From 2600fa884f6d71d325dfad995cc1d053106a88d6 Mon Sep 17 00:00:00 2001
From: Trek H <trek.hopton@gmail.com>
Date: Fri, 29 Mar 2019 16:08:10 +1030
Subject: [PATCH] adpcm: modified the adpcm encoding and decoding to not use
 blocks

---
 audio/adpcm/adpcm.go | 50 ++++++++++++++++++++++++++++++++++----------
 1 file changed, 39 insertions(+), 11 deletions(-)

diff --git a/audio/adpcm/adpcm.go b/audio/adpcm/adpcm.go
index 595728a2..3e1f48f1 100644
--- a/audio/adpcm/adpcm.go
+++ b/audio/adpcm/adpcm.go
@@ -300,15 +300,20 @@ func (d *decoder) decodeBlock(block []byte) (int, error) {
 // It writes its output to the encoder's dest.
 // The number of bytes written out is returned along with any error that occured.
 func (e *encoder) Write(inPcm []byte) (int, error) {
-	numBlocks := len(inPcm) / PcmBS
-	n := 0
-	for i := 0; i < numBlocks; i++ {
-		block := inPcm[PcmBS*i : PcmBS*(i+1)]
-		_n, err := e.encodeBlock(block)
-		n += _n
+
+	n, err := e.calcHead(inPcm[0:2])
+	if err != nil {
+		return n, err
+	}
+
+	for i := 3; i < len(inPcm); i += 4 {
+		nib1 := e.encodeSample(int16(binary.LittleEndian.Uint16(inPcm[i-1 : i+1])))
+		nib2 := e.encodeSample(int16(binary.LittleEndian.Uint16(inPcm[i+1 : i+3])))
+		err = e.dest.WriteByte(byte((nib2 << 4) | nib1))
 		if err != nil {
 			return n, err
 		}
+		n++
 	}
 
 	return n, nil
@@ -318,11 +323,34 @@ func (e *encoder) Write(inPcm []byte) (int, error) {
 // It writes its output to the decoder's dest.
 // The number of bytes written out is returned along with any error that occured.
 func (d *decoder) Write(inAdpcm []byte) (int, error) {
-	numBlocks := len(inAdpcm) / AdpcmBS
-	n := 0
-	for i := 0; i < numBlocks; i++ {
-		block := inAdpcm[AdpcmBS*i : AdpcmBS*(i+1)]
-		_n, err := d.decodeBlock(block)
+
+	// Initialize decoder with first 4 bytes of the inAdpcm.
+	d.pred = int16(binary.LittleEndian.Uint16(inAdpcm[0:2]))
+	d.index = int16(inAdpcm[2])
+	d.step = stepTable[d.index]
+	n, err := d.dest.Write(inAdpcm[0:2])
+	if err != nil {
+		return n, err
+	}
+
+	// For each byte, seperate it into two nibbles (each nibble is a compressed sample),
+	// then decode each nibble and output the resulting 16-bit samples.
+	for i := 4; i < len(inAdpcm); i++ {
+		twoNibs := inAdpcm[i]
+		nib2 := byte(twoNibs >> 4)
+		nib1 := byte((nib2 << 4) ^ twoNibs)
+
+		firstBytes := make([]byte, 2)
+		binary.LittleEndian.PutUint16(firstBytes, uint16(d.decodeSample(nib1)))
+		_n, err := d.dest.Write(firstBytes)
+		n += _n
+		if err != nil {
+			return n, err
+		}
+
+		secondBytes := make([]byte, 2)
+		binary.LittleEndian.PutUint16(secondBytes, uint16(d.decodeSample(nib2)))
+		_n, err = d.dest.Write(secondBytes)
 		n += _n
 		if err != nil {
 			return n, err