1 files changed, 71 insertions, 0 deletions

diff --git a/src/ascii.zig b/src/ascii.zig
index 6c28f25..5b91348 100644
--- a/src/ascii.zig
+++ b/src/ascii.zig
@@ -25,6 +25,77 @@ pub fn isAsciiOnly(str: []const u8) bool {
     return true;
 }
 
+/// Do a caseless comparison, with SIMD if possible.  Strings must be of equal
+/// length.  Returns how many bytes are case-fold-matched ASCII, this will be
+/// equal to the string length if they match.
+pub fn caselessCmpLen(str_a: []const u8, str_b: []const u8) usize {
+    std.debug.assert(str_a.len == str_b.len);
+    const vec_len = simd.suggestVectorLength(u8) orelse return caselessCmpNoSimd(str_a, str_b);
+    const Vec = @Vector(vec_len, u8);
+    const BVec = @Vector(vec_len, bool);
+
+    const msb: Vec = @splat(@as(u8, 0x80));
+    const case_bit: Vec = @splat(@as(u8, 0x20));
+    const low5: Vec = @splat(@as(u8, 0x1f));
+    const vec0: Vec = @splat(@as(u8, 0));
+    const vec1: Vec = @splat(@as(u8, 1));
+    const vec26: Vec = @splat(@as(u8, 26));
+
+    var rem_a = str_a;
+    var rem_b = str_b;
+
+    while (rem_a.len >= vec_len) {
+        const a: Vec = rem_a[0..vec_len].*;
+        const b: Vec = rem_b[0..vec_len].*;
+        // ASCII gate: MSB must be 0 in both.
+        const is_ascii: BVec = ((a | b) & msb) == vec0;
+
+        const xor: Vec = a ^ b;
+        const exact: BVec = xor == vec0;
+        const case_diff: BVec = xor == case_bit;
+
+        // Letter test (only needed when case_diff).
+        const x: Vec = (a | b) & low5;
+        const is_letter: BVec =
+            (x >= vec1) & (x <= vec26);
+
+        const matched: BVec = is_ascii & (exact | (case_diff & is_letter));
+
+        if (!@reduce(.And, matched)) break;
+        rem_a = rem_a[vec_len..];
+        rem_b = rem_b[vec_len..];
+    }
+
+    // Tail
+    return str_a.len - rem_a.len + caselessCmpNoSimd(rem_a, rem_b);
+}
+
+inline fn caselessCmpNoSimd(str_a: []const u8, str_b: []const u8) usize {
+    for (str_a, str_b, 0..) |a, b, i| {
+        // High?
+        if (((a | b) & 0x80) != 0) return i;
+        const xor = a ^ b;
+        if (xor == 0) continue; // Match
+        if (xor != 0x20) return i; // Not the upcase bit.
+
+        const lo = a | b;
+        const x = lo & 0x1f;
+        if (x < 1 or x > 26) return i; // Not a letter
+    } else return str_a.len;
+}
+
+test caselessCmpNoSimd {
+    const hi_l = "Hello, World!";
+    const hi_h = "HeLlO, wOrLd!";
+    try testing.expectEqual(hi_l.len, caselessCmpNoSimd(hi_l, hi_h));
+}
+
+test caselessCmpLen {
+    const hi_l = "Hello, World!" ** 25;
+    const hi_h = "HeLlO, wOrLd!" ** 25;
+    try testing.expectEqual(hi_l.len, caselessCmpLen(hi_l, hi_h));
+}
+
 test "isAsciiOnly" {
     const ascii_only = "Hello, World! 0123456789 !@#$%^&*()_-=+";
     try testing.expect(isAsciiOnly(ascii_only));