yoloyolo/synth/layer.go

package synth

import "math"

// EMAAlpha computes the per-sample smoothing coefficient for a given time constant.
// tau=1.0 at SR=44100 gives alpha ~0.0000227 (63% of target reached in 1 second). Per D-07.
func EMAAlpha(tau float64, sampleRate int) float64 {
	return 1.0 - math.Exp(-1.0/(tau*float64(sampleRate)))
}

// Layer combines an oscillator with ADSR envelope, LFO modulation,
// and EMA amplitude smoothing for one traffic class.
type Layer struct {
	Config     FreqConfig
	Osc        *Oscillator
	env        *Envelope
	pitchLFO   *LFO
	tremoloLFO *LFO
	seen       bool    // whether this class has ever had count > 0
	whisper    float64 // whisper floor amplitude (D-08)
}

// NewLayer creates a Layer for the given config using the specified sample rate and EMA time constant (tau in seconds).
// If cfg.WaveformType is not WaveformCustom, harmonics are resolved from the preset at construction time.
// LFO and ADSR parameters are derived from the protocol group and bursty flag.
func NewLayer(cfg FreqConfig, sampleRate int, tau float64) *Layer {
	if cfg.WaveformType != WaveformCustom {
		cfg.Harmonics = WaveformPresetHarmonics(cfg.WaveformType, cfg.BaseHz, sampleRate)
	}

	// Select envelope params based on bursty flag
	envParams := SustainedEnvParams
	if cfg.Bursty {
		envParams = BurstyEnvParams
	}

	// Get LFO config for this protocol group
	lfoCfg := LFOConfigForGroup(cfg.Group)

	return &Layer{
		Config:     cfg,
		Osc:        NewOscillator(cfg.BaseHz, sampleRate),
		env:        NewEnvelope(envParams, sampleRate, tau),
		pitchLFO:   NewLFO(lfoCfg.PitchRate, lfoCfg.PitchDepth, sampleRate),
		tremoloLFO: NewLFO(lfoCfg.TremoloRate, lfoCfg.TremoloDepth, sampleRate),
		whisper:    WhisperFloor,
	}
}

// UpdateTarget sets the envelope state and traffic rate from packet counts.
// Triggers the envelope on first appearance, releases when count drops to 0.
func (l *Layer) UpdateTarget(count int64, maxCount int64) {
	if count > 0 {
		if !l.seen {
			l.seen = true
			l.env.Trigger()
		}
		// Re-trigger if we were in release/idle
		if l.env.State() == EnvRelease || l.env.State() == EnvIdle {
			l.env.Trigger()
		}
	} else if l.seen && count == 0 {
		l.env.Release()
	}

	// Update traffic rate for sustain modulation
	normalizedRate := 0.0
	if maxCount > 0 {
		normalizedRate = float64(count) / float64(maxCount)
	}
	l.env.SetTrafficRate(normalizedRate)
}

// AdvanceSample renders one sample with LFO modulation and ADSR envelope.
func (l *Layer) AdvanceSample() float64 {
	// Pitch modulation: LFO shifts frequency by a few cents
	pitchMod := l.pitchLFO.Advance()
	l.Osc.freq = PitchLFOFreq(l.Config.BaseHz, pitchMod)

	// Generate waveform sample
	sample := l.Osc.Advance(l.Config.Harmonics)

	// Apply ADSR envelope
	envAmp := l.env.Advance()

	// Tremolo modulation: LFO modulates amplitude
	tremoloMod := 1.0 + l.tremoloLFO.Advance() // [1-depth, 1+depth]

	return sample * envAmp * tremoloMod
}

// CurrentAmp returns the current envelope level (for testing).
func (l *Layer) CurrentAmp() float64 {
	return l.env.Level()
}

// TargetAmp returns the current traffic amplitude in the envelope (for testing).
func (l *Layer) TargetAmp() float64 {
	return l.env.trafficAmp
}

// Seen returns whether this layer has ever received traffic (for testing).
func (l *Layer) Seen() bool {
	return l.seen
}