Module Hashtbl: hash tables and hash functions

Hash tables are hashed association tables, with inplace modification.
Generic interface
type ('a, 'b) t

The type of hash tables from type 'a to type 'b.
val create : int > ('a,'b) t

Hashtbl.create n creates a new, empty hash table, with
initial size n. The table grows as needed, so n is
just an initial guess. Better results are said to be
achieved when n is a prime number.
Raise Invalid_argument "hashtbl__new" if n is less than 1.
val clear : ('a, 'b) t > unit

Empty a hash table.
val add : ('a, 'b) t > 'a > 'b > unit

Hashtbl.add tbl x y adds a binding of x to y in table tbl.
Previous bindings for x are not removed, but simply
hidden. That is, after performing remove tbl x, the previous
binding for x, if any, is restored.
(This is the semantics of association lists.)
val find : ('a, 'b) t > 'a > 'b

Hashtbl.find tbl x returns the current binding of x in tbl,
or raises Not_found if no such binding exists.
val find_all : ('a, 'b) t > 'a > 'b list

Hashtbl.find_all tbl x returns the list of all data
associated with x in tbl.
The current binding is returned first, then the previous
bindings, in reverse order of introduction in the table.
val remove : ('a, 'b) t > 'a > unit

Hashtbl.remove tbl x removes the current binding of x in tbl,
restoring the previous binding if it exists.
It does nothing if x is not bound in tbl.
val iter : ('a > 'b > unit) > ('a, 'b) t > unit

Hashtbl.iter f tbl applies f to all bindings in table tbl.
f receives the key as first argument, and the associated value
as second argument. The order in which the bindings are passed to
f is unspecified. Each binding is presented exactly once
to f.
Functorial interface
module type HashedType =
sig
type t
val equal: t > t > bool
val hash: t > int
end

The input signature of the functor Hashtbl.Make.
t is the type of keys.
equal is the equality predicate used to compare keys.
hash is a hashing function on keys, returning a nonnegative
integer. It must be such that if two keys are equal according
to equal, then they must have identical hash values as computed
by hash.
Examples: suitable (equal, hash) pairs for arbitrary key
types include
((=), Hashtbl.hash) for comparing objects by structure,
((==), Hashtbl.hash) for comparing objects by addresses
(e.g. for mutable or cyclic keys).
module type S =
sig
type key
type 'a t
val create: int > 'a t
val clear: 'a t > unit
val add: 'a t > key > 'a > unit
val remove: 'a t > key > unit
val find: 'a t > key > 'a
val find_all: 'a t > key > 'a list
val iter: (key > 'a > unit) > 'a t > unit
end
module Make(H: HashedType): (S with type key = H.t)

The functor Hashtbl.Make returns a structure containing
a type key of keys and a type 'a t of hash tables
associating data of type 'a to keys of type key.
The operations perform similarly to those of the generic
interface, but use the hashing and equality functions
specified in the functor argument H instead of generic
equality and hashing.
The polymorphic hash primitive
val hash : 'a > int

Hashtbl.hash x associates a positive integer to any value of
any type. It is guaranteed that
if x = y, then hash x = hash y.
Moreover, hash always terminates, even on cyclic
structures.
val hash_param : int > int > 'a > int

Hashtbl.hash_param n m x computes a hash value for x, with the
same properties as for hash. The two extra parameters n and
m give more precise control over hashing. Hashing performs a
depthfirst, righttoleft traversal of the structure x, stopping
after n meaningful nodes were encountered, or m nodes,
meaningful or not, were encountered. Meaningful nodes are: integers;
floatingpoint numbers; strings; characters; booleans; and constant
constructors. Larger values of m and n means that more
nodes are taken into account to compute the final hash
value, and therefore collisions are less likely to happen.
However, hashing takes longer. The parameters m and n
govern the tradeoff between accuracy and speed.