From cbcab8ea4c74d5a1b36b8583571340ff728c2ad5 Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Thu, 9 Aug 2018 17:51:20 +0200
Subject: [PATCH] kernel: fix LogFFE on 32 bit systems

Fixes #2687
---
 src/finfield.c                       |  6 ++----
 tst/testbugfix/2011-01-16-t00231.tst |  3 ---
 tst/testinstall/ffe.tst              | 23 +++++++++++++++++++++--
 3 files changed, 23 insertions(+), 9 deletions(-)
 delete mode 100644 tst/testbugfix/2011-01-16-t00231.tst

diff --git a/src/finfield.c b/src/finfield.c
index b74bda4c74..22ab809092 100644
--- a/src/finfield.c
+++ b/src/finfield.c
@@ -1471,11 +1471,9 @@ Obj FuncLOG_FFE_DEFAULT (
 
     while (a < 0)
       a+= (qX -1)/c;
-    /* return the logarithm                                                */
-    
-
-    return INTOBJ_INT( (((Int) (vZ-1) / c) * a) % ((Int) (qX-1)) );
 
+    // return the logarithm
+    return INTOBJ_INT( (((UInt) (vZ-1) / c) * a) % ((UInt) (qX-1)) );
 }
 
 
diff --git a/tst/testbugfix/2011-01-16-t00231.tst b/tst/testbugfix/2011-01-16-t00231.tst
deleted file mode 100644
index fc94139b92..0000000000
--- a/tst/testbugfix/2011-01-16-t00231.tst
+++ /dev/null
@@ -1,3 +0,0 @@
-# Reported by MN on 2009/10/06, added by AK on 2011/01/16
-gap> (Z(65536)^2)^LogFFE(Z(65536)^16386,Z(65536)^2) = Z(65536)^16386;
-true
diff --git a/tst/testinstall/ffe.tst b/tst/testinstall/ffe.tst
index 2cebcea9ba..55dfa40cec 100644
--- a/tst/testinstall/ffe.tst
+++ b/tst/testinstall/ffe.tst
@@ -183,10 +183,29 @@ gap> DegreeFFE( [[Z(2),Z(8)],[Z(2), Z(4)]]);
 6
 
 #
-gap> n:=LogFFE(Z(25)^3, Z(25)^7);; (Z(25)^7)^n;
-Z(5^2)^3
+# LogFFE
+#
+gap> q:=25;; r:=Z(q)^7;; ForAll([0..q-2], i -> LogFFE(r^i,r)=i);
+true
 
+# test an issue reported by MN on 2009/10/06, added by AK on 2011/01/16
+gap> q:=2^16;; r:=Z(q)^2;; ForAll([0..q-2], i -> LogFFE(r^i,r)=i);
+true
+
+# test an edge case on 32 bit systems, where a kernel value could overflow
+# (see https://github.com/gap-system/gap/issues/2687)
+gap> q:=37^3;; r:=Z(q)^1055;; ForAll([0..q-2], i -> LogFFE(r^i,r)=i);
+true
+
+# error handling
+gap> LogFFE(0*Z(2), Z(2));
+Error, LogFFE: <z> must be a nonzero finite field element
+gap> LogFFE(Z(2), 0*Z(2));
+Error, LogFFE: <r> must be a nonzero finite field element
+
+#
 # RootFFE
+#
 gap> Rochambeau:=function(F)
 > local e,i,p,a,r;
 >  e:=Elements(F);