    const std = @import("../std.zig");
    const mem = std.mem;
    const math = std.math;

    const RoundParam = struct {
        a: usize,
        b: usize,
        c: usize,
        d: usize,
        e: usize,
        i: u32,
    };

    fn roundParam(a: usize, b: usize, c: usize, d: usize, e: usize, i: u32) RoundParam {
        return RoundParam{
            .a = a,
            .b = b,
            .c = c,
            .d = d,
            .e = e,
            .i = i,
        };
    }

    /// The SHA-1 function is now considered cryptographically broken.
    /// Namely, it is feasible to find multiple inputs producing the same hash.
    /// For a fast-performing, cryptographically secure hash function, see SHA512/256, BLAKE2 or BLAKE3.
    pub const Sha1 = struct {
        const Self = @This();
        pub const block_length = 64;
        pub const digest_length = 20;
        pub const Options = struct {};

        s: [5]u32,
        // Streaming Cache

        buf: [64]u8 = undefined,
        buf_len: u8 = 0,
        total_len: u64 = 0,

        pub fn init(options: Options) Self {
            _ = options;
            return Self{
                .s = [_]u32{
                    0x67452301,
                    0xEFCDAB89,
                    0x98BADCFE,
                    0x10325476,
                    0xC3D2E1F0,
                },
            };
        }

        pub fn hash(b: []const u8, out: *[digest_length]u8, options: Options) void {
            var d = Sha1.init(options);
            d.update(b);
            d.final(out);
        }

        pub fn update(d: *Self, b: []const u8) void {
            var off: usize = 0;

            // Partial buffer exists from previous update. Copy into buffer then hash.

            if (d.buf_len != 0 and d.buf_len + b.len >= 64) {
                off += 64 - d.buf_len;
                @memcpy(d.buf[d.buf_len..][0..off], b[0..off]);

                d.round(d.buf[0..]);
                d.buf_len = 0;
            }

            // Full middle blocks.

            while (off + 64 <= b.len) : (off += 64) {
                d.round(b[off..][0..64]);
            }

            // Copy any remainder for next pass.

            @memcpy(d.buf[d.buf_len..][0 .. b.len - off], b[off..]);
            d.buf_len += @as(u8, @intCast(b[off..].len));

            d.total_len += b.len;
        }

        pub fn peek(d: Self) [digest_length]u8 {
            var copy = d;
            return copy.finalResult();
        }

        pub fn final(d: *Self, out: *[digest_length]u8) void {
            // The buffer here will never be completely full.

            @memset(d.buf[d.buf_len..], 0);

            // Append padding bits.

            d.buf[d.buf_len] = 0x80;
            d.buf_len += 1;

            // > 448 mod 512 so need to add an extra round to wrap around.

            if (64 - d.buf_len < 8) {
                d.round(d.buf[0..]);
                @memset(d.buf[0..], 0);
            }

            // Append message length.

            var i: usize = 1;
            var len = d.total_len >> 5;
            d.buf[63] = @as(u8, @intCast(d.total_len & 0x1f)) << 3;
            while (i < 8) : (i += 1) {
                d.buf[63 - i] = @as(u8, @intCast(len & 0xff));
                len >>= 8;
            }

            d.round(d.buf[0..]);

            for (d.s, 0..) |s, j| {
                mem.writeIntBig(u32, out[4 * j ..][0..4], s);
            }
        }

        pub fn finalResult(d: *Self) [digest_length]u8 {
            var result: [digest_length]u8 = undefined;
            d.final(&result);
            return result;
        }

        fn round(d: *Self, b: *const [64]u8) void {
            var s: [16]u32 = undefined;

            var v: [5]u32 = [_]u32{
                d.s[0],
                d.s[1],
                d.s[2],
                d.s[3],
                d.s[4],
            };

            const round0a = comptime [_]RoundParam{
                roundParam(0, 1, 2, 3, 4, 0),
                roundParam(4, 0, 1, 2, 3, 1),
                roundParam(3, 4, 0, 1, 2, 2),
                roundParam(2, 3, 4, 0, 1, 3),
                roundParam(1, 2, 3, 4, 0, 4),
                roundParam(0, 1, 2, 3, 4, 5),
                roundParam(4, 0, 1, 2, 3, 6),
                roundParam(3, 4, 0, 1, 2, 7),
                roundParam(2, 3, 4, 0, 1, 8),
                roundParam(1, 2, 3, 4, 0, 9),
                roundParam(0, 1, 2, 3, 4, 10),
                roundParam(4, 0, 1, 2, 3, 11),
                roundParam(3, 4, 0, 1, 2, 12),
                roundParam(2, 3, 4, 0, 1, 13),
                roundParam(1, 2, 3, 4, 0, 14),
                roundParam(0, 1, 2, 3, 4, 15),
            };
            inline for (round0a) |r| {
                s[r.i] = mem.readIntBig(u32, b[r.i * 4 ..][0..4]);

                v[r.e] = v[r.e] +% math.rotl(u32, v[r.a], @as(u32, 5)) +% 0x5A827999 +% s[r.i & 0xf] +% ((v[r.b] & v[r.c]) | (~v[r.b] & v[r.d]));
                v[r.b] = math.rotl(u32, v[r.b], @as(u32, 30));
            }

            const round0b = comptime [_]RoundParam{
                roundParam(4, 0, 1, 2, 3, 16),
                roundParam(3, 4, 0, 1, 2, 17),
                roundParam(2, 3, 4, 0, 1, 18),
                roundParam(1, 2, 3, 4, 0, 19),
            };
            inline for (round0b) |r| {
                const t = s[(r.i - 3) & 0xf] ^ s[(r.i - 8) & 0xf] ^ s[(r.i - 14) & 0xf] ^ s[(r.i - 16) & 0xf];
                s[r.i & 0xf] = math.rotl(u32, t, @as(u32, 1));

                v[r.e] = v[r.e] +% math.rotl(u32, v[r.a], @as(u32, 5)) +% 0x5A827999 +% s[r.i & 0xf] +% ((v[r.b] & v[r.c]) | (~v[r.b] & v[r.d]));
                v[r.b] = math.rotl(u32, v[r.b], @as(u32, 30));
            }

            const round1 = comptime [_]RoundParam{
                roundParam(0, 1, 2, 3, 4, 20),
                roundParam(4, 0, 1, 2, 3, 21),
                roundParam(3, 4, 0, 1, 2, 22),
                roundParam(2, 3, 4, 0, 1, 23),
                roundParam(1, 2, 3, 4, 0, 24),
                roundParam(0, 1, 2, 3, 4, 25),
                roundParam(4, 0, 1, 2, 3, 26),
                roundParam(3, 4, 0, 1, 2, 27),
                roundParam(2, 3, 4, 0, 1, 28),
                roundParam(1, 2, 3, 4, 0, 29),
                roundParam(0, 1, 2, 3, 4, 30),
                roundParam(4, 0, 1, 2, 3, 31),
                roundParam(3, 4, 0, 1, 2, 32),
                roundParam(2, 3, 4, 0, 1, 33),
                roundParam(1, 2, 3, 4, 0, 34),
                roundParam(0, 1, 2, 3, 4, 35),
                roundParam(4, 0, 1, 2, 3, 36),
                roundParam(3, 4, 0, 1, 2, 37),
                roundParam(2, 3, 4, 0, 1, 38),
                roundParam(1, 2, 3, 4, 0, 39),
            };
            inline for (round1) |r| {
                const t = s[(r.i - 3) & 0xf] ^ s[(r.i - 8) & 0xf] ^ s[(r.i - 14) & 0xf] ^ s[(r.i - 16) & 0xf];
                s[r.i & 0xf] = math.rotl(u32, t, @as(u32, 1));

                v[r.e] = v[r.e] +% math.rotl(u32, v[r.a], @as(u32, 5)) +% 0x6ED9EBA1 +% s[r.i & 0xf] +% (v[r.b] ^ v[r.c] ^ v[r.d]);
                v[r.b] = math.rotl(u32, v[r.b], @as(u32, 30));
            }

            const round2 = comptime [_]RoundParam{
                roundParam(0, 1, 2, 3, 4, 40),
                roundParam(4, 0, 1, 2, 3, 41),
                roundParam(3, 4, 0, 1, 2, 42),
                roundParam(2, 3, 4, 0, 1, 43),
                roundParam(1, 2, 3, 4, 0, 44),
                roundParam(0, 1, 2, 3, 4, 45),
                roundParam(4, 0, 1, 2, 3, 46),
                roundParam(3, 4, 0, 1, 2, 47),
                roundParam(2, 3, 4, 0, 1, 48),
                roundParam(1, 2, 3, 4, 0, 49),
                roundParam(0, 1, 2, 3, 4, 50),
                roundParam(4, 0, 1, 2, 3, 51),
                roundParam(3, 4, 0, 1, 2, 52),
                roundParam(2, 3, 4, 0, 1, 53),
                roundParam(1, 2, 3, 4, 0, 54),
                roundParam(0, 1, 2, 3, 4, 55),
                roundParam(4, 0, 1, 2, 3, 56),
                roundParam(3, 4, 0, 1, 2, 57),
                roundParam(2, 3, 4, 0, 1, 58),
                roundParam(1, 2, 3, 4, 0, 59),
            };
            inline for (round2) |r| {
                const t = s[(r.i - 3) & 0xf] ^ s[(r.i - 8) & 0xf] ^ s[(r.i - 14) & 0xf] ^ s[(r.i - 16) & 0xf];
                s[r.i & 0xf] = math.rotl(u32, t, @as(u32, 1));

                v[r.e] = v[r.e] +% math.rotl(u32, v[r.a], @as(u32, 5)) +% 0x8F1BBCDC +% s[r.i & 0xf] +% ((v[r.b] & v[r.c]) ^ (v[r.b] & v[r.d]) ^ (v[r.c] & v[r.d]));
                v[r.b] = math.rotl(u32, v[r.b], @as(u32, 30));
            }

            const round3 = comptime [_]RoundParam{
                roundParam(0, 1, 2, 3, 4, 60),
                roundParam(4, 0, 1, 2, 3, 61),
                roundParam(3, 4, 0, 1, 2, 62),
                roundParam(2, 3, 4, 0, 1, 63),
                roundParam(1, 2, 3, 4, 0, 64),
                roundParam(0, 1, 2, 3, 4, 65),
                roundParam(4, 0, 1, 2, 3, 66),
                roundParam(3, 4, 0, 1, 2, 67),
                roundParam(2, 3, 4, 0, 1, 68),
                roundParam(1, 2, 3, 4, 0, 69),
                roundParam(0, 1, 2, 3, 4, 70),
                roundParam(4, 0, 1, 2, 3, 71),
                roundParam(3, 4, 0, 1, 2, 72),
                roundParam(2, 3, 4, 0, 1, 73),
                roundParam(1, 2, 3, 4, 0, 74),
                roundParam(0, 1, 2, 3, 4, 75),
                roundParam(4, 0, 1, 2, 3, 76),
                roundParam(3, 4, 0, 1, 2, 77),
                roundParam(2, 3, 4, 0, 1, 78),
                roundParam(1, 2, 3, 4, 0, 79),
            };
            inline for (round3) |r| {
                const t = s[(r.i - 3) & 0xf] ^ s[(r.i - 8) & 0xf] ^ s[(r.i - 14) & 0xf] ^ s[(r.i - 16) & 0xf];
                s[r.i & 0xf] = math.rotl(u32, t, @as(u32, 1));

                v[r.e] = v[r.e] +% math.rotl(u32, v[r.a], @as(u32, 5)) +% 0xCA62C1D6 +% s[r.i & 0xf] +% (v[r.b] ^ v[r.c] ^ v[r.d]);
                v[r.b] = math.rotl(u32, v[r.b], @as(u32, 30));
            }

            d.s[0] +%= v[0];
            d.s[1] +%= v[1];
            d.s[2] +%= v[2];
            d.s[3] +%= v[3];
            d.s[4] +%= v[4];
        }

        pub const Error = error{};
        pub const Writer = std.io.Writer(*Self, Error, write);

        fn write(self: *Self, bytes: []const u8) Error!usize {
            self.update(bytes);
            return bytes.len;
        }

        pub fn writer(self: *Self) Writer {
            return .{ .context = self };
        }
    };

    const htest = @import("test.zig");

    test "sha1 single" {
        try htest.assertEqualHash(Sha1, "da39a3ee5e6b4b0d3255bfef95601890afd80709", "");
        try htest.assertEqualHash(Sha1, "a9993e364706816aba3e25717850c26c9cd0d89d", "abc");
        try htest.assertEqualHash(Sha1, "a49b2446a02c645bf419f995b67091253a04a259", "abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu");
    }

    test "sha1 streaming" {
        var h = Sha1.init(.{});
        var out: [20]u8 = undefined;

        h.final(&out);
        try htest.assertEqual("da39a3ee5e6b4b0d3255bfef95601890afd80709", out[0..]);

        h = Sha1.init(.{});
        h.update("abc");
        h.final(&out);
        try htest.assertEqual("a9993e364706816aba3e25717850c26c9cd0d89d", out[0..]);

        h = Sha1.init(.{});
        h.update("a");
        h.update("b");
        h.update("c");
        h.final(&out);
        try htest.assertEqual("a9993e364706816aba3e25717850c26c9cd0d89d", out[0..]);
    }

    test "sha1 aligned final" {
        var block = [_]u8{0} ** Sha1.block_length;
        var out: [Sha1.digest_length]u8 = undefined;

        var h = Sha1.init(.{});
        h.update(&block);
        h.final(out[0..]);
    }