pperm: fix bug in PreImagePPermInt

This occasionally caused incorrect values to be returned when i == deg and by coincidence ptf2[i] == cpt. When ptf2[deg] is beyond the end of the bag containing f, and so is not valid. For reference, this bug caused errors in: semigroups/Semigroups#296 semigroups/Semigroups#472
gap-system · Apr 5, 2018 · c6174ff · fingolfin · Apr 5, 2018 · c6174ff
1 parent 37aee87
commit c6174ff
Showing 1 changed file with 2 additions and 2 deletions.
diff --git a/src/pperm.c b/src/pperm.c
@@ -318,15 +318,15 @@ static Obj PreImagePPermInt(Obj pt, Obj f)
         deg = DEG_PPERM2(f);
         while (ptf2[i] != cpt && i < deg)
             i++;
-        if (ptf2[i] != cpt)
+        if (i == deg || ptf2[i] != cpt)
             return Fail;
     }
     else {
         ptf4 = ADDR_PPERM4(f);
         deg = DEG_PPERM4(f);
         while (ptf4[i] != cpt && i < deg)
             i++;
-        if (ptf4[i] != cpt)
+        if (i == deg || ptf4[i] != cpt)
             return Fail;
     }
     return INTOBJ_INT(i + 1);