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(3).

• {premature_eof, FileName} - End-of-file was encountered inside some binary term.