queue (stdlib v7.0.2-rc0)
View SourceAbstract data type for FIFO queues.
This module provides (double-ended) FIFO queues in an efficient manner.
All functions fail with reason badarg if arguments are of wrong type, for
example, queue arguments are not queues, indexes are not integers, and list
arguments are not lists. Improper lists cause internal crashes. An index out of
range for a queue also causes a failure with reason badarg.
Some functions, where noted, fail with reason empty for an empty queue.
The data representing a queue as used by this module is to be regarded as opaque by other modules. In abstract terms, the representation is a composite type of existing Erlang terms. See note on data types. Any code assuming knowledge of the format is running on thin ice.
All operations have an amortized O(1) running time, except all/2, any/2,
delete/2, delete_r/2, delete_with/2, delete_with_r/2, filter/2,
filtermap/2, fold/3, join/2, len/1, member/2, split/2 that have
O(n). To minimize the size of a queue minimizing the amount of garbage built by
queue operations, the queues do not contain explicit length information, and
that is why len/1 is O(n). If better performance for this
particular operation is essential, it is easy for the caller to keep track of
the length.
Queues are double-ended. The mental picture of a queue is a line of people (items) waiting for their turn. The queue front is the end with the item that has waited the longest. The queue rear is the end an item enters when it starts to wait. If instead using the mental picture of a list, the front is called head and the rear is called tail.
Entering at the front and exiting at the rear are reverse operations on the queue.
This module has three sets of interface functions: the "Original API", the "Extended API", and the "Okasaki API".
The "Original API" and the "Extended API" both use the mental picture of a waiting line of items. Both have reverse operations suffixed "_r".
The "Original API" item removal functions return compound terms with both the removed item and the resulting queue. The "Extended API" contains alternative functions that build less garbage and functions for just inspecting the queue ends. Also the "Okasaki API" functions build less garbage.
The "Okasaki API" is inspired by "Purely Functional Data Structures" by Chris Okasaki. It regards queues as lists. This API is by many regarded as strange and avoidable. For example, many reverse operations have lexically reversed names, some with more readable but perhaps less understandable aliases.
Summary
Extended API
Returns a queue Q2 that is the result of removing the front item from Q1.
Returns a queue Q2 that is the result of removing the rear item from Q1.
Returns Item at the front of queue Q.
Returns Item at the rear of queue Q.
Returns tuple {value, Item}, where Item is the front item of Q, or empty
if Q is empty.
Returns tuple {value, Item}, where Item is the rear item of Q, or empty
if Q is empty.
Okasaki API
Inserts Item at the head of queue Q1. Returns the new queue Q2.
Returns the tail item of queue Q.
Returns Item from the head of queue Q.
Returns a queue Q2 that is the result of removing the tail item from Q1.
Returns a queue Q2 that is the result of removing the tail item from Q1.
Returns the tail item of queue Q.
Returns a queue Q2 that is the result of removing the tail item from Q1.
Inserts Item as the tail item of queue Q1. Returns the new queue Q2.
Returns a queue Q2 that is the result of removing the head item from Q1.
Original API
Returns true if Pred(Item) returns true for all items Item in Q,
otherwise false.
Returns true if Pred(Item) returns true for at least one item Item in
Q, otherwise false.
Returns a copy of Q1 where the first item matching Item is deleted, if there
is such an item.
Returns a copy of Q1 where the last item matching Item is deleted, if there
is such an item.
Returns a copy of Q1 where the first item for which Pred returns true is
deleted, if there is such an item.
Returns a copy of Q1 where the last item for which Pred returns true is
deleted, if there is such an item.
Returns a queue Q2 that is the result of calling Fun(Item) on all items in
Q1.
Returns a queue Q2 that is the result of calling Fun(Item) on all items in
Q1.
Calls Fun(Item, AccIn) on successive items Item of Queue, starting with
AccIn == Acc0. The queue is traversed in queue order, that is, from front to
rear. Fun/2 must return a new accumulator, which is passed to the next call.
The function returns the final value of the accumulator. Acc0 is returned if
the queue is empty.
Returns a queue containing the items in L in the same order; the head item of
the list becomes the front item of the queue.
Inserts Item at the rear of queue Q1. Returns the resulting queue Q2.
Inserts Item at the front of queue Q1. Returns the resulting queue Q2.
Tests if Q is empty and returns true if so, otherwise false.
Tests if Term is a queue and returns true if so, otherwise false. Note
that the test will return true for a term coinciding with the representation
of a queue, even when not constructed by thus module. See also note on
data types.
Returns a queue Q3 that is the result of joining Q1 and Q2 with Q1 in
front of Q2.
Calculates and returns the length of queue Q.
Returns true if Item matches some element in Q, otherwise false.
Returns an empty queue.
Removes the item at the front of queue Q1. Returns tuple
{{value, Item}, Q2}, where Item is the item removed and Q2 is the
resulting queue. If Q1 is empty, tuple {empty, Q1} is returned.
Removes the item at the rear of queue Q1. Returns tuple {{value, Item}, Q2},
where Item is the item removed and Q2 is the new queue. If Q1 is empty,
tuple {empty, Q1} is returned.
Returns a queue Q2 containing the items of Q1 in the reverse order.
Splits Q1 in two. The N front items are put in Q2 and the rest in Q3.
Returns a list of the items in the queue in the same order; the front item of the queue becomes the head of the list.
Types
Extended API
Returns a queue Q2 that is the result of removing the front item from Q1.
Fails with reason empty if Q1 is empty.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue = queue:drop(Queue).
{[5,4,3],[2]}
3> queue:to_list(Queue1).
[2,3,4,5]Returns a queue Q2 that is the result of removing the rear item from Q1.
Fails with reason empty if Q1 is empty.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue = queue:drop_r(Queue).
{[4,3],[1,2]}
3> queue:to_list(Queue1).
[1,2,3,4]-spec get(Q :: queue(Item)) -> Item.
Returns Item at the front of queue Q.
Fails with reason empty if Q is empty.
Example 1:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> 1 == queue:get(Queue).
true-spec get_r(Q :: queue(Item)) -> Item.
Returns Item at the rear of queue Q.
Fails with reason empty if Q is empty.
Example 1:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> 5 == queue:get_r(Queue).
true-spec peek(Q :: queue(Item)) -> empty | {value, Item}.
Returns tuple {value, Item}, where Item is the front item of Q, or empty
if Q is empty.
Example 1:
1> queue:peek(queue:new()).
empty
2> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
3> queue:peek(Queue).
{value, 1}-spec peek_r(Q :: queue(Item)) -> empty | {value, Item}.
Returns tuple {value, Item}, where Item is the rear item of Q, or empty
if Q is empty.
Example 1:
1> queue:peek_r(queue:new()).
empty
2> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
3> queue:peek_r(Queue).
{value, 5}Okasaki API
Inserts Item at the head of queue Q1. Returns the new queue Q2.
Example:
1> Queue = queue:cons(0, queue:from_list([1,2,3])).
{[3,2],[0,1]}
2> queue:to_list(Queue).
[0,1,2,3]-spec daeh(Q :: queue(Item)) -> Item.
Returns the tail item of queue Q.
Fails with reason empty if Q is empty.
Example 1:
1> queue:daeh(queue:from_list([1,2,3])).
3-spec head(Q :: queue(Item)) -> Item.
Returns Item from the head of queue Q.
Fails with reason empty if Q is empty.
Example 1:
1> queue:head(queue:from_list([1,2,3])).
1Returns a queue Q2 that is the result of removing the tail item from Q1.
Fails with reason empty if Q1 is empty.
Example:
1> Queue = queue:init(queue:from_list([1,2,3])).
{[2],[1]}
2> queue:to_list(Queue).
[1,2]Returns a queue Q2 that is the result of removing the tail item from Q1.
Fails with reason empty if Q1 is empty.
The name lait/1 is a misspelling - do not use it anymore.
-spec last(Q :: queue(Item)) -> Item.
Returns the tail item of queue Q.
Fails with reason empty if Q is empty.
Example:
1> queue:last(queue:from_list([1,2,3])).
3Returns a queue Q2 that is the result of removing the tail item from Q1.
Fails with reason empty if Q1 is empty.
Example:
1> Queue = queue:liat(queue:from_list([1,2,3])).
{[2],[1]}
2> queue:to_list(Queue).
[1,2]Inserts Item as the tail item of queue Q1. Returns the new queue Q2.
Example:
1> Queue = queue:snoc(queue:from_list([1,2,3]), 4).
{[4,3,2],[1]}
2> queue:to_list(Queue).
[1,2,3,4]Returns a queue Q2 that is the result of removing the head item from Q1.
Fails with reason empty if Q1 is empty.
Original API
Returns true if Pred(Item) returns true for all items Item in Q,
otherwise false.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
2> queue:all(fun (E) -> E > 3 end, Queue).
false
3> queue:all(fun (E) -> E > 0 end, Queue).
trueReturns true if Pred(Item) returns true for at least one item Item in
Q, otherwise false.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
2> queue:any(fun (E) -> E > 10 end, Queue).
false
3> queue:any(fun (E) -> E > 3 end, Queue).
trueReturns a copy of Q1 where the first item matching Item is deleted, if there
is such an item.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
2> Queue1 = queue:delete(3, Queue).
3> queue:member(3, Queue1).
falseReturns a copy of Q1 where the last item matching Item is deleted, if there
is such an item.
Example:
1> Queue = queue:from_list([1,2,3,4,3,5]).
2> Queue1 = queue:delete_r(3, Queue).
3> queue:to_list(Queue1).
[1,2,3,4,5]-spec delete_with(Pred, Q1) -> Q2 when Pred :: fun((Item) -> boolean()), Q1 :: queue(Item), Q2 :: queue(Item), Item :: term().
Returns a copy of Q1 where the first item for which Pred returns true is
deleted, if there is such an item.
Example:
1> Queue = queue:from_list([100,1,2,3,4,5]).
2> Queue1 = queue:delete_with(fun (E) -> E > 0, Queue).
3> queue:to_list(Queue1).
[1,2,3,4,5]-spec delete_with_r(Pred, Q1) -> Q2 when Pred :: fun((Item) -> boolean()), Q1 :: queue(Item), Q2 :: queue(Item), Item :: term().
Returns a copy of Q1 where the last item for which Pred returns true is
deleted, if there is such an item.
Example:
1> Queue = queue:from_list([1,2,3,4,5,100]).
2> Queue1 = queue:delete_with(fun (E) -> E > 10, Queue).
3> queue:to_list(Queue1).
[1,2,3,4,5]-spec filter(Fun, Q1 :: queue(Item)) -> Q2 :: queue(Item) when Fun :: fun((Item) -> boolean() | [Item]).
Returns a queue Q2 that is the result of calling Fun(Item) on all items in
Q1.
If Fun(Item) returns true, Item is copied to the result queue. If it
returns false, Item is not copied. If it returns a list, the list elements
are inserted instead of Item in the result queue.
Example 1:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:filter(fun (E) -> E > 2 end, Queue).
{[5],[3,4]}
3> queue:to_list(Queue1).
[3,4,5]So, Fun(Item) returning [Item] is thereby semantically equivalent to
returning true, just as returning [] is semantically equivalent to returning
false. But returning a list builds more garbage than returning an atom.
Example 2:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:filter(fun (E) -> [E, E+1] end, Queue).
{[6,5,5,4,4,3],[1,2,2,3]}
3> queue:to_list(Queue1).
[1,2,2,3,3,4,4,5,5,6]-spec filtermap(Fun, Q1) -> Q2 when Fun :: fun((Item) -> boolean() | {true, Value}), Q1 :: queue(Item), Q2 :: queue(Item | Value), Item :: term(), Value :: term().
Returns a queue Q2 that is the result of calling Fun(Item) on all items in
Q1.
If Fun(Item) returns true, Item is copied to the result queue. If it
returns false, Item is not copied. If it returns {true, NewItem}, the
queue element at this position is replaced with NewItem in the result queue.
Example 1:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:filtermap(fun (E) -> E > 2 end, Queue).
{[5],[3,4]}
3> queue:to_list(Queue1).
[3,4,5]
4> Queue1 = queue:filtermap(fun (E) -> {true, E+100} end, Queue).
{"ihg","ef"}
5> queue:to_list(Queue1).
"efghi-spec fold(Fun, Acc0, Q :: queue(Item)) -> Acc1 when Fun :: fun((Item, AccIn) -> AccOut), Acc0 :: term(), Acc1 :: term(), AccIn :: term(), AccOut :: term().
Calls Fun(Item, AccIn) on successive items Item of Queue, starting with
AccIn == Acc0. The queue is traversed in queue order, that is, from front to
rear. Fun/2 must return a new accumulator, which is passed to the next call.
The function returns the final value of the accumulator. Acc0 is returned if
the queue is empty.
Example:
1> queue:fold(fun(X, Sum) -> X + Sum end, 0, queue:from_list([1,2,3,4,5])).
15
2> queue:fold(fun(X, Prod) -> X * Prod end, 1, queue:from_list([1,2,3,4,5])).
120-spec from_list(L :: [Item]) -> queue(Item).
Returns a queue containing the items in L in the same order; the head item of
the list becomes the front item of the queue.
Inserts Item at the rear of queue Q1. Returns the resulting queue Q2.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:in(100, Queue).
{[100,5,4,3],[1,2]}
3> queue:to_list(Queue1).
[1,2,3,4,5,100]Inserts Item at the front of queue Q1. Returns the resulting queue Q2.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:in_r(100, Queue).
{[5,4,3],[100,1,2]}
3> queue:to_list(Queue1).
[100,1,2,3,4,5]Tests if Q is empty and returns true if so, otherwise false.
Tests if Term is a queue and returns true if so, otherwise false. Note
that the test will return true for a term coinciding with the representation
of a queue, even when not constructed by thus module. See also note on
data types.
Returns a queue Q3 that is the result of joining Q1 and Q2 with Q1 in
front of Q2.
Example:
1> Queue1 = queue:from_list([1,3]).
{[3],[1]}
2> Queue2 = queue:from_list([2,4]).
{[4],[2]}
3> queue:to_list(queue:join(Queue1, Queue2)).
[1,3,2,4]-spec len(Q :: queue()) -> non_neg_integer().
Calculates and returns the length of queue Q.
Returns true if Item matches some element in Q, otherwise false.
Returns an empty queue.
Removes the item at the front of queue Q1. Returns tuple
{{value, Item}, Q2}, where Item is the item removed and Q2 is the
resulting queue. If Q1 is empty, tuple {empty, Q1} is returned.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> {{value, 1=Item}, Queue1} = queue:out(Queue).
{{value,1},{[5,4,3],[2]}}
3> queue:to_list(Queue1).
[2,3,4,5]Removes the item at the rear of queue Q1. Returns tuple {{value, Item}, Q2},
where Item is the item removed and Q2 is the new queue. If Q1 is empty,
tuple {empty, Q1} is returned.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> {{value, 5=Item}, Queue1} = queue:out_r(Queue).
{{value,5},{[4,3],[1,2]}}
3> queue:to_list(Queue1).
[1,2,3,4]Returns a queue Q2 containing the items of Q1 in the reverse order.
-spec split(N :: non_neg_integer(), Q1 :: queue(Item)) -> {Q2 :: queue(Item), Q3 :: queue(Item)}.
Splits Q1 in two. The N front items are put in Q2 and the rest in Q3.
-spec to_list(Q :: queue(Item)) -> [Item].
Returns a list of the items in the queue in the same order; the front item of the queue becomes the head of the list.
Example:
1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> List == queue:to_list(Queue).
true