add extra xor "c"

this on an even number of rounds acts as a xor-encrypt-xor block, a common technique in cryptography. this helps reduce artifacts in the generic width implementation.

lib/std/hash.zig

@@ -36,13 +36,12 @@ const xxhash = @import("hash/xxhash.zig");
 pub const XxHash3 = xxhash.XxHash3;
 pub const XxHash64 = xxhash.XxHash64;
 pub const XxHash32 = xxhash.XxHash32;
-
 /// Deprecated in favor of `int`.
 pub fn uint32(input: u32) u32 {
     return int(input);
 }
 
-/// Applies a bit-mangling transformation to an unsigned integer type `T`.
+/// Applies a bit-mangling transformation to an integer type `T`.
 /// Optimized per type: for `u16` and `u32`, Skeeto's xorshift-multiply; for `u64`, Maiga's mx3.
 /// Falls back on an avalanche pattern for other integer types, ensuring high entropy.
 pub fn int(input: anytype) @TypeOf(input) {
@@ -90,25 +89,26 @@ pub fn int(input: anytype) @TypeOf(input) {
                 while (mul * 3 < max) mul *= 3;
                 break :blk ((mul ^ (mul >> hsize)) | 1);
             };
-            inline for (0..2) |_| {
-                x = (x ^ (x >> hsize + 1)) *% c;
-                x = (x ^ (x >> hsize - 1)) *% c;
+            const sh = comptime hsize | 1;
+            inline for (0..4) |_| {
+                //the extra xor here acts as a XEX, it helps greatly
+                x = ((x ^ (x >> sh)) *% c) ^ c;
             }
-            x ^= (x >> hsize);
+            x ^= (x >> sh);
         },
     }
     return x;
 }
 
 test int {
     const expectEqual = @import("std").testing.expectEqual;
-    try expectEqual(0xC, int(@as(u4, 1)));
-    try expectEqual(0x4F, int(@as(u8, 1)));
-    try expectEqual(0x4F, int(@as(i8, 1)));
+    try expectEqual(0xD, int(@as(u4, 1)));
+    try expectEqual(0x11, int(@as(u8, 3)));
+    try expectEqual(0x11, int(@as(i8, 3)));
     try expectEqual(0x2880, int(@as(u16, 1)));
     try expectEqual(0x42741D6, int(@as(u32, 1)));
     try expectEqual(0x71894DE00D9981F, int(@as(u64, 1)));
-    try expectEqual(0x50BC2BB18910C3DE0BAA2CE0D0C5B83E, int(@as(u128, 1)));
+    try expectEqual(0x555e8e1ae49daf5bfbe0787f46093d52, int(@as(u128, 1)));
 }
 
 test {