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import { DetachedPromise } from './detached-promise';
/**
 * A wrapper for a function that will only allow one call to the function to
 * execute at a time.
 */ export class Batcher {
    constructor(cacheKeyFn, /**
     * A function that will be called to schedule the wrapped function to be
     * executed. This defaults to a function that will execute the function
     * immediately.
     */ schedulerFn = (fn)=>fn()){
        this.cacheKeyFn = cacheKeyFn;
        this.schedulerFn = schedulerFn;
        this.pending = new Map();
    }
    static create(options) {
        return new Batcher(options == null ? void 0 : options.cacheKeyFn, options == null ? void 0 : options.schedulerFn);
    }
    /**
   * Wraps a function in a promise that will be resolved or rejected only once
   * for a given key. This will allow multiple calls to the function to be
   * made, but only one will be executed at a time. The result of the first
   * call will be returned to all callers.
   *
   * @param key the key to use for the cache
   * @param fn the function to wrap
   * @returns a promise that resolves to the result of the function
   */ async batch(key, fn) {
        const cacheKey = this.cacheKeyFn ? await this.cacheKeyFn(key) : key;
        if (cacheKey === null) {
            return fn({
                resolve: (value)=>Promise.resolve(value),
                key
            });
        }
        const pending = this.pending.get(cacheKey);
        if (pending) return pending;
        const { promise, resolve, reject } = new DetachedPromise();
        this.pending.set(cacheKey, promise);
        this.schedulerFn(async ()=>{
            try {
                const result = await fn({
                    resolve,
                    key
                });
                // Resolving a promise multiple times is a no-op, so we can safely
                // resolve all pending promises with the same result.
                resolve(result);
            } catch (err) {
                reject(err);
            } finally{
                this.pending.delete(cacheKey);
            }
        });
        return promise;
    }
}

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