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torvalds
GitHub Repository: torvalds/linux
 Path: blob/master/lib/crypto/mpi/mpi-sub-ui.c
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// SPDX-License-Identifier: GPL-2.0-or-later

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/* mpi-sub-ui.c - Subtract an unsigned integer from an MPI.

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 *

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 * Copyright 1991, 1993, 1994, 1996, 1999-2002, 2004, 2012, 2013, 2015

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 * Free Software Foundation, Inc.

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 *

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 * This file was based on the GNU MP Library source file:

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 * https://gmplib.org/repo/gmp-6.2/file/510b83519d1c/mpz/aors_ui.h

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 *

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 * The GNU MP Library is free software; you can redistribute it and/or modify

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 * it under the terms of either:

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 *

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 *   * the GNU Lesser General Public License as published by the Free

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 *     Software Foundation; either version 3 of the License, or (at your

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 *     option) any later version.

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 *

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 * or

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 *

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 *   * the GNU General Public License as published by the Free Software

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 *     Foundation; either version 2 of the License, or (at your option) any

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 *     later version.

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 *

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 * or both in parallel, as here.

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 *

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 * The GNU MP Library is distributed in the hope that it will be useful, but

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 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY

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 * or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

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 * for more details.

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 *

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 * You should have received copies of the GNU General Public License and the

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 * GNU Lesser General Public License along with the GNU MP Library.  If not,

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 * see https://www.gnu.org/licenses/.

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 */

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#include <linux/export.h>

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#include "mpi-internal.h"

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int mpi_sub_ui(MPI w, MPI u, unsigned long vval)

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{

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	if (u->nlimbs == 0) {

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		if (mpi_resize(w, 1) < 0)

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			return -ENOMEM;

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		w->d[0] = vval;

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		w->nlimbs = (vval != 0);

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		w->sign = (vval != 0);

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		return 0;

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	}

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	/* If not space for W (and possible carry), increase space. */

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	if (mpi_resize(w, u->nlimbs + 1))

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		return -ENOMEM;

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	if (u->sign) {

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		mpi_limb_t cy;

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		cy = mpihelp_add_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);

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		w->d[u->nlimbs] = cy;

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		w->nlimbs = u->nlimbs + cy;

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		w->sign = 1;

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	} else {

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		/* The signs are different.  Need exact comparison to determine

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		 * which operand to subtract from which.

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		 */

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		if (u->nlimbs == 1 && u->d[0] < vval) {

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			w->d[0] = vval - u->d[0];

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			w->nlimbs = 1;

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			w->sign = 1;

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		} else {

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			mpihelp_sub_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);

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			/* Size can decrease with at most one limb. */

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			w->nlimbs = (u->nlimbs - (w->d[u->nlimbs - 1] == 0));

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			w->sign = 0;

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		}

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	}

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	mpi_normalize(w);

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	return 0;

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}

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EXPORT_SYMBOL_GPL(mpi_sub_ui);

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