/*
* Reset decoder
*
* Set accumulated path metrics to zero. For termination other than
* tail-biting, initialize the zero state as the encoder starting state.
* Intialize with the maximum accumulated sum at length equal to the
* constraint length.
*/
static void reset_decoder(vdecoder dec, CONV_TERM term)
{
int ns = dec.trellis.num_states;
memset(dec.trellis.sums, 0, sizeof(short) * ns);
if (term != CONV_TERM.TAIL_BITING)
{
dec.trellis.sums[0] = (short)(sbyte.MaxValue * dec.n * dec.k);
}
}
/*
* Traceback and generate decoded output
*
* For tail biting, find the largest accumulated path metric at the final state
* followed by two trace back passes. For zero flushing the final state is
* always zero with a single traceback path.
*/
static int traceback(vdecoder dec, sbyte *o, CONV_TERM term, int len)
{
int i, sum, max_p = -1, max = -1;
int path, state = 0;
if (term == CONV_TERM.TAIL_BITING)
{
for (i = 0; i < dec.trellis.num_states; i++)
{
sum = dec.trellis.sums[i];
if (sum > max)
{
max_p = max;
max = sum;
state = (int)i;
}
}
if (max < 0)
{
throw new Exception();
}
for (i = dec.len - 1; i >= len + TAIL_BITING_EXTRA; i--)
{
path = (int)(dec.paths[i][state] + 1);
int prestate = state;
state = vstate_lshift(state, dec.k, (int)path);
//Console.WriteLine($"[Q] path={path}, state={state}, prestate={state}, k={dec.k}, i={i}");
}
}
else
{
for (i = dec.len - 1; i >= len; i--)
{
path = (int)(dec.paths[i][state] + 1);
state = vstate_lshift(state, dec.k, (int)path);
}
}
if (dec.recursive != 0)
{
_traceback_rec(dec, state, o, len);
}
else
{
state = _traceback(dec, state, o, len, term == CONV_TERM.TAIL_BITING ? TAIL_BITING_EXTRA : 0);
}
/* Don't handle the odd case of recursize tail-biting codes */
return(max - max_p);
}
/*
* Forward trellis recursion
*
* Generate branch metrics and path metrics with a combined function. Only
* accumulated path metric sums and path selections are stored. Normalize on
* the interval specified by the decoder.
*/
static void _conv_decode(vdecoder dec, sbyte *seq, CONV_TERM term, int len)
{
int i, j = 0;
vtrellis trellis = dec.trellis;
if (term == CONV_TERM.TAIL_BITING)
{
j = len - TAIL_BITING_EXTRA;
}
for (i = 0; i < dec.len; i++, j++)
{
if (term == CONV_TERM.TAIL_BITING && j == len)
{
j = 0;
}
gen_metrics_k7_n3(&seq[dec.n * j],
trellis.outputs,
trellis.sums,
dec.paths[i],
((i % dec.intrvl) != 0 ? 0 : 1)); //flipped on purpose
}
}
