// Package audio contains small, shared PCM16 transformations used by media and AI adapters.
package audio

import "encoding/binary"

// ResamplePCM16 linearly resamples mono signed little-endian PCM16.
// It preserves an empty input and returns a copy when the rates match.
func ResamplePCM16(src []byte, sourceRate, targetRate int) []byte {
	if sourceRate <= 0 || targetRate <= 0 || len(src) < 2 || sourceRate == targetRate {
		return append([]byte(nil), src...)
	}
	samples := len(src) / 2
	outSamples := samples * targetRate / sourceRate
	if outSamples < 1 {
		return nil
	}
	out := make([]byte, outSamples*2)
	for i := 0; i < outSamples; i++ {
		position := float64(i) * float64(sourceRate) / float64(targetRate)
		left := int(position)
		if left >= samples {
			left = samples - 1
		}
		right := left + 1
		if right >= samples {
			right = left
		}
		frac := position - float64(left)
		lv := int16(binary.LittleEndian.Uint16(src[left*2 : left*2+2]))
		rv := int16(binary.LittleEndian.Uint16(src[right*2 : right*2+2]))
		value := int16(float64(lv)*(1-frac) + float64(rv)*frac)
		binary.LittleEndian.PutUint16(out[i*2:i*2+2], uint16(value))
	}
	return out
}