source: frontend/node_modules/webpack/lib/util/StackedMap.js@ cdcff72

/*
        MIT License http://www.opensource.org/licenses/mit-license.php
        Author Tobias Koppers @sokra
*/

"use strict";

const TOMBSTONE = Symbol("tombstone");
const UNDEFINED_MARKER = Symbol("undefined");

/**
 * Public cell value exposed by `StackedMap`, where `undefined` is preserved as
 * a valid stored result.
 * @template T
 * @typedef {T | undefined} Cell<T>
 */

/**
 * Internal cell value used to distinguish deleted entries and explicit
 * `undefined` assignments while traversing stacked scopes.
 * @template T
 * @typedef {T | typeof TOMBSTONE | typeof UNDEFINED_MARKER} InternalCell<T>
 */

/**
 * Converts an internal key/value pair into the external representation returned
 * by iteration helpers.
 * @template K
 * @template V
 * @param {[K, InternalCell<V>]} pair the internal cell
 * @returns {[K, Cell<V>]} its “safe” representation
 */
const extractPair = (pair) => {
        const key = pair[0];
        const val = pair[1];
        if (val === UNDEFINED_MARKER || val === TOMBSTONE) {
                return [key, undefined];
        }
        return /** @type {[K, Cell<V>]} */ (pair);
};

/**
 * Layered map that supports child scopes while memoizing lookups from parent
 * scopes into the current layer.
 * @template K
 * @template V
 */
class StackedMap {
        /**
         * Creates a new map layer on top of an optional parent stack.
         * @param {Map<K, InternalCell<V>>[]=} parentStack an optional parent
         */
        constructor(parentStack) {
                /** @type {Map<K, InternalCell<V>>} */
                this.map = new Map();
                /** @type {Map<K, InternalCell<V>>[]} */
                this.stack = parentStack === undefined ? [] : [...parentStack];
                this.stack.push(this.map);
        }

        /**
         * Stores a value in the current layer, preserving explicit `undefined`
         * values with an internal marker.
         * @param {K} item the key of the element to add
         * @param {V} value the value of the element to add
         * @returns {void}
         */
        set(item, value) {
                this.map.set(item, value === undefined ? UNDEFINED_MARKER : value);
        }

        /**
         * Deletes a key from the current view, either by removing it outright in the
         * root layer or by recording a tombstone in child layers.
         * @param {K} item the item to delete
         * @returns {void}
         */
        delete(item) {
                if (this.stack.length > 1) {
                        this.map.set(item, TOMBSTONE);
                } else {
                        this.map.delete(item);
                }
        }

        /**
         * Checks whether a key exists in the current scope chain, caching any parent
         * lookup result in the current layer.
         * @param {K} item the item to test
         * @returns {boolean} true if the item exists in this set
         */
        has(item) {
                const topValue = this.map.get(item);
                if (topValue !== undefined) {
                        return topValue !== TOMBSTONE;
                }
                if (this.stack.length > 1) {
                        for (let i = this.stack.length - 2; i >= 0; i--) {
                                const value = this.stack[i].get(item);
                                if (value !== undefined) {
                                        this.map.set(item, value);
                                        return value !== TOMBSTONE;
                                }
                        }
                        this.map.set(item, TOMBSTONE);
                }
                return false;
        }

        /**
         * Returns the visible value for a key, caching parent hits and misses in the
         * current layer.
         * @param {K} item the key of the element to return
         * @returns {Cell<V>} the value of the element
         */
        get(item) {
                const topValue = this.map.get(item);
                if (topValue !== undefined) {
                        return topValue === TOMBSTONE || topValue === UNDEFINED_MARKER
                                ? undefined
                                : topValue;
                }
                if (this.stack.length > 1) {
                        for (let i = this.stack.length - 2; i >= 0; i--) {
                                const value = this.stack[i].get(item);
                                if (value !== undefined) {
                                        this.map.set(item, value);
                                        return value === TOMBSTONE || value === UNDEFINED_MARKER
                                                ? undefined
                                                : value;
                                }
                        }
                        this.map.set(item, TOMBSTONE);
                }
        }

        /**
         * Collapses the stacked layers into a single concrete map.
         */
        _compress() {
                if (this.stack.length === 1) return;
                this.map = new Map();
                for (const data of this.stack) {
                        for (const pair of data) {
                                if (pair[1] === TOMBSTONE) {
                                        this.map.delete(pair[0]);
                                } else {
                                        this.map.set(pair[0], pair[1]);
                                }
                        }
                }
                this.stack = [this.map];
        }

        /**
         * Returns the visible keys as an array after collapsing the stack.
         * @returns {K[]} array of keys
         */
        asArray() {
                this._compress();
                return [...this.map.keys()];
        }

        /**
         * Returns the visible keys as a `Set` after collapsing the stack.
         * @returns {Set<K>} set of keys
         */
        asSet() {
                this._compress();
                return new Set(this.map.keys());
        }

        /**
         * Returns visible key/value pairs using the external representation.
         * @returns {[K, Cell<V>][]} array of key/value pairs
         */
        asPairArray() {
                this._compress();
                return Array.from(this.map.entries(), extractPair);
        }

        /**
         * Returns the visible contents as a plain `Map`.
         * @returns {Map<K, Cell<V>>} materialized map
         */
        asMap() {
                return new Map(this.asPairArray());
        }

        /**
         * Returns the number of visible keys after collapsing the stack.
         * @returns {number} number of keys
         */
        get size() {
                this._compress();
                return this.map.size;
        }

        /**
         * Creates a child `StackedMap` that sees the current layers as its parent
         * scope.
         * @returns {StackedMap<K, V>} child map
         */
        createChild() {
                return new StackedMap(this.stack);
        }
}

module.exports = StackedMap;