file_sorter

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:

Other options are:

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: