computed.ts

#!/bin/ts-node

const SymbolCache = Symbol.for("cache");
let miss = 0;

function computed(...args) {
    const _cache = {};
    return function (
        target,
        propertyKey: string,
        descriptor: PropertyDescriptor
    ) {
        const _get = descriptor.get;
        descriptor.get = function () {
            this[SymbolCache] = this[SymbolCache] || {};
            // console.log(this.name, propertyKey, args);
            for (const key of args) {
                const cacheData = this[SymbolCache][key];
                const targetData = this[key];
                if (this[key] instanceof Object) {
                    throw new Error("Computed properties cannot depend on Reference type.");
                }
                if (!(key in this[SymbolCache]) || cacheData != targetData) {
                    this[SymbolCache][propertyKey] = _get.call(this);
                    this[SymbolCache][key] = targetData;
                    return this[SymbolCache][propertyKey];
                }
            }
            return this[SymbolCache][propertyKey];
        }
    };
}

let calcCounts = 0;

class TNode {
    name: string;
    childs?: TNode[];
    defaultHeight: number = 10;
    etag: number = Math.random();

    constructor(name, childs) {
        this.name = name;
        this.childs = childs;
    }

    @computed("etag")
    get childsHeight() {
        miss++;
        return this.childs &&
            this.childs.reduce((prev, childNode) => prev + childNode.height, 0);
    }

    @computed("defaultHeight", "childsHeight")
    get height() {
        miss++;
        return Math.max(this.defaultHeight, this.childs && this.childsHeight);
    }
}

function testNodes(depth) {
    if (depth == 5) return null;
    return Array.from(new Array(10)).map((_, i) => new TNode(`child-${depth}-${i}`, testNodes(depth + 1)))
}

const root = new TNode("root", testNodes(1));

console.time("first");
console.log(root.height, miss);
console.timeEnd("first");
console.time("change");
for (let index = 0; index < 10; index++) {
    miss = 0;
    root.childs[0].childs[0].childs[0].childs.pop();
    root.childs[0].childs[0].childs[0].etag = Math.random();
    root.childs[0].childs[0].etag = Math.random();
    root.childs[0].etag = Math.random();
    root.etag = Math.random();
    console.log(root.height, miss);
}
console.timeEnd("change");
miss = 0;
console.time("bc");
let tmp = 0;
for (let index = 0; index < 100000; index++) {
    tmp += root.height;
}
console.log(root.height, miss);
console.timeEnd("bc");

$ ts-node index.ts
100000 12222
first: 9.639ms
99990 17
99980 8
99970 8
99960 8
99950 8
99940 8
99930 8
99920 8
99910 8
99910 5
change: 6.732ms
99910 0
bc: 45.749ms