File sorter.
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}
, whereObjects
is a list of binaries or terms depending on the format, andFun
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 offile:posix()
, seefile(3)
. -
{premature_eof, FileName}
- End-of-file was encountered inside some binary term.
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_reply() = outfun() | output_reply()
object() = term() | binary()
outfun() = fun((o_command()) -> o_reply())
output() = file_name() | outfun()
output_reply() = term()
value() = term()
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()}
Functions
check(FileName) -> Reply
FileName = file_name()
Reply = {ok, [Result]} | {error, reason()}
Result = {FileName, TermPosition, term()}
TermPosition = integer() >= 1
check(FileNames, Options) -> Reply
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
KeyPos = key_pos()
FileName = file_name()
Reply = {ok, [Result]} | {error, reason()}
Result = {FileName, TermPosition, term()}
TermPosition = integer() >= 1
keycheck(KeyPos, FileNames, Options) -> Reply
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
KeyPos = key_pos()
FileNames = file_names()
Output = output()
Reply = ok | {error, reason()} | output_reply()
keymerge(KeyPos, FileNames, Output, Options) -> Reply
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
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
KeyPos = key_pos()
Input = input()
Output = output()
Reply = ok | {error, reason()} | input_reply() | output_reply()
keysort(KeyPos, Input, Output, Options) -> Reply
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
. If two
tuples compare equal (==
) on one element, the next
element according to
is compared. The sort is stable.
keysort(N, Input, Output)
is equivalent to
keysort(N, Input, Output, [])
.
merge(FileNames, Output) -> Reply
FileNames = file_names()
Output = output()
Reply = ok | {error, reason()} | output_reply()
merge(FileNames, Output, Options) -> Reply
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
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
Input = input()
Output = output()
Reply = ok | {error, reason()} | input_reply() | output_reply()
sort(Input, Output, Options) -> Reply
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, [])
.