54 lines
1.1 KiB
C++
54 lines
1.1 KiB
C++
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#include "fwht.h"
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#include <vector>
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using namespace std;
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/*
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* we count on that all integers are sufficiently large.
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* They should be, largest value occuring should be O(k*n) if initial vector is
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* consisted only from {0,1}^n, and we don't usually have codes of this size.
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*/
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static void fwht (vector<int> x, vector<int>&r)
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{
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int bs, s;
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s = x.size();
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r.resize (s);
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bs = s >> 1;
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r.swap (x);
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while (bs) {
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x.swap (r);
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for (uint i = 0; i < s; ++i) {
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if ( (i / bs) & 1)
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r[i] = x[i-bs] - x[i];
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else
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r[i] = x[i] + x[i+bs];
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}
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bs >>= 1;
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}
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}
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//we expect correct parameter size and preallocated out.
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void fwht_dyadic_multiply (const bvector& a, const bvector& b, bvector& out)
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{
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//lift everyting to Z.
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vector<int> t, A, B;
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uint i;
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for (i = 0; i < a.size(); ++i) t[i] = a[i];
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fwht (t, A);
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for (i = 0; i < b.size(); ++i) t[i] = b[i];
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fwht (t, B);
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//multiply diagonals to A
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for (i = 0; i < A.size(); ++i) A[i] *= B[i];
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fwht (A, t);
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uint bitpos = a.size(); //no problem as a.size() == 1<<m == 2^m
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for (i = 0; i < t.size(); ++i) out[i] = (t[i] & bitpos) ? 1 : 0;
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}
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