hash_fold_t state x mixes the content of
x into the
By default, all our
hash_fold_t functions (derived or not) should satisfy the following properties.
hash_fold_t state x should mix all the information present in
x in the state. That is, by default,
hash_fold_t will traverse the full term
x (this is a significant change for Hashtbl.hash which by default stops traversing the term after after considering a small number of "significant values").
hash_fold_t must not discard the
hash_fold_t must be compatible with the associated
compare function: that is, for all
compare x y = 0 must imply
hash_fold_t s x = hash_fold_t
3. To avoid avoid systematic collisions,
hash_fold_t should expand to different sequences of built-in mixing functions for different values of
x. No such sequence is allowed to be a prefix of another.
A common mistake is to implement
hash_fold_t of a collection by just folding all the elements. This makes the folding sequence of
a be a prefix of
a @ b, thereby violating the requirement. This creates large families of collisions: all of the following collections would hash the same:
[1; 2]; 
[1; 2; 3]; 
; ; ; ; ...
A good way to avoid this is to mix in the size of the collection to the beginning (
fold ~init:(hash_fold_int state length) ~f:hash_fold_elem). The default in our libraries is to mix the length of the structure before folding. To prevent the afforementioned collisions, one should respect this ordering.