fmul_block_width.c

/* Copyright 2008-2019 Douglas Wikstrom
 *
 * This file is part of Verificatum Elliptic Curve library (VEC).
 *
 * 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 <stdio.h>

/*
 * Computes a "theoretical" optimal block width for a given scalar
 * length.
 */
int
vec_fmul_block_width(int bit_length, int len) {

  int width = 2;
  double cost = 1.5 * bit_length;
  double oldCost;
  double t;
  double s;
  double m;

  do {

    oldCost = cost;

    /* Amortized cost for table. */
    t = ((double)(1 << width) - width + bit_length) / len;

    /* Cost for doubling. */
    s = ((double)bit_length) / width;

    /* Cost for adding. */
    m = ((double)bit_length) / width;

    cost = t + m + s;

    width++;

  } while (width < 17 && cost < oldCost);

  width--;

  return width;
}