source: imaps-frontend/node_modules/webpack/lib/util/StackedMap.js

/*
        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");

/**
 * @template T
 * @typedef {T | undefined} Cell<T>
 */

/**
 * @template T
 * @typedef {T | typeof TOMBSTONE | typeof UNDEFINED_MARKER} InternalCell<T>
 */

/**
 * @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);
};

/**
 * @template K
 * @template V
 */
class StackedMap {
        /**
         * @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.slice();
                this.stack.push(this.map);
        }

        /**
         * @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);
        }

        /**
         * @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);
                }
        }

        /**
         * @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;
        }

        /**
         * @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);
                }
        }

        _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];
        }

        asArray() {
                this._compress();
                return Array.from(this.map.keys());
        }

        asSet() {
                this._compress();
                return new Set(this.map.keys());
        }

        asPairArray() {
                this._compress();
                return Array.from(this.map.entries(), extractPair);
        }

        asMap() {
                return new Map(this.asPairArray());
        }

        get size() {
                this._compress();
                return this.map.size;
        }

        createChild() {
                return new StackedMap(this.stack);
        }
}

module.exports = StackedMap;