AtCoder.Internal.Barrett for 2^31 < m < 2^32

kzrnm · Sep 25, 2023 · 4176365 · 4176365
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commit 4176365
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Showing 3 changed files with 34 additions and 7 deletions.
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add `Deque<T>.Grow(int capacity)`
 
 ### Changed
+- `AtCoder.Internal.Barrett` for `2^31` < m < `2^32`
 - Fix empty `Deque<T>.GetEnumerator()`
 
 ## [3.1.0] - 2023-09-24
@@ -39,7 +40,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [3.0.0-pre5] - 2023-01-26
 ### Changed
-- Fix LCPArray ¨ LcpArray
+- Fix LCPArray → LcpArray
 
 ## [3.0.0-pre4] - 2023-01-26
 ### Added

diff --git a/Source/ac-library-csharp/Math/Internal/Barrett.cs b/Source/ac-library-csharp/Math/Internal/Barrett.cs
@@ -26,9 +26,9 @@ public Barrett(uint m)
         public uint Reduce(ulong z)
         {
             var x = InternalMath.Mul128Bit(z, IM);
-            var v = unchecked((uint)(z - x * Mod));
-            if (Mod <= v) v += Mod;
-            return v;
+            var y = x * Mod;
+            if (z < y) return (uint)(z - y + Mod);
+            return (uint)(z - y);
         }
 
         /// <summary>

diff --git a/Test/ac-library-csharp.Test/Internal/BarrettTest.cs b/Test/ac-library-csharp.Test/Internal/BarrettTest.cs
@@ -25,7 +25,7 @@ public void Barrett()
         }
 
         [Fact]
-        public void BarrettBorder()
+        public void BarrettInt32Border()
         {
             for (uint mod = int.MaxValue; mod >= int.MaxValue - 20; mod--)
             {
@@ -40,16 +40,42 @@ public void BarrettBorder()
                 }
                 foreach (var a in v)
                 {
-                    long a2 = a;
+                    ulong a2 = a;
                     bt.Mul(a, bt.Mul(a, a)).Should().Be((uint)(a2 * a2 % mod * a2 % mod));
                     foreach (var b in v)
                     {
-                        long b2 = b;
+                        ulong b2 = b;
                         bt.Mul(a, b).Should().Be((uint)(a2 * b2 % mod));
                     }
                 }
             }
         }
 
+        [Fact]
+        public void BarrettUInt32Border()
+        {
+            for (uint mod = uint.MaxValue; mod >= uint.MaxValue - 20; mod--)
+            {
+                var bt = new Barrett(mod);
+                var v = new List<uint>();
+                for (uint i = 0; i < 10; i++)
+                {
+                    v.Add(i);
+                    v.Add(mod - i);
+                    v.Add(mod / 2 + i);
+                    v.Add(mod / 2 - i);
+                }
+                foreach (var a in v)
+                {
+                    ulong a2 = a;
+                    bt.Mul(a, bt.Mul(a, a)).Should().Be((uint)(a2 * a2 % mod * a2 % mod));
+                    foreach (var b in v)
+                    {
+                        ulong b2 = b;
+                        bt.Mul(a, b).Should().Be((uint)(a2 * b2 % mod));
+                    }
+                }
+            }
+        }
     }
 }