Lesson 18 · Round-trip proof - Build a WAV/PCM Toolkit

Round-trip proof

The real test of a writer is that a reader gets back exactly what you put in. Today you write samples to a file and read them back, proving the whole read/write path is lossless.

The goal

Write samples to WAV bytes, read them back, and confirm the samples and format match.

Start here - the target

TO DO

Scenario: Writing then reading returns the original samples

Givenmono 8000 Hz 16-bit with samples [0, 32767, -32768, -1]

Whenthe samples are written to WAV bytes and then read back with ReadSamples

Thenthe read-back channel equals [0, 32767, -32768, -1] and the Format matches (numChannels 1, sampleRate 8000, bitsPerSample 16)

Anda stereo signal (left [100, 300], right [200, 400]) round-trips to the same two channels

Background

A writer you cannot verify is a writer you cannot trust, and the cleanest verification is a round trip: take samples, write them to bytes, read those bytes back, and assert you got the identical samples and format. WriteSamples ties the chapter together - interleave the channels, encode them to 16-bit bytes, wrap them in a file - and ReadSamples from lesson 13 unwinds all of it. If the two are true inverses, the loop is lossless.

Test it on values that stress the extremes and the channel logic at once: the [0, 32767, -32768, -1] mono case covers both signed edges, and a stereo case proves interleave and de-interleave undo each other while the format survives the trip. This is the payoff of the writing chapter - not a single new algorithm but the proof that everything built so far composes into a correct, symmetric codec. From here on you stop moving bytes and start changing the audio itself.

Make it work

// WriteSamples: interleave -> encode16 -> buildFile
func WriteSamples(f Format, channels [][]int) []byte {
  flat := interleave(channels)
  data := encode16(flat)
  return buildFile(f, data)
}
// round-trip: ReadSamples(WriteSamples(f, chans)) == (f, chans)

CheckpointDONE

Writing and reading are proven inverses. The writer is complete; commit and stop here.