graycode.h

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#ifndef GRAYCODE_H_
#define GRAYCODE_H_

#include "typedefs.h"

/******************************************************************************
 *
 *                 M4RI: Linear Algebra over GF(2)
 *
 *    Copyright (C) 2007 Gregory Bard <gregory.bard@ieee.org>
 *    Copyright (C) 2007 Martin Albrecht <malb@informatik.uni-bremen.de>
 *
 *  Distributed under the terms of the GNU General Public License (GPL)
 *  version 2 or higher.
 *
 *    This code is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *    General Public License for more details.
 *
 *  The full text of the GPL is available at:
 *
 *                  http://www.gnu.org/licenses/
 ******************************************************************************/

typedef struct {
    int *ord;
    int *inc;
} code;

int gray_code(int i, int l);

code* build_code(int l);

void destroy_code(code* codebook);

static inline int log2_floor(int v) {
    static unsigned const int b[] = { 0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000 };
    static unsigned const int S[] = { 1, 2, 4, 8, 16 };
    unsigned int r = 0;
    for (int i = 4; i >= 0; --i) {
        if ((v & b[i])) {
            v >>= S[i];
            r |= S[i];
        }
    }
    return r;
}

#endif /* GRAYCODE_H_ */