    // 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 {
                const n = nmax / 16; // note: 16 | nmax

                var i: usize = 0;

                while (i + nmax <= input.len) {
                    var rounds: usize = 0;
                    while (rounds < n) : (rounds += 1) {
                        comptime var j: usize = 0;
                        inline while (j < 16) : (j += 1) {
                            s1 +%= input[i + j];
                            s2 +%= s1;
                        }
                        i += 16;
                    }

                    s1 %= base;
                    s2 %= base;
                }

                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" {
        try testing.expectEqual(@as(u32, 0x620062), Adler32.hash("a"));
        try testing.expectEqual(@as(u32, 0xbc002ed), Adler32.hash("example"));
    }

    test "adler32 long" {
        const long1 = [_]u8{1} ** 1024;
        try testing.expectEqual(@as(u32, 0x06780401), Adler32.hash(long1[0..]));

        const long2 = [_]u8{1} ** 1025;
        try testing.expectEqual(@as(u32, 0x0a7a0402), Adler32.hash(long2[0..]));
    }

    test "adler32 very long" {
        const long = [_]u8{1} ** 5553;
        try testing.expectEqual(@as(u32, 0x707f15b2), Adler32.hash(long[0..]));
    }

    test "adler32 very long with variation" {
        const long = comptime blk: {
            @setEvalBranchQuota(7000);
            var result: [6000]u8 = undefined;

            var i: usize = 0;
            while (i < result.len) : (i += 1) {
                result[i] = @truncate(u8, i);
            }

            break :blk result;
        };

        try testing.expectEqual(@as(u32, 0x5af38d6e), std.hash.Adler32.hash(long[0..]));
    }