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third_party/sendspin-go/internal/server/resampler.go

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+// ABOUTME: Simple linear resampler for converting audio sample rates
+// ABOUTME: Used to convert MP3 files to 48kHz for Opus encoding
+package server
+
+// Resampler performs linear interpolation to convert between sample rates
+type Resampler struct {
+	inputRate  int
+	outputRate int
+	channels   int
+	ratio      float64
+	position   float64
+	lastSample []int32 // one sample per channel
+}
+
+func NewResampler(inputRate, outputRate, channels int) *Resampler {
+	return &Resampler{
+		inputRate:  inputRate,
+		outputRate: outputRate,
+		channels:   channels,
+		ratio:      float64(inputRate) / float64(outputRate),
+		position:   0.0,
+		lastSample: make([]int32, channels),
+	}
+}
+
+// Resample converts interleaved input samples at inputRate to outputRate via linear interpolation.
+func (r *Resampler) Resample(input []int32, output []int32) int {
+	if len(input) == 0 {
+		return 0
+	}
+
+	inputFrames := len(input) / r.channels
+	outputFrames := len(output) / r.channels
+
+	outIdx := 0
+
+	for outIdx < outputFrames {
+		// Calculate which input frame we need
+		inputPos := r.position
+		inputIdx := int(inputPos)
+
+		// If we've consumed all input, stop
+		if inputIdx >= inputFrames-1 {
+			break
+		}
+
+		frac := inputPos - float64(inputIdx)
+
+		for ch := 0; ch < r.channels; ch++ {
+			sample1 := input[inputIdx*r.channels+ch]
+			sample2 := input[(inputIdx+1)*r.channels+ch]
+
+			interpolated := float64(sample1)*(1.0-frac) + float64(sample2)*frac
+			output[outIdx*r.channels+ch] = int32(interpolated)
+		}
+
+		outIdx++
+		r.position += r.ratio
+	}
+
+	// Reset position for next chunk, keeping fractional part
+	r.position -= float64(int(r.position))
+
+	return outIdx * r.channels
+}
+
+func (r *Resampler) Reset() {
+	r.position = 0.0
+	for i := range r.lastSample {
+		r.lastSample[i] = 0
+	}
+}
+
+func (r *Resampler) OutputSamplesNeeded(inputSamples int) int {
+	inputFrames := inputSamples / r.channels
+	outputFrames := int(float64(inputFrames) / r.ratio)
+	return outputFrames * r.channels
+}
+
+func (r *Resampler) InputSamplesNeeded(outputSamples int) int {
+	outputFrames := outputSamples / r.channels
+	inputFrames := int(float64(outputFrames) * r.ratio)
+	return inputFrames * r.channels
+}