Fixing i257 is_zero() (#282)

Please check the type of change your PR introduces:

- [x] Bugfix
- [ ] Feature
- [ ] Code style update (formatting, renaming)
- [ ] Refactoring (no functional changes, no API changes)
- [ ] Build-related changes
- [ ] Documentation content changes
- [ ] Other (please describe):

Fixing is_zero behaviour that was causing is_zero to sometimes return
false when it was indeed zero

src/math/src/i257.cairo

@@ -1,3 +1,4 @@
+use core::zeroable::Zeroable;
 // ====================== INT 257 ======================
 
 // i257 represents a 129-bit integer.
@@ -259,10 +260,11 @@ impl i257Zeroable of Zeroable<i257> {
         i257_new(0, false)
     }
     fn is_zero(self: i257) -> bool {
-        self == Zeroable::zero()
+        assert(!self.is_negative, 'no negative zero');
+        self.abs == 0
     }
     fn is_non_zero(self: i257) -> bool {
-        self != Zeroable::zero()
+        !self.is_zero()
     }
 }
 

src/math/src/tests/i257_test.cairo

@@ -136,6 +136,19 @@ fn i257_test_mul() {
     assert!(!result.is_negative, "10 * 0 -> positive");
 }
 
+#[test]
+fn i257_test_is_zero() {
+    let a = i257 { abs: 0, is_negative: false };
+    assert!(a.is_zero(), "should be true");
+}
+
+#[test]
+#[should_panic(expected: ('no negative zero',))]
+fn i257_test_is_zero_panic() {
+    let a = i257 { abs: 0, is_negative: true };
+    let _x = a.is_zero();
+}
+
 #[test]
 fn i257_test_div_no_rem() {
     // Test division of positive integers