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NAME
file_sorter - File sorter.
DESCRIPTION
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.
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. Defaults to 4. The order of the header bytes is defined as follows: if B is a binary
containing a header only, size Size of the binary is calculated as <<Size:HeaderLength/unit:8>> = B.
{format, Format}:
Option Format determines the function that is applied to binaries to create the terms to be sorted.
Defaults to binary_term, which is equivalent to fun binary_to_term/1. Value binary is equivalent to
fun(X) -> X end, which means that the binaries are 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 option
header is allowed.
Option format 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.
Notice that the binary written is the same binary that was read; the results of applying function
Format are thrown away when the terms have been sorted. Reading and writing terms using the io module
is 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 value descending or by
specifying an ordering function Fun. An ordering function is antisymmetric, transitive, and total.
Fun(A, B) is to return true if A comes before B in the ordering, otherwise false. An example of a
typical ordering function is less than or equal to, =</2. Using an ordering function slows down the
sort considerably. Functions keysort, keymerge and keycheck 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. Defaults to 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 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 unique integer (erlang:unique_integer([positive])). A typical
name is fs_mynode@myhost_1763_4711.17, where 17 is a sequence number. Existing files are overwritten.
Temporary files are deleted unless some uncaught EXIT signal occurs.
{compressed, boolean()}:
Temporary files and the output file can be compressed. Defaults false, which implies that written
files are not compressed. Regardless of the value of option compressed, compressed files can always
be read. Notice that reading and writing compressed files are significantly slower than reading and
writing uncompressed files.
{size, Size}:
By default about 512*1024 bytes read from files are sorted internally. This option is rarely needed.
{no_files, NoFiles}:
By default 16 files are merged at a time. This option is rarely needed.
As an alternative to sorting files, a function of one argument can be specified as input. When called
with argument read, the function is assumed to return either of the following:
* end_of_input or {end_of_input, Value}} when there is no more input (Value is explained below).
* {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 is called exactly once. If an error occurs, the last function is called with argument close, the
reply of which is ignored.
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 close, and the reply is
returned as value of the current call to the sort or merge function.
If a function is specified as input and the last input function returns {end_of_input, Value}, the
function specified as output is called with 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.
For more examples of functions as input and output, see 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()} - For an explanation of file:posix(), see file(3erl).
* {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
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], []).
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, []).
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, the 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, []).
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, []).