add chunker transform stream (#44)

src/file-box.ts

@@ -43,6 +43,7 @@ import {
   bufferToQrValue,
   qrValueToStream,
 }                         from './qrcode'
+import { chunkerTransformStream } from './pure-functions/chunker-transform-stream'
 
 const EMPTY_META_DATA = Object.freeze({})
 
@@ -492,7 +493,12 @@ export class FileBox implements Pipeable {
   private transformBufferToStream (buffer?: Buffer): Readable {
     const bufferStream = new PassThrough()
     bufferStream.end(buffer || this.buffer)
-    return bufferStream
+
+    /**
+     * Use small `chunks` with `toStream()` #44
+     * https://github.com/huan/file-box/issues/44
+     */
+    return bufferStream.pipe(chunkerTransformStream())
   }
 
   private transformBase64ToStream (): Readable {

src/pure-functions/chunker-transform-stream.test.ts

@@ -0,0 +1,52 @@
+#!/usr/bin/env ts-node
+
+/* eslint @typescript-eslint/no-unused-vars:off */
+
+import { test } from 'tstest'
+
+import {
+  PassThrough,
+  Readable,
+}                     from 'stream'
+
+import { chunkerTransformStream } from './chunker-transform-stream'
+
+test('chunkerTransformStream()', async t => {
+  const DATA_LIST = [
+    'a',
+    'b',
+  ]
+  const DATA = DATA_LIST.join('')
+
+  const createStream = () => {
+    const stream = new PassThrough()
+    stream.end(DATA)
+    return stream
+  }
+
+  const getDataList = (stream: Readable) => {
+    return new Promise<any[]>(resolve => {
+      const list = [] as any[]
+      stream.on('end', () => resolve(list))
+      stream.on('data', chunk => list.push(chunk))
+    })
+  }
+
+  const newStream0 = createStream()
+  const dataList0 = await getDataList(newStream0)
+
+  t.equal(dataList0.length, 1, 'should get 1 chunks')
+  t.equal(dataList0[0].toString(), DATA, 'should get data')
+
+  const newStream1 = createStream().pipe(chunkerTransformStream(2))
+  const dataList1 = await getDataList(newStream1)
+
+  t.equal(dataList1.length, 1, 'should get 1 chunks')
+  t.equal(dataList1[0].toString(), DATA, 'should get data')
+
+  const newStream2 = createStream().pipe(chunkerTransformStream(1))
+  const dataList2 = await getDataList(newStream2)
+
+  t.equal(dataList2.length, 2, 'should get 2 chunks')
+  t.equal(dataList2.join(''), DATA, 'should get data')
+})

src/pure-functions/chunker-transform-stream.ts

@@ -0,0 +1,36 @@
+/**
+ * ChunkerTransformStream, a transform stream to take arbitrary chunk sizes and make them consistent
+ * https://codereview.stackexchange.com/q/57492/185709
+ */
+import stream from 'stream'
+
+function chunkerTransformStream (chunkSize = 16384) {
+  let buffer = Buffer.from([])
+
+  const chunker = new stream.Transform({
+    objectMode: true,
+  })
+  chunker._transform = function (chunk, _, done) {
+    buffer = Buffer.concat([buffer, chunk])
+
+    while (buffer.length >= chunkSize) {
+      this.push(buffer.slice(0, chunkSize))
+      buffer = buffer.slice(chunkSize)
+    }
+
+    done()
+  }
+
+  chunker._flush = function (done) {
+    if (buffer.length) {
+      this.push(buffer)
+    }
+    done()
+  }
+
+  return chunker
+}
+
+export {
+  chunkerTransformStream,
+}