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 EMA amplitude smoothing for one traffic class.
type Layer struct {
	Config     FreqConfig
	Osc        *Oscillator
	currentAmp float64
	targetAmp  float64
	alpha      float64 // EMA coefficient
	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.
func NewLayer(cfg FreqConfig, sampleRate int, tau float64) *Layer {
	if cfg.WaveformType != WaveformCustom {
		cfg.Harmonics = WaveformPresetHarmonics(cfg.WaveformType, cfg.BaseHz, sampleRate)
	}
	return &Layer{
		Config:  cfg,
		Osc:     NewOscillator(cfg.BaseHz, sampleRate),
		alpha:   EMAAlpha(tau, sampleRate),
		whisper: WhisperFloor,
	}
}

// UpdateTarget sets the target amplitude from a packet count and max count across all classes.
// Per D-07/D-08/D-09: once seen, floor is whisper; amplitude scales linearly with normalized rate.
func (l *Layer) UpdateTarget(count int64, maxCount int64) {
	if count > 0 {
		l.seen = true
	}
	if !l.seen {
		l.targetAmp = 0.0
		return
	}
	normalizedRate := 0.0
	if maxCount > 0 {
		normalizedRate = float64(count) / float64(maxCount)
	}
	l.targetAmp = l.whisper + (1.0-l.whisper)*normalizedRate
}

// AdvanceSample renders one sample and advances the EMA amplitude toward target.
// Returns the raw mono sample (before pan/gain).
func (l *Layer) AdvanceSample() float64 {
	sample := l.Osc.Advance(l.Config.Harmonics)
	l.currentAmp += l.alpha * (l.targetAmp - l.currentAmp)
	return sample * l.currentAmp
}

// CurrentAmp returns the current amplitude (for testing).
func (l *Layer) CurrentAmp() float64 {
	return l.currentAmp
}

// TargetAmp returns the target amplitude (for testing).
func (l *Layer) TargetAmp() float64 {
	return l.targetAmp
}

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