From: Mark Wooding <mdw@distorted.org.uk>
Date: Sat, 4 Feb 2006 16:19:03 +0000 (+0000)
Subject: gf: Fix gf_irreduciblep() for small-degree polynomials.
X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/commitdiff_plain/00f096fc078d1aa06927901d888261d4d13de495

gf: Fix gf_irreduciblep() for small-degree polynomials.

Fix division-by-zero error for argument zero, and segfaults for
arguments with degree less than 2 due to skipping the main loop.  Handle
these as a special case.
---

diff --git a/gf-arith.c b/gf-arith.c
index f380a35..eabbff8 100644
--- a/gf-arith.c
+++ b/gf-arith.c
@@ -179,11 +179,17 @@ void gf_div(mp **qq, mp **rr, mp *a, mp *b)
 
 int gf_irreduciblep(mp *f)
 {
-  unsigned long m = mp_bits(f) - 1;
+  unsigned long m;
   mp *u = MP_TWO;
   mp *v = MP_NEW;
 
-  m /= 2;
+  if (MP_ZEROP(f))
+    return (0);
+  else if (MP_LEN(f) == 1) {
+    if (f->v[0] < 2) return (0);
+    if (f->v[0] < 4) return (1);
+  }
+  m = (mp_bits(f) - 1)/2;
   while (m) {
     u = gf_sqr(u, u);
     gf_div(0, &u, u, f);
diff --git a/tests/gf b/tests/gf
index 801993d..fded680 100644
--- a/tests/gf
+++ b/tests/gf
@@ -59,6 +59,9 @@ exp {
 }
 
 irred {
+  0 0;
+  1 0;
+  2 1;
   0xc1a7bd3b4e853fc92d4e1588719986aa 0;
   0x800000000000000000000000000000000000000c9 1;
   0x2000000000000000000000000000000000000000000000000000000000000000000000000000000008000000000000000000001 1;