i15_fmont.c

/*
 * Copyright (c) 2017 Thomas Pornin <[email protected]>
 *
 * 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 */
void
br_i15_from_monty(uint16_t *x, const uint16_t *m, uint16_t m0i)
{
	size_t len, u, v;

	len = (m[0] + 15) >> 4;
	for (u = 0; u < len; u ++) {
		uint32_t f, cc;

		f = MUL15(x[1], m0i) & 0x7FFF;
		cc = 0;
		for (v = 0; v < len; v ++) {
			uint32_t z;

			z = (uint32_t)x[v + 1] + MUL15(f, m[v + 1]) + cc;
			cc = z >> 15;
			if (v != 0) {
				x[v] = z & 0x7FFF;
			}
		}
		x[len] = cc;
	}

	/*
	 * We may have to do an extra subtraction, but only if the
	 * value in x[] is indeed greater than or equal to that of m[],
	 * which is why we must do two calls (first call computes the
	 * carry, second call performs the subtraction only if the carry
	 * is 0).
	 */
	br_i15_sub(x, m, NOT(br_i15_sub(x, m, 0)));
}