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>2013</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>file_sorter</title>
    <prepared>Hans Bolinder</prepared>
    <responsible>nobody</responsible>
    <docno></docno>
    <approved>nobody</approved>
    <checked>no</checked>
    <date>2001-03-13</date>
    <rev>PA1</rev>
    <file>file_sorter.sgml</file>
  </header>
  <module>file_sorter</module>
  <modulesummary>File Sorter</modulesummary>
  <description>
    <p>The functions of this module sort terms on files, merge already
      sorted files, and check 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>
    <list type="bulleted">
      <item><c>{header, HeaderLength}</c>. HeaderLength determines the
       number of bytes preceding each binary and containing the
       length of the binary in bytes. Default is 4. The order of the
       header bytes is defined as follows: if <c>B</c> is a binary
       containing a header only, the size <c>Size</c> of the binary
       is calculated as
      <c><![CDATA[<<Size:HeaderLength/unit:8>> = B]]></c>.
      </item>
      <item><c>{format, Format}</c>. The format determines the
       function that is applied to binaries in order to create the
       terms that will be sorted. The default value is
      <c>binary_term</c>, which is equivalent to
      <c>fun&nbsp;binary_to_term/1</c>. The value <c>binary</c> is
       equivalent to <c>fun(X) -> X end</c>, which means that the
       binaries will be 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
       the <c>header</c> option is allowed. The <c>format</c> option
       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. Note that the binary written is the same binary
       that was read; the results of applying the <c>Format</c>
       function are thrown away as soon as the terms have been
       sorted. Reading and writing terms using the <c>io</c> module
       is very much slower than reading and writing binaries.
      </item>
    </list>
    <p>Other options are:
      </p>
    <list type="bulleted">
      <item><c>{order, Order}</c>. The default is to sort terms in
       ascending order, but that can be changed by the value
       <c>descending</c> or by giving an ordering function <c>Fun</c>.
       An ordering function is antisymmetric, transitive and total.
       <c>Fun(A,&nbsp;B)</c> should return <c>true</c> if <c>A</c>
       comes before <c>B</c> in the ordering, <c>false</c> otherwise.
       An example of a typical ordering function is less than or equal
       to, <c>=&lt;/2</c>.
       Using an ordering function will slow down the sort
       considerably. The <c>keysort</c>, <c>keymerge</c> and
       <c>keycheck</c> functions do not accept ordering functions.
      </item>
      <item><c>{unique, boolean()}</c>. 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>. The default
       value is <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>.
      </item>
      <item><c>{tmpdir, TempDirectory}</c>. The directory where
       temporary files are put can be chosen explicitly. The
       default, implied by the 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 timestamp
       (<c>erlang:now()</c>); a typical name would be
      <c>fs_mynode@myhost_1763_1043_337000_266005.17</c>, where
      <c>17</c> is a sequence number. Existing files will be
       overwritten. Temporary files are deleted unless some
       uncaught EXIT signal occurs.
      </item>
      <item><c>{compressed, boolean()}</c>. Temporary files and the
       output file may be compressed. The default value
      <c>false</c> implies that written files are not
       compressed. Regardless of the value of the <c>compressed</c>
       option, compressed files can always be read. Note that
       reading and writing compressed files is significantly slower
       than reading and writing uncompressed files.
      </item>
      <item><c>{size, Size}</c>. By default approximately 512*1024
       bytes read from files are sorted internally. This option
       should rarely be needed.
      </item>
      <item><c>{no_files, NoFiles}</c>. By default 16 files are
       merged at a time. This option should rarely be needed.
      </item>
    </list>
    <p>As an alternative to sorting files, a function of one argument
      can be given as input. When called with the argument <c>read</c>
      the function is assumed to return <c>end_of_input</c> or
      <c>{end_of_input, Value}}</c> when there is no more input
      (<c>Value</c> is explained below), or <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. Any other
      value is immediately returned as value of the current call to
      <c>sort</c> or <c>keysort</c>. Each input function will be
      called exactly once, and should an error occur, the last
      function is called with the argument <c>close</c>, the reply of
      which is ignored.
      </p>
    <p>A function of one argument can be given 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 the argument <c>close</c>, and the reply is returned
      as value of the current call to the sort or merge function. If a
      function is given as input and the last input function returns
      <c>{end_of_input, Value}</c>, the function given as output will
      be called with the 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>Further examples of functions as input and output can be found
      at 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>. See
          <c>file(3)</c> for an explanation of <c>file:posix()</c>.</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"/><br/>
    </datatype>
    <datatype>
      <name name="file_names"/><br/>
    </datatype>
    <datatype>
      <name name="i_command"/><br/>
    </datatype>
    <datatype>
      <name name="i_reply"/><br/>
    </datatype>
    <datatype>
      <name name="infun"/><br/>
    </datatype>
    <datatype>
      <name name="input"/><br/>
    </datatype>
    <datatype>
      <name name="input_reply"/><br/>
    </datatype>
    <datatype>
      <name name="o_command"/><br/>
    </datatype>
    <datatype>
      <name name="o_reply"/><br/>
    </datatype>
    <datatype>
      <name name="object"/><br/>
    </datatype>
    <datatype>
      <name name="outfun"/><br/>
    </datatype>
    <datatype>
      <name name="output"/><br/>
    </datatype>
    <datatype>
      <name name="output_reply"/><br/>
    </datatype>
    <datatype>
      <name name="value"/><br/>
    </datatype>
    <datatype>
      <name name="options"/><br/>
    </datatype>
    <datatype>
      <name name="option"/><br/>
    </datatype>
    <datatype>
      <name name="format"/><br/>
    </datatype>
    <datatype>
      <name name="format_fun"/><br/>
    </datatype>
    <datatype>
      <name name="header_length"/><br/>
    </datatype>
    <datatype>
      <name name="key_pos"/><br/>
    </datatype>
    <datatype>
      <name name="no_files"/><br/>
    </datatype>
    <datatype>
      <name name="order"/><br/>
    </datatype>
    <datatype>
      <name name="order_fun"/><br/>
    </datatype>
    <datatype>
      <name name="size"/><br/>
    </datatype>
    <datatype>
      <name name="tmp_directory"/><br/>
    </datatype>
    <datatype>
      <name name="reason"/><br/>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="sort" arity="1"/>
      <fsummary>Sort terms on files.</fsummary>
      <desc>
        <p>Sorts terms on files. <c>sort(FileName)</c> is equivalent
          to <c>sort([FileName], FileName)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="sort" arity="2"/>
      <name name="sort" arity="3"/>
      <fsummary>Sort terms on files.</fsummary>
      <desc>
        <p>Sorts terms on files. <c>sort(Input, Output)</c> is
          equivalent to <c>sort(Input, Output, [])</c>.</p>
      </desc>
    </func>
    <func>
      <name name="keysort" arity="2"/>
      <fsummary>Sort terms on files by key.</fsummary>
      <desc>
        <p>Sorts tuples on files. <c>keysort(N, FileName)</c> is
          equivalent to <c>keysort(N, [FileName], FileName)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="keysort" arity="3"/>
      <name name="keysort" arity="4"/>
      <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, 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"/>
      <name name="merge" arity="3"/>
      <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="keymerge" arity="3"/>
      <name name="keymerge" arity="4"/>
      <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="check" arity="1"/>
      <name name="check" arity="2"/>
      <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"/>
      <name name="keycheck" arity="3"/>
      <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>
  </funcs>
</erlref>