUT_PLACEHOLDER = new Set([...INPUT_TEXT, 'number']);
const KEYS_INPUT_RANGE = new Set([...INPUT_DATE, 'number', 'range']);
const KEYS_INPUT_REQUIRED = new Set([...INPUT_CHECK, ...INPUT_EDIT, 'file']);
const KEYS_INPUT_RESET = new Set(['button', 'reset']);
const KEYS_INPUT_SUBMIT = new Set(['image', 'submit']);
const KEYS_MODIFIER = new Set([
  'Alt',
  'AltGraph',
  'CapsLock',
  'Control',
  'Fn',
  'FnLock',
  'Hyper',
  'Meta',
  'NumLock',
  'ScrollLock',
  'Shift',
  'Super',
  'Symbol',
  'SymbolLock'
]);
const KEYS_PS_UNCACHE = new Set([
  'any-link',
  'defined',
  'dir',
  'link',
  'scope'
]);
const KEYS_PS_NTH_OF_TYPE = new Set([
  'first-of-type',
  'last-of-type',
  'only-of-type'
]);

/**
 * Finder
 * NOTE: #ast[i] corresponds to #nodes[i]
 */
export class Finder {
  /* private fields */
  #ast;
  #astCache;
  #check;
  #descendant;
  #document;
  #documentCache;
  #documentURL;
  #event;
  #eventHandlers;
  #focus;
  #invalidate;
  #invalidateResults;
  #lastFocusVisible;
  #node;
  #nodeWalker;
  #nodes;
  #noexcept;
  #pseudoElement;
  #results;
  #root;
  #rootWalker;
  #selector;
  #shadow;
  #verifyShadowHost;
  #walkers;
  #warn;
  #window;

  /**
   * constructor
   * @param {object} window - The window object.
   */
  constructor(window) {
    this.#window = window;
    this.#astCache = new WeakMap();
    this.#documentCache = new WeakMap();
    this.#event = null;
    this.#focus = null;
    this.#lastFocusVisible = null;
    this.#eventHandlers = new Set([
      {
        keys: ['focus', 'focusin'],
        handler: this._handleFocusEvent
      },
      {
        keys: ['keydown', 'keyup'],
        handler: this._handleKeyboardEvent
      },
      {
        keys: ['mouseover', 'mousedown', 'mouseup', 'click', 'mouseout'],
        handler: this._handleMouseEvent
      }
    ]);
    this._registerEventListeners();
    this.clearResults(true);
  }

  /**
   * Handles errors.
   * @param {Error} e - The error object.
   * @param {object} [opt] - Options.
   * @param {boolean} [opt.noexcept] - If true, exceptions are not thrown.
   * @throws {Error} Throws an error.
   * @returns {void}
   */
  onError = (e, opt = {}) => {
    const noexcept = opt.noexcept ?? this.#noexcept;
    if (noexcept) {
      return;
    }
    const isDOMException =
      e instanceof DOMException || e instanceof this.#window.DOMException;
    if (isDOMException) {
      if (e.name === NOT_SUPPORTED_ERR) {
        if (this.#warn) {
          console.warn(e.message);
        }
        return;
      }
      throw new this.#window.DOMException(e.message, e.name);
    }
    if (e.name in this.#window) {
      throw new this.#window[e.name](e.message, { cause: e });
    }
    throw e;
  };

  /**
   * Sets up the finder.
   * @param {string} selector - The CSS selector.
   * @param {object} node - Document, DocumentFragment, or Element.
   * @param {object} [opt] - Options.
   * @param {boolean} [opt.check] - Indicates if running in internal check().
   * @param {boolean} [opt.noexcept] - If true, exceptions are not thrown.
   * @param {boolean} [opt.warn] - If true, console warnings are enabled.
   * @returns {object} The finder instance.
   */
  setup = (selector, node, opt = {}) => {
    const { check, noexcept, warn } = opt;
    this.#check = !!check;
    this.#noexcept = !!noexcept;
    this.#warn = !!warn;
    [this.#document, this.#root, this.#shadow] = resolveContent(node);
    this.#documentURL = null;
    this.#node = node;
    this.#selector = selector;
    [this.#ast, this.#nodes] = this._correspond(selector);
    this.#pseudoElement = [];
    this.#walkers = new WeakMap();
    this.#nodeWalker = null;
    this.#rootWalker = null;
    this.#verifyShadowHost = null;
    this.clearResults();
    return this;
  };

  /**
   * Clear cached results.
   * @param {boolean} all - clear all results
   * @returns {void}
   */
  clearResults = (all = false) => {
    this.#invalidateResults = new WeakMap();
    if (all) {
      this.#results = new WeakMap();
    }
  };

  /**
   * Handles focus events.
   * @private
   * @param {Event} evt - The event object.
   * @returns {void}
   */
  _handleFocusEvent = evt => {
    this.#focus = evt;
  };

  /**
   * Handles keyboard events.
   * @private
   * @param {Event} evt - The event object.
   * @returns {void}
   */
  _handleKeyboardEvent = evt => {
    const { key } = evt;
    if (!KEYS_MODIFIER.has(key)) {
      this.#event = evt;
    }
  };

  /**
   * Handles mouse events.
   * @private
   * @param {Event} evt - The event object.
   * @returns {void}
   */
  _handleMouseEvent = evt => {
    this.#event = evt;
  };

  /**
   * Registers event listeners.
   * @private
   * @returns {Array.<void>} An array of return values from addEventListener.
   */
  _registerEventListeners = () => {
    const opt = {
      capture: true,
      passive: true
    };
    const func = [];
    for (const eventHandler of this.#eventHandlers) {
      const { keys, handler } = eventHandler;
      const l = keys.length;
      for (let i = 0; i < l; i++) {
        const key = keys[i];
        func.push(this.#window.addEventListener(key, handler, opt));
      }
    }
    return func;
  };

  /**
   * Processes selector branches into the internal AST structure.
   * @private
   * @param {Array.<Array.<object>>} branches - The branches from walkAST.
   * @param {string} selector - The original selector for error reporting.
   * @returns {{ast: Array, descendant: boolean}}
   * An object with the AST, descendant flag.
   */
  _processSelectorBranches = (branches, selector) => {
    let descendant = false;
    const ast = [];
    const l = branches.length;
    for (let i = 0; i < l; i++) {
      const items = [...branches[i]];
      const branch = [];
      let item = items.shift();
      if (item && item.type !== COMBINATOR) {
        const leaves = new Set();
        while (item) {
          if (item.type === COMBINATOR) {
            const [nextItem] = items;
            if (!nextItem || nextItem.type === COMBINATOR) {
              const msg = `Invalid selector ${selector}`;
              this.onError(generateException(msg, SYNTAX_ERR, this.#window));
              // Stop processing on invalid selector.
              return { ast: [], descendant: false, invalidate: false };
            }
            if (item.name === ' ' || item.name === '>') {
              descendant = true;
            }
            branch.push({ combo: item, leaves: sortAST(leaves) });
            leaves.clear();
          } else {
            if (item.name && typeof item.name === 'string') {
              const unescapedName = unescapeSelector(item.name);
              if (unescapedName !== item.name) {
                item.name = unescapedName;
              }
              if (/[|:]/.test(unescapedName)) {
                item.namespace = true;
              }
            }
            leaves.add(item);
          }
          if (items.length) {
            item = items.shift();
          } else {
            branch.push({ combo: null, leaves: sortAST(leaves) });
            leaves.clear();
            break;
          }
        }
      }
      ast.push({ branch, dir: null, filtered: false, find: false });
    }
    return { ast, descendant };
  };

  /**
   * Corresponds AST and nodes.
   * @private
   * @param {string} selector - The CSS selector.
   * @returns {Array.<Array.<object>>} An array with the AST and nodes.
   */
  _correspond = selector => {
    const nodes = [];
    this.#descendant = false;
    this.#invalidate = false;
    let ast;
    if (this.#documentCache.has(this.#document)) {
      const cachedItem = this.#documentCache.get(this.#document);
      if (cachedItem && cachedItem.has(`${selector}`)) {
        const item = cachedItem.get(`${selector}`);
        ast = item.ast;
        this.#descendant = item.descendant;
        this.#invalidate = item.invalidate;
      }
    }
    if (ast) {
      const l = ast.length;
      for (let i = 0; i < l; i++) {
        ast[i].dir = null;
        ast[i].filtered = false;
        ast[i].find = false;
        nodes[i] = [];
      }
    } else {
      let cssAst;
      try {
        cssAst = parseSelector(selector);
      } catch (e) {
        return this.onError(e);
      }
      const { branches, info } = walkAST(cssAst);
      const {
        hasHasPseudoFunc,
        hasLogicalPseudoFunc,
        hasNthChildOfSelector,
        hasStatePseudoClass
      } = info;
      this.#invalidate =
        hasHasPseudoFunc ||
        hasStatePseudoClass ||
        !!(hasLogicalPseudoFunc && hasNthChildOfSelector);
      const processed = this._processSelectorBranches(branches, selector);
      ast = processed.ast;
      this.#descendant = processed.descendant;
      let cachedItem;
      if (this.#documentCache.has(this.#document)) {
        cachedItem = this.#documentCache.get(this.#document);
      } else {
        cachedItem = new Map();
      }
      cachedItem.set(`${selector}`, {
        ast,
        descendant: this.#descendant,
        invalidate: this.#invalidate
      });
      this.#documentCache.set(this.#document, cachedItem);
      // Initialize nodes array for each branch.
      for (let i = 0; i < ast.length; i++) {
        nodes[i] = [];
      }
    }
    return [ast, nodes];
  };

  /**
   * Creates a TreeWalker.
   * @private
   * @param {object} node - The Document, DocumentFragment, or Element node.
   * @param {object} [opt] - Options.
   * @param {boolean} [opt.force] - Force creation of a new TreeWalker.
   * @param {number} [opt.whatToShow] - The NodeFilter whatToShow value.
   * @returns {object} The TreeWalker object.
   */
  _createTreeWalker = (node, opt = {}) => {
    const { force = false, whatToShow = SHOW_CONTAINER } = opt;
    if (force) {
      return this.#document.createTreeWalker(node, whatToShow);
    } else if (this.#walkers.has(node)) {
      return this.#walkers.get(node);
    }
    const walker = this.#document.createTreeWalker(node, whatToShow);
    this.#walkers.set(node, walker);
    return walker;
  };

  /**
   * Gets selector branches from cache or parses them.
   * @private
   * @param {object} selector - The AST.
   * @returns {Array.<Array.<object>>} The selector branches.
   */
  _getSelectorBranches = selector => {
    if (this.#astCache.has(selector)) {
      return this.#astCache.get(selector);
    }
    const { branches } = walkAST(selector);
    this.#astCache.set(selector, branches);
    return branches;
  };

  /**
   * Gets the children of a node, optionally filtered by a selector.
   * @private
   * @param {object} parentNode - The parent element.
   * @param {Array.<Array.<object>>} selectorBranches - The selector branches.
   * @param {object} [opt] - Options.
   * @returns {Array.<object>} An array of child nodes.
   */
  _getFilteredChildren = (parentNode, selectorBranches, opt = {}) => {
    const children = [];
    const walker = this._createTreeWalker(parentNode, { force: true });
    let childNode = walker.firstChild();
    while (childNode) {
      if (selectorBranches) {
        if (isVisible(childNode)) {
          let isMatch = false;
          const l = selectorBranches.length;
          for (let i = 0; i < l; i++) {
            const leaves = selectorBranches[i];
            if (this._matchLeaves(leaves, childNode, opt)) {
              isMatch = true;
              break;
            }
          }
          if (isMatch) {
            children.push(childNode);
          }
        }
      } else {
        children.push(childNode);
      }
      childNode = walker.nextSibling();
    }
    return children;
  };

  /**
   * Collects nth-child nodes.
   * @private
   * @param {object} anb - An+B options.
   * @param {number} anb.a - The 'a' value.
   * @param {number} anb.b - The 'b' value.
   * @param {boolean} [anb.reverse] - If true, reverses the order.
   * @param {object} [anb.selector] - The AST.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _collectNthChild = (anb, node, opt = {}) => {
    const { a, b, selector } = anb;
    const { parentNode } = node;
    if (!parentNode) {
      const matchedNode = new Set();
      if (node === this.#root && a * 1 + b * 1 === 1) {
        if (selector) {
          const selectorBranches = this._getSelectorBranches(selector);
          const l = selectorBranches.length;
          for (let i = 0; i < l; i++) {
            const leaves = selectorBranches[i];
            if (this._matchLeaves(leaves, node, opt)) {
              matchedNode.add(node);
              break;
            }
          }
        } else {
          matchedNode.add(node);
        }
      }
      return matchedNode;
    }
    const selectorBranches = selector
      ? this._getSelectorBranches(selector)
      : null;
    const children = this._getFilteredChildren(
      parentNode,
      selectorBranches,
      opt
    );
    const matchedNodes = filterNodesByAnB(children, anb);
    return new Set(matchedNodes);
  };

  /**
   * Collects nth-of-type nodes.
   * @private
   * @param {object} anb - An+B options.
   * @param {number} anb.a - The 'a' value.
   * @param {number} anb.b - The 'b' value.
   * @param {boolean} [anb.reverse] - If true, reverses the order.
   * @param {object} node - The Element node.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _collectNthOfType = (anb, node) => {
    const { parentNode } = node;
    if (!parentNode) {
      if (node === this.#root && anb.a * 1 + anb.b * 1 === 1) {
        return new Set([node]);
      }
      return new Set();
    }
    const typedSiblings = [];
    const walker = this._createTreeWalker(parentNode, { force: true });
    let sibling = walker.firstChild();
    while (sibling) {
      if (
        sibling.localName === node.localName &&
        sibling.namespaceURI === node.namespaceURI &&
        sibling.prefix === node.prefix
      ) {
        typedSiblings.push(sibling);
      }
      sibling = walker.nextSibling();
    }
    const matchedNodes = filterNodesByAnB(typedSiblings, anb);
    return new Set(matchedNodes);
  };

  /**
   * Matches An+B.
   * @private
   * @param {object} ast - The AST.
   * @param {object} node - The Element node.
   * @param {string} nthName - The name of the nth pseudo-class.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchAnPlusB = (ast, node, nthName, opt = {}) => {
    const {
      nth: { a, b, name: nthIdentName },
      selector
    } = ast;
    const anbMap = new Map();
    if (nthIdentName) {
      if (nthIdentName === 'even') {
        anbMap.set('a', 2);
        anbMap.set('b', 0);
      } else if (nthIdentName === 'odd') {
        anbMap.set('a', 2);
        anbMap.set('b', 1);
      }
      if (nthName.indexOf('last') > -1) {
        anbMap.set('reverse', true);
      }
    } else {
      if (typeof a === 'string' && /-?\d+/.test(a)) {
        anbMap.set('a', a * 1);
      } else {
        anbMap.set('a', 0);
      }
      if (typeof b === 'string' && /-?\d+/.test(b)) {
        anbMap.set('b', b * 1);
      } else {
        anbMap.set('b', 0);
      }
      if (nthName.indexOf('last') > -1) {
        anbMap.set('reverse', true);
      }
    }
    if (nthName === 'nth-child' || nthName === 'nth-last-child') {
      if (selector) {
        anbMap.set('selector', selector);
      }
      const anb = Object.fromEntries(anbMap);
      const nodes = this._collectNthChild(anb, node, opt);
      return nodes;
    } else if (nthName === 'nth-of-type' || nthName === 'nth-last-of-type') {
      const anb = Object.fromEntries(anbMap);
      const nodes = this._collectNthOfType(anb, node);
      return nodes;
    }
    return new Set();
  };

  /**
   * Matches the :has() pseudo-class function.
   * @private
   * @param {Array.<object>} astLeaves - The AST leaves.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {boolean} The result.
   */
  _matchHasPseudoFunc = (astLeaves, node, opt = {}) => {
    if (Array.isArray(astLeaves) && astLeaves.length) {
      // Prepare a copy to avoid astLeaves being consumed.
      const leaves = [...astLeaves];
      const [leaf] = leaves;
      const { type: leafType } = leaf;
      let combo;
      if (leafType === COMBINATOR) {
        combo = leaves.shift();
      } else {
        combo = {
          name: ' ',
          type: COMBINATOR
        };
      }
      const twigLeaves = [];
      while (leaves.length) {
        const [item] = leaves;
        const { type: itemType } = item;
        if (itemType === COMBINATOR) {
          break;
        } else {
          twigLeaves.push(leaves.shift());
        }
      }
      const twig = {
        combo,
        leaves: twigLeaves
      };
      opt.dir = DIR_NEXT;
      const nodes = this._matchCombinator(twig, node, opt);
      if (nodes.size) {
        if (leaves.length) {
          let bool = false;
          for (const nextNode of nodes) {
            bool = this._matchHasPseudoFunc(leaves, nextNode, opt);
            if (bool) {
              break;
            }
          }
          return bool;
        }
        return true;
      }
    }
    return false;
  };

  /**
   * Evaluates the :has() pseudo-class.
   * @private
   * @param {object} astData - The AST data.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {?object} The matched node.
   */
  _evaluateHasPseudo = (astData, node, opt) => {
    const { branches } = astData;
    let bool = false;
    const l = branches.length;
    for (let i = 0; i < l; i++) {
      const leaves = branches[i];
      bool = this._matchHasPseudoFunc(leaves, node, opt);
      if (bool) {
        break;
      }
    }
    if (!bool) {
      return null;
    }
    if (
      (opt.isShadowRoot || this.#shadow) &&
      node.nodeType === DOCUMENT_FRAGMENT_NODE
    ) {
      return this.#verifyShadowHost ? node : null;
    }
    return node;
  };

  /**
   * Matches logical pseudo-class functions.
   * @private
   * @param {object} astData - The AST data.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {?object} The matched node.
   */
  _matchLogicalPseudoFunc = (astData, node, opt = {}) => {
    const { astName, branches, twigBranches } = astData;
    // Handle :has().
    if (astName === 'has') {
      return this._evaluateHasPseudo(astData, node, opt);
    }
    // Handle :is(), :not(), :where().
    const isShadowRoot =
      (opt.isShadowRoot || this.#shadow) &&
      node.nodeType === DOCUMENT_FRAGMENT_NODE;
    // Check for invalid shadow root.
    if (isShadowRoot) {
      let invalid = false;
      for (const branch of branches) {
        if (branch.length > 1) {
          invalid = true;
          break;
        } else if (astName === 'not') {
          const [{ type: childAstType }] = branch;
          if (childAstType !== PS_CLASS_SELECTOR) {
            invalid = true;
            break;
          }
        }
      }
      if (invalid) {
        return null;
      }
    }
    opt.forgive = astName === 'is' || astName === 'where';
    const l = twigBranches.length;
    let bool;
    for (let i = 0; i < l; i++) {
      const branch = twigBranches[i];
      const lastIndex = branch.length - 1;
      const { leaves } = branch[lastIndex];
      bool = this._matchLeaves(leaves, node, opt);
      if (bool && lastIndex > 0) {
        let nextNodes = new Set([node]);
        for (let j = lastIndex - 1; j >= 0; j--) {
          const twig = branch[j];
          const arr = [];
          opt.dir = DIR_PREV;
          for (const nextNode of nextNodes) {
            const m = this._matchCombinator(twig, nextNode, opt);
            if (m.size) {
              arr.push(...m);
            }
          }
          if (arr.length) {
            if (j === 0) {
              bool = true;
            } else {
              nextNodes = new Set(arr);
            }
          } else {
            bool = false;
            break;
          }
        }
      }
      if (bool) {
        break;
      }
    }
    if (astName === 'not') {
      if (bool) {
        return null;
      }
      return node;
    } else if (bool) {
      return node;
    }
    return null;
  };

  /**
   * match pseudo-class selector
   * @private
   * @see https://html.spec.whatwg.org/#pseudo-classes
   * @param {object} ast - AST
   * @param {object} node - Element node
   * @param {object} [opt] - options
   * @returns {Set.<object>} - collection of matched nodes
   */
  _matchPseudoClassSelector(ast, node, opt = {}) {
    const { children: astChildren, name: astName } = ast;
    const { localName, parentNode } = node;
    const { forgive, warn = this.#warn } = opt;
    const matched = new Set();
    // :has(), :is(), :not(), :where()
    if (Array.isArray(astChildren) && KEYS_LOGICAL.has(astName)) {
      if (!astChildren.length && astName !== 'is' && astName !== 'where') {
        const css = generateCSS(ast);
        const msg = `Invalid selector ${css}`;
        return this.onError(generateException(msg, SYNTAX_ERR, this.#window));
      }
      let astData;
      if (this.#astCache.has(ast)) {
        astData = this.#astCache.get(ast);
      } else {
        const { branches } = walkAST(ast);
        if (astName === 'has') {
          // Check for nested :has().
          let forgiven = false;
          const l = astChildren.length;
          for (let i = 0; i < l; i++) {
            const child = astChildren[i];
            const item = findAST(child, findLogicalWithNestedHas);
            if (item) {
              const itemName = item.name;
              if (itemName === 'is' || itemName === 'where') {
                forgiven = true;
                break;
              } else {
                const css = generateCSS(ast);
                const msg = `Invalid selector ${css}`;
                return this.onError(
                  generateException(msg, SYNTAX_ERR, this.#window)
                );
              }
            }
          }
          if (forgiven) {
            return matched;
          }
          astData = {
            astName,
            branches
          };
        } else {
          const twigBranches = [];
          const l = branches.length;
          for (let i = 0; i < l; i++) {
            const [...leaves] = branches[i];
            const branch = [];
            const leavesSet = new Set();
            let item = leaves.shift();
            while (item) {
              if (item.type === COMBINATOR) {
                branch.push({
                  combo: item,
                  leaves: [...leavesSet]
                });
                leavesSet.clear();
              } else if (item) {
                leavesSet.add(item);
              }
              if (leaves.length) {
                item = leaves.shift();
              } else {
                branch.push({
                  combo: null,
                  leaves: [...leavesSet]
                });
                leavesSet.clear();
                break;
              }
            }
            twigBranches.push(branch);
          }
          astData = {
            astName,
            branches,
            twigBranches
          };
          this.#astCache.set(ast, astData);
        }
      }
      const res = this._matchLogicalPseudoFunc(astData, node, opt);
      if (res) {
        matched.add(res);
      }
    } else if (Array.isArray(astChildren)) {
      // :nth-child(), :nth-last-child(), nth-of-type(), :nth-last-of-type()
      if (/^nth-(?:last-)?(?:child|of-type)$/.test(astName)) {
        if (astChildren.length !== 1) {
          const css = generateCSS(ast);
          return this.onError(
            generateException(
              `Invalid selector ${css}`,
              SYNTAX_ERR,
              this.#window
            )
          );
        }
        const [branch] = astChildren;
        const nodes = this._matchAnPlusB(branch, node, astName, opt);
        return nodes;
      } else {
        switch (astName) {
          // :dir()
          case 'dir': {
            if (astChildren.length !== 1) {
              const css = generateCSS(ast);
              return this.onError(
                generateException(
                  `Invalid selector ${css}`,
                  SYNTAX_ERR,
                  this.#window
                )
              );
            }
            const [astChild] = astChildren;
            const res = matchDirectionPseudoClass(astChild, node);
            if (res) {
              matched.add(node);
            }
            break;
          }
          // :lang()
          case 'lang': {
            if (!astChildren.length) {
              const css = generateCSS(ast);
              return this.onError(
                generateException(
                  `Invalid selector ${css}`,
                  SYNTAX_ERR,
                  this.#window
                )
              );
            }
            let bool;
            for (const astChild of astChildren) {
              bool = matchLanguagePseudoClass(astChild, node);
              if (bool) {
                break;
              }
            }
            if (bool) {
              matched.add(node);
            }
            break;
          }
          // :state()
          case 'state': {
            if (isCustomElement(node)) {
              const [{ value: stateValue }] = astChildren;
              if (stateValue) {
                if (node[stateValue]) {
                  matched.add(node);
                } else {
                  for (const i in node) {
                    const prop = node[i];
                    if (prop instanceof this.#window.ElementInternals) {
                      if (prop?.states?.has(stateValue)) {
                        matched.add(node);
                      }
                      break;
                    }
                  }
                }
              }
            }
            break;
          }
          case 'current':
          case 'heading':
          case 'nth-col':
          case 'nth-last-col': {
            if (warn) {
              this.onError(
                generateException(
                  `Unsupported pseudo-class :${astName}()`,
                  NOT_SUPPORTED_ERR,
                  this.#window
                )
              );
            }
            break;
          }
          // Ignore :host() and :host-context().
          case 'host':
          case 'host-context': {
            break;
          }
          // Deprecated in CSS Selectors 3.
          case 'contains': {
            if (warn) {
              this.onError(
                generateException(
                  `Unknown pseudo-class :${astName}()`,
                  NOT_SUPPORTED_ERR,
                  this.#window
                )
              );
            }
            break;
          }
          default: {
            if (!forgive) {
              this.onError(
                generateException(
                  `Unknown pseudo-class :${astName}()`,
                  SYNTAX_ERR,
                  this.#window
                )
              );
            }
          }
        }
      }
    } else if (KEYS_PS_NTH_OF_TYPE.has(astName)) {
      if (node === this.#root) {
        matched.add(node);
      } else if (parentNode) {
        switch (astName) {
          case 'first-of-type': {
            const [node1] = this._collectNthOfType(
              {
                a: 0,
                b: 1
              },
              node
            );
            if (node1) {
              matched.add(node1);
            }
            break;
          }
          case 'last-of-type': {
            const [node1] = this._collectNthOfType(
              {
                a: 0,
                b: 1,
                reverse: true
              },
              node
            );
            if (node1) {
              matched.add(node1);
            }
            break;
          }
          // 'only-of-type' is handled by default.
          default: {
            const [node1] = this._collectNthOfType(
              {
                a: 0,
                b: 1
              },
              node
            );
            if (node1 === node) {
              const [node2] = this._collectNthOfType(
                {
                  a: 0,
                  b: 1,
                  reverse: true
                },
                node
              );
              if (node2 === node) {
                matched.add(node);
              }
            }
          }
        }
      }
    } else {
      switch (astName) {
        case 'disabled':
        case 'enabled': {
          const isMatch = matchDisabledPseudoClass(astName, node);
          if (isMatch) {
            matched.add(node);
          }
          break;
        }
        case 'read-only':
        case 'read-write': {
          const isMatch = matchReadOnlyPseudoClass(astName, node);
          if (isMatch) {
            matched.add(node);
          }
          break;
        }
        case 'any-link':
        case 'link': {
          if (
            (localName === 'a' || localName === 'area') &&
            node.hasAttribute('href')
          ) {
            matched.add(node);
          }
          break;
        }
        case 'local-link': {
          if (
            (localName === 'a' || localName === 'area') &&
            node.hasAttribute('href')
          ) {
            if (!this.#documentURL) {
              this.#documentURL = new URL(this.#document.URL);
            }
            const { href, origin, pathname } = this.#documentURL;
            const attrURL = new URL(node.getAttribute('href'), href);
            if (attrURL.origin === origin && attrURL.pathname === pathname) {
              matched.add(node);
            }
          }
          break;
        }
        case 'visited': {
          // prevent fingerprinting
          break;
        }
        case 'hover': {
          const { target, type } = this.#event ?? {};
          if (
            /^(?:click|mouse(?:down|over|up))$/.test(type) &&
            node.contains(target)
          ) {
            matched.add(node);
          }
          break;
        }
        case 'active': {
          const { buttons, target, type } = this.#event ?? {};
          if (type === 'mousedown' && buttons & 1 && node.contains(target)) {
            matched.add(node);
          }
          break;
        }
        case 'target': {
          if (!this.#documentURL) {
            this.#documentURL = new URL(this.#document.URL);
          }
          const { hash } = this.#documentURL;
          if (
            node.id &&
            hash === `#${node.id}` &&
            this.#document.contains(node)
          ) {
            matched.add(node);
          }
          break;
        }
        case 'target-within': {
          if (!this.#documentURL) {
            this.#documentURL = new URL(this.#document.URL);
          }
          const { hash } = this.#documentURL;
          if (hash) {
            const id = hash.replace(/^#/, '');
            let current = this.#document.getElementById(id);
            while (current) {
              if (current === node) {
                matched.add(node);
                break;
              }
              current = current.parentNode;
            }
          }
          break;
        }
        case 'scope': {
          if (this.#node.nodeType === ELEMENT_NODE) {
            if (!this.#shadow && node === this.#node) {
              matched.add(node);
            }
          } else if (node === this.#document.documentElement) {
            matched.add(node);
          }
          break;
        }
        case 'focus': {
          if (node === this.#document.activeElement && isFocusableArea(node)) {
            matched.add(node);
          }
          break;
        }
        case 'focus-visible': {
          if (node === this.#document.activeElement && isFocusableArea(node)) {
            let bool;
            if (isFocusVisible(node)) {
              bool = true;
            } else if (this.#focus) {
              const { relatedTarget, target: focusTarget } = this.#focus;
              if (focusTarget === node) {
                if (isFocusVisible(relatedTarget)) {
                  bool = true;
                } else if (this.#event) {
                  const {
                    altKey: eventAltKey,
                    ctrlKey: eventCtrlKey,
                    key: eventKey,
                    metaKey: eventMetaKey,
                    target: eventTarget,
                    type: eventType
                  } = this.#event;
                  // this.#event is irrelevant if eventTarget === relatedTarget
                  if (eventTarget === relatedTarget) {
                    if (this.#lastFocusVisible === null) {
                      bool = true;
                    } else if (focusTarget === this.#lastFocusVisible) {
                      bool = true;
                    }
                  } else if (eventKey === 'Tab') {
                    if (
                      (eventType === 'keydown' && eventTarget !== node) ||
                      (eventType === 'keyup' && eventTarget === node)
                    ) {
                      if (eventTarget === focusTarget) {
                        if (this.#lastFocusVisible === null) {
                          bool = true;
                        } else if (
                          eventTarget === this.#lastFocusVisible &&
                          relatedTarget === null
                        ) {
                          bool = true;
                        }
                      } else {
                        bool = true;
                      }
                    }
                  } else if (eventKey) {
                    if (
                      (eventType === 'keydown' || eventType === 'keyup') &&
                      !eventAltKey &&
                      !eventCtrlKey &&
                      !eventMetaKey &&
                      eventTarget === node
                    ) {
                      bool = true;
                    }
                  }
                } else if (
                  relatedTarget === null ||
                  relatedTarget === this.#lastFocusVisible
                ) {
                  bool = true;
                }
              }
            }
            if (bool) {
              this.#lastFocusVisible = node;
              matched.add(node);
            } else if (this.#lastFocusVisible === node) {
              this.#lastFocusVisible = null;
            }
          }
          break;
        }
        case 'focus-within': {
          let bool;
          let current = this.#document.activeElement;
          if (isFocusableArea(current)) {
            while (current) {
              if (current === node) {
                bool = true;
                break;
              }
              current = current.parentNode;
            }
          }
          if (bool) {
            matched.add(node);
          }
          break;
        }
        case 'open':
        case 'closed': {
          if (localName === 'details' || localName === 'dialog') {
            if (node.hasAttribute('open')) {
              if (astName === 'open') {
                matched.add(node);
              }
            } else if (astName === 'closed') {
              matched.add(node);
            }
          }
          break;
        }
        case 'placeholder-shown': {
          let placeholder;
          if (node.placeholder) {
            placeholder = node.placeholder;
          } else if (node.hasAttribute('placeholder')) {
            placeholder = node.getAttribute('placeholder');
          }
          if (typeof placeholder === 'string' && !/[\r\n]/.test(placeholder)) {
            let targetNode;
            if (localName === 'textarea') {
              targetNode = node;
            } else if (localName === 'input') {
              if (node.hasAttribute('type')) {
                if (KEYS_INPUT_PLACEHOLDER.has(node.getAttribute('type'))) {
                  targetNode = node;
                }
              } else {
                targetNode = node;
              }
            }
            if (targetNode && node.value === '') {
              matched.add(node);
            }
          }
          break;
        }
        case 'checked': {
          const attrType = node.getAttribute('type');
          if (
            (node.checked &&
              localName === 'input' &&
              (attrType === 'checkbox' || attrType === 'radio')) ||
            (node.selected && localName === 'option')
          ) {
            matched.add(node);
          }
          break;
        }
        case 'indeterminate': {
          if (
            (node.indeterminate &&
              localName === 'input' &&
              node.type === 'checkbox') ||
            (localName === 'progress' && !node.hasAttribute('value'))
          ) {
            matched.add(node);
          } else if (
            localName === 'input' &&
            node.type === 'radio' &&
            !node.hasAttribute('checked')
          ) {
            const nodeName = node.name;
            let parent = node.parentNode;
            while (parent) {
              if (parent.localName === 'form') {
                break;
              }
              parent = parent.parentNode;
            }
            if (!parent) {
              parent = this.#document.documentElement;
            }
            const walker = this._createTreeWalker(parent);
            let refNode = traverseNode(parent, walker);
            refNode = walker.firstChild();
            let checked;
            while (refNode) {
              if (
                refNode.localName === 'input' &&
                refNode.getAttribute('type') === 'radio'
              ) {
                if (refNode.hasAttribute('name')) {
                  if (refNode.getAttribute('name') === nodeName) {
                    checked = !!refNode.checked;
                  }
                } else {
                  checked = !!refNode.checked;
                }
                if (checked) {
                  break;
                }
              }
              refNode = walker.nextNode();
            }
            if (!checked) {
              matched.add(node);
            }
          }
          break;
        }
        case 'default': {
          // button[type="submit"], input[type="submit"], input[type="image"]
          const attrType = node.getAttribute('type');
          if (
            (localName === 'button' &&
              !(node.hasAttribute('type') && KEYS_INPUT_RESET.has(attrType))) ||
            (localName === 'input' &&
              node.hasAttribute('type') &&
              KEYS_INPUT_SUBMIT.has(attrType))
          ) {
            let form = node.parentNode;
            while (form) {
              if (form.localName === 'form') {
                break;
              }
              form = form.parentNode;
            }
            if (form) {
              const walker = this._createTreeWalker(form);
              let refNode = traverseNode(form, walker);
              refNode = walker.firstChild();
              while (refNode) {
                const nodeName = refNode.localName;
                const nodeAttrType = refNode.getAttribute('type');
                let m;
                if (nodeName === 'button') {
                  m = !(
                    refNode.hasAttribute('type') &&
                    KEYS_INPUT_RESET.has(nodeAttrType)
                  );
                } else if (nodeName === 'input') {
                  m =
                    refNode.hasAttribute('type') &&
                    KEYS_INPUT_SUBMIT.has(nodeAttrType);
                }
                if (m) {
                  if (refNode === node) {
                    matched.add(node);
                  }
                  break;
                }
                refNode = walker.nextNode();
              }
            }
            // input[type="checkbox"], input[type="radio"]
          } else if (
            localName === 'input' &&
            node.hasAttribute('type') &&
            node.hasAttribute('checked') &&
            KEYS_INPUT_CHECK.has(attrType)
          ) {
            matched.add(node);
            // option
          } else if (localName === 'option' && node.hasAttribute('selected')) {
            matched.add(node);
          }
          break;
        }
        case 'valid':
        case 'invalid': {
          if (KEYS_FORM_PS_VALID.has(localName)) {
            let valid;
            if (node.checkValidity()) {
              if (node.maxLength >= 0) {
                if (node.maxLength >= node.value.length) {
                  valid = true;
                }
              } else {
                valid = true;
              }
            }
            if (valid) {
              if (astName === 'valid') {
                matched.add(node);
              }
            } else if (astName === 'invalid') {
              matched.add(node);
            }
          } else if (localName === 'fieldset') {
            const walker = this._createTreeWalker(node);
            let refNode = traverseNode(node, walker);
            refNode = walker.firstChild();
            let valid;
            if (!refNode) {
              valid = true;
            } else {
              while (refNode) {
                if (KEYS_FORM_PS_VALID.has(refNode.localName)) {
                  if (refNode.checkValidity()) {
                    if (refNode.maxLength >= 0) {
                      valid = refNode.maxLength >= refNode.value.length;
                    } else {
                      valid = true;
                    }
                  } else {
                    valid = false;
                  }
                  if (!valid) {
                    break;
                  }
                }
                refNode = walker.nextNode();
              }
            }
            if (valid) {
              if (astName === 'valid') {
                matched.add(node);
              }
            } else if (astName === 'invalid') {
              matched.add(node);
            }
          }
          break;
        }
        case 'in-range':
        case 'out-of-range': {
          const attrType = node.getAttribute('type');
          if (
            localName === 'input' &&
            !(node.readonly || node.hasAttribute('readonly')) &&
            !(node.disabled || node.hasAttribute('disabled')) &&
            KEYS_INPUT_RANGE.has(attrType)
          ) {
            const flowed =
              node.validity.rangeUnderflow || node.validity.rangeOverflow;
            if (astName === 'out-of-range' && flowed) {
              matched.add(node);
            } else if (
              astName === 'in-range' &&
              !flowed &&
              (node.hasAttribute('min') ||
                node.hasAttribute('max') ||
                attrType === 'range')
            ) {
              matched.add(node);
            }
          }
          break;
        }
        case 'required':
        case 'optional': {
          let required;
          let optional;
          if (localName === 'select' || localName === 'textarea') {
            if (node.required || node.hasAttribute('required')) {
              required = true;
            } else {
              optional = true;
            }
          } else if (localName === 'input') {
            if (node.hasAttribute('type')) {
              const attrType = node.getAttribute('type');
              if (KEYS_INPUT_REQUIRED.has(attrType)) {
                if (node.required || node.hasAttribute('required')) {
                  required = true;
                } else {
                  optional = true;
                }
              } else {
                optional = true;
              }
            } else if (node.required || node.hasAttribute('required')) {
              required = true;
            } else {
              optional = true;
            }
          }
          if (astName === 'required' && required) {
            matched.add(node);
          } else if (astName === 'optional' && optional) {
            matched.add(node);
          }
          break;
        }
        case 'root': {
          if (node === this.#document.documentElement) {
            matched.add(node);
          }
          break;
        }
        case 'empty': {
          if (node.hasChildNodes()) {
            const walker = this._createTreeWalker(node, {
              force: true,
              whatToShow: SHOW_ALL
            });
            let refNode = walker.firstChild();
            let bool;
            while (refNode) {
              bool =
                refNode.nodeType !== ELEMENT_NODE &&
                refNode.nodeType !== TEXT_NODE;
              if (!bool) {
                break;
              }
              refNode = walker.nextSibling();
            }
            if (bool) {
              matched.add(node);
            }
          } else {
            matched.add(node);
          }
          break;
        }
        case 'first-child': {
          if (
            (parentNode && node === parentNode.firstElementChild) ||
            node === this.#root
          ) {
            matched.add(node);
          }
          break;
        }
        case 'last-child': {
          if (
            (parentNode && node === parentNode.lastElementChild) ||
            node === this.#root
          ) {
            matched.add(node);
          }
          break;
        }
        case 'only-child': {
          if (
            (parentNode &&
              node === parentNode.firstElementChild &&
              node === parentNode.lastElementChild) ||
            node === this.#root
          ) {
            matched.add(node);
          }
          break;
        }
        case 'defined': {
          if (node.hasAttribute('is') || localName.includes('-')) {
            if (isCustomElement(node)) {
              matched.add(node);
            }
            // NOTE: MathMLElement is not implemented in jsdom.
          } else if (
            node instanceof this.#window.HTMLElement ||
            node instanceof this.#window.SVGElement
          ) {
            matched.add(node);
          }
          break;
        }
        case 'popover-open': {
          if (node.popover && isVisible(node)) {
            matched.add(node);
          }
          break;
        }
        // Ignore :host.
        case 'host': {
          break;
        }
        // Legacy pseudo-elements.
        case 'after':
        case 'before':
        case 'first-letter':
        case 'first-line': {
          if (warn) {
            this.onError(
              generateException(
                `Unsupported pseudo-element ::${astName}`,
                NOT_SUPPORTED_ERR,
                this.#window
              )
            );
          }
          break;
        }
        // Not supported.
        case 'autofill':
        case 'blank':
        case 'buffering':
        case 'current':
        case 'fullscreen':
        case 'future':
        case 'has-slotted':
        case 'heading':
        case 'modal':
        case 'muted':
        case 'past':
        case 'paused':
        case 'picture-in-picture':
        case 'playing':
        case 'seeking':
        case 'stalled':
        case 'user-invalid':
        case 'user-valid':
        case 'volume-locked':
        case '-webkit-autofill': {
          if (warn) {
            this.onError(
              generateException(
                `Unsupported pseudo-class :${astName}`,
                NOT_SUPPORTED_ERR,
                this.#window
              )
            );
          }
          break;
        }
        default: {
          if (astName.startsWith('-webkit-')) {
            if (warn) {
              this.onError(
                generateException(
                  `Unsupported pseudo-class :${astName}`,
                  NOT_SUPPORTED_ERR,
                  this.#window
                )
              );
            }
          } else if (!forgive) {
            this.onError(
              generateException(
                `Unknown pseudo-class :${astName}`,
                SYNTAX_ERR,
                this.#window
              )
            );
          }
        }
      }
    }
    return matched;
  }

  /**
   * Evaluates the :host() pseudo-class.
   * @private
   * @param {Array.<object>} leaves - The AST leaves.
   * @param {object} host - The host element.
   * @param {object} ast - The original AST for error reporting.
   * @returns {boolean} True if matched.
   */
  _evaluateHostPseudo = (leaves, host, ast) => {
    const l = leaves.length;
    for (let i = 0; i < l; i++) {
      const leaf = leaves[i];
      if (leaf.type === COMBINATOR) {
        const css = generateCSS(ast);
        const msg = `Invalid selector ${css}`;
        this.onError(generateException(msg, SYNTAX_ERR, this.#window));
        return false;
      }
      if (!this._matchSelector(leaf, host).has(host)) {
        return false;
      }
    }
    return true;
  };

  /**
   * Evaluates the :host-context() pseudo-class.
   * @private
   * @param {Array.<object>} leaves - The AST leaves.
   * @param {object} host - The host element.
   * @param {object} ast - The original AST for error reporting.
   * @returns {boolean} True if matched.
   */
  _evaluateHostContextPseudo = (leaves, host, ast) => {
    let parent = host;
    while (parent) {
      let bool;
      const l = leaves.length;
      for (let i = 0; i < l; i++) {
        const leaf = leaves[i];
        if (leaf.type === COMBINATOR) {
          const css = generateCSS(ast);
          const msg = `Invalid selector ${css}`;
          this.onError(generateException(msg, SYNTAX_ERR, this.#window));
          return false;
        }
        bool = this._matchSelector(leaf, parent).has(parent);
        if (!bool) {
          break;
        }
      }
      if (bool) {
        return true;
      }
      parent = parent.parentNode;
    }
    return false;
  };

  /**
   * Matches shadow host pseudo-classes.
   * @private
   * @param {object} ast - The AST.
   * @param {object} node - The DocumentFragment node.
   * @returns {?object} The matched node.
   */
  _matchShadowHostPseudoClass = (ast, node) => {
    const { children: astChildren, name: astName } = ast;
    // Handle simple pseudo-class (no arguments).
    if (!Array.isArray(astChildren)) {
      if (astName === 'host') {
        return node;
      }
      const msg = `Invalid selector :${astName}`;
      return this.onError(generateException(msg, SYNTAX_ERR, this.#window));
    }
    // Handle functional pseudo-class like :host(...).
    if (astName !== 'host' && astName !== 'host-context') {
      const msg = `Invalid selector :${astName}()`;
      return this.onError(generateException(msg, SYNTAX_ERR, this.#window));
    }
    if (astChildren.length !== 1) {
      const css = generateCSS(ast);
      const msg = `Invalid selector ${css}`;
      return this.onError(generateException(msg, SYNTAX_ERR, this.#window));
    }
    const { host } = node;
    const { branches } = walkAST(astChildren[0]);
    const [branch] = branches;
    const [...leaves] = branch;
    let isMatch = false;
    if (astName === 'host') {
      isMatch = this._evaluateHostPseudo(leaves, host, ast);
      // astName === 'host-context'.
    } else {
      isMatch = this._evaluateHostContextPseudo(leaves, host, ast);
    }
    return isMatch ? node : null;
  };

  /**
   * Matches a selector for element nodes.
   * @private
   * @param {object} ast - The AST.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchSelectorForElement = (ast, node, opt = {}) => {
    const { type: astType } = ast;
    const astName = unescapeSelector(ast.name);
    const matched = new Set();
    switch (astType) {
      case ATTR_SELECTOR: {
        if (matchAttributeSelector(ast, node, opt)) {
          matched.add(node);
        }
        break;
      }
      case ID_SELECTOR: {
        if (node.id === astName) {
          matched.add(node);
        }
        break;
      }
      case CLASS_SELECTOR: {
        if (node.classList.contains(astName)) {
          matched.add(node);
        }
        break;
      }
      case PS_CLASS_SELECTOR: {
        return this._matchPseudoClassSelector(ast, node, opt);
      }
      case TYPE_SELECTOR: {
        if (matchTypeSelector(ast, node, opt)) {
          matched.add(node);
        }
        break;
      }
      // PS_ELEMENT_SELECTOR is handled by default.
      default: {
        try {
          if (opt.check) {
            const css = generateCSS(ast);
            this.#pseudoElement.push(css);
            matched.add(node);
          } else {
            matchPseudoElementSelector(astName, astType, opt);
          }
        } catch (e) {
          this.onError(e);
        }
      }
    }
    return matched;
  };

  /**
   * Matches a selector for a shadow root.
   * @private
   * @param {object} ast - The AST.
   * @param {object} node - The DocumentFragment node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchSelectorForShadowRoot = (ast, node, opt = {}) => {
    const { name: astName } = ast;
    if (KEYS_LOGICAL.has(astName)) {
      opt.isShadowRoot = true;
      return this._matchPseudoClassSelector(ast, node, opt);
    }
    const matched = new Set();
    if (astName === 'host' || astName === 'host-context') {
      const res = this._matchShadowHostPseudoClass(ast, node, opt);
      if (res) {
        this.#verifyShadowHost = true;
        matched.add(res);
      }
    }
    return matched;
  };

  /**
   * Matches a selector.
   * @private
   * @param {object} ast - The AST.
   * @param {object} node - The Document, DocumentFragment, or Element node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchSelector = (ast, node, opt = {}) => {
    if (node.nodeType === ELEMENT_NODE) {
      return this._matchSelectorForElement(ast, node, opt);
    }
    if (
      this.#shadow &&
      node.nodeType === DOCUMENT_FRAGMENT_NODE &&
      ast.type === PS_CLASS_SELECTOR
    ) {
      return this._matchSelectorForShadowRoot(ast, node, opt);
    }
    return new Set();
  };

  /**
   * Matches leaves.
   * @private
   * @param {Array.<object>} leaves - The AST leaves.
   * @param {object} node - The node.
   * @param {object} [opt] - Options.
   * @returns {boolean} The result.
   */
  _matchLeaves = (leaves, node, opt = {}) => {
    const results = this.#invalidate ? this.#invalidateResults : this.#results;
    let result = results.get(leaves);
    if (result && result.has(node)) {
      const { matched } = result.get(node);
      return matched;
    }
    let cacheable = true;
    if (node.nodeType === ELEMENT_NODE && KEYS_FORM.has(node.localName)) {
      cacheable = false;
    }
    let bool;
    const l = leaves.length;
    for (let i = 0; i < l; i++) {
      const leaf = leaves[i];
      switch (leaf.type) {
        case ATTR_SELECTOR:
        case ID_SELECTOR: {
          cacheable = false;
          break;
        }
        case PS_CLASS_SELECTOR: {
          if (KEYS_PS_UNCACHE.has(leaf.name)) {
            cacheable = false;
          }
          break;
        }
        default: {
          // No action needed for other types.
        }
      }
      bool = this._matchSelector(leaf, node, opt).has(node);
      if (!bool) {
        break;
      }
    }
    if (cacheable) {
      if (!result) {
        result = new WeakMap();
      }
      result.set(node, {
        matched: bool
      });
      results.set(leaves, result);
    }
    return bool;
  };

  /**
   * Traverses all descendant nodes and collects matches.
   * @private
   * @param {object} baseNode - The base Element node or Element.shadowRoot.
   * @param {Array.<object>} leaves - The AST leaves.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _traverseAllDescendants = (baseNode, leaves, opt = {}) => {
    const walker = this._createTreeWalker(baseNode);
    traverseNode(baseNode, walker);
    let currentNode = walker.firstChild();
    const nodes = new Set();
    while (currentNode) {
      if (this._matchLeaves(leaves, currentNode, opt)) {
        nodes.add(currentNode);
      }
      currentNode = walker.nextNode();
    }
    return nodes;
  };

  /**
   * Finds descendant nodes.
   * @private
   * @param {Array.<object>} leaves - The AST leaves.
   * @param {object} baseNode - The base Element node or Element.shadowRoot.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _findDescendantNodes = (leaves, baseNode, opt = {}) => {
    const [leaf, ...filterLeaves] = leaves;
    const { type: leafType } = leaf;
    switch (leafType) {
      case ID_SELECTOR: {
        const canUseGetElementById =
          !this.#shadow &&
          baseNode.nodeType === ELEMENT_NODE &&
          this.#root.nodeType !== ELEMENT_NODE;
        if (canUseGetElementById) {
          const leafName = unescapeSelector(leaf.name);
          const nodes = new Set();
          const foundNode = this.#root.getElementById(leafName);
          if (
            foundNode &&
            foundNode !== baseNode &&
            baseNode.contains(foundNode)
          ) {
            const isCompoundSelector = filterLeaves.length > 0;
            if (
              !isCompoundSelector ||
              this._matchLeaves(filterLeaves, foundNode, opt)
            ) {
              nodes.add(foundNode);
            }
          }
          return nodes;
        }
        // Fallback to default traversal if fast path is not applicable.
        return this._traverseAllDescendants(baseNode, leaves, opt);
      }
      case PS_ELEMENT_SELECTOR: {
        const leafName = unescapeSelector(leaf.name);
        matchPseudoElementSelector(leafName, leafType, opt);
        return new Set();
      }
      default: {
        return this._traverseAllDescendants(baseNode, leaves, opt);
      }
    }
  };

  /**
   * Matches the descendant combinator ' '.
   * @private
   * @param {object} twig - The twig object.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchDescendantCombinator = (twig, node, opt = {}) => {
    const { leaves } = twig;
    const { parentNode } = node;
    const { dir } = opt;
    if (dir === DIR_NEXT) {
      return this._findDescendantNodes(leaves, node, opt);
    }
    // DIR_PREV
    const ancestors = [];
    let refNode = parentNode;
    while (refNode) {
      if (this._matchLeaves(leaves, refNode, opt)) {
        ancestors.push(refNode);
      }
      refNode = refNode.parentNode;
    }
    if (ancestors.length) {
      // Reverse to maintain document order.
      return new Set(ancestors.reverse());
    }
    return new Set();
  };

  /**
   * Matches the child combinator '>'.
   * @private
   * @param {object} twig - The twig object.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchChildCombinator = (twig, node, opt = {}) => {
    const { leaves } = twig;
    const { dir } = opt;
    const { parentNode } = node;
    const matched = new Set();
    if (dir === DIR_NEXT) {
      let refNode = node.firstElementChild;
      while (refNode) {
        if (this._matchLeaves(leaves, refNode, opt)) {
          matched.add(refNode);
        }
        refNode = refNode.nextElementSibling;
      }
    } else {
      // DIR_PREV
      if (parentNode && this._matchLeaves(leaves, parentNode, opt)) {
        matched.add(parentNode);
      }
    }
    return matched;
  };

  /**
   * Matches the adjacent sibling combinator '+'.
   * @private
   * @param {object} twig - The twig object.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchAdjacentSiblingCombinator = (twig, node, opt = {}) => {
    const { leaves } = twig;
    const { dir } = opt;
    const matched = new Set();
    const refNode =
      dir === DIR_NEXT ? node.nextElementSibling : node.previousElementSibling;
    if (refNode && this._matchLeaves(leaves, refNode, opt)) {
      matched.add(refNode);
    }
    return matched;
  };

  /**
   * Matches the general sibling combinator '~'.
   * @private
   * @param {object} twig - The twig object.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchGeneralSiblingCombinator = (twig, node, opt = {}) => {
    const { leaves } = twig;
    const { dir } = opt;
    const matched = new Set();
    let refNode =
      dir === DIR_NEXT ? node.nextElementSibling : node.previousElementSibling;
    while (refNode) {
      if (this._matchLeaves(leaves, refNode, opt)) {
        matched.add(refNode);
      }
      refNode =
        dir === DIR_NEXT
          ? refNode.nextElementSibling
          : refNode.previousElementSibling;
    }
    return matched;
  };

  /**
   * Matches a combinator.
   * @private
   * @param {object} twig - The twig object.
   * @param {object} node - The Element node.
   * @param {object} [opt] - Options.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  _matchCombinator = (twig, node, opt = {}) => {
    const {
      combo: { name: comboName }
    } = twig;
    switch (comboName) {
      case '+': {
        return this._matchAdjacentSiblingCombinator(twig, node, opt);
      }
      case '~': {
        return this._matchGeneralSiblingCombinator(twig, node, opt);
      }
      case '>': {
        return this._matchChildCombinator(twig, node, opt);
      }
      case ' ':
      default: {
        return this._matchDescendantCombinator(twig, node, opt);
      }
    }
  };

  /**
   * Traverses with a TreeWalker and collects nodes matching the leaves.
   * @private
   * @param {TreeWalker} walker - The TreeWalker instance to use.
   * @param {Array} leaves - The AST leaves to match against.
   * @param {object} options - Traversal options.
   * @param {Node} options.startNode - The node to start traversal from.
   * @param {string} options.targetType - The type of target ('all' or 'first').
   * @param {Node} [options.boundaryNode] - The node to stop traversal at.
   * @param {boolean} [options.force] - Force traversal to the next node.
   * @returns {Array.<Node>} An array of matched nodes.
   */
  _traverseAndCollectNodes = (walker, leaves, options) => {
    const { boundaryNode, force, startNode, targetType } = options;
    const collectedNodes = [];
    let currentNode = traverseNode(startNode, walker, !!force);
    if (!currentNode) {
      return [];
    }
    // Adjust starting node.
    if (currentNode.nodeType !== ELEMENT_NODE) {
      currentNode = walker.nextNode();
    } else if (currentNode === startNode && currentNode !== this.#root) {
      currentNode = walker.nextNode();
    }
    const matchOpt = {
      warn: this.#warn
    };
    while (currentNode) {
      // Stop when we reach the boundary.
      if (boundaryNode) {
        if (currentNode === boundaryNode) {
          break;
        } else if (
          targetType === TARGET_ALL &&
          !boundaryNode.contains(currentNode)
        ) {
          break;
        }
      }
      if (
        this._matchLeaves(leaves, currentNode, matchOpt) &&
        currentNode !== this.#node
      ) {
        collectedNodes.push(currentNode);
        // Stop after the first match if not collecting all.
        if (targetType !== TARGET_ALL) {
          break;
        }
      }
      currentNode = walker.nextNode();
    }
    return collectedNodes;
  };

  /**
   * Finds matched node(s) preceding this.#node.
   * @private
   * @param {Array.<object>} leaves - The AST leaves.
   * @param {object} node - The node to start from.
   * @param {object} opt - Options.
   * @param {boolean} [opt.force] - If true, traverses only to the next node.
   * @param {string} [opt.targetType] - The target type.
   * @returns {Array.<object>} A collection of matched nodes.
   */
  _findPrecede = (leaves, node, opt = {}) => {
    const { force, targetType } = opt;
    if (!this.#rootWalker) {
      this.#rootWalker = this._createTreeWalker(this.#root);
    }
    return this._traverseAndCollectNodes(this.#rootWalker, leaves, {
      force,
      targetType,
      boundaryNode: this.#node,
      startNode: node
    });
  };

  /**
   * Finds matched node(s) in #nodeWalker.
   * @private
   * @param {Array.<object>} leaves - The AST leaves.
   * @param {object} node - The node to start from.
   * @param {object} opt - Options.
   * @param {boolean} [opt.precede] - If true, finds preceding nodes.
   * @returns {Array.<object>} A collection of matched nodes.
   */
  _findNodeWalker = (leaves, node, opt = {}) => {
    const { precede, ...traversalOpts } = opt;
    if (precede) {
      const precedeNodes = this._findPrecede(leaves, this.#root, opt);
      if (precedeNodes.length) {
        return precedeNodes;
      }
    }
    if (!this.#nodeWalker) {
      this.#nodeWalker = this._createTreeWalker(this.#node);
    }
    return this._traverseAndCollectNodes(this.#nodeWalker, leaves, {
      startNode: node,
      ...traversalOpts
    });
  };

  /**
   * Matches the node itself.
   * @private
   * @param {Array} leaves - The AST leaves.
   * @param {boolean} check - Indicates if running in internal check().
   * @returns {Array} An array containing [nodes, filtered, pseudoElement].
   */
  _matchSelf = (leaves, check = false) => {
    const options = { check, warn: this.#warn };
    const matched = this._matchLeaves(leaves, this.#node, options);
    const nodes = matched ? [this.#node] : [];
    return [nodes, matched, this.#pseudoElement];
  };

  /**
   * Finds lineal nodes (self and ancestors).
   * @private
   * @param {Array} leaves - The AST leaves.
   * @param {object} opt - Options.
   * @returns {Array} An array containing [nodes, filtered].
   */
  _findLineal = (leaves, opt) => {
    const { complex } = opt;
    const nodes = [];
    const options = { warn: this.#warn };
    const selfMatched = this._matchLeaves(leaves, this.#node, options);
    if (selfMatched) {
      nodes.push(this.#node);
    }
    if (!selfMatched || complex) {
      let currentNode = this.#node.parentNode;
      while (currentNode) {
        if (this._matchLeaves(leaves, currentNode, options)) {
          nodes.push(currentNode);
        }
        currentNode = currentNode.parentNode;
      }
    }
    const filtered = nodes.length > 0;
    return [nodes, filtered];
  };

  /**
   * Finds entry nodes for pseudo-element selectors.
   * @private
   * @param {object} leaf - The pseudo-element leaf from the AST.
   * @param {Array.<object>} filterLeaves - Leaves for compound selectors.
   * @param {string} targetType - The type of target to find.
   * @returns {object} The result { nodes, filtered, pending }.
   */
  _findEntryNodesForPseudoElement = (leaf, filterLeaves, targetType) => {
    let nodes = [];
    let filtered = false;
    if (targetType === TARGET_SELF && this.#check) {
      const css = generateCSS(leaf);
      this.#pseudoElement.push(css);
      if (filterLeaves.length) {
        [nodes, filtered] = this._matchSelf(filterLeaves, this.#check);
      } else {
        nodes.push(this.#node);
        filtered = true;
      }
    } else {
      matchPseudoElementSelector(leaf.name, leaf.type, { warn: this.#warn });
    }
    return { nodes, filtered, pending: false };
  };

  /**
   * Finds entry nodes for ID selectors.
   * @private
   * @param {object} twig - The current twig from the AST branch.
   * @param {string} targetType - The type of target to find.
   * @param {object} opt - Additional options for finding nodes.
   * @returns {object} The result { nodes, filtered, pending }.
   */
  _findEntryNodesForId = (twig, targetType, opt) => {
    const { leaves } = twig;
    const [leaf, ...filterLeaves] = leaves;
    const { complex, precede } = opt;
    let nodes = [];
    let filtered = false;
    if (targetType === TARGET_SELF) {
      [nodes, filtered] = this._matchSelf(leaves);
    } else if (targetType === TARGET_LINEAL) {
      [nodes, filtered] = this._findLineal(leaves, { complex });
    } else if (
      targetType === TARGET_FIRST &&
      this.#root.nodeType !== ELEMENT_NODE
    ) {
      const node = this.#root.getElementById(leaf.name);
      if (node) {
        if (filterLeaves.length) {
          if (this._matchLeaves(filterLeaves, node, { warn: this.#warn })) {
            nodes.push(node);
            filtered = true;
          }
        } else {
          nodes.push(node);
          filtered = true;
        }
      }
    } else {
      nodes = this._findNodeWalker(leaves, this.#node, { precede, targetType });
      filtered = nodes.length > 0;
    }
    return { nodes, filtered, pending: false };
  };

  /**
   * Finds entry nodes for class selectors.
   * @private
   * @param {Array.<object>} leaves - The AST leaves for the selector.
   * @param {string} targetType - The type of target to find.
   * @param {object} opt - Additional options for finding nodes.
   * @returns {object} The result { nodes, filtered, pending }.
   */
  _findEntryNodesForClass = (leaves, targetType, opt) => {
    const { complex, precede } = opt;
    let nodes = [];
    let filtered = false;
    if (targetType === TARGET_SELF) {
      [nodes, filtered] = this._matchSelf(leaves);
    } else if (targetType === TARGET_LINEAL) {
      [nodes, filtered] = this._findLineal(leaves, { complex });
    } else {
      nodes = this._findNodeWalker(leaves, this.#node, { precede, targetType });
      filtered = nodes.length > 0;
    }
    return { nodes, filtered, pending: false };
  };

  /**
   * Finds entry nodes for type selectors.
   * @private
   * @param {Array.<object>} leaves - The AST leaves for the selector.
   * @param {string} targetType - The type of target to find.
   * @param {object} opt - Additional options for finding nodes.
   * @returns {object} The result { nodes, filtered, pending }.
   */
  _findEntryNodesForType = (leaves, targetType, opt) => {
    const { complex, precede } = opt;
    let nodes = [];
    let filtered = false;
    if (targetType === TARGET_SELF) {
      [nodes, filtered] = this._matchSelf(leaves);
    } else if (targetType === TARGET_LINEAL) {
      [nodes, filtered] = this._findLineal(leaves, { complex });
    } else {
      nodes = this._findNodeWalker(leaves, this.#node, { precede, targetType });
      filtered = nodes.length > 0;
    }
    return { nodes, filtered, pending: false };
  };

  /**
   * Finds entry nodes for other selector types (default case).
   * @private
   * @param {object} twig - The current twig from the AST branch.
   * @param {string} targetType - The type of target to find.
   * @param {object} opt - Additional options for finding nodes.
   * @returns {object} The result { nodes, filtered, pending }.
   */
  _findEntryNodesForOther = (twig, targetType, opt) => {
    const { leaves } = twig;
    const [leaf, ...filterLeaves] = leaves;
    const { complex, precede } = opt;
    let nodes = [];
    let filtered = false;
    let pending = false;
    if (targetType !== TARGET_LINEAL && /host(?:-context)?/.test(leaf.name)) {
      let shadowRoot = null;
      if (this.#shadow && this.#node.nodeType === DOCUMENT_FRAGMENT_NODE) {
        shadowRoot = this._matchShadowHostPseudoClass(leaf, this.#node);
      } else if (filterLeaves.length && this.#node.nodeType === ELEMENT_NODE) {
        shadowRoot = this._matchShadowHostPseudoClass(
          leaf,
          this.#node.shadowRoot
        );
      }
      if (shadowRoot) {
        let bool = true;
        const l = filterLeaves.length;
        for (let i = 0; i < l; i++) {
          const filterLeaf = filterLeaves[i];
          switch (filterLeaf.name) {
            case 'host':
            case 'host-context': {
              const matchedNode = this._matchShadowHostPseudoClass(
                filterLeaf,
                shadowRoot
              );
              bool = matchedNode === shadowRoot;
              break;
            }
            case 'has': {
              bool = this._matchPseudoClassSelector(
                filterLeaf,
                shadowRoot,
                {}
              ).has(shadowRoot);
              break;
            }
            default: {
              bool = false;
            }
          }
          if (!bool) {
            break;
          }
        }
        if (bool) {
          nodes.push(shadowRoot);
          filtered = true;
        }
      }
    } else if (targetType === TARGET_SELF) {
      [nodes, filtered] = this._matchSelf(leaves);
    } else if (targetType === TARGET_LINEAL) {
      [nodes, filtered] = this._findLineal(leaves, { complex });
    } else if (targetType === TARGET_FIRST) {
      nodes = this._findNodeWalker(leaves, this.#node, { precede, targetType });
      filtered = nodes.length > 0;
    } else {
      pending = true;
    }
    return { nodes, filtered, pending };
  };

  /**
   * Finds entry nodes.
   * @private
   * @param {object} twig - The twig object.
   * @param {string} targetType - The target type.
   * @param {object} [opt] - Options.
   * @param {boolean} [opt.complex] - If true, the selector is complex.
   * @param {string} [opt.dir] - The find direction.
   * @returns {object} An object with nodes and their state.
   */
  _findEntryNodes = (twig, targetType, opt = {}) => {
    const { leaves } = twig;
    const [leaf, ...filterLeaves] = leaves;
    const { complex = false, dir = DIR_PREV } = opt;
    const precede =
      dir === DIR_NEXT &&
      this.#node.nodeType === ELEMENT_NODE &&
      this.#node !== this.#root;
    let result;
    switch (leaf.type) {
      case PS_ELEMENT_SELECTOR: {
        result = this._findEntryNodesForPseudoElement(
          leaf,
          filterLeaves,
          targetType
        );
        break;
      }
      case ID_SELECTOR: {
        result = this._findEntryNodesForId(twig, targetType, {
          complex,
          precede
        });
        break;
      }
      case CLASS_SELECTOR: {
        result = this._findEntryNodesForClass(leaves, targetType, {
          complex,
          precede
        });
        break;
      }
      case TYPE_SELECTOR: {
        result = this._findEntryNodesForType(leaves, targetType, {
          complex,
          precede
        });
        break;
      }
      default: {
        result = this._findEntryNodesForOther(twig, targetType, {
          complex,
          precede
        });
      }
    }
    return {
      compound: filterLeaves.length > 0,
      filtered: result.filtered,
      nodes: result.nodes,
      pending: result.pending
    };
  };

  /**
   * Determines the direction and starting twig for a selector branch.
   * @private
   * @param {Array.<object>} branch - The AST branch.
   * @param {string} targetType - The type of target to find.
   * @returns {object} An object with the direction and starting twig.
   */
  _determineTraversalStrategy = (branch, targetType) => {
    const branchLen = branch.length;
    const firstTwig = branch[0];
    const lastTwig = branch[branchLen - 1];
    if (branchLen === 1) {
      return { dir: DIR_PREV, twig: firstTwig };
    }
    // Complex selector (branchLen > 1).
    const {
      leaves: [{ name: firstName, type: firstType }]
    } = firstTwig;
    const {
      leaves: [{ name: lastName, type: lastType }]
    } = lastTwig;
    const { combo: firstCombo } = firstTwig;
    if (
      this.#selector.includes(':scope') ||
      lastType === PS_ELEMENT_SELECTOR ||
      lastType === ID_SELECTOR
    ) {
      return { dir: DIR_PREV, twig: lastTwig };
    }
    if (firstType === ID_SELECTOR) {
      return { dir: DIR_NEXT, twig: firstTwig };
    }
    if (firstName === '*' && firstType === TYPE_SELECTOR) {
      return { dir: DIR_PREV, twig: lastTwig };
    }
    if (lastName === '*' && lastType === TYPE_SELECTOR) {
      return { dir: DIR_NEXT, twig: firstTwig };
    }
    if (branchLen === 2) {
      if (targetType === TARGET_FIRST) {
        return { dir: DIR_PREV, twig: lastTwig };
      }
      const { name: comboName } = firstCombo;
      if (comboName === '+' || comboName === '~') {
        return { dir: DIR_PREV, twig: lastTwig };
      }
    }
    // Default strategy for complex selectors.
    return { dir: DIR_NEXT, twig: firstTwig };
  };

  /**
   * Processes pending items not resolved with a direct strategy.
   * @private
   * @param {Set.<Map>} pendingItems - The set of pending items.
   */
  _processPendingItems = pendingItems => {
    if (!pendingItems.size) {
      return;
    }
    if (!this.#rootWalker) {
      this.#rootWalker = this._createTreeWalker(this.#root);
    }
    const isScopedContext =
      this.#node !== this.#root && this.#node.nodeType === ELEMENT_NODE;
    const walker = this.#rootWalker;
    let node = this.#root;
    if (isScopedContext) {
      node = this.#node;
    }
    let nextNode = traverseNode(node, walker);
    while (nextNode) {
      const isWithinScope =
        this.#node.nodeType !== ELEMENT_NODE ||
        nextNode === this.#node ||
        this.#node.contains(nextNode);
      if (isWithinScope) {
        for (const pendingItem of pendingItems) {
          const { leaves } = pendingItem.get('twig');
          if (this._matchLeaves(leaves, nextNode, { warn: this.#warn })) {
            const index = pendingItem.get('index');
            this.#ast[index].filtered = true;
            this.#ast[index].find = true;
            this.#nodes[index].push(nextNode);
          }
        }
      } else if (isScopedContext) {
        break;
      }
      nextNode = walker.nextNode();
    }
  };

  /**
   * Collects nodes.
   * @private
   * @param {string} targetType - The target type.
   * @returns {Array.<Array.<object>>} An array containing the AST and nodes.
   */
  _collectNodes = targetType => {
    const ast = this.#ast.values();
    if (targetType === TARGET_ALL || targetType === TARGET_FIRST) {
      const pendingItems = new Set();
      let i = 0;
      for (const { branch } of ast) {
        const complex = branch.length > 1;
        const { dir, twig } = this._determineTraversalStrategy(
          branch,
          targetType
        );
        const { compound, filtered, nodes, pending } = this._findEntryNodes(
          twig,
          targetType,
          { complex, dir }
        );
        if (nodes.length) {
          this.#ast[i].find = true;
          this.#nodes[i] = nodes;
        } else if (pending) {
          pendingItems.add(
            new Map([
              ['index', i],
              ['twig', twig]
            ])
          );
        }
        this.#ast[i].dir = dir;
        this.#ast[i].filtered = filtered || !compound;
        i++;
      }
      this._processPendingItems(pendingItems);
    } else {
      let i = 0;
      for (const { branch } of ast) {
        const twig = branch[branch.length - 1];
        const complex = branch.length > 1;
        const dir = DIR_PREV;
        const { compound, filtered, nodes } = this._findEntryNodes(
          twig,
          targetType,
          { complex, dir }
        );
        if (nodes.length) {
          this.#ast[i].find = true;
          this.#nodes[i] = nodes;
        }
        this.#ast[i].dir = dir;
        this.#ast[i].filtered = filtered || !compound;
        i++;
      }
    }
    return [this.#ast, this.#nodes];
  };

  /**
   * Gets combined nodes.
   * @private
   * @param {object} twig - The twig object.
   * @param {object} nodes - A collection of nodes.
   * @param {string} dir - The direction.
   * @returns {Array.<object>} A collection of matched nodes.
   */
  _getCombinedNodes = (twig, nodes, dir) => {
    const arr = [];
    const options = {
      dir,
      warn: this.#warn
    };
    for (const node of nodes) {
      const matched = this._matchCombinator(twig, node, options);
      if (matched.size) {
        arr.push(...matched);
      }
    }
    return arr;
  };

  /**
   * Matches a node in the 'next' direction.
   * @private
   * @param {Array} branch - The branch.
   * @param {Set.<object>} nodes - A collection of Element nodes.
   * @param {object} opt - Options.
   * @param {object} opt.combo - The combo object.
   * @param {number} opt.index - The index.
   * @returns {?object} The matched node.
   */
  _matchNodeNext = (branch, nodes, opt) => {
    const { combo, index } = opt;
    const { combo: nextCombo, leaves } = branch[index];
    const twig = {
      combo,
      leaves
    };
    const nextNodes = new Set(this._getCombinedNodes(twig, nodes, DIR_NEXT));
    if (nextNodes.size) {
      if (index === branch.length - 1) {
        const [nextNode] = sortNodes(nextNodes);
        return nextNode;
      }
      return this._matchNodeNext(branch, nextNodes, {
        combo: nextCombo,
        index: index + 1
      });
    }
    return null;
  };

  /**
   * Matches a node in the 'previous' direction.
   * @private
   * @param {Array} branch - The branch.
   * @param {object} node - The Element node.
   * @param {object} opt - Options.
   * @param {number} opt.index - The index.
   * @returns {?object} The node.
   */
  _matchNodePrev = (branch, node, opt) => {
    const { index } = opt;
    const twig = branch[index];
    const nodes = new Set([node]);
    const nextNodes = new Set(this._getCombinedNodes(twig, nodes, DIR_PREV));
    if (nextNodes.size) {
      if (index === 0) {
        return node;
      }
      let matched;
      for (const nextNode of nextNodes) {
        matched = this._matchNodePrev(branch, nextNode, {
          index: index - 1
        });
        if (matched) {
          break;
        }
      }
      if (matched) {
        return node;
      }
    }
    return null;
  };

  /**
   * Processes a complex selector branch to find all matching nodes.
   * @private
   * @param {Array} branch - The selector branch from the AST.
   * @param {Array} entryNodes - The initial set of nodes to start from.
   * @param {string} dir - The direction of traversal ('next' or 'prev').
   * @returns {Set.<object>} A set of all matched nodes.
   */
  _processComplexBranchAll = (branch, entryNodes, dir) => {
    const matchedNodes = new Set();
    const branchLen = branch.length;
    const lastIndex = branchLen - 1;

    if (dir === DIR_NEXT) {
      const { combo: firstCombo } = branch[0];
      for (const node of entryNodes) {
        let combo = firstCombo;
        let nextNodes = new Set([node]);
        for (let j = 1; j < branchLen; j++) {
          const { combo: nextCombo, leaves } = branch[j];
          const twig = { combo, leaves };
          const nodesArr = this._getCombinedNodes(twig, nextNodes, dir);
          if (nodesArr.length) {
            if (j === lastIndex) {
              for (const nextNode of nodesArr) {
                matchedNodes.add(nextNode);
              }
            }
            combo = nextCombo;
            nextNodes = new Set(nodesArr);
          } else {
            // No further matches down this path.
            nextNodes.clear();
            break;
          }
        }
      }
      // DIR_PREV
    } else {
      for (const node of entryNodes) {
        let nextNodes = new Set([node]);
        for (let j = lastIndex - 1; j >= 0; j--) {
          const twig = branch[j];
          const nodesArr = this._getCombinedNodes(twig, nextNodes, dir);
          if (nodesArr.length) {
            // The entry node is the final match
            if (j === 0) {
              matchedNodes.add(node);
            }
            nextNodes = new Set(nodesArr);
          } else {
            // No further matches down this path.
            nextNodes.clear();
            break;
          }
        }
      }
    }
    return matchedNodes;
  };

  /**
   * Find a node contained by this.#node.
   * @private
   * @param {Array} nodesArr - The set of nodes to find from.
   * @returns {?object} The matched node, or null.
   */
  _findChildNodeContainedByNode = nodesArr => {
    let matchedNode = null;
    if (Array.isArray(nodesArr)) {
      const l = nodesArr.length;
      for (let i = 0; i < l; i++) {
        const node = nodesArr[i];
        if (this.#node.contains(node)) {
          matchedNode = node;
          break;
        }
      }
    }
    return matchedNode;
  };

  /**
   * Processes a complex selector branch to find the first matching node.
   * @private
   * @param {Array} branch - The selector branch from the AST.
   * @param {Array} entryNodes - The initial set of nodes to start from.
   * @param {string} dir - The direction of traversal ('next' or 'prev').
   * @param {string} targetType - The type of search (e.g., 'first').
   * @returns {?object} The first matched node, or null.
   */
  _processComplexBranchFirst = (branch, entryNodes, dir, targetType) => {
    const branchLen = branch.length;
    const lastIndex = branchLen - 1;
    // DIR_NEXT logic for finding the first match.
    if (dir === DIR_NEXT) {
      const { combo: entryCombo } = branch[0];
      for (const node of entryNodes) {
        const matchedNode = this._matchNodeNext(branch, new Set([node]), {
          combo: entryCombo,
          index: 1
        });
        if (matchedNode) {
          if (this.#node.nodeType === ELEMENT_NODE) {
            if (
              matchedNode !== this.#node &&
              this.#node.contains(matchedNode)
            ) {
              return matchedNode;
            }
          } else {
            return matchedNode;
          }
        }
      }
      // Fallback logic if no direct match found.
      const { leaves: entryLeaves } = branch[0];
      const [entryNode] = entryNodes;
      if (this.#node.contains(entryNode)) {
        let [refNode] = this._findNodeWalker(entryLeaves, entryNode, {
          targetType
        });
        while (refNode) {
          const matchedNode = this._matchNodeNext(branch, new Set([refNode]), {
            combo: entryCombo,
            index: 1
          });
          if (matchedNode) {
            if (this.#node.nodeType === ELEMENT_NODE) {
              if (
                matchedNode !== this.#node &&
                this.#node.contains(matchedNode)
              ) {
                return matchedNode;
              }
            } else {
              return matchedNode;
            }
          }
          [refNode] = this._findNodeWalker(entryLeaves, refNode, {
            targetType,
            force: true
          });
        }
      } else {
        const { combo: firstCombo } = branch[0];
        let combo = firstCombo;
        let nextNodes = new Set([entryNode]);
        for (let j = 1; j < branchLen; j++) {
          const { combo: nextCombo, leaves } = branch[j];
          const twig = { combo, leaves };
          const nodesArr = this._getCombinedNodes(twig, nextNodes, dir);
          if (nodesArr.length) {
            if (j === lastIndex) {
              return this._findChildNodeContainedByNode(nodesArr);
            }
            combo = nextCombo;
            nextNodes = new Set(nodesArr);
          } else {
            break;
          }
        }
      }
      // DIR_PREV logic for finding the first match.
    } else {
      for (const node of entryNodes) {
        const matchedNode = this._matchNodePrev(branch, node, {
          index: lastIndex - 1
        });
        if (matchedNode) {
          return matchedNode;
        }
      }
      // Fallback for TARGET_FIRST.
      if (targetType === TARGET_FIRST) {
        const { leaves: entryLeaves } = branch[lastIndex];
        const [entryNode] = entryNodes;
        let [refNode] = this._findNodeWalker(entryLeaves, entryNode, {
          targetType
        });
        while (refNode) {
          const matchedNode = this._matchNodePrev(branch, refNode, {
            index: lastIndex - 1
          });
          if (matchedNode) {
            return refNode;
          }
          [refNode] = this._findNodeWalker(entryLeaves, refNode, {
            targetType,
            force: true
          });
        }
      }
    }
    return null;
  };

  /**
   * Finds matched nodes.
   * @param {string} targetType - The target type.
   * @returns {Set.<object>} A collection of matched nodes.
   */
  find = targetType => {
    const [[...branches], collectedNodes] = this._collectNodes(targetType);
    const l = branches.length;
    let sort = false;
    let nodes = new Set();
    for (let i = 0; i < l; i++) {
      const { branch, dir, find } = branches[i];
      if (!branch.length || !find) {
        continue;
      }
      const entryNodes = collectedNodes[i];
      const lastIndex = branch.length - 1;
      // Handle simple selectors (no combinators).
      if (lastIndex === 0) {
        if (
          (targetType === TARGET_ALL || targetType === TARGET_FIRST) &&
          this.#node.nodeType === ELEMENT_NODE
        ) {
          for (const node of entryNodes) {
            if (node !== this.#node && this.#node.contains(node)) {
              nodes.add(node);
              if (targetType === TARGET_FIRST) {
                break;
              }
            }
          }
        } else if (targetType === TARGET_ALL) {
          if (nodes.size) {
            for (const node of entryNodes) {
              nodes.add(node);
            }
            sort = true;
          } else {
            nodes = new Set(entryNodes);
          }
        } else {
          if (entryNodes.length) {
            nodes.add(entryNodes[0]);
          }
        }
        // Handle complex selectors.
      } else {
        if (targetType === TARGET_ALL) {
          const newNodes = this._processComplexBranchAll(
            branch,
            entryNodes,
            dir
          );
          if (nodes.size) {
            for (const newNode of newNodes) {
              nodes.add(newNode);
            }
            sort = true;
          } else {
            nodes = newNodes;
          }
        } else {
          const matchedNode = this._processComplexBranchFirst(
            branch,
            entryNodes,
            dir,
            targetType
          );
          if (matchedNode) {
            nodes.add(matchedNode);
          }
        }
      }
    }
    if (this.#check) {
      const match = !!nodes.size;
      let pseudoElement;
      if (this.#pseudoElement.length) {
        pseudoElement = this.#pseudoElement.join('');
      } else {
        pseudoElement = null;
      }
      return { match, pseudoElement };
    }
    if (targetType === TARGET_FIRST || targetType === TARGET_ALL) {
      nodes.delete(this.#node);
    }
    if ((sort || targetType === TARGET_FIRST) && nodes.size > 1) {
      return new Set(sortNodes(nodes));
    }
    return nodes;
  };
}
