019.手写系列：promise 限制最大并发

export class SchedulerPromiseWithMaxConcurrency<T> {
    // 最大并发数
    private readonly _maxConcurrency;
    // 正在运行的任务数
    private _runningCnt = 0;
    // 还未运行的任务队列
    private _tasks: {
        promiseCreator: () => Promise<T>;
        resolve: (value: T | PromiseLike<T>) => void;
        reject: (reason?: any) => void;
    }[] = [];
 
    constructor(maxConcurrency: number) {
        if (maxConcurrency <= 0) {
            throw new Error('maxConcurrency must be greater than 0');
        }
        this._maxConcurrency = maxConcurrency;
    }
 
    /**
     * 添加任务
     */
    add(promiseCreator: () => Promise<T>) {
        return new Promise<T>((resolve, reject) => {
            this._tasks.push({
                promiseCreator,
                resolve,
                reject,
            });
            this._run();
        });
    }
 
    private _run() {
        // 已经达到最大并发
        if (this._runningCnt >= this._maxConcurrency) {
            return;
        }
        // 还未运行的任务队列为空
        if (this._tasks.length === 0) {
            return;
        }
        this._runningCnt++;
        const { promiseCreator, resolve, reject } = this._tasks.shift()!;
        promiseCreator()
            .then(resolve, reject)
            .finally(() => {
                this._runningCnt--;
                this._run();
            });
    }
}
 
export function promiseAllSettledWithMaxConcurrency<T>(tasks: (() => Promise<T>)[], maxConcurrency: number) {
    const scheduler = new SchedulerPromiseWithMaxConcurrency(maxConcurrency);
    return Promise.allSettled(tasks.map(task => scheduler.add(task)));
}