/**
 * CachedFileReader will deduplicate requests made to the same folder structure
 * to scan for files.
 */ export class BatchedFileReader {
    constructor(reader){
        this.reader = reader;
    }
    schedule(callback) {
        if (!this.schedulePromise) {
            this.schedulePromise = Promise.resolve();
        }
        this.schedulePromise.then(()=>{
            process.nextTick(callback);
        });
    }
    getOrCreateBatch() {
        // If there is an existing batch and it's not completed, then reuse it.
        if (this.batch && !this.batch.completed) {
            return this.batch;
        }
        const batch = {
            completed: false,
            directories: [],
            callbacks: []
        };
        this.batch = batch;
        this.schedule(async ()=>{
            batch.completed = true;
            if (batch.directories.length === 0) return;
            // Collect all the results for each of the directories. If any error
            // occurs, send the results back to the loaders.
            let values;
            try {
                values = await this.load(batch.directories);
            } catch (err) {
                // Reject all the callbacks.
                for (const { reject  } of batch.callbacks){
                    reject(err);
                }
                return;
            }
            // Loop over all the callbacks and send them their results.
            for(let i = 0; i < batch.callbacks.length; i++){
                const value = values[i];
                if (value instanceof Error) {
                    batch.callbacks[i].reject(value);
                } else {
                    batch.callbacks[i].resolve(value);
                }
            }
        });
        return batch;
    }
    async load(directories) {
        // Make a unique array of directories. This is what lets us de-duplicate
        // loads for the same directory.
        const unique = [
            ...new Set(directories)
        ];
        const results = await Promise.all(unique.map(async (directory)=>{
            let files;
            let error;
            try {
                files = await this.reader.read(directory);
            } catch (err) {
                if (err instanceof Error) error = err;
            }
            return {
                directory,
                files,
                error
            };
        }));
        return directories.map((directory)=>{
            const found = results.find((result)=>result.directory === directory);
            if (!found) return [];
            if (found.files) return found.files;
            if (found.error) return found.error;
            return [];
        });
    }
    async read(dir) {
        // Get or create a new file reading batch.
        const batch = this.getOrCreateBatch();
        // Push this directory into the batch to resolve.
        batch.directories.push(dir);
        // Push the promise handles into the batch (under the same index) so it can
        // be resolved later when it's scheduled.
        const promise = new Promise((resolve, reject)=>{
            batch.callbacks.push({
                resolve,
                reject
            });
        });
        return promise;
    }
}

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