mirror of https://bitbucket.org/ausocean/av.git
261 lines
7.1 KiB
Go
261 lines
7.1 KiB
Go
/*
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NAME
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adpcm.go
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DESCRIPTION
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adpcm.go contains functions for encoding/compressing pcm into adpcm and decoding/decompressing back to pcm.
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AUTHOR
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Trek Hopton <trek@ausocean.org>
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LICENSE
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adpcm.go is Copyright (C) 2018 the Australian Ocean Lab (AusOcean)
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It is free software: you can redistribute it and/or modify them
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under the terms of the GNU General Public License as published by the
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Free Software Foundation, either version 3 of the License, or (at your
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option) any later version.
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It is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more details.
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You should have received a copy of the GNU General Public License in gpl.txt.
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If not, see [GNU licenses](http://www.gnu.org/licenses).
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*/
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/*
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Original IMA/DVI ADPCM specification: (http://www.cs.columbia.edu/~hgs/audio/dvi/IMA_ADPCM.pdf).
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Reference algorithms for ADPCM compression and decompression are in part 6.
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*/
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package adpcm
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import (
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"bytes"
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"encoding/binary"
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"fmt"
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)
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// Encoder is used to encode to ADPCM from PCM data.
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// pred and index hold state that persists between calls to encodeSample and calcHead.
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// dest is the output buffer that implements io.writer and io.bytewriter, ie. where the encoded ADPCM data is written to.
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type Encoder struct {
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dest *bytes.Buffer
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pred int16
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index int16
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}
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// Decoder is used to decode from ADPCM to PCM data.
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// pred, index, and step hold state that persists between calls to decodeSample.
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// dest is the output buffer that implements io.writer and io.bytewriter, ie. where the decoded PCM data is written to.
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type Decoder struct {
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dest *bytes.Buffer
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pred int16
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index int16
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step int16
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}
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// Table of index changes (see spec).
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var indexTable = []int16{
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-1, -1, -1, -1, 2, 4, 6, 8,
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-1, -1, -1, -1, 2, 4, 6, 8,
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}
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// Quantize step size table (see spec).
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var stepTable = []int16{
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7, 8, 9, 10, 11, 12, 13, 14,
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16, 17, 19, 21, 23, 25, 28, 31,
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34, 37, 41, 45, 50, 55, 60, 66,
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73, 80, 88, 97, 107, 118, 130, 143,
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157, 173, 190, 209, 230, 253, 279, 307,
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337, 371, 408, 449, 494, 544, 598, 658,
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724, 796, 876, 963, 1060, 1166, 1282, 1411,
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1552, 1707, 1878, 2066, 2272, 2499, 2749, 3024,
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3327, 3660, 4026, 4428, 4871, 5358, 5894, 6484,
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7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
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15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794,
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32767,
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}
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// NewEncoder retuns a new ADPCM encoder.
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func NewEncoder(dst *bytes.Buffer) *Encoder {
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e := Encoder{
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dest: dst,
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}
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return &e
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}
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// NewDecoder retuns a new ADPCM decoder.
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func NewDecoder(dst *bytes.Buffer) *Decoder {
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d := Decoder{
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step: stepTable[0],
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dest: dst,
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}
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return &d
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}
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// encodeSample takes a single 16 bit PCM sample and
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// returns a byte of which the last 4 bits are an encoded ADPCM nibble.
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func (e *Encoder) encodeSample(sample int16) byte {
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// Find difference of actual sample from encoder's prediction.
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delta := sample - e.pred
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// Create and set sign bit for nibble and find absolute value of difference.
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var nib byte
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if delta < 0 {
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nib = 8
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delta = -delta
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}
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step := stepTable[e.index]
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diff := step >> 3
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var mask byte = 4
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for i := 0; i < 3; i++ {
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if delta > step {
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nib |= mask
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delta -= step
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diff += step
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}
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mask >>= 1
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step >>= 1
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}
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// Adjust predicted sample based on calculated difference.
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if nib&8 != 0 {
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e.pred -= diff
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} else {
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e.pred += diff
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}
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e.index += indexTable[nib&7]
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// Check for underflow and overflow.
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if e.index < 0 {
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e.index = 0
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} else if e.index > int16(len(stepTable)-1) {
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e.index = int16(len(stepTable) - 1)
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}
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return nib
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}
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// decodeSample takes a byte, the last 4 bits of which contain a single
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// 4 bit ADPCM nibble, and returns a 16 bit decoded PCM sample.
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func (d *Decoder) decodeSample(nibble byte) int16 {
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// Calculate difference.
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var diff int16
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if nibble&4 != 0 {
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diff += d.step
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}
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if nibble&2 != 0 {
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diff += d.step >> 1
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}
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if nibble&1 != 0 {
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diff += d.step >> 2
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}
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diff += d.step >> 3
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// Account for sign bit.
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if nibble&8 != 0 {
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diff = -diff
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}
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// Adjust predicted sample based on calculated difference.
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d.pred += diff
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// Adjust index into step size lookup table using nibble.
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d.index += indexTable[nibble]
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// Check for overflow and underflow.
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if d.index < 0 {
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d.index = 0
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} else if d.index > int16(len(stepTable)-1) {
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d.index = int16(len(stepTable) - 1)
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}
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// Find new quantizer step size.
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d.step = stepTable[d.index]
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return d.pred
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}
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// calcHead sets the state for the encoder by running the first sample through
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// the encoder, and writing the first sample.
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func (e *Encoder) calcHead(sample []byte) error {
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// Check that we are given 1 16-bit sample (2 bytes).
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sampSize := 2
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if len(sample) != sampSize {
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return fmt.Errorf("length of given byte array is: %v, expected: %v", len(sample), sampSize)
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}
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intSample := int16(binary.LittleEndian.Uint16(sample))
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e.encodeSample(intSample)
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e.dest.Write(sample)
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e.dest.WriteByte(byte(uint16(e.index)))
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e.dest.WriteByte(byte(0x00))
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return nil
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}
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// EncodeBlock takes a slice of 1010 bytes (505 16-bit PCM samples).
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// It outputs encoded (compressed) bytes (each byte containing two ADPCM nibbles) to the encoder's dest writer.
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// Note: nibbles are output in little endian order, eg. n1n0 n3n2 n5n4...
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// Note: first 4 bytes are for initializing the decoder before decoding a block.
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// - First two bytes contain the first 16-bit sample uncompressed.
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// - Third byte is the decoder's starting index for the block, the fourth is padding and ignored.
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func (e *Encoder) EncodeBlock(block []byte) error {
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bSize := 1010
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if len(block) != bSize {
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return fmt.Errorf("unsupported block size. Given: %v, expected: %v, ie. 505 16-bit PCM samples", len(block), bSize)
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}
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err := e.calcHead(block[0:2])
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if err != nil {
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return err
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}
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for i := 3; i < bSize; i += 4 {
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nib1 := e.encodeSample(int16(binary.LittleEndian.Uint16(block[i-1 : i+1])))
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nib2 := e.encodeSample(int16(binary.LittleEndian.Uint16(block[i+1 : i+3])))
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e.dest.WriteByte(byte((nib2 << 4) | nib1))
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}
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return nil
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}
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// DecodeBlock takes a slice of 256 bytes, each byte after the first 4 should contain two ADPCM encoded nibbles.
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// It outputs the resulting decoded (decompressed) 16-bit PCM samples to the decoder's dest writer.
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func (d *Decoder) DecodeBlock(block []byte) error {
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bSize := 256
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if len(block) != bSize {
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return fmt.Errorf("unsupported block size. Given: %v, expected: %v", len(block), bSize)
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}
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// Initialize decoder with first 4 bytes of the block.
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d.pred = int16(binary.LittleEndian.Uint16(block[0:2]))
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d.index = int16(block[2])
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d.step = stepTable[d.index]
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d.dest.Write(block[0:2])
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// For each byte, seperate it into two nibbles (each nibble is a compressed sample),
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// then decode each nibble and output the resulting 16-bit samples.
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for i := 4; i < bSize; i++ {
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twoNibs := block[i]
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nib2 := byte(twoNibs >> 4)
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nib1 := byte((nib2 << 4) ^ twoNibs)
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firstBytes := make([]byte, 2)
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binary.LittleEndian.PutUint16(firstBytes, uint16(d.decodeSample(nib1)))
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d.dest.Write(firstBytes)
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secondBytes := make([]byte, 2)
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binary.LittleEndian.PutUint16(secondBytes, uint16(d.decodeSample(nib2)))
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d.dest.Write(secondBytes)
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}
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return nil
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}
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