Provided by: erlang-manpages_16.b.3-dfsg-1ubuntu2.2_all 
NAME
file_sorter - File Sorter
DESCRIPTION
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.
On a file, a term is represented by a header and a binary. Two options define the format
of terms on files:
* {header, HeaderLength}. 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 B is a binary containing a header only, the
size Size of the binary is calculated as <<Size:HeaderLength/unit:8>> = B.
* {format, Format}. The format determines the function that is applied to binaries in
order to create the terms that will be sorted. The default value is binary_term, which
is equivalent to fun binary_to_term/1. The value binary is equivalent to fun(X) -> X
end, which means that the binaries will be sorted as they are. This is the fastest
format. If Format is term, io:read/2 is called to read terms. In that case only the
default value of the header option is allowed. The format option also determines what
is written to the sorted output file: if Format is term then io:format/3 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 Format
function are thrown away as soon as the terms have been sorted. Reading and writing
terms using the io module is very much slower than reading and writing binaries.
Other options are:
* {order, Order}. The default is to sort terms in ascending order, but that can be
changed by the value descending or by giving an ordering function Fun. An ordering
function is antisymmetric, transitive and total. Fun(A, B) should return true if A
comes before B in the ordering, false otherwise. An example of a typical ordering
function is less than or equal to, =</2. Using an ordering function will slow down the
sort considerably. The keysort, keymerge and keycheck functions do not accept ordering
functions.
* {unique, boolean()}. When sorting or merging files, only the first of a sequence of
terms that compare equal (==) is output if this option is set to true. The default
value is false 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 true.
* {tmpdir, TempDirectory}. The directory where temporary files are put can be chosen
explicitly. The default, implied by the value "", 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 file:get_cwd() is used instead. The names of temporary files are
derived from the Erlang nodename (node()), the process identifier of the current
Erlang emulator (os:getpid()), and a timestamp (erlang:now()); a typical name would be
fs_mynode@myhost_1763_1043_337000_266005.17, where 17 is a sequence number. Existing
files will be overwritten. Temporary files are deleted unless some uncaught EXIT
signal occurs.
* {compressed, boolean()}. Temporary files and the output file may be compressed. The
default value false implies that written files are not compressed. Regardless of the
value of the compressed option, compressed files can always be read. Note that reading
and writing compressed files is significantly slower than reading and writing
uncompressed files.
* {size, Size}. By default approximately 512*1024 bytes read from files are sorted
internally. This option should rarely be needed.
* {no_files, NoFiles}. By default 16 files are merged at a time. This option should
rarely be needed.
As an alternative to sorting files, a function of one argument can be given as input. When
called with the argument read the function is assumed to return end_of_input or
{end_of_input, Value}} when there is no more input (Value is explained below), or
{Objects, Fun}, where Objects is a list of binaries or terms depending on the format and
Fun is a new input function. Any other value is immediately returned as value of the
current call to sort or keysort. Each input function will be called exactly once, and
should an error occur, the last function is called with the argument close, the reply of
which is ignored.
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 close, 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 {end_of_input, Value}, the function given as output
will be called with the argument {value, Value}. This makes it easy to initiate the
sequence of output functions with a value calculated by the input functions.
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.
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.
Further examples of functions as input and output can be found at the end of the
file_sorter module; the term format is implemented with functions.
The possible values of Reason returned when an error occurs are:
* bad_object, {bad_object, FileName}. Applying the format function failed for some
binary, or the key(s) could not be extracted from some term.
* {bad_term, FileName}. io:read/2 failed to read some term.
* {file_error, FileName, file:posix()}. See file(3erl) for an explanation of
file:posix().
* {premature_eof, FileName}. End-of-file was encountered inside some binary term.
DATA TYPES
file_name() = file:name()
file_names() = [file:name()]
i_command() = read | close
i_reply() = end_of_input
| {end_of_input, value()}
| {[object()], infun()}
| input_reply()
infun() = fun((i_command()) -> i_reply())
input() = file_names() | infun()
input_reply() = term()
o_command() = {value, value()} | [object()] | close
o_reply() = outfun() | output_reply()
object() = term() | binary()
outfun() = fun((o_command()) -> o_reply())
output() = file_name() | outfun()
output_reply() = term()
value() = term()
options() = [option()] | option()
option() = {compressed, boolean()}
| {header, header_length()}
| {format, format()}
| {no_files, no_files()}
| {order, order()}
| {size, size()}
| {tmpdir, tmp_directory()}
| {unique, boolean()}
format() = binary_term | term | binary | format_fun()
format_fun() = fun((binary()) -> term())
header_length() = integer() >= 1
key_pos() = integer() >= 1 | [integer() >= 1]
no_files() = integer() >= 1
order() = ascending | descending | order_fun()
order_fun() = fun((term(), term()) -> boolean())
size() = integer() >= 0
tmp_directory() = [] | file:name()
reason() = bad_object
| {bad_object, file_name()}
| {bad_term, file_name()}
| {file_error,
file_name(),
file:posix() | badarg | system_limit}
| {premature_eof, file_name()}
EXPORTS
sort(FileName) -> Reply
Types:
FileName = file_name()
Reply = ok | {error, reason()} | input_reply() | output_reply()
Sorts terms on files. sort(FileName) is equivalent to sort([FileName], FileName).
sort(Input, Output) -> Reply
sort(Input, Output, Options) -> Reply
Types:
Input = input()
Output = output()
Options = options()
Reply = ok | {error, reason()} | input_reply() | output_reply()
Sorts terms on files. sort(Input, Output) is equivalent to sort(Input, Output, []).
keysort(KeyPos, FileName) -> Reply
Types:
KeyPos = key_pos()
FileName = file_name()
Reply = ok | {error, reason()} | input_reply() | output_reply()
Sorts tuples on files. keysort(N, FileName) is equivalent to keysort(N, [FileName],
FileName).
keysort(KeyPos, Input, Output) -> Reply
keysort(KeyPos, Input, Output, Options) -> Reply
Types:
KeyPos = key_pos()
Input = input()
Output = output()
Options = options()
Reply = ok | {error, reason()} | input_reply() | output_reply()
Sorts tuples on files. The sort is performed on the element(s) mentioned in KeyPos.
If two tuples compare equal (==) on one element, next element according to KeyPos
is compared. The sort is stable.
keysort(N, Input, Output) is equivalent to keysort(N, Input, Output, []).
merge(FileNames, Output) -> Reply
merge(FileNames, Output, Options) -> Reply
Types:
FileNames = file_names()
Output = output()
Options = options()
Reply = ok | {error, reason()} | output_reply()
Merges terms on files. Each input file is assumed to be sorted.
merge(FileNames, Output) is equivalent to merge(FileNames, Output, []).
keymerge(KeyPos, FileNames, Output) -> Reply
keymerge(KeyPos, FileNames, Output, Options) -> Reply
Types:
KeyPos = key_pos()
FileNames = file_names()
Output = output()
Options = options()
Reply = ok | {error, reason()} | output_reply()
Merges tuples on files. Each input file is assumed to be sorted on key(s).
keymerge(KeyPos, FileNames, Output) is equivalent to keymerge(KeyPos, FileNames,
Output, []).
check(FileName) -> Reply
check(FileNames, Options) -> Reply
Types:
FileNames = file_names()
Options = options()
Reply = {ok, [Result]} | {error, reason()}
Result = {FileName, TermPosition, term()}
FileName = file_name()
TermPosition = integer() >= 1
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.
check(FileName) is equivalent to check([FileName], []).
keycheck(KeyPos, FileName) -> Reply
keycheck(KeyPos, FileNames, Options) -> Reply
Types:
KeyPos = key_pos()
FileNames = file_names()
Options = options()
Reply = {ok, [Result]} | {error, reason()}
Result = {FileName, TermPosition, term()}
FileName = file_name()
TermPosition = integer() >= 1
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.
keycheck(KeyPos, FileName) is equivalent to keycheck(KeyPos, [FileName], []).