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  <header>
    <copyright>
      <year>1996</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>queue</title>
    <prepared>Joe</prepared>
    <responsible>Bjarne D&auml;cker</responsible>
    <docno>1</docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked></checked>
    <date>1997-01-15</date>
    <rev>B</rev>
    <file>queue.xml</file>
  </header>
  <module>queue</module>
  <modulesummary>Abstract data type for FIFO queues.</modulesummary>
  <description>
    <p>This module provides (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, 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 <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
      is to 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
      <seealso marker="#filter/2"><c>filter/2</c></seealso>,
      <seealso marker="#join/2"><c>join/2</c></seealso>,
      <seealso marker="#len/1"><c>len/1</c></seealso>,
      <seealso marker="#member/2"><c>member/2</c></seealso>,
      <seealso marker="#split/2"><c>split/2</c></seealso> 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>This module has three 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
      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" contains alternative functions that build
      less garbage and 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.
      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.</p>
  </description>

  <section>
    <title>Original API</title>
  </section>

  <datatypes>
    <datatype>
      <name name="queue" n_vars="1"/>
      <desc><p>As returned by
        <seealso marker="#new/0"><c>new/0</c></seealso>.</p></desc>
    </datatype>
    <datatype>
      <name name="queue" n_vars="0"/>
    </datatype>
  </datatypes>

  <funcs>
    <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="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 becomes the front
          item of the queue.</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="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, otherwise otherwise.</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, otherwise <c>false</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="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="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>

    <func>
      <name name="new" arity="0"/>
      <fsummary>Create an empty queue.</fsummary>
      <desc>
        <p>Returns an empty queue.</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 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, 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 queue <c><anno>Q1</anno></c>.
          Returns 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, tuple <c>{empty, <anno>Q1</anno>}</c> is returned.</p>
      </desc>
    </func>

    <func>
      <name name="reverse" arity="1"/>
      <fsummary>Reverse a queue.</fsummary>
      <desc>
        <p>Returns a queue <c><anno>Q2</anno></c> containing 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="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 becomes the head of the list.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Extended API</title>
  </section>

  <funcs>
    <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="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="peek" arity="1"/>
      <fsummary>Return the front item of a queue.</fsummary>
      <desc>
        <p>Returns 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 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="daeh" 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="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="init" 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>
      </desc>
    </func>

    <func>
      <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>

    <func>
      <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"/>
      <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>
      </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="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>
  </funcs>
</erlref>

