pub const CityHash64 = struct {
    const Self = @This();

    // Some primes between 2^63 and 2^64 for various uses.
    const k0: u64 = 0xc3a5c85c97cb3127;
    const k1: u64 = 0xb492b66fbe98f273;
    const k2: u64 = 0x9ae16a3b2f90404f;

    // Rotate right helper
    fn rotr64(x: u64, comptime r: u64) u64 {
        return (x >> r) | (x << (64 - r));
    }

    fn shiftmix(v: u64) u64 {
        return v ^ (v >> 47);
    }

    fn hashLen16(u: u64, v: u64) u64 {
        return @call(.always_inline, hash128To64, .{ u, v });
    }

    fn hashLen16Mul(low: u64, high: u64, mul: u64) u64 {
        var a: u64 = (low ^ high) *% mul;
        a ^= (a >> 47);
        var b: u64 = (high ^ a) *% mul;
        b ^= (b >> 47);
        b *%= mul;
        return b;
    }

    fn hash128To64(low: u64, high: u64) u64 {
        return @call(.always_inline, hashLen16Mul, .{ low, high, 0x9ddfea08eb382d69 });
    }

    fn hashLen0To16(str: []const u8) u64 {
        const len: u64 = @as(u64, str.len);
        if (len >= 8) {
            const mul: u64 = k2 +% len *% 2;
            const a: u64 = fetch64(str.ptr, 0) +% k2;
            const b: u64 = fetch64(str.ptr, str.len - 8);
            const c: u64 = rotr64(b, 37) *% mul +% a;
            const d: u64 = (rotr64(a, 25) +% b) *% mul;
            return hashLen16Mul(c, d, mul);
        }
        if (len >= 4) {
            const mul: u64 = k2 +% len *% 2;
            const a: u64 = fetch32(str.ptr, 0);
            return hashLen16Mul(len +% (a << 3), fetch32(str.ptr, str.len - 4), mul);
        }
        if (len > 0) {
            const a: u8 = str[0];
            const b: u8 = str[str.len >> 1];
            const c: u8 = str[str.len - 1];
            const y: u32 = @as(u32, @intCast(a)) +% (@as(u32, @intCast(b)) << 8);
            const z: u32 = @as(u32, @truncate(str.len)) +% (@as(u32, @intCast(c)) << 2);
            return shiftmix(@as(u64, @intCast(y)) *% k2 ^ @as(u64, @intCast(z)) *% k0) *% k2;
        }
        return k2;
    }

    fn hashLen17To32(str: []const u8) u64 {
        const len: u64 = @as(u64, str.len);
        const mul: u64 = k2 +% len *% 2;
        const a: u64 = fetch64(str.ptr, 0) *% k1;
        const b: u64 = fetch64(str.ptr, 8);
        const c: u64 = fetch64(str.ptr, str.len - 8) *% mul;
        const d: u64 = fetch64(str.ptr, str.len - 16) *% k2;

        return hashLen16Mul(rotr64(a +% b, 43) +% rotr64(c, 30) +% d, a +% rotr64(b +% k2, 18) +% c, mul);
    }

    fn hashLen33To64(str: []const u8) u64 {
        const len: u64 = @as(u64, str.len);
        const mul: u64 = k2 +% len *% 2;
        const a: u64 = fetch64(str.ptr, 0) *% k2;
        const b: u64 = fetch64(str.ptr, 8);
        const c: u64 = fetch64(str.ptr, str.len - 24);
        const d: u64 = fetch64(str.ptr, str.len - 32);
        const e: u64 = fetch64(str.ptr, 16) *% k2;
        const f: u64 = fetch64(str.ptr, 24) *% 9;
        const g: u64 = fetch64(str.ptr, str.len - 8);
        const h: u64 = fetch64(str.ptr, str.len - 16) *% mul;

        const u: u64 = rotr64(a +% g, 43) +% (rotr64(b, 30) +% c) *% 9;
        const v: u64 = ((a +% g) ^ d) +% f +% 1;
        const w: u64 = @byteSwap((u +% v) *% mul) +% h;
        const x: u64 = rotr64(e +% f, 42) +% c;
        const y: u64 = (@byteSwap((v +% w) *% mul) +% g) *% mul;
        const z: u64 = e +% f +% c;
        const a1: u64 = @byteSwap((x +% z) *% mul +% y) +% b;
        const b1: u64 = shiftmix((z +% a1) *% mul +% d +% h) *% mul;
        return b1 +% x;
    }

    const WeakPair = struct {
        first: u64,
        second: u64,
    };

    fn weakHashLen32WithSeedsHelper(w: u64, x: u64, y: u64, z: u64, a: u64, b: u64) WeakPair {
        var a1: u64 = a;
        var b1: u64 = b;
        a1 +%= w;
        b1 = rotr64(b1 +% a1 +% z, 21);
        const c: u64 = a1;
        a1 +%= x;
        a1 +%= y;
        b1 +%= rotr64(a1, 44);
        return WeakPair{ .first = a1 +% z, .second = b1 +% c };
    }

    fn weakHashLen32WithSeeds(ptr: [*]const u8, a: u64, b: u64) WeakPair {
        return @call(.always_inline, weakHashLen32WithSeedsHelper, .{
            fetch64(ptr, 0),
            fetch64(ptr, 8),
            fetch64(ptr, 16),
            fetch64(ptr, 24),
            a,
            b,
        });
    }

    pub fn hash(str: []const u8) u64 {
        if (str.len <= 32) {
            if (str.len <= 16) {
                return hashLen0To16(str);
            } else {
                return hashLen17To32(str);
            }
        } else if (str.len <= 64) {
            return hashLen33To64(str);
        }

        var len: u64 = @as(u64, str.len);

        var x: u64 = fetch64(str.ptr, str.len - 40);
        var y: u64 = fetch64(str.ptr, str.len - 16) +% fetch64(str.ptr, str.len - 56);
        var z: u64 = hashLen16(fetch64(str.ptr, str.len - 48) +% len, fetch64(str.ptr, str.len - 24));
        var v: WeakPair = weakHashLen32WithSeeds(offsetPtr(str.ptr, str.len - 64), len, z);
        var w: WeakPair = weakHashLen32WithSeeds(offsetPtr(str.ptr, str.len - 32), y +% k1, x);

        x = x *% k1 +% fetch64(str.ptr, 0);
        len = (len - 1) & ~@as(u64, @intCast(63));

        var ptr: [*]const u8 = str.ptr;
        while (true) {
            x = rotr64(x +% y +% v.first +% fetch64(ptr, 8), 37) *% k1;
            y = rotr64(y +% v.second +% fetch64(ptr, 48), 42) *% k1;
            x ^= w.second;
            y +%= v.first +% fetch64(ptr, 40);
            z = rotr64(z +% w.first, 33) *% k1;
            v = weakHashLen32WithSeeds(ptr, v.second *% k1, x +% w.first);
            w = weakHashLen32WithSeeds(offsetPtr(ptr, 32), z +% w.second, y +% fetch64(ptr, 16));
            const t: u64 = z;
            z = x;
            x = t;

            ptr = offsetPtr(ptr, 64);
            len -= 64;
            if (len == 0)
                break;
        }

        return hashLen16(hashLen16(v.first, w.first) +% shiftmix(y) *% k1 +% z, hashLen16(v.second, w.second) +% x);
    }

    pub fn hashWithSeed(str: []const u8, seed: u64) u64 {
        return @call(.always_inline, Self.hashWithSeeds, .{ str, k2, seed });
    }

    pub fn hashWithSeeds(str: []const u8, seed0: u64, seed1: u64) u64 {
        return hashLen16(hash(str) -% seed0, seed1);
    }
}