# Architecture Patterns

**Domain:** Network traffic sonification CLI (Go) — v1.1 Custom Sound Mappings
**Researched:** 2026-03-26
**Confidence:** HIGH — based on direct code inspection of the existing v1.0 codebase

---

## v1.1 Integration Overview

This document supersedes the pre-implementation v1.0 architecture research. It is grounded in the actual codebase (3,254 lines, 6 packages) and answers: what changes, what's new, and in what order.

---

## Existing Package Map (v1.0 Baseline)

```
cmd/netsynth/main.go      CLI, pipeline wiring, Cobra flags
capture/                  go-pcap live capture + pcap file reader + BPF
classify/
  types.go                TrafficClass, ClassifiedPacket, WindowSnapshot
  classifier.go           NewClassifier(rules []Rule) — first-match-wins
  rules.go                DefaultRules []Rule (12 hardcoded rules)
aggregate/
  window.go               500ms time-windowed snapshot accumulation
synth/
  config.go               ClassFreqConfigs — fixed map[TrafficClass]FreqConfig
  oscillator.go           Phase-accumulator oscillator — sine only
  layer.go                EMA amplitude smoothing per layer
  bank.go                 NewBank(tau) — one Layer per AllClasses()
  mixer.go                PanGains, StereoFramesToInt16Bytes
encode/
  mp3.go                  RunSynthesis(snapshots, path) — NewBank + EncodeMP3
```

---

## What v1.1 Adds

Three independent but related features:

1. **TOML config file** — override frequencies and waveforms per built-in class
2. **Additional waveforms** — square, sawtooth, triangle alongside existing sine
3. **User-defined classification rules** — TOML-defined rules prepended before DefaultRules

---

## Integration Point Analysis

### Feature 1: TOML Config File

**Where config is consumed today:** `synth/config.go` holds a package-level `var ClassFreqConfigs`. `synth/bank.go:NewBank()` reads it directly with `ClassFreqConfigs[class]`. No config is passed through `encode.RunSynthesis` or `main.go`.

**Required change:** `NewBank` must accept a config parameter instead of reading the global. `encode.RunSynthesis` must accept and forward a config. `main.go` must load config from disk and pass it in.

**New package: `config/`**

This package does not exist yet in the codebase (the pre-implementation research anticipated it but it was deferred). It should own:
- TOML struct definitions
- File discovery logic (auto-detect `./netsynth.toml`, then `~/.config/netsynth/config.toml`)
- Merging: loaded config overlays defaults, does not replace them entirely

```
config/
  config.go       Config struct, Load(path string) (*Config, error)
  defaults.go     DefaultConfig() — wraps existing ClassFreqConfigs values
```

**TOML struct shape:**

```toml
[[class]]
name = "HTTPS"
frequency_hz = 200.0
waveform = "sawtooth"

[[class]]
name = "myservice"           # user-defined class (Feature 3)
frequency_hz = 350.0
waveform = "triangle"
```

The `Config` struct passed into `NewBank` should merge with `ClassFreqConfigs`:

```go
// config/config.go
type ClassConfig struct {
    Name        string  `toml:"name"`
    FrequencyHz float64 `toml:"frequency_hz"`
    Waveform    string  `toml:"waveform"`   // "sine" | "square" | "sawtooth" | "triangle"
}

type Config struct {
    Classes []ClassConfig `toml:"class"`
    Rules   []RuleConfig  `toml:"rule"`    // Feature 3
}
```

**TOML library:** Use `github.com/BurntSushi/toml`. It is the de-facto standard for TOML in Go (used by Hugo, dep, buf, etc.). Already a transitive dependency in many Go module graphs. Provides struct-tag-based decode, good error messages.

---

### Feature 2: Additional Waveforms

**Where waveform logic lives today:** `synth/oscillator.go:Advance()` — pure sine via `math.Sin`. The `HarmonicDef.Ratio` and `HarmonicDef.Amplitude` fields are stored in `FreqConfig.Harmonics` but the waveform function is hardcoded.

**Required change:** `Oscillator.Advance` must dispatch on a waveform type. Two clean approaches:

**Option A (recommended): Waveform enum on Oscillator**

Add a `waveform` field to `Oscillator`. `Advance` switches on it. `NewOscillator` gains a waveform parameter.

```go
type Waveform int

const (
    WaveformSine     Waveform = iota
    WaveformSquare
    WaveformSawtooth
    WaveformTriangle
)

type Oscillator struct {
    phase    float64
    freq     float64
    sr       float64
    waveform Waveform
}

func (o *Oscillator) sampleAt(phase, ratio float64) float64 {
    p := phase * float64(ratio)
    p -= math.Floor(p)          // wrap to [0, 1)
    switch o.waveform {
    case WaveformSquare:
        if p < 0.5 { return 1.0 }
        return -1.0
    case WaveformSawtooth:
        return 2.0*p - 1.0
    case WaveformTriangle:
        if p < 0.5 { return 4.0*p - 1.0 }
        return 3.0 - 4.0*p
    default: // WaveformSine
        return math.Sin(2 * math.Pi * p)
    }
}
```

**Option B: Function field on Oscillator**

Store `waveFn func(phase float64) float64`. More flexible but harder to serialize/configure.

Option A is preferred because waveform type maps cleanly to the TOML `waveform` string field without reflection tricks.

**`FreqConfig` change:** Add `Waveform` field:

```go
type FreqConfig struct {
    BaseHz    float64
    Harmonics []HarmonicDef
    Pan       float64
    Waveform  Waveform      // NEW: defaults to WaveformSine
}
```

`NewLayer` passes `cfg.Waveform` to `NewOscillator`. `NewOscillator` signature changes to accept the waveform.

**What does NOT change:** `HarmonicDef`, `EMAAlpha`, `Layer.UpdateTarget`, `Layer.AdvanceSample`, `OscillatorBank.RenderWindow`, `mixer.go`, `encode/mp3.go`. The waveform change is contained to `oscillator.go` and the `FreqConfig` struct.

---

### Feature 3: User-Defined Classification Rules

**Where rules are wired today:** `main.go` lines 111, 175 — both `runLiveMode` and `runPcapMode` call `classify.NewClassifier(classify.DefaultRules)` directly. No config is passed.

**Required change:** User rules from TOML prepend before `DefaultRules`. `Classifier` already supports arbitrary `[]Rule` — `NewClassifier(rules []Rule)` is the constructor. No change to `classifier.go` itself.

**`RuleConfig` TOML struct:**

```toml
[[rule]]
protocol = "tcp"
dst_port = 8443
class    = "myservice"
```

```go
// config/config.go
type RuleConfig struct {
    Protocol string `toml:"protocol"`
    DstPort  uint16 `toml:"dst_port"`
    Class    string `toml:"class"`    // must match a name in [[class]] or a builtin class name
}
```

**Merging in main.go:**

```go
userRules := config.ToClassifyRules(cfg.Rules)      // []classify.Rule
allRules := append(userRules, classify.DefaultRules...)
classifier := classify.NewClassifier(allRules)
```

**New `TrafficClass` values:** User-defined classes in TOML produce new `TrafficClass` string values (e.g., `"myservice"`). `AllClasses()` in `classify/types.go` is currently a hardcoded slice. For user-defined classes, `AllClasses()` cannot be the source of truth for bank layer construction. `NewBank` must instead iterate over whatever classes have a `FreqConfig` entry.

This is a critical integration point: `bank.go:NewBank` currently ranges over `classify.AllClasses()`. If user classes can appear, `NewBank` must accept the full config map and range over that instead.

---

## New vs Modified Components

### New

| Component | Location | Purpose |
|-----------|----------|---------|
| `config` package | `config/config.go` | TOML struct, `Load()`, file discovery, merge with defaults |
| `config/defaults.go` | optional split | `DefaultConfig()` wrapping existing `ClassFreqConfigs` values |

### Modified

| Component | Change | Impact |
|-----------|--------|--------|
| `synth/oscillator.go` | Add `Waveform` type + `waveform` field; dispatch in `Advance` | Self-contained; no caller signature breaks except `NewOscillator` |
| `synth/config.go` | Add `Waveform Waveform` field to `FreqConfig`; default to `WaveformSine` | Requires `NewLayer` to pass waveform to `NewOscillator` |
| `synth/layer.go` | Pass `cfg.Waveform` to `NewOscillator` | One-line change |
| `synth/bank.go` | Accept `map[classify.TrafficClass]FreqConfig` param instead of reading global; range over param keys not `AllClasses()` | Decouples bank from global; enables user classes |
| `encode/mp3.go` | Accept `*config.Config` or merged `FreqConfig` map; pass to `NewBank` | Thin forwarding change |
| `cmd/netsynth/main.go` | Add `--config` flag; load config; prepend user rules; pass config to `RunSynthesis` | Touches both `runLiveMode` and `runPcapMode` |
| `classify/rules.go` | No change — `DefaultRules` stays as the fallback | Unchanged |
| `classify/classifier.go` | No change — already accepts `[]Rule` | Unchanged |
| `classify/types.go` | `AllClasses()` may need a note that it returns only builtins; bank no longer relies on it | Low risk; document only |

---

## Data Flow Changes

### v1.0 Flow (config hardcoded)

```
main.go
  └─ classify.NewClassifier(classify.DefaultRules)
  └─ encode.RunSynthesis(snapshots, path)
       └─ synth.NewBank(1.0)
            └─ ClassFreqConfigs[class]  ← global, hardcoded
```

### v1.1 Flow (config injected)

```
main.go
  └─ config.Load(configPath)         ← NEW: resolve path, parse TOML, merge defaults
       └─ cfg *config.Config
  └─ classify.NewClassifier(
         append(config.ToClassifyRules(cfg.Rules), classify.DefaultRules...)
     )                                ← user rules prepend built-ins
  └─ encode.RunSynthesis(snapshots, path, cfg.FreqConfigs())
       └─ synth.NewBank(1.0, freqConfigs)   ← map passed in, not read from global
            └─ freqConfigs[class]    ← merged: user overrides + defaults
```

---

## Suggested Build Order

The following order minimizes integration risk. Each step is independently testable before the next begins.

### Step 1: Waveform types in `synth/oscillator.go`

No external dependencies. Pure math. Testable with golden-sample unit tests (square wave sample at phase 0.25 should be 1.0, etc.). Does not affect `Layer`, `Bank`, or `encode` yet.

**Files changed:** `synth/oscillator.go` only.

### Step 2: Wire `Waveform` through `FreqConfig` and `Layer`

Add `Waveform` to `FreqConfig`. Update `NewLayer` to pass it to `NewOscillator`. `ClassFreqConfigs` entries default to `WaveformSine` (zero value — valid if `WaveformSine = 0`).

Existing tests continue to pass without modification since all existing configs use the zero-value waveform.

**Files changed:** `synth/config.go`, `synth/layer.go`.

### Step 3: Decouple `NewBank` from the global

Change `NewBank(tau float64)` to `NewBank(tau float64, cfgs map[classify.TrafficClass]FreqConfig)`. Update `encode/mp3.go:RunSynthesis` to pass `synth.ClassFreqConfigs` as default.

At this point the system is functionally identical to v1.0 but `NewBank` no longer reads a global.

**Files changed:** `synth/bank.go`, `encode/mp3.go`.

### Step 4: `config` package — TOML structs and file discovery

Implement `config.Load()`, file discovery, and the `DefaultConfig()` function that wraps `synth.ClassFreqConfigs`. No TOML parsing yet — start with the struct definitions and the merge logic.

Add `github.com/BurntSushi/toml` dependency (`go get`).

**Files added:** `config/config.go`.

### Step 5: `--config` flag and user rule merging in `main.go`

Wire `config.Load()` into `run()`. Pass user rules to both `runLiveMode` and `runPcapMode`. Pass merged `FreqConfig` map to `RunSynthesis`.

At this point a minimal TOML config (empty file, or `[[rule]]` only) can be validated end-to-end.

**Files changed:** `cmd/netsynth/main.go`.

### Step 6: Custom frequency and waveform overrides in config

Implement the `[[class]]` TOML section parsing. Add `FreqConfigs()` method to `Config` that returns the merged map (user overrides applied over defaults). Write table-driven tests: "TOML sets HTTPS to 200 Hz sawtooth, bank layer for HTTPS uses 200 Hz sawtooth."

**Files changed:** `config/config.go`.

### Step 7: User-defined classes end-to-end

Support `[[class]]` entries with names not in `classify.AllClasses()`. These become new `TrafficClass` values. User `[[rule]]` entries pointing to these classes are prepended to `DefaultRules`. The bank creates layers for all classes in the merged `FreqConfig` map.

This step requires the most cross-package coordination but by this point each piece is already in place.

**Files changed:** `config/config.go`, `cmd/netsynth/main.go` (verification that unknown class names don't panic).

---

## Component Boundaries After v1.1

| Component | Responsibility | Communicates With |
|-----------|---------------|-------------------|
| `config` | TOML parsing, file discovery, merge logic, `DefaultConfig()` | `synth` (FreqConfig type), `classify` (Rule type) |
| `synth/oscillator` | Phase-accumulator for sine/square/sawtooth/triangle | Used by `Layer` |
| `synth/bank` | Accepts freq config map, constructs one `Layer` per entry | `encode` passes config map in |
| `encode` | Receives config map from `main`, passes to `NewBank` | Thin pass-through |
| `cmd/netsynth/main` | Loads config, merges rules, wires all stages | All packages |
| `classify` | Rules engine (unchanged); `DefaultRules` stays as package-level var | `main` constructs with merged rules |

---

## Critical Integration Constraints

### `AllClasses()` Is Not the Source of Truth for Bank Construction

`bank.go` currently iterates `classify.AllClasses()` to construct layers. After v1.1, the bank must iterate the keys of the `FreqConfig` map passed to it. User-defined classes will not appear in `AllClasses()`. If this is not changed, user-defined class packets will be aggregated in `WindowSnapshot.Counts` but have no corresponding layer — they will produce silence and no error.

**Fix:** `NewBank` iterates `maps.Keys(cfgs)` (or equivalent range over the map), not `classify.AllClasses()`.

### Class Name Validation Must Happen at Config Load Time

If a `[[rule]]` references a class name that has no corresponding `[[class]]` entry and is not a builtin, the system will silently mis-classify packets into a layer that doesn't exist. Validate at `config.Load()` time: every class name in `[[rule]]` must resolve to either a builtin `TrafficClass` or a `[[class]]` entry in the same config.

### `encode.RunSynthesis` Signature Change Is a Breaking API Change

`encode.RunSynthesis(snapshots []classify.WindowSnapshot, outputPath string)` will need to accept the config. If any external code (tests, future callers) uses this signature, they will break. Keep the change to a single place and update all call sites in the same commit.

---

## Anti-Patterns to Avoid

### Anti-Pattern: Reading Global `ClassFreqConfigs` from Multiple Places

If `NewBank`, `encode.RunSynthesis`, and config loading all reference the package-level `synth.ClassFreqConfigs`, the merge point becomes ambiguous. The fix (Step 3 above) centralizes config reading to one place: `config.DefaultConfig()` reads from `ClassFreqConfigs` once when building defaults; everything downstream receives the already-merged map.

### Anti-Pattern: Storing `Waveform` as a String Everywhere

Keeping `waveform` as a `string` from TOML all the way into `Oscillator` means every advance call parses or switches on a string. Parse the string to a `Waveform` int type at config-load time. The `Oscillator` field should be a typed `Waveform`, not `string`.

### Anti-Pattern: User Rules Appended After DefaultRules

User rules must **prepend** `DefaultRules`, not append. `DefaultRules` ends with catch-all rules (`DstPort: 0`) that match any TCP or UDP packet. Appending user rules after these catch-alls means they will never be reached.

---

## Sources

- Direct code inspection: `synth/config.go`, `synth/oscillator.go`, `synth/bank.go`, `synth/layer.go`, `classify/classifier.go`, `classify/rules.go`, `classify/types.go`, `encode/mp3.go`, `cmd/netsynth/main.go` — HIGH confidence
- BurntSushi/toml usage in production Go projects (Hugo, dep): MEDIUM confidence (well-known in Go ecosystem)
- Phase-accumulator waveform synthesis formulas (square, sawtooth, triangle): HIGH confidence (standard DSP, textbook formulas)

---

*Architecture research for: NetSynth v1.1 — custom sound mappings integration*
*Researched: 2026-03-26*