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<?xml version="1.0" encoding="utf-8" ?>
<!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>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 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, 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>=</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"/>
<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>
<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="keysort" arity="2"/>
<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"/>
<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, 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"/>
<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="sort" arity="1"/>
<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"/>
<name name="sort" arity="3"/>
<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>
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