Is there a safe value that is unreachable?

[ashtul]

While collisions are very unlikely when we use a 64 bits hash value, I would like to have a value with absolute safety to use as sentinel value.
Is 0 safe?

[arthurprs]

AFAIK there isn't. But you can use 0 (or any value) as a sentinel. Just make sure to add a branch to your actual hash calculation and return something else (e.g. 1), it'll be like any other collision.