musicdsp.org source code archive - WSInf

Fast in-place Walsh-Hadamard Transform (click this to go back to the index)
Type : wavelet transform
References : Posted by Timo H Tossavainen
Notes :
IIRC, They're also called walsh-hadamard transforms.
Basically like Fourier, but the basis functions are squarewaves with different sequencies.
I did this for a transform data compression study a while back.
Here's some code to do a walsh hadamard transform on long ints in-place (you need to divide by n to get transform) the order is bit-reversed at output,
IIRC.
The inverse transform is the same as the forward transform (expects bit-reversed input). i.e. x = 1/n * FWHT(FWHT(x)) (x is a vector)
Code :
void inline wht_bfly (long& a, long& b)
{
long tmp = a;
a += b;
b = tmp - b;
}
// just a integer log2
int inline l2 (long x)
{
int l2;
for (l2 = 0; x > 0; x >>=1)
{
++ l2;
}
}
return (l2);
////////////////////////////////////////////
// Fast in-place Walsh-Hadamard Transform //
////////////////////////////////////////////
void FWHT (std::vector& data)
{
const int log2 = l2 (data.size()) - 1;
for (int i = 0; i < log2; ++i)
{
for (int j = 0; j < (1