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43 lines
1.3 KiB
43 lines
1.3 KiB
const { PassThrough: ThroughStream } = require('stream');
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// eslint-ignore-next-line node/no-extraneous-require
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const duplexify = require('duplexify').obj;
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module.exports = {
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streamFlatMap,
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asyncGeneratorToStream,
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};
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// returns an async generator that maps each chunk to a stream with the specified
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// "entryToStream" mapping fn, and forwards child streams out
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// useable with streams.pipeline
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function streamFlatMap(entryToStream) {
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const duplex = asyncGeneratorToStream(flatMapGenerator);
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return duplex;
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async function* flatMapGenerator(source) {
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for await (const entry of source) {
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const subStream = entryToStream(entry);
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yield* subStream;
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}
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}
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}
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// this stupid utility turns an async iterator factory into a duplex stream
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function asyncGeneratorToStream(factoryFn) {
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const writableStream = new ThroughStream({ objectMode: true });
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const readableStream = new ThroughStream({ objectMode: true });
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const duplex = duplexify(writableStream, readableStream);
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const asyncIter = factoryFn(writableStream);
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// drain iterator into readable stream
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process.nextTick(async () => {
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try {
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for await (const item of asyncIter) {
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readableStream.write(item);
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}
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readableStream.end();
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} catch (err) {
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readableStream.destroy(err);
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}
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});
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return duplex;
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}
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