Fix BlistList for two ranges

Several results were wrong, and had been wrong since at least GAP 4.4.
E.g. before this commit:

    gap> BlistList([0..3], [-3..-1]);
    [ true, true, true, false ]

Now we get the correct result

    gap> BlistList([0,1,2,3], [-3..-1]);
    [ false, false, false, false ]

One major issue was the use of unsigned variables to store signed data. Two
variables were only of type `long` instead of `Int`, but on 64bit, this could
lead to further issues.

The logic handling the intersection was broken and overly complicated. The
rewritten logic should be simpler.

Finally, there is a tiny optimization enabled by switching from 1-based
indexing to 0-based.

src/blister.c

@@ -1286,9 +1286,9 @@ static Obj FuncUNITE_BLIST_LIST(Obj self, Obj list, Obj blist, Obj sub)
     UInt                bit;            /* one bit of block                */
     Int                 lenList;        /* logical length of the list      */
     const Obj *         ptrSub;         /* pointer to the sublist          */
-    UInt                lenSub;         /* logical length of sublist       */
-    UInt                i, j, k = 0, l;     /* loop variables                  */
-    long                s, t;           /* elements of a range             */
+    Int                 lenSub;         /* logical length of sublist       */
+    Int                 i, j, k, l;     /* loop variables                  */
+    Int                 s, t;           /* elements of a range             */
 
     RequireSmallList("UniteBlistList", list);
     RequireBlist("UniteBlistList", blist);
@@ -1313,19 +1313,22 @@ static Obj FuncUNITE_BLIST_LIST(Obj self, Obj list, Obj blist, Obj sub)
         /* get the bounds of the subset with respect to the boolean list   */
         s = INT_INTOBJ( GET_ELM_RANGE( list, 1 ) );
         t = INT_INTOBJ( GET_ELM_RANGE( sub, 1 ) );
-        if ( s <= t )  i = t - s + 1;
-        else           i = 1;
 
-        if ( i + lenSub - 1 <= lenList )  j = i + lenSub - 1;
-        else                              j = lenList;
+        // compute bounds
+        i = t - s;
+        j = lenSub + i;
+        if (i < 0)
+            i = 0;
+        if (j > lenList)
+            j = lenList;
 
         /* set the corresponding entries to 'true'                         */
-        for ( k = i; k <= j && (k-1)%BIPEB != 0; k++ )
-            ptrBlist[(k-1)/BIPEB] |= (1UL << (k-1)%BIPEB);
-        for ( ; k+BIPEB <= j; k += BIPEB )
-            ptrBlist[(k-1)/BIPEB] = ~(UInt)0;
-        for ( ; k <= j; k++ )
-            ptrBlist[(k-1)/BIPEB] |= (1UL << (k-1)%BIPEB);
+        for ( k = i; k < j && k%BIPEB != 0; k++ )
+            ptrBlist[k/BIPEB] |= (1UL << k%BIPEB);
+        for ( ; k+BIPEB < j; k += BIPEB )
+            ptrBlist[k/BIPEB] = ~(UInt)0;
+        for ( ; k < j; k++ )
+            ptrBlist[k/BIPEB] |= (1UL << k%BIPEB);
 
     }
 

tst/testinstall/kernel/blister.tst

@@ -149,15 +149,35 @@ gap> List(l, IsSSortedList);
 gap> List(l, SizeBlist);
 [ 1, 1, 2, 2 ]
 
+#
 # FuncBLIST_LIST
+#
 gap> BlistList(fail, fail);
 Error, BlistList: <list> must be a small list (not the value 'fail')
 gap> BlistList([1,2,3], fail);
 Error, BlistList: <sub> must be a small list (not the value 'fail')
 gap> BlistList([1,2,3], [1,3]);
 [ true, false, true ]
 
+# ranges with increment 1
+gap> BlistList([0..3], [-3..-1]);
+[ false, false, false, false ]
+gap> BlistList([0..3], [-3..1]);
+[ true, true, false, false ]
+gap> BlistList([0..3], [-3..6]);
+[ true, true, true, true ]
+gap> BlistList([0..3], [0..1]);
+[ true, true, false, false ]
+gap> BlistList([0..3], [0..6]);
+[ true, true, true, true ]
+gap> BlistList([0..3], [2..6]);
+[ false, false, true, true ]
+gap> BlistList([0..3], [4..6]);
+[ false, false, false, false ]
+
+#
 # FuncLIST_BLIST
+#
 gap> ListBlist(fail, fail);
 Error, ListBlist: <list> must be a small list (not the value 'fail')
 gap> ListBlist([1,2,3], fail);