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use in-place fast Walsh–Hadamard transform

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This commit is contained in:
Ahmet Inan 2020-06-14 00:23:11 +02:00
commit 9b4998a3c5
1 changed files with 20 additions and 25 deletions
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45
simplex_decoder.hh
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@ -12,38 +12,33 @@ template <int K>
class SimplexDecoder
{
static const int W = 1 << K;
static const int N = (1 << K) - 1;
int8_t mod[W*N];
static bool parity(unsigned x)
{
x ^= x >> 16;
x ^= x >> 8;
x ^= x >> 4;
x ^= x >> 2;
x ^= x >> 1;
return x & 1;
}
public:
SimplexDecoder()
{
for (int msg = 0; msg < W; ++msg)
for (int i = 0; i < N; ++i)
mod[msg*N+i] = 1 - 2 * parity(msg&(i+1));
}
int operator()(const int8_t *code)
{
int sum[W] = { code[0], -code[0], };
for (int i = 1; i < W; i += 2) {
sum[i+1] = code[i] + code[i+1];
sum[i+2] = code[i] - code[i+1];
}
for (int h = 2; h < W; h *= 2) {
for (int i = 0; i < W; i += 2 * h) {
for (int j = i; j < i + h; ++j) {
int x = sum[j] + sum[j+h];
int y = sum[j] - sum[j+h];
sum[j] = x;
sum[j+h] = y;
}
}
}
int word = 0, best = 0, next = 0;
for (int msg = 0; msg < W; ++msg) {
int sum = 0;
for (int i = 0; i < N; ++i)
sum += mod[msg*N+i] * code[i];
if (sum > best) {
int mag = sum[msg];
if (mag > best) {
next = best;
best = sum;
best = mag;
word = msg;
} else if (sum > next) {
next = sum;
} else if (mag > next) {
next = mag;
}
}
if (best == next)