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From a7f911140e7d0a0125653a40aa2c5fe257bd78f5 Mon Sep 17 00:00:00 2001
From: Fredrik Johansson <fredrik.johansson@gmail.com>
Date: Thu, 18 Sep 2014 14:49:05 +0200
Subject: [PATCH] redefine fmpz_invmod to consider any integer invertible mod 1
(for gmp 6.0 compatibility)
---
fmpz/doc/fmpz.txt | 3 ++-
fmpz/invmod.c | 11 +++++++++--
fmpz/test/t-invmod.c | 21 +++++++++++++++++----
3 files changed, 28 insertions(+), 7 deletions(-)
diff --git a/fmpz/doc/fmpz.txt b/fmpz/doc/fmpz.txt
index fb422d8..2ada719 100644
--- a/fmpz/doc/fmpz.txt
+++ b/fmpz/doc/fmpz.txt
@@ -899,7 +899,8 @@ int fmpz_invmod(fmpz_t f, const fmpz_t g, const fmpz_t h)
Sets $f$ to the inverse of $g$ modulo $h$. The value of $h$ may
not be $0$ otherwise an exception results. If the inverse exists
the return value will be non-zero, otherwise the return value will
- be $0$ and the value of $f$ undefined.
+ be $0$ and the value of $f$ undefined. As a special case, we
+ consider any number invertible modulo $h = \pm 1$, with inverse 0.
void fmpz_negmod(fmpz_t f, const fmpz_t g, const fmpz_t h)
diff --git a/fmpz/invmod.c b/fmpz/invmod.c
index a0cf601..0e20f39 100644
--- a/fmpz/invmod.c
+++ b/fmpz/invmod.c
@@ -67,7 +67,11 @@ fmpz_invmod(fmpz_t f, const fmpz_t g, const fmpz_t h)
if (c2 < WORD(0))
c2 = -c2;
if (c2 == WORD(1))
- return 0; /* special case not handled by n_invmod */
+ {
+ fmpz_zero(f);
+ return 1; /* special case not handled by n_invmod */
+ }
+
gcd = z_gcdinv(&inv, c1, c2);
return (gcd == UWORD(1) ? fmpz_set_si(f, inv), 1 : 0);
@@ -106,7 +110,10 @@ fmpz_invmod(fmpz_t f, const fmpz_t g, const fmpz_t h)
if (c2 < WORD(0))
c2 = -c2;
if (c2 == WORD(1))
- return 0; /* special case not handled by z_gcd_invert */
+ {
+ fmpz_zero(f);
+ return 1; /* special case not handled by z_gcd_invert */
+ }
/* reduce g mod h first */
r = flint_mpz_fdiv_ui(COEFF_TO_PTR(c1), c2);
diff --git a/fmpz/test/t-invmod.c b/fmpz/test/t-invmod.c
index aea236e..8ff1c7f 100644
--- a/fmpz/test/t-invmod.c
+++ b/fmpz/test/t-invmod.c
@@ -30,6 +30,19 @@
#include "ulong_extras.h"
#include "fmpz.h"
+/* Use the definiton of GMP versions >= 6.0 */
+int
+mpz_invert2(mpz_t a, const mpz_t b, const mpz_t c)
+{
+ if (mpz_cmpabs_ui(c, 1) == 0)
+ {
+ mpz_set_ui(a, 0);
+ return 1;
+ }
+ else
+ return mpz_invert(a, b, c);
+}
+
int
main(void)
{
@@ -63,7 +76,7 @@ main(void)
fmpz_get_mpz(e, b);
r1 = fmpz_invmod(c, a, b);
- r2 = mpz_invert(f, d, e);
+ r2 = mpz_invert2(f, d, e);
fmpz_get_mpz(g, c);
@@ -106,7 +119,7 @@ main(void)
fmpz_get_mpz(d, a);
r1 = fmpz_invmod(c, a, a);
- r2 = mpz_invert(f, d, d);
+ r2 = mpz_invert2(f, d, d);
fmpz_get_mpz(g, c);
@@ -149,7 +162,7 @@ main(void)
fmpz_get_mpz(e, b);
r1 = fmpz_invmod(a, a, b);
- r2 = mpz_invert(f, d, e);
+ r2 = mpz_invert2(f, d, e);
fmpz_get_mpz(g, a);
@@ -192,7 +205,7 @@ main(void)
fmpz_get_mpz(e, b);
r1 = fmpz_invmod(b, a, b);
- r2 = mpz_invert(f, d, e);
+ r2 = mpz_invert2(f, d, e);
fmpz_get_mpz(g, b);
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