source: imaps-frontend/node_modules/webpack/lib/optimize/AggressiveMergingPlugin.js

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

"use strict";

const { STAGE_ADVANCED } = require("../OptimizationStages");

/** @typedef {import("../Chunk")} Chunk */
/** @typedef {import("../Compiler")} Compiler */

/**
 * @typedef {object} AggressiveMergingPluginOptions
 * @property {number=} minSizeReduce minimal size reduction to trigger merging
 */

class AggressiveMergingPlugin {
        /**
         * @param {AggressiveMergingPluginOptions=} [options] options object
         */
        constructor(options) {
                if (
                        (options !== undefined && typeof options !== "object") ||
                        Array.isArray(options)
                ) {
                        throw new Error(
                                "Argument should be an options object. To use defaults, pass in nothing.\nFor more info on options, see https://webpack.js.org/plugins/"
                        );
                }
                this.options = options || {};
        }

        /**
         * Apply the plugin
         * @param {Compiler} compiler the compiler instance
         * @returns {void}
         */
        apply(compiler) {
                const options = this.options;
                const minSizeReduce = options.minSizeReduce || 1.5;

                compiler.hooks.thisCompilation.tap(
                        "AggressiveMergingPlugin",
                        compilation => {
                                compilation.hooks.optimizeChunks.tap(
                                        {
                                                name: "AggressiveMergingPlugin",
                                                stage: STAGE_ADVANCED
                                        },
                                        chunks => {
                                                const chunkGraph = compilation.chunkGraph;
                                                /** @type {{a: Chunk, b: Chunk, improvement: number}[]} */
                                                const combinations = [];
                                                for (const a of chunks) {
                                                        if (a.canBeInitial()) continue;
                                                        for (const b of chunks) {
                                                                if (b.canBeInitial()) continue;
                                                                if (b === a) break;
                                                                if (!chunkGraph.canChunksBeIntegrated(a, b)) {
                                                                        continue;
                                                                }
                                                                const aSize = chunkGraph.getChunkSize(b, {
                                                                        chunkOverhead: 0
                                                                });
                                                                const bSize = chunkGraph.getChunkSize(a, {
                                                                        chunkOverhead: 0
                                                                });
                                                                const abSize = chunkGraph.getIntegratedChunksSize(b, a, {
                                                                        chunkOverhead: 0
                                                                });
                                                                const improvement = (aSize + bSize) / abSize;
                                                                combinations.push({
                                                                        a,
                                                                        b,
                                                                        improvement
                                                                });
                                                        }
                                                }

                                                combinations.sort((a, b) => b.improvement - a.improvement);

                                                const pair = combinations[0];

                                                if (!pair) return;
                                                if (pair.improvement < minSizeReduce) return;

                                                chunkGraph.integrateChunks(pair.b, pair.a);
                                                compilation.chunks.delete(pair.a);
                                                return true;
                                        }
                                );
                        }
                );
        }
}

module.exports = AggressiveMergingPlugin;