trellis: decoder

trellis/trellis.go

@@ -0,0 +1,234 @@
+package trellis
+
+import (
+	"errors"
+	"fmt"
+	"log"
+
+	"github.com/pd0mz/go-dmr"
+)
+
+var (
+	// See DMR AI protocol spec. page 130.
+	interleaveMatrix = []uint8{
+		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,
+		2, 3, 10, 11, 18, 19, 26, 27, 34, 35, 42, 43, 50, 51, 58, 59, 66, 67, 74, 75, 82, 83, 90, 91,
+		4, 5, 12, 13, 20, 21, 28, 29, 36, 37, 44, 45, 52, 53, 60, 61, 68, 69, 76, 77, 84, 85, 92, 93,
+		6, 7, 14, 15, 22, 23, 30, 31, 38, 39, 46, 47, 54, 55, 62, 63, 70, 71, 78, 79, 86, 87, 94, 95,
+	}
+
+	// See DMR AI protocol spec. page 129.
+	encoderStateTransition = []uint8{
+		0, 8, 4, 12, 2, 10, 6, 14,
+		4, 12, 2, 10, 6, 14, 0, 8,
+		1, 9, 5, 13, 3, 11, 7, 15,
+		5, 13, 3, 11, 7, 15, 1, 9,
+		3, 11, 7, 15, 1, 9, 5, 13,
+		7, 15, 1, 9, 5, 13, 3, 11,
+		2, 10, 6, 14, 0, 8, 4, 12,
+		6, 14, 0, 8, 4, 12, 2, 10,
+	}
+)
+
+func init() {
+	log.Printf("interleave matrix has %d points\n", len(interleaveMatrix))
+}
+
+// Decode is a convenience function that takes 196 Info bits and decodes them to 18 bytes (144 bits) binary using Trellis decoding.
+func Decode(bits []byte, bytes []byte) error {
+	if bytes == nil {
+		return errors.New("trellis: bytes can't be nil")
+	}
+	if len(bytes) < 18 {
+		return fmt.Errorf("trellis: need buffer of at least 18 bytes, got %d", len(bytes))
+	}
+	dibits, err := ExtractDibits(bits)
+	if err != nil {
+		return err
+	}
+	deinterleaved, err := Deinterleave(dibits)
+	if err != nil {
+		return err
+	}
+	points, err := ConstellationPoints(deinterleaved)
+	if err != nil {
+		return err
+	}
+	tribits, err := ExtractTribits(points)
+	if err != nil {
+		return err
+	}
+	binary, err := ExtractBinary(tribits)
+	if err != nil {
+		return err
+	}
+	copy(bytes, dmr.BitsToBytes(binary))
+	return nil
+}
+
+// ExtractDibits extracts dibits from bits.
+func ExtractDibits(bits []byte) ([]int8, error) {
+	if len(bits) != dmr.InfoBits {
+		return nil, fmt.Errorf("trellis: expected %d bits, got %d", dmr.InfoBits, len(bits))
+	}
+	var dibits = make([]int8, 98)
+
+	for i := 0; i < 196; i += 2 {
+		o := i / 2
+		switch {
+		case bits[i] == 0 && bits[i+1] == 1:
+			dibits[o] = 3
+			break
+		case bits[i] == 0 && bits[i+1] == 0:
+			dibits[o] = 1
+			break
+		case bits[i] == 1 && bits[i+1] == 0:
+			dibits[o] = -1
+			break
+		case bits[i] == 1 && bits[i+1] == 1:
+			dibits[o] = -3
+			break
+		}
+	}
+
+	return dibits, nil
+}
+
+// Deinterleave the dibits according to DMR AI protocol spec. page 130.
+func Deinterleave(dibits []int8) ([]int8, error) {
+	if dibits == nil {
+		return nil, errors.New("trellis: dibits can't be nil")
+	}
+	if len(dibits) != 98 {
+		return nil, fmt.Errorf("trellis: expected 98 dibits, got %d", len(dibits))
+	}
+
+	var deinterleaved = make([]int8, 98)
+	for i := 0; i < 98; i++ {
+		deinterleaved[interleaveMatrix[i]] = dibits[i]
+	}
+	return deinterleaved, nil
+}
+
+// ConstellationPoints decodes the constellation points according to DMR AI protocol spec. page 129.
+func ConstellationPoints(dibits []int8) ([]uint8, error) {
+	if dibits == nil {
+		return nil, errors.New("trellis: dibits can't be nil")
+	}
+	if len(dibits) != 98 {
+		return nil, fmt.Errorf("trellis: expected 98 dibits, got %d", len(dibits))
+	}
+
+	var points = make([]uint8, 49)
+	for i := 0; i < 98; i += 2 {
+		o := i / 2
+		switch {
+		case dibits[i] == +1 && dibits[i+1] == -1:
+			points[o] = 0
+			break
+		case dibits[i] == -1 && dibits[i+1] == -1:
+			points[o] = 1
+			break
+		case dibits[i] == +3 && dibits[i+1] == -3:
+			points[o] = 2
+			break
+		case dibits[i] == -3 && dibits[i+1] == -3:
+			points[o] = 3
+			break
+		case dibits[i] == -3 && dibits[i+1] == -1:
+			points[o] = 4
+			break
+		case dibits[i] == +3 && dibits[i+1] == -1:
+			points[o] = 5
+			break
+		case dibits[i] == -1 && dibits[i+1] == -3:
+			points[o] = 6
+			break
+		case dibits[i] == +1 && dibits[i+1] == -3:
+			points[o] = 7
+			break
+		case dibits[i] == -3 && dibits[i+1] == +3:
+			points[o] = 8
+			break
+		case dibits[i] == +3 && dibits[i+1] == +3:
+			points[o] = 9
+			break
+		case dibits[i] == -1 && dibits[i+1] == +1:
+			points[o] = 10
+			break
+		case dibits[i] == +1 && dibits[i+1] == +1:
+			points[o] = 11
+			break
+		case dibits[i] == +1 && dibits[i+1] == +3:
+			points[o] = 12
+			break
+		case dibits[i] == -1 && dibits[i+1] == +3:
+			points[o] = 13
+			break
+		case dibits[i] == +3 && dibits[i+1] == +1:
+			points[o] = 14
+			break
+		case dibits[i] == -3 && dibits[i+1] == +1:
+			points[o] = 15
+			break
+		}
+	}
+
+	return points, nil
+}
+
+// ExtractTribits maps constellation points to Trellis tribits according to DMR AI protocol spec. page 129.
+func ExtractTribits(points []uint8) ([]uint8, error) {
+	var (
+		match   bool
+		last    uint8
+		start   int
+		tribits = make([]uint8, 48)
+	)
+
+	for i := 0; i < 48; i++ {
+		start = int(last) * 8
+		match = false
+		for j := start; j < (start + 8); j++ {
+			// Check if this constellation point matches an element of this row of the state table.
+			if points[i] == encoderStateTransition[j] {
+				match = true
+				last = uint8(j - start)
+				tribits[i] = uint8(last)
+			}
+		}
+
+		if !match {
+			return nil, fmt.Errorf("trellis: trellis tribit extract error at point %d, data is corrupted", i)
+		}
+	}
+
+	return tribits, nil
+}
+
+// ExtractBinary maps the tribits back to bits.
+func ExtractBinary(tribits []uint8) ([]byte, error) {
+	if tribits == nil {
+		return nil, errors.New("trellis: tribits can't be nill")
+	}
+	if len(tribits) != 48 {
+		return nil, fmt.Errorf("trellis: tribits length is %d, expected 48", len(tribits))
+	}
+
+	var bits = make([]byte, 196)
+	for i := 0; i < 144; i += 3 {
+		o := i / 3
+
+		if tribits[o]&dmr.B00000100 > 0 {
+			bits[i] = 1
+		}
+		if tribits[o]&dmr.B00000010 > 0 {
+			bits[i+1] = 1
+		}
+		if tribits[o]&dmr.B00000001 > 0 {
+			bits[i+2] = 1
+		}
+	}
+
+	return bits, nil
+}