server/server -> server/core

server/core/helpers/stream-replacer.ts

@@ -0,0 +1,58 @@
+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
+}