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fix issue #33 Negating most-negative-fixnum doesn't. (#35)

* fix issue #33 Negating most-negative-fixnum doesn't.

* Add unit test for issue #33 Negating most-negative-fixnum doesn't.

--- a/cvalues.c

+++ b/cvalues.c

@@ -1115,7 +1115,11 @@

 static value_t fl_neg(value_t n)

 {

     if (isfixnum(n)) {

-        return fixnum(-numval(n));

+        fixnum_t s = fixnum(-numval(n));

+        if (__unlikely(s == n))

+            return mk_long(-numval(n)); // negate overflows

+        else

+            return s;

     }

     else if (iscprim(n)) {

         cprim_t *cp = (cprim_t*)ptr(n);

--- a/flisp.c

+++ b/flisp.c

@@ -1396,8 +1396,13 @@

             NEXT_OP;

         OP(OP_NEG)

         do_neg:

-            if (isfixnum(Stack[SP-1]))

-                Stack[SP-1] = fixnum(-numval(Stack[SP-1]));

+            if (isfixnum(Stack[SP-1])) {

+                s = fixnum(-numval(Stack[SP-1]));

+                if (__unlikely(s == Stack[SP-1]))

+                  Stack[SP-1] = mk_long(-numval(Stack[SP-1])); // negate overflows

+                else

+                  Stack[SP-1] = s;

+            }

             else

                 Stack[SP-1] = fl_neg(Stack[SP-1]);

             NEXT_OP;

--- a/tests/unittest.lsp

+++ b/tests/unittest.lsp

@@ -65,6 +65,9 @@

 (assert (> (- #int32(0x80000000)) 0))

 (assert (< (- #uint64(0x8000000000000000)) 0))

 (assert (> (- #int64(0x8000000000000000)) 0))

+; fixnum versions

+(assert (= (- -536870912) 536870912))

+(assert (= (- -2305843009213693952) 2305843009213693952))

 (assert (not (equal? #int64(0x8000000000000000) #uint64(0x8000000000000000))))

 (assert (equal? (+ #int64(0x4000000000000000) #int64(0x4000000000000000))