// 1 is perfect and 0 means all the bits were 0.5
static float getEnergy(const SoftVector &vec,float &low)
{
int len = vec.size();
avg = 0; low = 1;
for (int i = 0; i < len; i++) {
float bit = vec[i];
float energy = 2*((bit < 0.5) ? (0.5-bit) : (bit-0.5));
if (energy < low) low = energy;
avg += energy/len;
}
}
void ViterbiTCH_AFS7_4::decode(const SoftVector &in, BitVector& target)
{
ViterbiTCH_AFS7_4 &decoder = *this;
const size_t sz = in.size() - 12;
const unsigned deferral = decoder.deferral();
const size_t ctsz = sz + deferral*decoder.iRate();
assert(sz == decoder.iRate()*target.size());
// Build a "history" array where each element contains the full history.
uint32_t history[ctsz];
{
BitVector bits = in.sliced();
uint32_t accum = 0;
for (size_t i=0; i<sz; i++) {
accum = (accum<<1) | bits.bit(i);
history[i] = accum;
}
// Repeat last bit at the end.
for (size_t i=sz; i<ctsz; i++) {
accum = (accum<<1) | (accum & 0x01);
history[i] = accum;
}
}
// Precompute metric tables.
float matchCostTable[ctsz];
float mismatchCostTable[ctsz];
{
const float *dp = in.begin();
for (size_t i=0; i<sz; i++) {
// pVal is the probability that a bit is correct.
// ipVal is the probability that a bit is incorrect.
float pVal = dp[i];
if (pVal>0.5F) pVal = 1.0F-pVal;
float ipVal = 1.0F-pVal;
// This is a cheap approximation to an ideal cost function.
if (pVal<0.01F) pVal = 0.01;
if (ipVal<0.01F) ipVal = 0.01;
matchCostTable[i] = 0.25F/ipVal;
mismatchCostTable[i] = 0.25F/pVal;
}
// pad end of table with unknowns
for (size_t i=sz; i<ctsz; i++) {
matchCostTable[i] = 0.5F;
mismatchCostTable[i] = 0.5F;
}
}
{
decoder.initializeStates();
// Each sample of history[] carries its history.
// So we only have to process every iRate-th sample.
const unsigned step = decoder.iRate();
// input pointer
const uint32_t *ip = history + step - 1;
// output pointers
char *op = target.begin();
const char *const opt = target.end();
// table pointers
const float* match = matchCostTable;
const float* mismatch = mismatchCostTable;
size_t oCount = 0;
while (op<opt) {
// Viterbi algorithm
assert(match-matchCostTable<(int)(sizeof(matchCostTable)/sizeof(matchCostTable[0])-1));
assert(mismatch-mismatchCostTable<(int)(sizeof(mismatchCostTable)/sizeof(mismatchCostTable[0])-1));
const ViterbiTCH_AFS7_4::vCand &minCost = decoder.step(*ip, match, mismatch);
ip += step;
match += step;
mismatch += step;
// output
if (oCount>=deferral) *op++ = (minCost.iState >> deferral)&0x01;
oCount++;
}
}
}