1 files changed, 107 insertions, 0 deletions
diff --git a/lib/std/hash/adler.zig b/lib/std/hash/adler.zig
new file mode 100644
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+++ b/lib/std/hash/adler.zig
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+// Adler32 checksum.
+//
+// https://tools.ietf.org/html/rfc1950#section-9
+// https://github.com/madler/zlib/blob/master/adler32.c
+
+const std = @import("../std.zig");
+const testing = std.testing;
+
+pub const Adler32 = struct {
+    const base = 65521;
+    const nmax = 5552;
+
+    adler: u32,
+
+    pub fn init() Adler32 {
+        return Adler32{ .adler = 1 };
+    }
+
+    // This fast variant is taken from zlib. It reduces the required modulos and unrolls longer
+    // buffer inputs and should be much quicker.
+    pub fn update(self: *Adler32, input: []const u8) void {
+        var s1 = self.adler & 0xffff;
+        var s2 = (self.adler >> 16) & 0xffff;
+
+        if (input.len == 1) {
+            s1 +%= input[0];
+            if (s1 >= base) {
+                s1 -= base;
+            }
+            s2 +%= s1;
+            if (s2 >= base) {
+                s2 -= base;
+            }
+        } else if (input.len < 16) {
+            for (input) |b| {
+                s1 +%= b;
+                s2 +%= s1;
+            }
+            if (s1 >= base) {
+                s1 -= base;
+            }
+
+            s2 %= base;
+        } else {
+            var i: usize = 0;
+            while (i + nmax <= input.len) : (i += nmax) {
+                const n = nmax / 16; // note: 16 | nmax
+
+                var rounds: usize = 0;
+                while (rounds < n) : (rounds += 1) {
+                    comptime var j: usize = 0;
+                    inline while (j < 16) : (j += 1) {
+                        s1 +%= input[i + n * j];
+                        s2 +%= s1;
+                    }
+                }
+            }
+
+            if (i < input.len) {
+                while (i + 16 <= input.len) : (i += 16) {
+                    comptime var j: usize = 0;
+                    inline while (j < 16) : (j += 1) {
+                        s1 +%= input[i + j];
+                        s2 +%= s1;
+                    }
+                }
+                while (i < input.len) : (i += 1) {
+                    s1 +%= input[i];
+                    s2 +%= s1;
+                }
+
+                s1 %= base;
+                s2 %= base;
+            }
+        }
+
+        self.adler = s1 | (s2 << 16);
+    }
+
+    pub fn final(self: *Adler32) u32 {
+        return self.adler;
+    }
+
+    pub fn hash(input: []const u8) u32 {
+        var c = Adler32.init();
+        c.update(input);
+        return c.final();
+    }
+};
+
+test "adler32 sanity" {
+    testing.expect(Adler32.hash("a") == 0x620062);
+    testing.expect(Adler32.hash("example") == 0xbc002ed);
+}
+
+test "adler32 long" {
+    const long1 = [_]u8{1} ** 1024;
+    testing.expect(Adler32.hash(long1[0..]) == 0x06780401);
+
+    const long2 = [_]u8{1} ** 1025;
+    testing.expect(Adler32.hash(long2[0..]) == 0x0a7a0402);
+}
+
+test "adler32 very long" {
+    const long = [_]u8{1} ** 5553;
+    testing.expect(Adler32.hash(long[0..]) == 0x707f15b2);
+}