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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. Fun(A, B) should return true
if A comes before B in the ordering,
false otherwise. Using an ordering function will slow
down the sort considerably. The keysort,
keymerge and keycheck functions do not accept
ordering functions.
{unique, bool()}. 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, bool()}. 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.
To summarize, here is the syntax of the options:
Options = [Option] | Option
Option = {header, HeaderLength}
| {format, Format}
| {order, Order}
| {unique, bool()}
| {tmpdir, TempDirectory}
| {compressed, bool()}
| {size, Size}
| {no_files, NoFiles}
HeaderLength = int() > 0
Format = binary_term | term | binary | FormatFun
FormatFun = fun(Binary) -> Term
Order = ascending | descending | OrderFun
OrderFun = fun(Term, Term) -> bool()
TempDirectory = "" | file_name()
Size = int() > 0
NoFiles = int() > 1
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, Reason2}. See
file(3) for an explanation of Reason2.
{premature_eof, FileName}. End-of-file was
encountered inside some binary term.
{not_a_directory, FileName}. The file
supplied with the tmpdir option is not a directory.
Types
Binary = binary()
FileName = file_name()
FileNames = [FileName]
ICommand = read | close
IReply = end_of_input | {end_of_input, Value} | {[Object], Infun} | InputReply
Infun = fun(ICommand) -> IReply
Input = FileNames | Infun
InputReply = Term
KeyPos = int() > 0 | [int() > 0]
OCommand = {value, Value} | [Object] | close
OReply = Outfun | OutputReply
Object = Term | Binary
Outfun = fun(OCommand) -> OReply
Output = FileName | Outfun
OutputReply = Term
Term = term()
Value = Term
sort(FileName) -> Reply
sort(Input, Output) -> Reply
sort(Input, Output, Options) -> Reply
Reply = ok | {error, Reason} | InputReply | OutputReplySorts terms on files.
sort(FileName) is equivalent to
sort([FileName], FileName).
sort(Input, Output) is equivalent to
sort(Input, Output, []).
keysort(KeyPos, FileName) -> Reply
keysort(KeyPos, Input, Output) -> Reply
keysort(KeyPos, Input, Output, Options) -> Reply
Reply = ok | {error, Reason} | InputReply | OutputReplySorts 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, FileName) is equivalent to
keysort(N, [FileName], FileName).
keysort(N, Input, Output) is equivalent to
keysort(N, Input, Output, []).
merge(FileNames, Output) -> Reply
merge(FileNames, Output, Options) -> Reply
Reply = ok | {error, Reason} | OutputReplyMerges 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
Reply = ok | {error, Reason} | OutputReplyMerges 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
Reply = {ok, [Result]} | {error, Reason}Result = {FileName, TermPosition, Term}TermPosition = int() > 1Checks 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) -> CheckReply
keycheck(KeyPos, FileNames, Options) -> Reply
Reply = {ok, [Result]} | {error, Reason}Result = {FileName, TermPosition, Term}TermPosition = int() > 1Checks 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], []).