'a
to type 'b
.Hashtbl.create n
creates a new, empty hash table, with
initial size n
. For best results, n
should be on the
order of the expected number of elements that will be in
the table. The table grows as needed, so n
is just an
initial guess.
random
parameter (a boolean) controls whether
the internal organization of the hash table is randomized at each
execution of Hashtbl.create
or deterministic over all executions.
~random:false
uses a
fixed hash function (Hashtbl.hash
) to distribute keys among
buckets. As a consequence, collisions between keys happen
deterministically. In Web-facing applications or other
security-sensitive applications, the deterministic collision
patterns can be exploited by a malicious user to create a
denial-of-service attack: the attacker sends input crafted to
create many collisions in the table, slowing the application down.
~random:true
uses the seeded
hash function Hashtbl.seeded_hash
with a seed that is randomly
chosen at hash table creation time. In effect, the hash function
used is randomly selected among 2^{30}
different hash functions.
All these hash functions have different collision patterns,
rendering ineffective the denial-of-service attack described above.
However, because of randomization, enumerating all elements of the
hash table using Hashtbl.fold
or Hashtbl.iter
is no longer
deterministic: elements are enumerated in different orders at
different runs of the program.
~random
parameter is given, hash tables are created
in non-random mode by default. This default can be changed
either programmatically by calling Hashtbl.randomize
or by
setting the R
flag in the OCAMLRUNPARAM
environment variable.random
parameter was not present and all
hash tables were created in non-randomized mode.reset
instead of clear
to shrink the
size of the bucket table to its initial size.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 Hashtbl.remove
tbl x
,
the previous binding for x
, if any, is restored.
(Same behavior as with association lists.)Hashtbl.find tbl x
returns the current binding of x
in tbl
,
or raises Not_found
if no such binding exists.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.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
.Hashtbl.replace tbl x y
replaces the current binding of x
in tbl
by a binding of x
to y
. If x
is unbound in tbl
,
a binding of x
to y
is added to tbl
.
This is functionally equivalent to Hashtbl.remove
tbl x
followed by Hashtbl.add
tbl x y
.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. Each binding is presented exactly once to f
.
f
is unspecified.
However, if the table contains several bindings for the same key,
they are passed to f
in reverse order of introduction, that is,
the most recent binding is passed first.
Hashtbl.fold f tbl init
computes
(f kN dN ... (f k1 d1 init)...)
,
where k1 ... kN
are the keys of all bindings in tbl
,
and d1 ... dN
are the associated values.
Each binding is presented exactly once to f
.
f
is unspecified.
However, if the table contains several bindings for the same key,
they are passed to f
in reverse order of introduction, that is,
the most recent binding is passed first.
Hashtbl.length tbl
returns the number of bindings in tbl
.
It takes constant time. Multiple bindings are counted once each, so
Hashtbl.length
gives the number of times Hashtbl.iter
calls its
first argument.Hashtbl.randomize()
, hash tables are created in
randomized mode by default: Hashtbl.create
returns randomized
hash tables, unless the ~random:false
optional parameter is given.
The same effect can be achieved by setting the R
parameter in
the OCAMLRUNPARAM
environment variable.
Hashtbl.create
call Hashtbl.randomize()
at initialization
time.
Hashtbl.randomize()
was called, there is no way
to revert to the non-randomized default behavior of Hashtbl.create
.
This is intentional. Non-randomized hash tables can still be
created using Hashtbl.create ~random:false
.num_bindings : int; | (* | Number of bindings present in the table.
Same value as returned by Hashtbl.length . | *) | |
num_buckets : int; | (* | Number of buckets in the table. | *) | |
max_bucket_length : int; | (* | Maximal number of bindings per bucket. | *) | |
bucket_histogram : int array; | (* | Histogram of bucket sizes. This array histo has
length max_bucket_length + 1 . The value of
histo.(i) is the number of buckets whose size is i . | *) |
Hashtbl.stats tbl
returns statistics about the table tbl
:
number of buckets, size of the biggest bucket, distribution of
buckets by size.Hashtbl.hash x
associates a nonnegative integer to any value of
any type. It is guaranteed that
if x = y
or Pervasives.compare x y = 0
, then hash x = hash y
.
Moreover, hash
always terminates, even on cyclic structures.Hashtbl.hash_param meaningful total x
computes a hash value for x
,
with the same properties as for hash
. The two extra integer
parameters meaningful
and total
give more precise control over
hashing. Hashing performs a breadth-first, left-to-right traversal
of the structure x
, stopping after meaningful
meaningful nodes
were encountered, or total
nodes (meaningful or not) were
encountered. Meaningful nodes are: integers; floating-point
numbers; strings; characters; booleans; and constant
constructors. Larger values of meaningful
and total
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 meaningful
and total
govern the tradeoff between accuracy and speed. As default
choices, Hashtbl.hash
and Hashtbl.seeded_hash
take
meaningful = 10
and total = 100
.Hashtbl.hash_param
that is further parameterized by
an integer seed. Usage:
Hashtbl.seeded_hash_param meaningful total seed x
.