yoloyolo/encode/mp3.go

package encode

import (
	"fmt"
	"os"

	lame "github.com/sjzar/go-lame"

	"github.com/netsynth/netsynth/classify"
	"github.com/netsynth/netsynth/synth"
)

// EncodeMP3 writes stereo PCM frames to an MP3 file at the given path.
// Encoding parameters: sampleRate Hz, 128 kbps, stereo, quality 5.
// Per D-13 (44100 Hz CD quality) and D-14 (128 kbps bitrate).
// CRITICAL: InitParams() is called after all Set* calls (go-lame Pitfall 1).
func EncodeMP3(outputPath string, frames [][2]float64, sampleRate int) error {
	f, err := os.Create(outputPath)
	if err != nil {
		return fmt.Errorf("create output file: %w", err)
	}
	defer f.Close()

	wr := lame.NewWriter(f)
	wr.Encoder.SetInSamplerate(sampleRate)
	wr.Encoder.SetOutSamplerate(sampleRate)
	wr.Encoder.SetNumChannels(2)
	wr.Encoder.SetBitrate(128)
	wr.Encoder.SetQuality(5)
	wr.Encoder.InitParams() // MUST be called after all Set* calls

	pcmBytes := synth.StereoFramesToInt16Bytes(frames)
	if _, err := wr.Write(pcmBytes); err != nil {
		return fmt.Errorf("write PCM to LAME encoder: %w", err)
	}
	if err := wr.Close(); err != nil {
		return fmt.Errorf("finalize MP3: %w", err)
	}
	return nil
}

// RunSynthesis consumes a slice of WindowSnapshots, renders audio via OscillatorBank,
// and encodes to MP3 at outputPath.
// freqCfgs is the merged config map from config.Load (D-10: injected, not hardcoded).
// Returns an error if zero packets were captured (D-16 / OUT-03).
// The zero-packet guard runs BEFORE file creation to avoid leaving an empty file on disk.
func RunSynthesis(snapshots []classify.WindowSnapshot, outputPath string, freqCfgs map[classify.TrafficClass]synth.FreqConfig) error {
	// D-16 / OUT-03: Zero-packet guard — check BEFORE creating output file
	var totalPackets int64
	for _, snap := range snapshots {
		totalPackets += snap.TotalPackets
	}
	if totalPackets == 0 {
		return fmt.Errorf("no packets captured: output MP3 not written (empty capture produces no audio)")
	}

	// Render all windows to stereo frames
	bank := synth.NewBank(1.0, freqCfgs) // tau=1.0s per D-07
	var allFrames [][2]float64
	for _, snap := range snapshots {
		frames := bank.RenderWindow(snap)
		allFrames = append(allFrames, frames...)
	}

	// Encode to MP3
	return EncodeMP3(outputPath, allFrames, synth.SampleRate)
}