talker alias stuff

talkeralias.go

package session

import (
	"strings"

	"dmr"
	"golang.org/x/text/encoding/unicode"
)

var (
	talkerAliasBlocks = [][]uint8{
		[]uint8{7, 15, 23, 31},
		[]uint8{6, 13, 20, 27},
		[]uint8{6, 13, 20, 27},
		[]uint8{3, 6, 10, 13},
	}
	zeroRaw dmr.Raw
)

type talkerAlias struct {
	format uint8
	length uint8
	blocks uint8
	text   [28]byte
}

func (ta *talkerAlias) Decode(meta *dmr.Meta) (string, bool) {
	if fid := meta.Raw[1]; fid != 0 {
		logger.Debugf("skip embedded data, not ETSI FID; got %#06b", fid)
		return "", false
	}

	//logger.Debugf("embedded data flco=%s", dmr.FLCO(meta.Raw[0]&0x7f))
	switch flco := dmr.FLCO(meta.Raw[0] & 0x7f); flco {
	case dmr.TalkerAliasHeaderValue:
		// byte 0: code
		// byte 1: feature
		// byte 2: options
		// byte 3-9: data
		ta.format = (meta.Raw[2] >> 6) & 0x03
		ta.length = (meta.Raw[2] >> 1) & 0x1f
		copy(ta.text[:], zeroRaw[:])
		copy(ta.text[1:], meta.Raw[3:9])

		if ta.length <= talkerAliasBlocks[ta.format][0] {
			return ta.decodeString()
		}

	case dmr.TalkerAliasBlock1Value, dmr.TalkerAliasBlock2Value, dmr.TalkerAliasBlock3Value:
		// byte 0: code
		// byte 1: feature
		// bute 2-9: data
		block := uint8(flco - dmr.TalkerAliasHeaderValue)
		copy(ta.text[7*block:], meta.Raw[2:9])

		if ta.length <= talkerAliasBlocks[ta.format][block] {
			return ta.decodeString()
		}

	default:
	}

	return "", false
}

// Character encodings
var (
	utf8           = unicode.UTF8
	utf8Decoder    = utf8.NewDecoder()
	utf16be        = unicode.UTF16(unicode.BigEndian, unicode.IgnoreBOM)
	utf16beDecoder = utf16be.NewDecoder()
)

func (ta *talkerAlias) decodeString() (s string, ok bool) {
	switch ta.format {
	case 0x00: // US-ASCII 7-bit
		var data = make([]byte, 32)
		for i := 0; i < 4; i++ {
			decode7bit(data[i*8:], ta.text[i*7:])
		}
		s = string(data[1:])

	case 0x01: // US-ASCII 8-bit
		var data = make([]rune, ta.length)
		for i := range data {
			data[i] = rune(ta.text[i+1])
		}
		s = string(data)

	case 0x02: // UTF-8
		s, _ = utf8Decoder.String(string(ta.text[1:]))

	case 0x03: // UTF-16BE
		s, _ = utf16beDecoder.String(string(ta.text[1:]))

	default:
		return
	}

	// Cleanup cruft left by cheap radios. Some radios re-use the talker alias
	// internal buffer without clearing the old buffer. Thank you China.
	return strings.SplitN(strings.TrimRight(s, " "), "\x00", 2)[0], true
}

// decode7bit decodes a block of 7-bit bytes to 8-bit bytes in chunks of 7
func decode7bit(dst, src []byte) {
	dst[0] |= (src[0]>>1)&0x7f | 0
	dst[1] |= (src[0]<<6)&0x40 | (src[1]>>2)&0x3f
	dst[2] |= (src[1]<<5)&0x60 | (src[2]>>3)&0x1f
	dst[3] |= (src[2]<<4)&0x70 | (src[3]>>4)&0x0f
	dst[4] |= (src[3]<<3)&0x78 | (src[4]>>5)&0x07
	dst[5] |= (src[4]<<2)&0x7c | (src[5]>>6)&0x03
	dst[6] |= (src[5]<<1)&0x7e | (src[6]>>7)&0x01
	dst[7] |= (src[6]<<0)&0x7f | 0
}