trellis: decoder
Wijnand Modderman-Lenstra
9 years ago
parent
a4db831098
commit
caa595110d
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package trellis
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import (
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"errors"
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"fmt"
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"log"
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"github.com/pd0mz/go-dmr"
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)
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var (
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// See DMR AI protocol spec. page 130.
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interleaveMatrix = []uint8{
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0, 1, 8, 9, 16, 17, 24, 25, 32, 33, 40, 41, 48, 49, 56, 57, 64, 65, 72, 73, 80, 81, 88, 89, 96, 97,
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2, 3, 10, 11, 18, 19, 26, 27, 34, 35, 42, 43, 50, 51, 58, 59, 66, 67, 74, 75, 82, 83, 90, 91,
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4, 5, 12, 13, 20, 21, 28, 29, 36, 37, 44, 45, 52, 53, 60, 61, 68, 69, 76, 77, 84, 85, 92, 93,
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6, 7, 14, 15, 22, 23, 30, 31, 38, 39, 46, 47, 54, 55, 62, 63, 70, 71, 78, 79, 86, 87, 94, 95,
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}
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// See DMR AI protocol spec. page 129.
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encoderStateTransition = []uint8{
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0, 8, 4, 12, 2, 10, 6, 14,
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4, 12, 2, 10, 6, 14, 0, 8,
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1, 9, 5, 13, 3, 11, 7, 15,
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5, 13, 3, 11, 7, 15, 1, 9,
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3, 11, 7, 15, 1, 9, 5, 13,
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7, 15, 1, 9, 5, 13, 3, 11,
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2, 10, 6, 14, 0, 8, 4, 12,
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6, 14, 0, 8, 4, 12, 2, 10,
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}
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)
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func init() {
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log.Printf("interleave matrix has %d points\n", len(interleaveMatrix))
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}
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// Decode is a convenience function that takes 196 Info bits and decodes them to 18 bytes (144 bits) binary using Trellis decoding.
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func Decode(bits []byte, bytes []byte) error {
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if bytes == nil {
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return errors.New("trellis: bytes can't be nil")
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}
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if len(bytes) < 18 {
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return fmt.Errorf("trellis: need buffer of at least 18 bytes, got %d", len(bytes))
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}
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dibits, err := ExtractDibits(bits)
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if err != nil {
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return err
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}
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deinterleaved, err := Deinterleave(dibits)
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if err != nil {
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return err
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}
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points, err := ConstellationPoints(deinterleaved)
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if err != nil {
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return err
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}
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tribits, err := ExtractTribits(points)
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if err != nil {
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return err
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}
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binary, err := ExtractBinary(tribits)
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if err != nil {
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return err
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}
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copy(bytes, dmr.BitsToBytes(binary))
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return nil
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}
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// ExtractDibits extracts dibits from bits.
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func ExtractDibits(bits []byte) ([]int8, error) {
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if len(bits) != dmr.InfoBits {
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return nil, fmt.Errorf("trellis: expected %d bits, got %d", dmr.InfoBits, len(bits))
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}
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var dibits = make([]int8, 98)
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for i := 0; i < 196; i += 2 {
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o := i / 2
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switch {
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case bits[i] == 0 && bits[i+1] == 1:
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dibits[o] = 3
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break
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case bits[i] == 0 && bits[i+1] == 0:
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dibits[o] = 1
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break
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case bits[i] == 1 && bits[i+1] == 0:
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dibits[o] = -1
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break
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case bits[i] == 1 && bits[i+1] == 1:
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dibits[o] = -3
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break
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}
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}
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return dibits, nil
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}
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// Deinterleave the dibits according to DMR AI protocol spec. page 130.
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func Deinterleave(dibits []int8) ([]int8, error) {
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if dibits == nil {
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return nil, errors.New("trellis: dibits can't be nil")
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}
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if len(dibits) != 98 {
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return nil, fmt.Errorf("trellis: expected 98 dibits, got %d", len(dibits))
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}
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var deinterleaved = make([]int8, 98)
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for i := 0; i < 98; i++ {
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deinterleaved[interleaveMatrix[i]] = dibits[i]
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}
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return deinterleaved, nil
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}
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// ConstellationPoints decodes the constellation points according to DMR AI protocol spec. page 129.
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func ConstellationPoints(dibits []int8) ([]uint8, error) {
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if dibits == nil {
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return nil, errors.New("trellis: dibits can't be nil")
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}
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if len(dibits) != 98 {
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return nil, fmt.Errorf("trellis: expected 98 dibits, got %d", len(dibits))
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}
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var points = make([]uint8, 49)
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for i := 0; i < 98; i += 2 {
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o := i / 2
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switch {
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case dibits[i] == +1 && dibits[i+1] == -1:
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points[o] = 0
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break
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case dibits[i] == -1 && dibits[i+1] == -1:
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points[o] = 1
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break
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case dibits[i] == +3 && dibits[i+1] == -3:
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points[o] = 2
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break
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case dibits[i] == -3 && dibits[i+1] == -3:
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points[o] = 3
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break
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case dibits[i] == -3 && dibits[i+1] == -1:
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points[o] = 4
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break
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case dibits[i] == +3 && dibits[i+1] == -1:
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points[o] = 5
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break
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case dibits[i] == -1 && dibits[i+1] == -3:
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points[o] = 6
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break
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case dibits[i] == +1 && dibits[i+1] == -3:
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points[o] = 7
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break
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case dibits[i] == -3 && dibits[i+1] == +3:
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points[o] = 8
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break
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case dibits[i] == +3 && dibits[i+1] == +3:
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points[o] = 9
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break
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case dibits[i] == -1 && dibits[i+1] == +1:
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points[o] = 10
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break
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case dibits[i] == +1 && dibits[i+1] == +1:
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points[o] = 11
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break
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case dibits[i] == +1 && dibits[i+1] == +3:
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points[o] = 12
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break
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case dibits[i] == -1 && dibits[i+1] == +3:
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points[o] = 13
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break
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case dibits[i] == +3 && dibits[i+1] == +1:
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points[o] = 14
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break
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case dibits[i] == -3 && dibits[i+1] == +1:
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points[o] = 15
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break
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}
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}
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return points, nil
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}
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// ExtractTribits maps constellation points to Trellis tribits according to DMR AI protocol spec. page 129.
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func ExtractTribits(points []uint8) ([]uint8, error) {
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var (
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match bool
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last uint8
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start int
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tribits = make([]uint8, 48)
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)
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for i := 0; i < 48; i++ {
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start = int(last) * 8
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match = false
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for j := start; j < (start + 8); j++ {
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// Check if this constellation point matches an element of this row of the state table.
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if points[i] == encoderStateTransition[j] {
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match = true
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last = uint8(j - start)
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tribits[i] = uint8(last)
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}
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}
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if !match {
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return nil, fmt.Errorf("trellis: trellis tribit extract error at point %d, data is corrupted", i)
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}
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}
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return tribits, nil
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}
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// ExtractBinary maps the tribits back to bits.
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func ExtractBinary(tribits []uint8) ([]byte, error) {
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if tribits == nil {
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return nil, errors.New("trellis: tribits can't be nill")
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}
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if len(tribits) != 48 {
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return nil, fmt.Errorf("trellis: tribits length is %d, expected 48", len(tribits))
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}
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var bits = make([]byte, 196)
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for i := 0; i < 144; i += 3 {
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o := i / 3
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if tribits[o]&dmr.B00000100 > 0 {
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bits[i] = 1
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}
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if tribits[o]&dmr.B00000010 > 0 {
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bits[i+1] = 1
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}
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if tribits[o]&dmr.B00000001 > 0 {
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bits[i+2] = 1
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}
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}
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return bits, nil
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}
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