--- /dev/null
+/* mpz_combit -- complement a specified bit.
+
+Copyright 2002, 2003 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library.
+
+The GNU MP Library is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 3 of the License, or (at your
+option) any later version.
+
+The GNU MP Library 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 Lesser General Public
+License for more details.
+
+You should have received a copy of the GNU Lesser General Public License
+along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+
+void
+mpz_combit (mpz_ptr d, unsigned long int bit_index)
+{
+ mp_size_t dsize = ABSIZ(d);
+ mp_ptr dp = LIMBS(d);
+
+ mp_size_t limb_index = bit_index / GMP_NUMB_BITS;
+ mp_limb_t bit = ((mp_limb_t) 1 << (bit_index % GMP_NUMB_BITS));
+
+ if (limb_index >= dsize)
+ {
+ MPZ_REALLOC(d, limb_index + 1);
+ dp = LIMBS(d);
+
+ MPN_ZERO(dp + dsize, limb_index + 1 - dsize);
+ dsize = limb_index + 1;
+ }
+
+ if (SIZ(d) >= 0)
+ {
+ dp[limb_index] ^= bit;
+ MPN_NORMALIZE (dp, dsize);
+ SIZ(d) = dsize;
+ }
+ else
+ {
+ mp_limb_t x = -dp[limb_index];
+ mp_size_t i;
+
+ /* non-zero limb below us means ones-complement */
+ for (i = limb_index-1; i >= 0; i--)
+ if (dp[i] != 0)
+ {
+ x--; /* change twos comp to ones comp */
+ break;
+ }
+
+ if (x & bit)
+ {
+ mp_limb_t c;
+
+ /* Clearing the bit increases the magitude. We might need a carry. */
+ MPZ_REALLOC(d, dsize + 1);
+ dp = LIMBS(d);
+
+ __GMPN_ADD_1 (c, dp+limb_index, dp+limb_index,
+ dsize - limb_index, bit);
+ dp[dsize] = c;
+ dsize += c;
+ }
+ else
+ /* Setting the bit decreases the magnitude */
+ mpn_sub_1(dp+limb_index, dp+limb_index, dsize + limb_index, bit);
+
+ MPN_NORMALIZE (dp, dsize);
+ SIZ(d) = -dsize;
+ }
+}