Stdlib.Queue
First-in first-out queues.
This module implements queues (FIFOs), with in-place modification. See the example section below.
Unsynchronized accesses
Unsynchronized accesses to a queue may lead to an invalid queue state. Thus, concurrent accesses to queues must be synchronized (for instance with a Mutex.t
).
Raised when Queue.take
or Queue.peek
is applied to an empty queue.
let create: unit => t('a);
Return a new queue, initially empty.
let add: 'a => t('a) => unit;
add x q
adds the element x
at the end of the queue q
.
let push: 'a => t('a) => unit;
push
is a synonym for add
.
let take: t('a) => 'a;
take q
removes and returns the first element in queue q
, or raises Empty
if the queue is empty.
let take_opt: t('a) => option('a);
take_opt q
removes and returns the first element in queue q
, or returns None
if the queue is empty.
let pop: t('a) => 'a;
pop
is a synonym for take
.
let peek: t('a) => 'a;
peek q
returns the first element in queue q
, without removing it from the queue, or raises Empty
if the queue is empty.
let peek_opt: t('a) => option('a);
peek_opt q
returns the first element in queue q
, without removing it from the queue, or returns None
if the queue is empty.
let top: t('a) => 'a;
top
is a synonym for peek
.
let clear: t('a) => unit;
Discard all elements from a queue.
let is_empty: t('a) => bool;
Return true
if the given queue is empty, false
otherwise.
let length: t('a) => int;
Return the number of elements in a queue.
let iter: ('a => unit) => t('a) => unit;
iter f q
applies f
in turn to all elements of q
, from the least recently entered to the most recently entered. The queue itself is unchanged.
let fold: ('acc => 'a => 'acc) => 'acc => t('a) => 'acc;
fold f accu q
is equivalent to List.fold_left f accu l
, where l
is the list of q
's elements. The queue remains unchanged.
transfer q1 q2
adds all of q1
's elements at the end of the queue q2
, then clears q1
. It is equivalent to the sequence iter (fun x -> add x q2) q1; clear q1
, but runs in constant time.
Iterate on the queue, in front-to-back order. The behavior is not specified if the queue is modified during the iteration.
A basic example:
# let q = Queue.create ()
val q : '_weak1 Queue.t = <abstr>
# Queue.push 1 q; Queue.push 2 q; Queue.push 3 q
- : unit = ()
# Queue.length q
- : int = 3
# Queue.pop q
- : int = 1
# Queue.pop q
- : int = 2
# Queue.pop q
- : int = 3
# Queue.pop q
Exception: Stdlib.Queue.Empty.
For a more elaborate example, a classic algorithmic use of queues is to implement a BFS (breadth-first search) through a graph.
type graph = {
edges: (int, int list) Hashtbl.t
}
(* Search in graph [g] using BFS, starting from node [start].
It returns the first node that satisfies [p], or [None] if
no node reachable from [start] satisfies [p].
*)
let search_for ~(g:graph) ~(start:int) (p:int -> bool) : int option =
let to_explore = Queue.create() in
let explored = Hashtbl.create 16 in
Queue.push start to_explore;
let rec loop () =
if Queue.is_empty to_explore then None
else
(* node to explore *)
let node = Queue.pop to_explore in
explore_node node
and explore_node node =
if not (Hashtbl.mem explored node) then (
if p node then Some node (* found *)
else (
Hashtbl.add explored node ();
let children =
Hashtbl.find_opt g.edges node
|> Option.value ~default:[]
in
List.iter (fun child -> Queue.push child to_explore) children;
loop()
)
) else loop()
in
loop()
(* a sample graph *)
let my_graph: graph =
let edges =
List.to_seq [
1, [2;3];
2, [10; 11];
3, [4;5];
5, [100];
11, [0; 20];
]
|> Hashtbl.of_seq
in {edges}
# search_for ~g:my_graph ~start:1 (fun x -> x = 30)
- : int option = None
# search_for ~g:my_graph ~start:1 (fun x -> x >= 15)
- : int option = Some 20
# search_for ~g:my_graph ~start:1 (fun x -> x >= 50)
- : int option = Some 100