getIndex

        fn getIndex(self: Self, key: anytype, ctx: anytype) ?usize {
            if (self.size == 0) {
                // We use cold instead of unlikely to force a jump to this case,
                // no matter the weight of the opposing side.
                @branchHint(.cold);
                return null;
            }

            // If you get a compile error on this line, it means that your generic hash
            // function is invalid for these parameters.
            const hash: Hash = ctx.hash(key);

            const mask = self.capacity() - 1;
            const fingerprint = Metadata.takeFingerprint(hash);
            // Don't loop indefinitely when there are no empty slots.
            var limit = self.capacity();
            var idx = @as(usize, @truncate(hash & mask));

            var metadata = self.metadata.? + idx;
            while (!metadata[0].isFree() and limit != 0) {
                if (metadata[0].isUsed() and metadata[0].fingerprint == fingerprint) {
                    const test_key = &self.keys()[idx];

                    if (ctx.eql(key, test_key.*)) {
                        return idx;
                    }
                }

                limit -= 1;
                idx = (idx + 1) & mask;
                metadata = self.metadata.? + idx;
            }

            return null;
        }