fm-rds-tx/internal/audio/resample.go

package audio

// ResampledSource adapts a WAVSource to a different sample rate using
// linear interpolation between adjacent samples.
type ResampledSource struct {
	src      *WAVSource
	ratio    float64 // source rate / target rate
	position float64 // fractional position in source frames
}

// NewResampledSource wraps a WAV source with rate conversion.
func NewResampledSource(src *WAVSource, targetSampleRate float64) *ResampledSource {
	ratio := 1.0
	if src != nil && src.SampleRate > 0 && targetSampleRate > 0 {
		ratio = float64(src.SampleRate) / targetSampleRate
	}
	return &ResampledSource{src: src, ratio: ratio}
}

// NextFrame returns the next interpolated stereo frame.
func (s *ResampledSource) NextFrame() Frame {
	if s.src == nil || len(s.src.frames) == 0 {
		return NewFrame(0, 0)
	}

	n := len(s.src.frames)
	idx0 := int(s.position) % n
	idx1 := (idx0 + 1) % n
	frac := s.position - float64(int(s.position))

	f0 := s.src.frames[idx0]
	f1 := s.src.frames[idx1]

	l := float64(f0.L)*(1-frac) + float64(f1.L)*frac
	r := float64(f0.R)*(1-frac) + float64(f1.R)*frac

	s.position += s.ratio
	for s.position >= float64(n) {
		s.position -= float64(n)
	}

	return NewFrame(Sample(l), Sample(r))
}