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# queue

### MODULE

queue

### MODULE SUMMARY

Abstract Data Type for FIFO Queues

### DESCRIPTION

This module implements (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, 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 should be regarded as opaque by other modules. Any code assuming knowledge of the format is running on thin ice.

All operations has an amortized O(1) running time, except len/1, join/2, split/2, filter/2 and member/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.

The module has several 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 also 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" contain alternative functions that build less garbage as well as 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. The 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.

### Original API

### DATA TYPES

queue()

As returned by new/0.

### EXPORTS

new() -> queue()

Returns an empty queue.

is_queue(Term :: term()) -> boolean()

Tests if Term is a queue and returns true if so and false otherwise.

is_empty(Q :: queue()) -> boolean()

Tests if Q is empty and returns true if so and false otherwise.

len(Q :: queue()) -> integer() >= 0

Calculates and returns the length of queue Q.

in(Item :: term(), Q1 :: queue()) -> Q2 :: queue()

Inserts Item at the rear of queue Q1. Returns the resulting queue Q2.

in_r(Item :: term(), Q1 :: queue()) -> Q2 :: queue()

Inserts Item at the front of queue Q1. Returns the resulting queue Q2.

out(Q1 :: queue()) ->
{{value, Item :: term()}, Q2 :: queue()} |
{empty, Q1 :: queue()}

Removes the item at the front of queue Q1. Returns the tuple {{value, Item}, Q2}, where Item is the item removed and Q2 is the resulting queue. If Q1 is empty, the tuple {empty, Q1} is returned.

out_r(Q1 :: queue()) ->
{{value, Item :: term()}, Q2 :: queue()} |
{empty, Q1 :: queue()}

Removes the item at the rear of the queue Q1. Returns the tuple {{value, Item}, Q2}, where Item is the item removed and Q2 is the new queue. If Q1 is empty, the tuple {empty, Q1} is returned.

from_list(L :: list()) -> queue()

Returns a queue containing the items in L in the same order; the head item of the list will become the front item of the queue.

to_list(Q :: queue()) -> list()

Returns a list of the items in the queue in the same order; the front item of the queue will become the head of the list.

reverse(Q1 :: queue()) -> Q2 :: queue()

Returns a queue Q2 that contains the items of Q1 in the reverse order.

split(N :: integer() >= 0, Q1 :: queue()) ->
{Q2 :: queue(), Q3 :: queue()}

Splits Q1 in two. The N front items are put in Q2 and the rest in Q3

join(Q1 :: queue(), Q2 :: queue()) -> Q3 :: queue()

Returns a queue Q3 that is the result of joining Q1 and Q2 with Q1 in front of Q2.

filter(Fun, Q1 :: queue()) -> Q2 :: queue()

Types:

Fun = fun((Item :: term()) -> boolean() | list())

Returns a queue Q2 that is the result of calling Fun(Item) on all items in Q1, in order from front to rear.

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.

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.

member(Item :: term(), Q :: queue()) -> boolean()

Returns true if Item matches some element in Q, otherwise false.

### Extended API

### EXPORTS

get(Q :: queue()) -> Item :: term()

Returns Item at the front of queue Q.

Fails with reason empty if Q is empty.

get_r(Q :: queue()) -> Item :: term()

Returns Item at the rear of queue Q.

Fails with reason empty if Q is empty.

drop(Q1 :: queue()) -> Q2 :: queue()

Returns a queue Q2 that is the result of removing the front item from Q1.

Fails with reason empty if Q1 is empty.

drop_r(Q1 :: queue()) -> Q2 :: queue()

Returns a queue Q2 that is the result of removing the rear item from Q1.

Fails with reason empty if Q1 is empty.

peek(Q :: queue()) -> empty | {value, Item :: term()}

Returns the tuple {value, Item} where Item is the front item of Q, or empty if Q is empty.

peek_r(Q :: queue()) -> empty | {value, Item :: term()}

Returns the tuple {value, Item} where Item is the rear item of Q, or empty if Q is empty.

### Okasaki API

### EXPORTS

cons(Item :: term(), Q1 :: queue()) -> Q2 :: queue()

Inserts Item at the head of queue Q1. Returns the new queue Q2.

head(Q :: queue()) -> Item :: term()

Returns Item from the head of queue Q.

Fails with reason empty if Q is empty.

tail(Q1 :: queue()) -> Q2 :: queue()

Returns a queue Q2 that is the result of removing the head item from Q1.

Fails with reason empty if Q1 is empty.

snoc(Q1 :: queue(), Item :: term()) -> Q2 :: queue()

Inserts Item as the tail item of queue Q1. Returns the new queue Q2.

daeh(Q :: queue()) -> Item :: term()
last(Q :: queue()) -> Item :: term()

Returns the tail item of queue Q.

Fails with reason empty if Q is empty.

liat(Q1 :: queue()) -> Q2 :: queue()
init(Q1 :: queue()) -> Q2 :: queue()
lait(Q1 :: queue()) -> Q2 :: queue()

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.