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import { Transform, TransformCallback } from 'stream'
// Thanks: https://stackoverflow.com/a/45126242
class StreamReplacer extends Transform {
private pendingChunk: Buffer
constructor (private readonly replacer: (line: string) => string) {
super()
}
_transform (chunk: Buffer, _encoding: BufferEncoding, done: TransformCallback) {
try {
this.pendingChunk = this.pendingChunk?.length
? Buffer.concat([ this.pendingChunk, chunk ])
: chunk
let index: number
// As long as we keep finding newlines, keep making slices of the buffer and push them to the
// readable side of the transform stream
while ((index = this.pendingChunk.indexOf('\n')) !== -1) {
// The `end` parameter is non-inclusive, so increase it to include the newline we found
const line = this.pendingChunk.slice(0, ++index)
// `start` is inclusive, but we are already one char ahead of the newline -> all good
this.pendingChunk = this.pendingChunk.slice(index)
// We have a single line here! Prepend the string we want
this.push(this.doReplace(line))
}
return done()
} catch (err) {
return done(err)
}
}
_flush (done: TransformCallback) {
// If we have any remaining data in the cache, send it out
if (!this.pendingChunk?.length) return done()
try {
return done(null, this.doReplace(this.pendingChunk))
} catch (err) {
return done(err)
}
}
private doReplace (buffer: Buffer) {
const line = this.replacer(buffer.toString('utf8'))
return Buffer.from(line, 'utf8')
}
}
export {
StreamReplacer
}
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