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<?xml version="1.0" encoding="latin1" ?>
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<!DOCTYPE erlref SYSTEM "erlref.dtd">
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<holder>Ericsson AB, All Rights Reserved</holder>
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The contents of this file are subject to the Erlang Public License,
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Version 1.1, (the "License"); you may not use this file except in
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compliance with the License. You should have received a copy of the
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Erlang Public License along with this software. If not, it can be
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retrieved online at http://www.erlang.org/.
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Software distributed under the License is distributed on an "AS IS"
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basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
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the License for the specific language governing rights and limitations
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The Initial Developer of the Original Code is Ericsson AB.
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<title>file_sorter</title>
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<prepared>Hans Bolinder</prepared>
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<responsible>nobody</responsible>
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<approved>nobody</approved>
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<date>2001-03-13</date>
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<file>file_sorter.sgml</file>
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<module>file_sorter</module>
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<modulesummary>File Sorter</modulesummary>
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<p>The functions of this module sort terms on files, merge already
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sorted files, and check files for sortedness. Chunks containing
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binary terms are read from a sequence of files, sorted
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internally in memory and written on temporary files, which are
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merged producing one sorted file as output. Merging is provided
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as an optimization; it is faster when the files are already
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sorted, but it always works to sort instead of merge.
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<p>On a file, a term is represented by a header and a binary. Two
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options define the format of terms on files:
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<list type="bulleted">
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<item><c>{header, HeaderLength}</c>. HeaderLength determines the
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number of bytes preceding each binary and containing the
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length of the binary in bytes. Default is 4. The order of the
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header bytes is defined as follows: if <c>B</c> is a binary
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containing a header only, the size <c>Size</c> of the binary
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<c><![CDATA[<<Size:HeaderLength/unit:8>> = B]]></c>.
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<item><c>{format, Format}</c>. The format determines the
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function that is applied to binaries in order to create the
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terms that will be sorted. The default value is
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<c>binary_term</c>, which is equivalent to
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<c>fun binary_to_term/1</c>. The value <c>binary</c> is
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equivalent to <c>fun(X) -> X end</c>, which means that the
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binaries will be sorted as they are. This is the fastest
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format. If <c>Format</c> is <c>term</c>, <c>io:read/2</c> is
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called to read terms. In that case only the default value of
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the <c>header</c> option is allowed. The <c>format</c> option
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also determines what is written to the sorted output file: if
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<c>Format</c> is <c>term</c> then <c>io:format/3</c> is called
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to write each term, otherwise the binary prefixed by a header
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is written. Note that the binary written is the same binary
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that was read; the results of applying the <c>Format</c>
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function are thrown away as soon as the terms have been
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sorted. Reading and writing terms using the <c>io</c> module
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is very much slower than reading and writing binaries.
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<list type="bulleted">
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<item><c>{order, Order}</c>. The default is to sort terms in
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ascending order, but that can be changed by the value
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<c>descending</c> or by giving an ordering function <c>Fun</c>.
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An ordering function is antisymmetric, transitive and total.
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<c>Fun(A, B)</c> should return <c>true</c> if <c>A</c>
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comes before <c>B</c> in the ordering, <c>false</c> otherwise.
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Using an ordering function will slow down the sort
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considerably. The <c>keysort</c>, <c>keymerge</c> and
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<c>keycheck</c> functions do not accept ordering functions.
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<item><c>{unique, bool()}</c>. When sorting or merging files,
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only the first of a sequence of terms that compare equal is
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output if this option is set to <c>true</c>. The default
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value is <c>false</c> which implies that all terms that
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compare equal are output. When checking files for
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sortedness, a check that no pair of consecutive terms
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compares equal is done if this option is set to <c>true</c>.
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<item><c>{tmpdir, TempDirectory}</c>. The directory where
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temporary files are put can be chosen explicitly. The
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default, implied by the value <c>""</c>, is to put temporary
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files on the same directory as the sorted output file. If
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output is a function (see below), the directory returned by
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<c>file:get_cwd()</c> is used instead. The names of
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temporary files are derived from the Erlang nodename
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(<c>node()</c>), the process identifier of the current Erlang
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emulator (<c>os:getpid()</c>), and a timestamp
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(<c>erlang:now()</c>); a typical name would be
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<c>fs_mynode@myhost_1763_1043_337000_266005.17</c>, where
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<c>17</c> is a sequence number. Existing files will be
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overwritten. Temporary files are deleted unless some
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uncaught EXIT signal occurs.
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<item><c>{compressed, bool()}</c>. Temporary files and the
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output file may be compressed. The default value
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<c>false</c> implies that written files are not
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compressed. Regardless of the value of the <c>compressed</c>
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option, compressed files can always be read. Note that
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reading and writing compressed files is significantly slower
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than reading and writing uncompressed files.
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<item><c>{size, Size}</c>. By default approximately 512*1024
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bytes read from files are sorted internally. This option
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should rarely be needed.
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<item><c>{no_files, NoFiles}</c>. By default 16 files are
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merged at a time. This option should rarely be needed.
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<p>To summarize, here is the syntax of the options:</p>
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<list type="bulleted">
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<p><c>Options = [Option] | Option</c></p>
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<p><c>Option = {header, HeaderLength} | {format, Format} | {order, Order} | {unique, bool()} | {tmpdir, TempDirectory} | {compressed, bool()} | {size, Size} | {no_files, NoFiles}</c></p>
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<p><c>HeaderLength = int() > 0</c></p>
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<p><c>Format = binary_term | term | binary | FormatFun</c></p>
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<p><c>FormatFun = fun(Binary) -> Term</c></p>
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<p><c>Order = ascending | descending | OrderFun</c></p>
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<p><c>OrderFun = fun(Term, Term) -> bool()</c></p>
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<p><c>TempDirectory = "" | file_name()</c></p>
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<p><c>Size = int() >= 0</c></p>
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<p><c>NoFiles = int() > 1</c></p>
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<p>As an alternative to sorting files, a function of one argument
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can be given as input. When called with the argument <c>read</c>
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the function is assumed to return <c>end_of_input</c> or
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<c>{end_of_input, Value}}</c> when there is no more input
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(<c>Value</c> is explained below), or <c>{Objects, Fun}</c>,
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where <c>Objects</c> is a list of binaries or terms depending on
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the format and <c>Fun</c> is a new input function. Any other
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value is immediately returned as value of the current call to
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<c>sort</c> or <c>keysort</c>. Each input function will be
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called exactly once, and should an error occur, the last
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function is called with the argument <c>close</c>, the reply of
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<p>A function of one argument can be given as output. The results
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of sorting or merging the input is collected in a non-empty
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sequence of variable length lists of binaries or terms depending
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on the format. The output function is called with one list at a
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time, and is assumed to return a new output function. Any other
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return value is immediately returned as value of the current
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call to the sort or merge function. Each output function is
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called exactly once. When some output function has been applied
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to all of the results or an error occurs, the last function is
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called with the argument <c>close</c>, and the reply is returned
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as value of the current call to the sort or merge function. If a
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function is given as input and the last input function returns
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<c>{end_of_input, Value}</c>, the function given as output will
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be called with the argument <c>{value, Value}</c>. This makes it
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easy to initiate the sequence of output functions with a value
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calculated by the input functions.
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<p>As an example, consider sorting the terms on a disk log file.
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A function that reads chunks from the disk log and returns a
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list of binaries is used as input. The results are collected in
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{ok, _} = disk_log:open([{name,Log}, {mode,read_only}]),
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Input = input(Log, start),
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Reply = file_sorter:sort(Input, Output, {format,term}),
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ok = disk_log:close(Log),
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\011 case disk_log:chunk(Log, Cont) of
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\011\011{error, Reason} ->
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\011\011 {error, Reason};
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\011\011{Cont2, Terms} ->
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\011\011 {Terms, input(Log, Cont2)};
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\011\011{Cont2, Terms, _Badbytes} ->
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\011\011 {Terms, input(Log, Cont2)};
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\011\011 end_of_input
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\011 lists:append(lists:reverse(L));
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\011 output([Terms | L])
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<p>Further examples of functions as input and output can be found
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at the end of the <c>file_sorter</c> module; the <c>term</c>
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format is implemented with functions.
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<p>The possible values of <c>Reason</c> returned when an error
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<list type="bulleted">
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<p><c>bad_object</c>, <c>{bad_object, FileName}</c>.
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Applying the format function failed for some binary,
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or the key(s) could not be extracted from some term.</p>
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<p><c>{bad_term, FileName}</c>. <c>io:read/2</c> failed
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to read some term.\011 </p>
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<p><c>{file_error, FileName, Reason2}</c>. See
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<c>file(3)</c> for an explanation of <c>Reason2</c>.</p>
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<p><c>{premature_eof, FileName}</c>. End-of-file was
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encountered inside some binary term.</p>
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<p><em>Types</em></p>
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FileName = file_name()
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FileNames = [FileName]
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ICommand = read | close
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IReply = end_of_input | {end_of_input, Value} | {[Object], Infun} | InputReply
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Infun = fun(ICommand) -> IReply
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Input = FileNames | Infun
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KeyPos = int() > 0 | [int() > 0]
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OCommand = {value, Value} | [Object] | close
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OReply = Outfun | OutputReply
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Object = Term | Binary
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Outfun = fun(OCommand) -> OReply
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Output = FileName | Outfun
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<name>sort(FileName) -> Reply</name>
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<name>sort(Input, Output) -> Reply</name>
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<name>sort(Input, Output, Options) -> Reply</name>
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<fsummary>Sort terms on files.</fsummary>
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<v>Reply = ok | {error, Reason} | InputReply | OutputReply</v>
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<p>Sorts terms on files.
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<p><c>sort(FileName)</c> is equivalent to
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<c>sort([FileName], FileName)</c>.
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<p><c>sort(Input, Output)</c> is equivalent to
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<c>sort(Input, Output, [])</c>.
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<name>keysort(KeyPos, FileName) -> Reply</name>
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<name>keysort(KeyPos, Input, Output) -> Reply</name>
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<name>keysort(KeyPos, Input, Output, Options) -> Reply</name>
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<fsummary>Sort terms on files by key.</fsummary>
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<v>Reply = ok | {error, Reason} | InputReply | OutputReply</v>
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<p>Sorts tuples on files. The sort is performed on the
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element(s) mentioned in <c>KeyPos</c>. If two tuples
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compare equal on one element, next element according to
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<c>KeyPos</c> is compared. The sort is stable.
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<p><c>keysort(N, FileName)</c> is equivalent to
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<c>keysort(N, [FileName], FileName)</c>.
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<p><c>keysort(N, Input, Output)</c> is equivalent to
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<c>keysort(N, Input, Output, [])</c>.
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<name>merge(FileNames, Output) -> Reply</name>
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<name>merge(FileNames, Output, Options) -> Reply</name>
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<fsummary>Merge terms on files.</fsummary>
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<v>Reply = ok | {error, Reason} | OutputReply</v>
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<p>Merges terms on files. Each input file is assumed to be
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<p><c>merge(FileNames, Output)</c> is equivalent to
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<c>merge(FileNames, Output, [])</c>.
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<name>keymerge(KeyPos, FileNames, Output) -> Reply</name>
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<name>keymerge(KeyPos, FileNames, Output, Options) -> Reply</name>
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<fsummary>Merge terms on files by key.</fsummary>
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<v>Reply = ok | {error, Reason} | OutputReply</v>
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<p>Merges tuples on files. Each input file is assumed to be
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<p><c>keymerge(KeyPos, FileNames, Output)</c> is equivalent
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to <c>keymerge(KeyPos, FileNames, Output, [])</c>.
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<name>check(FileName) -> Reply</name>
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<name>check(FileNames, Options) -> Reply</name>
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<fsummary>Check whether terms on files are sorted.</fsummary>
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<v>Reply = {ok, [Result]} | {error, Reason}</v>
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<v>Result = {FileName, TermPosition, Term}</v>
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<v>TermPosition = int() > 1</v>
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<p>Checks files for sortedness. If a file is not sorted, the
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first out-of-order element is returned. The first term on a
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<p><c>check(FileName)</c> is equivalent to
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<c>check([FileName], [])</c>.
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<name>keycheck(KeyPos, FileName) -> CheckReply</name>
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<name>keycheck(KeyPos, FileNames, Options) -> Reply</name>
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<fsummary>Check whether terms on files are sorted by key.</fsummary>
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<v>Reply = {ok, [Result]} | {error, Reason}</v>
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<v>Result = {FileName, TermPosition, Term}</v>
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<v>TermPosition = int() > 1</v>
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<p>Checks files for sortedness. If a file is not sorted, the
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first out-of-order element is returned. The first term on a
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<p><c>keycheck(KeyPos, FileName)</c> is equivalent
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to <c>keycheck(KeyPos, [FileName], [])</c>.
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lib/stdlib/doc/src/file_sorter.xml
