modint: add/sub impl fix for 2^31<modulus<2^32

Author: mizar

src/modint.rs

@@ -793,20 +793,20 @@ trait InternalImplementations: ModIntBase {
     #[inline]
     fn add_impl(lhs: Self, rhs: Self) -> Self {
         let modulus = Self::modulus();
-        let mut val = lhs.val() + rhs.val();
-        if val >= modulus {
-            val -= modulus;
-        }
+        let val = match lhs.val().overflowing_sub(modulus - rhs.val()) {
+            (v, true) => v.wrapping_add(modulus),
+            (v, false) => v,
+        };
         Self::raw(val)
     }
 
     #[inline]
     fn sub_impl(lhs: Self, rhs: Self) -> Self {
         let modulus = Self::modulus();
-        let mut val = lhs.val().wrapping_sub(rhs.val());
-        if val >= modulus {
-            val = val.wrapping_add(modulus)
-        }
+        let val = match lhs.val().overflowing_sub(rhs.val()) {
+            (v, true) => v.wrapping_add(modulus),
+            (v, false) => v,
+        };
         Self::raw(val)
     }
 
@@ -1050,6 +1050,7 @@ impl_folding! {
 
 #[cfg(test)]
 mod tests {
+    use crate::modint::ModInt;
     use crate::modint::ModInt1000000007;
 
     #[test]
@@ -1157,4 +1158,28 @@ mod tests {
         c /= b;
         assert_eq!(expected, c);
     }
+
+    // test `2^31 < modulus < 2^32` case.
+    #[test]
+    fn dynamic_modint_m32() {
+        let m = 3221225471;
+        ModInt::set_modulus(m);
+        let f = ModInt::new::<u32>;
+        assert_eq!(f(1398188832) + f(3184083880), f(1361047241));
+        assert_eq!(f(3013899062) + f(2238406135), f(2031079726));
+        assert_eq!(f(2699997885) + f(2745140255), f(2223912669));
+        assert_eq!(f(2824399978) + f(2531872141), f(2135046648));
+        assert_eq!(f(36496612) - f(2039504668), f(1218217415));
+        assert_eq!(f(266176802) - f(1609833977), f(1877568296));
+        assert_eq!(f(713535382) - f(2153383999), f(1781376854));
+        assert_eq!(f(1249965147) - f(3144251805), f(1326938813));
+        assert_eq!(f(2692223381) * f(2935379475), f(2084179397));
+        assert_eq!(f(2800462205) * f(2822998916), f(2089431198));
+        assert_eq!(f(3061947734) * f(3210920667), f(1962208034));
+        assert_eq!(f(3138997926) * f(2994465129), f(1772479317));
+        assert_eq!(f(2947552629) / f(576466398), f(2041593039));
+        assert_eq!(f(2914694891) / f(399734126), f(1983162347));
+        assert_eq!(f(2202862138) / f(1154428799), f(2139936238));
+        assert_eq!(f(3037207894) / f(2865447143), f(1894581230));
+    }
 }