file_sorter
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:
{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: ifB
is a binary containing a header only, the sizeSize
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 isbinary_term
, which is equivalent tofun binary_to_term/1
. The valuebinary
is equivalent tofun(X) -> X end
, which means that the binaries will be sorted as they are. This is the fastest format. IfFormat
isterm
,io:read/2
is called to read terms. In that case only the default value of theheader
option is allowed. Theformat
option also determines what is written to the sorted output file: ifFormat
isterm
thenio: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 theFormat
function are thrown away as soon as the terms have been sorted. Reading and writing terms using theio
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 valuedescending
or by giving an ordering functionFun
. An ordering function is antisymmetric, transitive and total.Fun(A, B)
should returntrue
ifA
comes beforeB
in the ordering,false
otherwise. Using an ordering function will slow down the sort considerably. Thekeysort
,keymerge
andkeycheck
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 totrue
. The default value isfalse
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 totrue
.{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 byfile: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 befs_mynode@myhost_1763_1043_337000_266005.17
, where17
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 valuefalse
implies that written files are not compressed. Regardless of the value of thecompressed
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}
. Seefile(3)
for an explanation ofReason2
. -
{premature_eof, FileName}
. End-of-file was encountered inside some binary term.
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
Functions
sort(FileName) -> Reply
sort(Input, Output) -> Reply
sort(Input, Output, Options) -> Reply
Reply = ok | {error, Reason} | InputReply | OutputReply
Sorts 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 | OutputReply
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, 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} | OutputReply
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
Reply = ok | {error, Reason} | OutputReply
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
Reply = {ok, [Result]} | {error, Reason}
Result = {FileName, TermPosition, Term}
TermPosition = int() > 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) -> CheckReply
keycheck(KeyPos, FileNames, Options) -> Reply
Reply = {ok, [Result]} | {error, Reason}
Result = {FileName, TermPosition, Term}
TermPosition = int() > 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], [])
.