Add tests from #1282

Author: tarcieri

src/uint/boxed/mul_mod.rs

@@ -98,7 +98,7 @@ fn mac_by_limb(a: &UintRef, b: &UintRef, c: Limb, carry: Limb) -> (BoxedUint, Li
 
 #[cfg(all(test, feature = "rand_core"))]
 mod tests {
-    use crate::{BoxedUint, ConcatenatingMul, Limb, NonZero, Random, RandomMod};
+    use crate::{BoxedUint, ConcatenatingMul, Limb, NonZero, Random, RandomMod, Resize};
     use rand_core::SeedableRng;
 
     #[test]
@@ -153,4 +153,18 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn mul_mod_special_zero_c_is_wrapping_multiplication() {
+        for bits in [Limb::BITS, 2 * Limb::BITS, 4 * Limb::BITS] {
+            let a = BoxedUint::from(0x1234_5678u32).resize(bits);
+            let b = BoxedUint::from(0xfedc_ba91u32).resize(bits);
+
+            assert_eq!(
+                a.mul_mod_special(&b, Limb::ZERO),
+                a.wrapping_mul(&b),
+                "c = 0 represents the power-of-two modulus"
+            );
+        }
+    }
 }

src/uint/mul_mod.rs

@@ -168,4 +168,25 @@ mod tests {
             test_size::<16>();
         }
     }
+
+    #[test]
+    fn mul_mod_special_zero_c_is_wrapping_multiplication() {
+        let a = Uint::<1>::from_u32(0x1234_5678);
+        let b = Uint::<1>::from_u32(0xfedc_ba91);
+
+        assert_eq!(
+            a.mul_mod_special(&b, Limb::ZERO),
+            a.wrapping_mul(&b),
+            "c = 0 represents the power-of-two modulus"
+        );
+
+        let a = Uint::<2>::from_u32(0x1234_5678);
+        let b = Uint::<2>::from_u32(0xfedc_ba91);
+
+        assert_eq!(
+            a.mul_mod_special(&b, Limb::ZERO),
+            a.wrapping_mul(&b),
+            "c = 0 represents the power-of-two modulus"
+        );
+    }
 }