From 207c44fc53cacddf8760cea6fc3ee36eb0c097f1 Mon Sep 17 00:00:00 2001
From: Mizar <mizar.jp@gmail.com>
Date: Sat, 21 Jan 2023 16:38:25 +0900
Subject: [PATCH] Corner cases of "modint" when mod = 1
 https://github.com/rust-lang-ja/ac-library-rs/issues/110

---
 src/modint.rs | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/src/modint.rs b/src/modint.rs
index c52b24b..19483f4 100644
--- a/src/modint.rs
+++ b/src/modint.rs
@@ -673,7 +673,7 @@ pub trait ModIntBase:
     #[inline]
     fn pow(self, mut n: u64) -> Self {
         let mut x = self;
-        let mut r = Self::raw(1);
+        let mut r = Self::raw(u32::from(Self::modulus() > 1));
         while n > 0 {
             if n & 1 == 1 {
                 r *= x;
@@ -1046,9 +1046,9 @@ macro_rules! impl_folding {
 
 impl_folding! {
     impl<M: Modulus> Sum<_>     for StaticModInt<M>  { fn sum(_)     -> _ { _(Self::raw(0), Add::add) } }
-    impl<M: Modulus> Product<_> for StaticModInt<M>  { fn product(_) -> _ { _(Self::raw(1), Mul::mul) } }
+    impl<M: Modulus> Product<_> for StaticModInt<M>  { fn product(_) -> _ { _(Self::raw(u32::from(Self::modulus() > 1)), Mul::mul) } }
     impl<I: Id     > Sum<_>     for DynamicModInt<I> { fn sum(_)     -> _ { _(Self::raw(0), Add::add) } }
-    impl<I: Id     > Product<_> for DynamicModInt<I> { fn product(_) -> _ { _(Self::raw(1), Mul::mul) } }
+    impl<I: Id     > Product<_> for DynamicModInt<I> { fn product(_) -> _ { _(Self::raw(u32::from(Self::modulus() > 1)), Mul::mul) } }
 }
 
 #[cfg(test)]
@@ -1163,6 +1163,19 @@ mod tests {
         assert_eq!(expected, c);
     }
 
+    // Corner cases of "modint" when mod = 1
+    // https://github.com/rust-lang-ja/ac-library-rs/issues/110
+    #[test]
+    fn mod1_corner_case() {
+        ModInt::set_modulus(1); // !!
+
+        let x: ModInt = std::iter::empty::<ModInt>().product();
+        assert_eq!(x.val(), 0);
+
+        let y = ModInt::new(123).pow(0);
+        assert_eq!(y.val(), 0);
+    }
+
     // test `2^31 < modulus < 2^32` case
     // https://github.com/rust-lang-ja/ac-library-rs/issues/111
     #[test]