bucketForAddress

    fn bucketForAddress(address: usize) *Bucket {
        const global = struct {
            /// Length must be a power of two. The longer this array, the less likely contention is
            /// between different futexes. This length seems like it'll provide a reasonable balance
            /// between contention and memory usage: assuming a 128-byte `Bucket` (due to cache line
            /// alignment), this uses 32 KiB of memory.
            var buckets: [256]Bucket = @splat(.init);
        };

        // Here we use Fibonacci hashing: the golden ratio can be used to evenly redistribute input
        // values across a range, giving a poor, but extremely quick to compute, hash.

        // This literal is the rounded value of '2^64 / phi' (where 'phi' is the golden ratio). The
        // shift then converts it to '2^b / phi', where 'b' is the pointer bit width.
        const fibonacci_multiplier = 0x9E3779B97F4A7C15 >> (64 - @bitSizeOf(usize));
        const hashed = address *% fibonacci_multiplier;

        comptime assert(std.math.isPowerOfTwo(global.buckets.len));
        // The high bits of `hashed` have better entropy than the low bits.
        const index = hashed >> (@bitSizeOf(usize) - @ctz(global.buckets.len));

        return &global.buckets[index];
    }