Handle signs and carry overflow addition for multiplication

src/bigint.rs

@@ -260,19 +260,14 @@ impl Mul for BigInt {
             if overflowed {
                 todo!()
             } else {
-                match res.checked_add(carry) {
-                    Some(res) => {
-                        out.inner.push(res);
-                        carry = 0;
-                    }
-                    None => {
-                        // Well, we have to deal with overflow again
-                        todo!();
-                    }
-                }
+                let (res, overflowed) = res.overflowing_add(carry);
+
+                out.inner.push(res);
+                carry = if overflowed { 1 } else { 0 };
             }
         }
 
+        out.sign = Self::get_sign(self, rhs, FracOp::Other);
         out.shrink_to_fit();
 
         out
@@ -568,6 +563,33 @@ mod tests {
         assert_eq!(product.inner[0], 65536usize * 4);
     }
 
+    #[test]
+    fn test_mul_signs() {
+        let a = BigInt::from(2);
+        let b = BigInt::from(-2);
+        let product = a * b;
+        assert_eq!(product.inner[0], 4usize);
+        assert_eq!(product.sign, Negative);
+
+        let a = -BigInt::from(2);
+        let b = BigInt::from(2);
+        let product = a * b;
+        assert_eq!(product.inner[0], 4usize);
+        assert_eq!(product.sign, Negative);
+
+        let a = BigInt::from(-2);
+        let b = BigInt::from(-2);
+        let product = a * b;
+        assert_eq!(product.inner[0], 4usize);
+        assert_eq!(product.sign, Positive);
+
+        let a = BigInt::from(2);
+        let b = BigInt::from(2);
+        let product = a * b;
+        assert_eq!(product.inner[0], 4usize);
+        assert_eq!(product.sign, Positive);
+    }
+
     #[test]
     #[should_panic]
     fn test_mul_overflow() {