---
phase: 02-audio-synthesis-engine
plan: 03
type: execute
wave: 3
depends_on: ["02-02"]
files_modified:
  - encode/mp3.go
  - encode/mp3_test.go
  - cmd/netsynth/main.go
autonomous: true
requirements: [OUT-01, OUT-02, OUT-03]

must_haves:
  truths:
    - "Synthetic WindowSnapshots produce a valid MP3 file that passes ffprobe validation"
    - "Zero-packet input produces a clear error message and no file on disk"
    - "User can specify -o flag for output path; default includes timestamp"
    - "MP3 is encoded at 44100 Hz, 128 kbps, stereo via go-lame"
  artifacts:
    - path: "encode/mp3.go"
      provides: "EncodeMP3 function and RunSynthesis orchestrator"
      contains: "func EncodeMP3"
    - path: "encode/mp3_test.go"
      provides: "Integration test with ffprobe validation and zero-packet guard test"
      contains: "TestMP3Valid"
    - path: "cmd/netsynth/main.go"
      provides: "-o flag wiring with timestamp default, snapshot slice accumulator stub"
      contains: "outputPath"
  key_links:
    - from: "encode/mp3.go"
      to: "synth/bank.go"
      via: "RunSynthesis calls bank.RenderWindow for each snapshot"
      pattern: "bank\\.RenderWindow"
    - from: "encode/mp3.go"
      to: "synth/mixer.go"
      via: "Uses StereoFramesToInt16Bytes for PCM conversion"
      pattern: "StereoFramesToInt16Bytes"
    - from: "cmd/netsynth/main.go"
      to: "encode/mp3.go"
      via: "Will wire RunSynthesis in Phase 3; -o flag defined now"
      pattern: "outputPath"
---

<objective>
Build the MP3 encoder package and wire the -o output flag into the CLI. This completes OUT-01 (output path flag), OUT-02 (valid MP3 encoding), and OUT-03 (zero-packet guard). An integration test synthesizes from synthetic snapshots and validates the MP3 with ffprobe.

Purpose: The encoder is the final stage of the audio pipeline. After this plan, the complete synthesis chain (WindowSnapshot -> OscillatorBank -> stereo PCM -> MP3 file) is validated end-to-end against synthetic data.

Output: `encode/mp3.go`, `encode/mp3_test.go`, updated `cmd/netsynth/main.go` with -o flag.
</objective>

<execution_context>
@$HOME/.claude/get-shit-done/workflows/execute-plan.md
@$HOME/.claude/get-shit-done/templates/summary.md
</execution_context>

<context>
@.planning/PROJECT.md
@.planning/ROADMAP.md
@.planning/phases/02-audio-synthesis-engine/02-CONTEXT.md
@.planning/phases/02-audio-synthesis-engine/02-RESEARCH.md
@.planning/phases/02-audio-synthesis-engine/02-01-SUMMARY.md
@.planning/phases/02-audio-synthesis-engine/02-02-SUMMARY.md

@classify/types.go
@cmd/netsynth/main.go

<interfaces>
<!-- From synth/bank.go (created in Plan 02) -->
From synth/bank.go:
```go
func NewBank(tau float64) *OscillatorBank
func (b *OscillatorBank) RenderWindow(snap classify.WindowSnapshot) [][2]float64
```

From synth/mixer.go:
```go
func StereoFramesToInt16Bytes(frames [][2]float64) []byte
```

From synth/config.go:
```go
const SampleRate = 44100
const SamplesPerWindow = 22050
```

From classify/types.go:
```go
type WindowSnapshot struct {
    Counts       map[TrafficClass]int64
    TotalPackets int64
    WindowIndex  int
}
```

<!-- From go-lame (external dependency) -->
From github.com/sjzar/go-lame:
```go
func NewWriter(w io.Writer) *LameWriter
wr.Encoder.SetInSamplerate(int)
wr.Encoder.SetOutSamplerate(int)
wr.Encoder.SetNumChannels(int)
wr.Encoder.SetBitrate(int)
wr.Encoder.SetQuality(int)
wr.Encoder.InitParams()  // MUST call after all Set* and before Write
wr.Write([]byte) (int, error)
wr.Close() error
```
</interfaces>
</context>

<tasks>

<task type="auto" tdd="true">
  <name>Task 1: MP3 encoder package with zero-packet guard and integration test</name>
  <files>encode/mp3.go, encode/mp3_test.go</files>
  <read_first>
    - synth/bank.go (NewBank, RenderWindow signatures)
    - synth/mixer.go (StereoFramesToInt16Bytes)
    - synth/config.go (SampleRate constant)
    - classify/types.go (WindowSnapshot, TrafficClass)
    - .planning/phases/02-audio-synthesis-engine/02-RESEARCH.md (Pattern 4: go-lame API, zero-packet guard, anti-patterns)
    - .planning/phases/02-audio-synthesis-engine/02-CONTEXT.md (D-14: 128 kbps, D-16: zero-packet error)
  </read_first>
  <behavior>
    - TestMP3Valid: Create 3 synthetic WindowSnapshots with varied traffic (ICMP=50, DNS=30 in snap1; HTTPS=200 in snap2; SSH=10, HTTP=80 in snap3). Run RunSynthesis into a temp file. Assert: file exists, file size > 0, `ffprobe -v error -show_entries format=format_name,duration,nb_streams -of csv=p=0 <file>` outputs "mp3" in format_name, duration > 0, nb_streams=1.
    - TestZeroPacketError: Pass empty slice of snapshots (or snapshots where all TotalPackets=0). Assert: returns non-nil error containing "no packets", output file does NOT exist on disk.
    - TestEncodeMP3DirectBytes: Call EncodeMP3 with known [][2]float64 frames (e.g., 44100 frames of 440Hz sine). Assert: output file > 0 bytes, ffprobe validates it as MP3.
  </behavior>
  <action>
**encode/mp3.go** — MP3 encoding and synthesis orchestrator:

```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.
// sampleRate=44100, bitrate=128 kbps, stereo, quality=5. Per D-13, D-14.
// CRITICAL: InitParams() must be called after all Set* calls (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 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. Returns error if zero packets were captured (D-16/OUT-03).
// NOTE: Accepts a slice, not a channel — caller collects snapshots before calling this.
// The zero-packet check MUST happen before file creation (Pitfall 5).
func RunSynthesis(snapshots []classify.WindowSnapshot, outputPath string) 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) // 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)
}
```

Key design:
- `RunSynthesis` takes a `[]classify.WindowSnapshot` slice (not a channel). The caller (main.go in Phase 3) collects from the channel, then passes the slice. This enables the zero-packet check before file creation.
- `EncodeMP3` is a separate function so it can be tested independently with raw frames.
- PCM bytes are written in a single `Write` call (batch output, not streaming — per project constraints).

Write tests FIRST, using `os.CreateTemp` for output files and `exec.Command("ffprobe", ...)` for validation. Clean up temp files in test teardown.
  </action>
  <verify>
    <automated>cd /home/dev/workspace/yoloyolo && CGO_ENABLED=1 go test ./encode/... -v -count=1</automated>
  </verify>
  <acceptance_criteria>
    - encode/mp3.go contains `func EncodeMP3(outputPath string, frames [][2]float64, sampleRate int) error`
    - encode/mp3.go contains `func RunSynthesis(snapshots []classify.WindowSnapshot, outputPath string) error`
    - encode/mp3.go contains `wr.Encoder.InitParams()` AFTER all `Set*` calls
    - encode/mp3.go contains `if totalPackets == 0` guard BEFORE `os.Create`
    - encode/mp3.go contains the error string "no packets captured"
    - encode/mp3.go imports `github.com/sjzar/go-lame`
    - encode/mp3_test.go contains `TestMP3Valid`
    - encode/mp3_test.go contains `TestZeroPacketError`
    - encode/mp3_test.go uses `ffprobe` to validate MP3 output
    - `CGO_ENABLED=1 go test ./encode/...` exits 0
  </acceptance_criteria>
  <done>EncodeMP3 produces a valid MP3 file validated by ffprobe. RunSynthesis renders synthetic snapshots through the full pipeline. Zero-packet input returns error without creating a file. All encode tests pass.</done>
</task>

<task type="auto">
  <name>Task 2: Wire -o output flag and snapshot accumulator stub into CLI</name>
  <files>cmd/netsynth/main.go</files>
  <read_first>
    - cmd/netsynth/main.go (current Cobra flag setup, run function)
    - .planning/phases/02-audio-synthesis-engine/02-CONTEXT.md (D-15: -o flag, timestamp default)
    - .planning/phases/02-audio-synthesis-engine/02-RESEARCH.md (output flag wiring example)
  </read_first>
  <action>
Add the `-o` / `--output` flag to main.go. Per D-15: defaults to `netsynth-<timestamp>.mp3` when omitted. The flag is defined now but NOT wired to the synthesis pipeline yet — that happens in Phase 3 when capture and synthesis are integrated.

1. Add a package-level var:
   ```go
   var outputPath string
   ```

2. Add the flag registration in `main()` after the existing flags:
   ```go
   rootCmd.Flags().StringVarP(&outputPath, "output", "o", "", "Output MP3 file path (default: netsynth-<timestamp>.mp3)")
   ```

3. Add the timestamp default resolution at the top of `run()`, after the `--list-interfaces` check and before the interface check:
   ```go
   // Resolve default output path (D-15 / OUT-01)
   if outputPath == "" {
       outputPath = fmt.Sprintf("netsynth-%s.mp3", time.Now().Format("20060102-150405"))
   }
   ```

4. Add `"time"` to the imports.

5. Add a snapshot slice accumulator alongside the existing snapshot consumption loop, and a TODO for Phase 3 wiring. Replace the current `for snap := range snapshots` block to also collect snapshots into a slice:
   ```go
   // Accumulate snapshots for synthesis (Phase 3 will pass to encode.RunSynthesis)
   var collectedSnapshots []classify.WindowSnapshot
   for snap := range snapshots {
       collectedSnapshots = append(collectedSnapshots, snap)
       aggregate.AccumulateTotals(totals, snap)
   }

   // TODO(phase-3): Pass collectedSnapshots to encode.RunSynthesis(collectedSnapshots, outputPath)
   ```

Do NOT import the encode package yet — Phase 3 handles that. The flag must be functional (parseable, shows in --help) even though the synthesis pipeline isn't wired.
  </action>
  <verify>
    <automated>cd /home/dev/workspace/yoloyolo && go build -o /dev/null ./cmd/netsynth/ && go run ./cmd/netsynth/ --help 2>&1 | grep -q "\-o.*output" && echo "OK"</automated>
  </verify>
  <acceptance_criteria>
    - cmd/netsynth/main.go contains `var outputPath string`
    - cmd/netsynth/main.go contains `StringVarP(&outputPath, "output", "o",`
    - cmd/netsynth/main.go contains `netsynth-%s.mp3`
    - cmd/netsynth/main.go contains `time.Now().Format("20060102-150405")`
    - cmd/netsynth/main.go imports `"time"`
    - cmd/netsynth/main.go contains `var collectedSnapshots []classify.WindowSnapshot`
    - cmd/netsynth/main.go contains `TODO(phase-3)` referencing `encode.RunSynthesis`
    - `go build ./cmd/netsynth/` exits 0
    - `go run ./cmd/netsynth/ --help` output contains `-o, --output`
  </acceptance_criteria>
  <done>The -o/--output flag is registered in Cobra, defaults to netsynth-<timestamp>.mp3, visible in --help. Snapshot slice accumulator stub is in place for Phase 3 wiring. Build succeeds.</done>
</task>

</tasks>

<verification>
- `CGO_ENABLED=1 go test ./...` passes all tests (synth + encode + existing)
- `ffprobe` validates the test-generated MP3 (within encode test)
- `go run ./cmd/netsynth/ --help` shows `-o, --output` flag
- `grep "no packets captured" encode/mp3.go` confirms zero-packet error message
- `grep "collectedSnapshots" cmd/netsynth/main.go` confirms snapshot accumulator stub
</verification>

<success_criteria>
- encode/mp3.go produces valid MP3 files from synthetic WindowSnapshots (validated by ffprobe)
- Zero-packet guard prevents file creation and returns descriptive error (OUT-03)
- -o flag registered in CLI with timestamp default (OUT-01)
- Snapshot slice accumulator ready for Phase 3 wiring
- Full test suite passes: `CGO_ENABLED=1 go test ./...`
- Complete Phase 2 audio pipeline validated end-to-end against synthetic data
</success_criteria>

<output>
After completion, create `.planning/phases/02-audio-synthesis-engine/02-03-SUMMARY.md`
</output>