<?xml version="1.0" encoding="latin1" ?> <!DOCTYPE erlref SYSTEM "erlref.dtd"> <erlref> <header> <copyright> <year>1996</year><year>2011</year> <holder>Ericsson AB. All Rights Reserved.</holder> </copyright> <legalnotice> The contents of this file are subject to the Erlang Public License, Version 1.1, (the "License"); you may not use this file except in compliance with the License. You should have received a copy of the Erlang Public License along with this software. If not, it can be retrieved online at http://www.erlang.org/. Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License. </legalnotice> <title>queue</title> <prepared>Joe</prepared> <responsible>Bjarne Däcker</responsible> <docno>1</docno> <approved>Bjarne Däcker</approved> <checked></checked> <date>97-01-15</date> <rev>B</rev> <file>queue.sgml</file> </header> <module>queue</module> <modulesummary>Abstract Data Type for FIFO Queues</modulesummary> <description> <p>This module implements (double ended) FIFO queues in an efficient manner.</p> <p>All functions fail with reason <c>badarg</c> 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 <c>badarg</c>.</p> <p>Some functions, where noted, fail with reason <c>empty</c> for an empty queue.</p> <p>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.</p> <p>All operations has an amortized O(1) running time, except <c>len/1</c>, <c>join/2</c>, <c>split/2</c>, <c>filter/2</c> and <c>member/2</c> 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 <c>len/1</c> is O(n). If better performance for this particular operation is essential, it is easy for the caller to keep track of the length.</p> <p>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.</p> <p>Entering at the front and exiting at the rear are reverse operations on the queue.</p> <p>The module has several sets of interface functions. The "Original API", the "Extended API" and the "Okasaki API".</p> <p>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".</p> <p>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.</p> <p>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.</p> </description> <section> <title>Original API</title> </section> <datatypes> <datatype> <name><marker id="type-queue">queue()</marker></name> <desc><p>As returned by <c>new/0</c>.</p></desc> </datatype> </datatypes> <funcs> <func> <name name="new" arity="0"/> <fsummary>Create an empty queue</fsummary> <desc> <p>Returns an empty queue.</p> </desc> </func> <func> <name name="is_queue" arity="1"/> <fsummary>Test if a term is a queue</fsummary> <desc> <p>Tests if <c><anno>Term</anno></c> is a queue and returns <c>true</c> if so and <c>false</c> otherwise.</p> </desc> </func> <func> <name name="is_empty" arity="1"/> <fsummary>Test if a queue is empty</fsummary> <desc> <p>Tests if <c><anno>Q</anno></c> is empty and returns <c>true</c> if so and <c>false</c> otherwise.</p> </desc> </func> <func> <name name="len" arity="1"/> <fsummary>Get the length of a queue</fsummary> <desc> <p>Calculates and returns the length of queue <c><anno>Q</anno></c>.</p> </desc> </func> <func> <name name="in" arity="2"/> <fsummary>Insert an item at the rear of a queue</fsummary> <desc> <p>Inserts <c><anno>Item</anno></c> at the rear of queue <c><anno>Q1</anno></c>. Returns the resulting queue <c><anno>Q2</anno></c>.</p> </desc> </func> <func> <name name="in_r" arity="2"/> <fsummary>Insert an item at the front of a queue</fsummary> <desc> <p>Inserts <c><anno>Item</anno></c> at the front of queue <c><anno>Q1</anno></c>. Returns the resulting queue <c><anno>Q2</anno></c>.</p> </desc> </func> <func> <name name="out" arity="1"/> <fsummary>Remove the front item from a queue</fsummary> <desc> <p>Removes the item at the front of queue <c><anno>Q1</anno></c>. Returns the tuple <c>{{value, <anno>Item</anno>}, <anno>Q2</anno>}</c>, where <c><anno>Item</anno></c> is the item removed and <c><anno>Q2</anno></c> is the resulting queue. If <c><anno>Q1</anno></c> is empty, the tuple <c>{empty, <anno>Q1</anno>}</c> is returned.</p> </desc> </func> <func> <name name="out_r" arity="1"/> <fsummary>Remove the rear item from a queue</fsummary> <desc> <p>Removes the item at the rear of the queue <c><anno>Q1</anno></c>. Returns the tuple <c>{{value, <anno>Item</anno>}, <anno>Q2</anno>}</c>, where <c><anno>Item</anno></c> is the item removed and <c><anno>Q2</anno></c> is the new queue. If <c><anno>Q1</anno></c> is empty, the tuple <c>{empty, <anno>Q1</anno>}</c> is returned. </p> </desc> </func> <func> <name name="from_list" arity="1"/> <fsummary>Convert a list to a queue</fsummary> <desc> <p>Returns a queue containing the items in <c><anno>L</anno></c> in the same order; the head item of the list will become the front item of the queue.</p> </desc> </func> <func> <name name="to_list" arity="1"/> <fsummary>Convert a queue to a list</fsummary> <desc> <p>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.</p> </desc> </func> <func> <name name="reverse" arity="1"/> <fsummary>Reverse a queue</fsummary> <desc> <p>Returns a queue <c><anno>Q2</anno></c> that contains the items of <c><anno>Q1</anno></c> in the reverse order.</p> </desc> </func> <func> <name name="split" arity="2"/> <fsummary>Split a queue in two</fsummary> <desc> <p>Splits <c><anno>Q1</anno></c> in two. The <c><anno>N</anno></c> front items are put in <c><anno>Q2</anno></c> and the rest in <c><anno>Q3</anno></c></p> </desc> </func> <func> <name name="join" arity="2"/> <fsummary>Join two queues</fsummary> <desc> <p>Returns a queue <c><anno>Q3</anno></c> that is the result of joining <c><anno>Q1</anno></c> and <c><anno>Q2</anno></c> with <c><anno>Q1</anno></c> in front of <c><anno>Q2</anno></c>.</p> </desc> </func> <func> <name name="filter" arity="2"/> <fsummary>Filter a queue</fsummary> <desc> <p>Returns a queue <c><anno>Q2</anno></c> that is the result of calling <c><anno>Fun</anno>(<anno>Item</anno>)</c> on all items in <c><anno>Q1</anno></c>, in order from front to rear.</p> <p>If <c><anno>Fun</anno>(<anno>Item</anno>)</c> returns <c>true</c>, <c>Item</c> is copied to the result queue. If it returns <c>false</c>, <c><anno>Item</anno></c> is not copied. If it returns a list the list elements are inserted instead of <c>Item</c> in the result queue.</p> <p>So, <c><anno>Fun</anno>(<anno>Item</anno>)</c> returning <c>[<anno>Item</anno>]</c> is thereby semantically equivalent to returning <c>true</c>, just as returning <c>[]</c> is semantically equivalent to returning <c>false</c>. But returning a list builds more garbage than returning an atom.</p> </desc> </func> <func> <name name="member" arity="2"/> <fsummary>Test if an item is in a queue</fsummary> <desc> <p>Returns <c>true</c> if <c><anno>Item</anno></c> matches some element in <c><anno>Q</anno></c>, otherwise <c>false</c>.</p> </desc> </func> </funcs> <section> <title>Extended API</title> </section> <funcs> <func> <name name="get" arity="1"/> <fsummary>Return the front item of a queue</fsummary> <desc> <p>Returns <c><anno>Item</anno></c> at the front of queue <c><anno>Q</anno></c>.</p> <p>Fails with reason <c>empty</c> if <c><anno>Q</anno></c> is empty.</p> </desc> </func> <func> <name name="get_r" arity="1"/> <fsummary>Return the rear item of a queue</fsummary> <desc> <p>Returns <c><anno>Item</anno></c> at the rear of queue <c><anno>Q</anno></c>.</p> <p>Fails with reason <c>empty</c> if <c><anno>Q</anno></c> is empty.</p> </desc> </func> <func> <name name="drop" arity="1"/> <fsummary>Remove the front item from a queue</fsummary> <desc> <p>Returns a queue <c><anno>Q2</anno></c> that is the result of removing the front item from <c><anno>Q1</anno></c>.</p> <p>Fails with reason <c>empty</c> if <c><anno>Q1</anno></c> is empty.</p> </desc> </func> <func> <name name="drop_r" arity="1"/> <fsummary>Remove the rear item from a queue</fsummary> <desc> <p>Returns a queue <c><anno>Q2</anno></c> that is the result of removing the rear item from <c><anno>Q1</anno></c>.</p> <p>Fails with reason <c>empty</c> if <c><anno>Q1</anno></c> is empty.</p> </desc> </func> <func> <name name="peek" arity="1"/> <fsummary>Return the front item of a queue</fsummary> <desc> <p>Returns the tuple <c>{value, <anno>Item</anno>}</c> where <c><anno>Item</anno></c> is the front item of <c><anno>Q</anno></c>, or <c>empty</c> if <c><anno>Q</anno></c> is empty.</p> </desc> </func> <func> <name name="peek_r" arity="1"/> <fsummary>Return the rear item of a queue</fsummary> <desc> <p>Returns the tuple <c>{value, <anno>Item</anno>}</c> where <c><anno>Item</anno></c> is the rear item of <c><anno>Q</anno></c>, or <c>empty</c> if <c><anno>Q</anno></c> is empty.</p> </desc> </func> </funcs> <section> <title>Okasaki API</title> </section> <funcs> <func> <name name="cons" arity="2"/> <fsummary>Insert an item at the head of a queue</fsummary> <desc> <p>Inserts <c><anno>Item</anno></c> at the head of queue <c><anno>Q1</anno></c>. Returns the new queue <c><anno>Q2</anno></c>.</p> </desc> </func> <func> <name name="head" arity="1"/> <fsummary>Return the item at the head of a queue</fsummary> <desc> <p>Returns <c><anno>Item</anno></c> from the head of queue <c><anno>Q</anno></c>.</p> <p>Fails with reason <c>empty</c> if <c><anno>Q</anno></c> is empty.</p> </desc> </func> <func> <name name="tail" arity="1"/> <fsummary>Remove the head item from a queue</fsummary> <desc> <p>Returns a queue <c><anno>Q2</anno></c> that is the result of removing the head item from <c><anno>Q1</anno></c>.</p> <p>Fails with reason <c>empty</c> if <c><anno>Q1</anno></c> is empty.</p> </desc> </func> <func> <name name="snoc" arity="2"/> <fsummary>Insert an item at the tail of a queue</fsummary> <desc> <p>Inserts <c><anno>Item</anno></c> as the tail item of queue <c><anno>Q1</anno></c>. Returns the new queue <c><anno>Q2</anno></c>.</p> </desc> </func> <func> <name name="daeh" arity="1"/> <name name="last" arity="1"/> <fsummary>Return the tail item of a queue</fsummary> <desc> <p>Returns the tail item of queue <c><anno>Q</anno></c>.</p> <p>Fails with reason <c>empty</c> if <c><anno>Q</anno></c> is empty.</p> </desc> </func> <func> <name name="liat" arity="1"/> <name name="init" arity="1"/> <name name="lait" arity="1"/> <fsummary>Remove the tail item from a queue</fsummary> <desc> <p>Returns a queue <c><anno>Q2</anno></c> that is the result of removing the tail item from <c><anno>Q1</anno></c>.</p> <p>Fails with reason <c>empty</c> if <c><anno>Q1</anno></c> is empty.</p> <p>The name <c>lait/1</c> is a misspelling - do not use it anymore.</p> </desc> </func> </funcs> </erlref>