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use in-place fast Walsh–Hadamard transform

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This commit is contained in:
Ahmet Inan 2020-06-13 21:35:59 +02:00
commit 3596f2943b
1 changed files with 23 additions and 26 deletions
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49
hadamard_decoder.hh
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@ -13,37 +13,34 @@ class HadamardDecoder
{
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:
HadamardDecoder()
{
for (int msg = 0; msg < W; ++msg)
for (int i = 0; i < N; ++i)
mod[msg*N+i] = 1 - 2 * parity(msg&(i|N));
}
int operator()(const int8_t *code)
int operator()(const int8_t *c)
{
int sum[N];
for (int i = 0; i < N; i += 2) {
sum[i] = c[i] + c[i+1];
sum[i+1] = c[i] - c[i+1];
}
for (int h = 2; h < N; h *= 2) {
for (int i = 0; i < N; 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) {
for (int i = 0; i < N; ++i) {
int mag = std::abs(sum[i]);
int msg = i + N * (sum[i] < 0);
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)