path: root/lib/stdlib/doc/src/file_sorter.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>2001</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>file_sorter</title>
    <prepared>Hans Bolinder</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>2001-03-13</date>
    <rev>PA1</rev>
    <file>file_sorter.xml</file>
  </header>
  <module since="">file_sorter</module>
  <modulesummary>File sorter.</modulesummary>
  <description>
    <p>This module contains functions for sorting terms on files, merging
      already sorted files, and checking files for sortedness. Chunks
      containing binary terms are read from a sequence of files, sorted
      internally in memory and written on temporary files, which are
      merged producing one sorted file as output. Merging is provided
      as an optimization; it is faster when the files are already
      sorted, but it always works to sort instead of merge.</p>

    <p>On a file, a term is represented by a header and a binary. Two
      options define the format of terms on files:</p>

    <taglist>
      <tag><c>{header, HeaderLength}</c></tag>
      <item>
        <p><c>HeaderLength</c> determines the
          number of bytes preceding each binary and containing the
          length of the binary in bytes. Defaults to 4. The order of the
          header bytes is defined as follows: if <c>B</c> is a binary
          containing a header only, size <c>Size</c> of the binary
          is calculated as
          <c><![CDATA[<<Size:HeaderLength/unit:8>> = B]]></c>.</p>
      </item>
      <tag><c>{format, Format}</c></tag>
      <item>
        <p>Option <c>Format</c> determines the
          function that is applied to binaries to create the
          terms to be sorted. Defaults to
          <c>binary_term</c>, which is equivalent to
          <c>fun&nbsp;binary_to_term/1</c>. Value <c>binary</c> is
          equivalent to <c>fun(X) -> X end</c>, which means that the
          binaries are sorted as they are. This is the fastest
          format. If <c>Format</c> is <c>term</c>, <c>io:read/2</c> is
          called to read terms. In that case, only the default value of
          option <c>header</c> is allowed.</p>
        <p>Option <c>format</c> also determines what is written to the
          sorted output file: if
          <c>Format</c> is <c>term</c>, then <c>io:format/3</c> is called
          to write each term, otherwise the binary prefixed by a header
          is written. Notice that the binary written is the same binary
          that was read; the results of applying function <c>Format</c>
          are thrown away when the terms have been sorted.
          Reading and writing terms using the <c>io</c> module
          is much slower than reading and writing binaries.</p>
      </item>
    </taglist>

    <p>Other options are:</p>

    <taglist>
      <tag><c>{order, Order}</c></tag>
      <item>
        <p>The default is to sort terms in
          ascending order, but that can be changed by value
          <c>descending</c> or by specifying an ordering function <c>Fun</c>.
          An ordering function is antisymmetric, transitive, and total.
          <c>Fun(A,&nbsp;B)</c> is to return <c>true</c> if <c>A</c>
          comes before <c>B</c> in the ordering, otherwise <c>false</c>.
          An example of a typical ordering function is less than or equal
          to, <c>=&lt;/2</c>. Using an ordering function slows down the sort
          considerably. Functions <c>keysort</c>, <c>keymerge</c> and
          <c>keycheck</c> do not accept ordering functions.</p>
      </item>
      <tag><c>{unique, boolean()}</c></tag>
      <item>
        <p>When sorting or merging files,
           only the first of a sequence of terms that compare equal (<c>==</c>)
           is output if this option is set to <c>true</c>. Defaults
           to <c>false</c>, which implies that all terms that
           compare equal are output. When checking files for
           sortedness, a check that no pair of consecutive terms
           compares equal is done if this option is set to <c>true</c>.</p>
      </item>
      <tag><c>{tmpdir, TempDirectory}</c></tag>
      <item>
        <p>The directory where
           temporary files are put can be chosen explicitly. The
           default, implied by value <c>""</c>, is to put temporary
           files on the same directory as the sorted output file. If
           output is a function (see below), the directory returned by
           <c>file:get_cwd()</c> is used instead. The names of
           temporary files are derived from the Erlang nodename
           (<c>node()</c>), the process identifier of the current Erlang
           emulator (<c>os:getpid()</c>), and a unique integer
           (<c>erlang:unique_integer([positive])</c>). A typical name is
           <c>fs_mynode@myhost_1763_4711.17</c>, where
           <c>17</c> is a sequence number. Existing files are
           overwritten. Temporary files are deleted unless some
           uncaught <c>EXIT</c> signal occurs.</p>
      </item>
      <tag><c>{compressed, boolean()}</c></tag>
      <item>
        <p>Temporary files and the output file can be compressed. Defaults
          <c>false</c>, which implies that written files are not
          compressed. Regardless of the value of option <c>compressed</c>,
          compressed files can always be read. Notice that
          reading and writing compressed files are significantly slower
          than reading and writing uncompressed files.</p>
      </item>
      <tag><c>{size, Size}</c></tag>
      <item>
        <p>By default about 512*1024 bytes read from files are sorted
          internally. This option is rarely needed.</p>
      </item>
      <tag><c>{no_files, NoFiles}</c></tag>
      <item>
        <p>By default 16 files are merged at a time. This option is rarely
          needed.</p>
      </item>
    </taglist>

    <p>As an alternative to sorting files, a function of one argument
      can be specified as input. When called with argument <c>read</c>,
      the function is assumed to return either of the following:</p>

    <list type="bulleted">
      <item>
        <p><c>end_of_input</c> or <c>{end_of_input, Value}}</c> when there
          is no more input (<c>Value</c> is explained below).</p>
      </item>
      <item>
        <p><c>{Objects, Fun}</c>, where <c>Objects</c> is a list of binaries
          or terms depending on the format, and <c>Fun</c> is a new input
          function.</p>
      </item>
    </list>

    <p>Any other value is immediately returned as value of the current call
      to <c>sort</c> or <c>keysort</c>. Each input function is
      called exactly once. If an error occurs, the last
      function is called with argument <c>close</c>, the reply of
      which is ignored.</p>

    <p>A function of one argument can be specified as output. The results
      of sorting or merging the input is collected in a non-empty
      sequence of variable length lists of binaries or terms depending
      on the format. The output function is called with one list at a
      time, and is assumed to return a new output function. Any other
      return value is immediately returned as value of the current
      call to the sort or merge function. Each output function is
      called exactly once. When some output function has been applied
      to all of the results or an error occurs, the last function is
      called with argument <c>close</c>, and the reply is returned
      as value of the current call to the sort or merge function.</p>

    <p>If a function is specified as input and the last input function
      returns <c>{end_of_input, Value}</c>, the function specified as output
      is called with argument <c>{value, Value}</c>. This makes it
      easy to initiate the sequence of output functions with a value
      calculated by the input functions.</p>

    <p>As an example, consider sorting the terms on a disk log file.
      A function that reads chunks from the disk log and returns a
      list of binaries is used as input. The results are collected in
      a list of terms.</p>

    <pre>
sort(Log) ->
    {ok, _} = disk_log:open([{name,Log}, {mode,read_only}]),
    Input = input(Log, start),
    Output = output([]),
    Reply = file_sorter:sort(Input, Output, {format,term}),
    ok = disk_log:close(Log),
    Reply.

input(Log, Cont) ->
    fun(close) ->
            ok;
       (read) ->
            case disk_log:chunk(Log, Cont) of
                {error, Reason} ->
                    {error, Reason};
                {Cont2, Terms} ->
                    {Terms, input(Log, Cont2)};
                {Cont2, Terms, _Badbytes} ->
                    {Terms, input(Log, Cont2)};
                eof ->
                    end_of_input
            end
    end.

output(L) ->
    fun(close) ->
            lists:append(lists:reverse(L));
       (Terms) ->
            output([Terms | L])
    end.</pre>

    <p>For more examples of functions as input and output, see
      the end of the <c>file_sorter</c> module; the <c>term</c>
      format is implemented with functions.</p>

    <p>The possible values of <c>Reason</c> returned when an error
      occurs are:</p>

    <list type="bulleted">
      <item>
        <p><c>bad_object</c>, <c>{bad_object, FileName}</c> -
          Applying the format function failed for some binary,
          or the key(s) could not be extracted from some term.</p>
      </item>
      <item>
        <p><c>{bad_term, FileName}</c> - <c>io:read/2</c> failed
          to read some term.</p>
      </item>
      <item>
        <p><c>{file_error, FileName, file:posix()}</c> - For an
          explanation of <c>file:posix()</c>, see
          <seealso marker="kernel:file"><c>file(3)</c></seealso>.</p>
      </item>
      <item>
        <p><c>{premature_eof, FileName}</c> - End-of-file was
          encountered inside some binary term.</p>
      </item>
    </list>
  </description>

  <datatypes>
    <datatype>
      <name name="file_name"/>
    </datatype>
    <datatype>
      <name name="file_names"/>
    </datatype>
    <datatype>
      <name name="i_command"/>
    </datatype>
    <datatype>
      <name name="i_reply"/>
    </datatype>
    <datatype>
      <name name="infun"/>
    </datatype>
    <datatype>
      <name name="input"/>
    </datatype>
    <datatype>
      <name name="input_reply"/>
    </datatype>
    <datatype>
      <name name="o_command"/>
    </datatype>
    <datatype>
      <name name="o_reply"/>
    </datatype>
    <datatype>
      <name name="object"/>
    </datatype>
    <datatype>
      <name name="outfun"/>
    </datatype>
    <datatype>
      <name name="output"/>
    </datatype>
    <datatype>
      <name name="output_reply"/>
    </datatype>
    <datatype>
      <name name="value"/>
    </datatype>
    <datatype>
      <name name="options"/>
    </datatype>
    <datatype>
      <name name="option"/>
    </datatype>
    <datatype>
      <name name="format"/>
    </datatype>
    <datatype>
      <name name="format_fun"/>
    </datatype>
    <datatype>
      <name name="header_length"/>
    </datatype>
    <datatype>
      <name name="key_pos"/>
    </datatype>
    <datatype>
      <name name="no_files"/>
    </datatype>
    <datatype>
      <name name="order"/>
    </datatype>
    <datatype>
      <name name="order_fun"/>
    </datatype>
    <datatype>
      <name name="size"/>
    </datatype>
    <datatype>
      <name name="tmp_directory"/>
    </datatype>
    <datatype>
      <name name="reason"/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="check" arity="1" since=""/>
      <name name="check" arity="2" since=""/>
      <fsummary>Check whether terms on files are sorted.</fsummary>
      <desc>
        <p>Checks files for sortedness. If a file is not sorted, the
          first out-of-order element is returned. The first term on a
          file has position 1.</p>
        <p><c>check(FileName)</c> is equivalent to
          <c>check([FileName], [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="keycheck" arity="2" since=""/>
      <name name="keycheck" arity="3" since=""/>
      <fsummary>Check whether terms on files are sorted by key.</fsummary>
      <desc>
        <p>Checks files for sortedness. If a file is not sorted, the
          first out-of-order element is returned. The first term on a
          file has position 1.</p>
        <p><c>keycheck(KeyPos, FileName)</c> is equivalent
          to <c>keycheck(KeyPos, [FileName], [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="keymerge" arity="3" since=""/>
      <name name="keymerge" arity="4" since=""/>
      <fsummary>Merge terms on files by key.</fsummary>
      <desc>
        <p>Merges tuples on files. Each input file is assumed to be
          sorted on key(s).</p>
        <p><c>keymerge(KeyPos, FileNames, Output)</c> is equivalent
          to <c>keymerge(KeyPos, FileNames, Output, [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="keysort" arity="2" since=""/>
      <fsummary>Sort terms on files by key.</fsummary>
      <desc>
        <p>Sorts tuples on files.</p>
        <p><c>keysort(N, FileName)</c> is
          equivalent to <c>keysort(N, [FileName], FileName)</c>.</p>
      </desc>
    </func>

    <func>
      <name name="keysort" arity="3" since=""/>
      <name name="keysort" arity="4" since=""/>
      <fsummary>Sort terms on files by key.</fsummary>
      <desc>
        <p>Sorts tuples on files. The sort is performed on the
          element(s) mentioned in <c><anno>KeyPos</anno></c>. If two
          tuples compare equal (<c>==</c>) on one element, the next
          element according to <c><anno>KeyPos</anno></c>
          is compared. The sort is stable.</p>
        <p><c>keysort(N, Input, Output)</c> is equivalent to
          <c>keysort(N, Input, Output, [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="merge" arity="2" since=""/>
      <name name="merge" arity="3" since=""/>
      <fsummary>Merge terms on files.</fsummary>
      <desc>
        <p>Merges terms on files. Each input file is assumed to be
          sorted.</p>
        <p><c>merge(FileNames, Output)</c> is equivalent to
          <c>merge(FileNames, Output, [])</c>.</p>
      </desc>
    </func>

    <func>
      <name name="sort" arity="1" since=""/>
      <fsummary>Sort terms on files.</fsummary>
      <desc>
        <p>Sorts terms on files.</p>
        <p><c>sort(FileName)</c> is equivalent
          to <c>sort([FileName], FileName)</c>.</p>
      </desc>
    </func>

    <func>
      <name name="sort" arity="2" since=""/>
      <name name="sort" arity="3" since=""/>
      <fsummary>Sort terms on files.</fsummary>
      <desc>
        <p>Sorts terms on files.</p>
        <p><c>sort(Input, Output)</c> is
          equivalent to <c>sort(Input, Output, [])</c>.</p>
      </desc>
    </func>
  </funcs>
</erlref>