/*

 * Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>

 *

 * Permission is hereby granted, free of charge, to any person obtaining 

 * a copy of this software and associated documentation files (the

 * "Software"), to deal in the Software without restriction, including

 * without limitation the rights to use, copy, modify, merge, publish,

 * distribute, sublicense, and/or sell copies of the Software, and to

 * permit persons to whom the Software is furnished to do so, subject to

 * the following conditions:

 *

 * The above copyright notice and this permission notice shall be 

 * included in all copies or substantial portions of the Software.

 *

 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 

 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF

 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 

 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS

 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN

 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN

 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE

 * SOFTWARE.

 */

#include "inner.h"

/* see inner.h */

uint32_t

br_i31_decode_mod(uint32_t *x, const void *src, size_t len, const uint32_t *m)

{

  /*

   * Two-pass algorithm: in the first pass, we determine whether the

   * value fits; in the second pass, we do the actual write.

   *

   * During the first pass, 'r' contains the comparison result so

   * far:

   *  0x00000000   value is equal to the modulus

   *  0x00000001   value is greater than the modulus

   *  0xFFFFFFFF   value is lower than the modulus

   *

   * Since we iterate starting with the least significant bytes (at

   * the end of src[]), each new comparison overrides the previous

   * except when the comparison yields 0 (equal).

   *

   * During the second pass, 'r' is either 0xFFFFFFFF (value fits)

   * or 0x00000000 (value does not fit).

   *

   * We must iterate over all bytes of the source, _and_ possibly

   * some extra virtual bytes (with value 0) so as to cover the

   * complete modulus as well. We also add 4 such extra bytes beyond

   * the modulus length because it then guarantees that no accumulated

   * partial word remains to be processed.

   */

  const unsigned char *buf;

  size_t mlen, tlen;

  int pass;

  uint32_t r;

  buf = src;

  mlen = (m[0] + 31) >> 5;

  tlen = (mlen << 2);

  if (tlen < len) {

    tlen = len;

  }

  tlen += 4;

  r = 0;

  for (pass = 0; pass < 2; pass ++) {

    size_t u, v;

    uint32_t acc;

    int acc_len;

    v = 1;

    acc = 0;

    acc_len = 0;

    for (u = 0; u < tlen; u ++) {

      uint32_t b;

      if (u < len) {

        b = buf[len - 1 - u];

      } else {

        b = 0;

      }

      acc |= (b << acc_len);

      acc_len += 8;

      if (acc_len >= 31) {

        uint32_t xw;

        xw = acc & (uint32_t)0x7FFFFFFF;

        acc_len -= 31;

        acc = b >> (8 - acc_len);

        if (v <= mlen) {

          if (pass) {

            x[v] = r & xw;

          } else {

            uint32_t cc;

            cc = (uint32_t)CMP(xw, m[v]);

            r = MUX(EQ(cc, 0), r, cc);

          }

        } else {

          if (!pass) {

            r = MUX(EQ(xw, 0), r, 1);

          }

        }

        v ++;

      }

    }

    /*

     * When we reach this point at the end of the first pass:

     * r is either 0, 1 or -1; we want to set r to 0 if it

     * is equal to 0 or 1, and leave it to -1 otherwise.

     *

     * When we reach this point at the end of the second pass:

     * r is either 0 or -1; we want to leave that value

     * untouched. This is a subcase of the previous.

     */

    r >>= 1;

    r |= (r << 1);

  }

  x[0] = m[0];

  return r & (uint32_t)1;

}