yoloyolo/synth/oscillator.go

package synth

import "math"

// Oscillator is a phase-accumulator oscillator that generates additive sine waveforms.
type Oscillator struct {
	phase float64
	freq  float64
	sr    float64
}

// NewOscillator creates an oscillator at the given frequency and sample rate.
func NewOscillator(freq float64, sampleRate int) *Oscillator {
	return &Oscillator{freq: freq, sr: float64(sampleRate)}
}

// Advance returns one sample: fundamental + harmonics summed and normalized to [-1, 1].
// Normalization uses sum of absolute amplitudes so that alternating-sign harmonic series
// (e.g. triangle wave) are correctly bounded. Without math.Abs, signed cancellation
// produces an inflated normalization denominator that causes output to exceed [-1, 1].
func (o *Oscillator) Advance(harmonics []HarmonicDef) float64 {
	sum := 0.0
	totalWeight := 0.0
	for _, h := range harmonics {
		sum += h.Amplitude * math.Sin(2*math.Pi*o.phase*float64(h.Ratio))
		totalWeight += math.Abs(h.Amplitude)
	}
	o.phase += o.freq / o.sr
	if o.phase >= 1.0 {
		o.phase -= 1.0
	}
	if totalWeight > 0 {
		return sum / totalWeight
	}
	return 0
}

// Phase returns the current phase (for testing).
func (o *Oscillator) Phase() float64 {
	return o.phase
}