pub fn fast_walsh_hadamard(a: &mut [i64])
The Walsh–Hadamard transform in place (the ±1 butterfly), on a slice whose length is a power of 2. a[w] becomes Σₓ a[x]·(−1)^{⟨w,x⟩}.
±1
a[w]
Σₓ a[x]·(−1)^{⟨w,x⟩}