package audio

import "encoding/binary"

const muLawBias = 0x84
const muLawClip = 32635

func EncodeULaw(pcm []byte) []byte {
	out := make([]byte, len(pcm)/2)
	for i := range out {
		out[i] = LinearToULaw(int16(binary.LittleEndian.Uint16(pcm[i*2:])))
	}
	return out
}

func DecodeULaw(data []byte) []byte {
	out := make([]byte, len(data)*2)
	for i, sample := range data {
		binary.LittleEndian.PutUint16(out[i*2:], uint16(ULawToLinear(sample)))
	}
	return out
}

func LinearToULaw(sample int16) byte {
	pcm := int(sample)
	sign := byte(0)
	if pcm < 0 {
		pcm = -pcm
		sign = 0x80
	}
	if pcm > muLawClip {
		pcm = muLawClip
	}
	pcm += muLawBias
	exponent := 7
	for mask := 0x4000; (pcm&mask) == 0 && exponent > 0; mask >>= 1 {
		exponent--
	}
	mantissa := (pcm >> (exponent + 3)) & 0x0f
	return ^(sign | byte(exponent<<4) | byte(mantissa))
}

func ULawToLinear(sample byte) int16 {
	u := ^sample
	sign := u & 0x80
	exponent := (u >> 4) & 0x07
	mantissa := u & 0x0f
	pcm := ((int(mantissa) << 3) + muLawBias) << exponent
	pcm -= muLawBias
	if sign != 0 {
		pcm = -pcm
	}
	return int16(pcm)
}