Accumulator

    const Accumulator = struct {
        acc1: u64,
        acc2: u64,
        acc3: u64,
        acc4: u64,

        fn init(seed: u64) Accumulator {
            return .{
                .acc1 = seed +% prime_1 +% prime_2,
                .acc2 = seed +% prime_2,
                .acc3 = seed,
                .acc4 = seed -% prime_1,
            };
        }

        fn updateEmpty(self: *Accumulator, input: anytype, comptime unroll_count: usize) usize {
            var i: usize = 0;

            if (unroll_count > 0) {
                const unrolled_bytes = unroll_count * 32;
                while (i + unrolled_bytes <= input.len) : (i += unrolled_bytes) {
                    inline for (0..unroll_count) |j| {
                        self.processStripe(input[i + j * 32 ..][0..32]);
                    }
                }
            }

            while (i + 32 <= input.len) : (i += 32) {
                self.processStripe(input[i..][0..32]);
            }

            return i;
        }

        fn processStripe(self: *Accumulator, buf: *const [32]u8) void {
            self.acc1 = round(self.acc1, mem.readInt(u64, buf[0..8], .little));
            self.acc2 = round(self.acc2, mem.readInt(u64, buf[8..16], .little));
            self.acc3 = round(self.acc3, mem.readInt(u64, buf[16..24], .little));
            self.acc4 = round(self.acc4, mem.readInt(u64, buf[24..32], .little));
        }

        fn merge(self: Accumulator) u64 {
            var acc = rotl(u64, self.acc1, 1) +% rotl(u64, self.acc2, 7) +%
                rotl(u64, self.acc3, 12) +% rotl(u64, self.acc4, 18);
            acc = mergeAccumulator(acc, self.acc1);
            acc = mergeAccumulator(acc, self.acc2);
            acc = mergeAccumulator(acc, self.acc3);
            acc = mergeAccumulator(acc, self.acc4);
            return acc;
        }

        fn mergeAccumulator(acc: u64, other: u64) u64 {
            const a = acc ^ round(0, other);
            const b = a *% prime_1;
            return b +% prime_4;
        }
    }