eep-0031.txt

EEP:            31
Title:          Binary manipulation and searching module
Version:        1.2
Last-Modified:  18-Dec-2009 12:12
Author:         Patrik Nyblom, Fredrik Svahn
Status:         Draft
Type:           Standards Track
Content-Type:   text/x-rst
Created:        28-Nov-2009
Erlang-Version: R13B03
Post-History:	


Abstract
========

This EEP contains developed suggestions regarding the module ``binary``
first suggested in EEP 9. 

EEP 9 suggests several modules and is partially superseded by later
EEP's (i.e. EEP 11), while still containing valuable suggestions not
yet implemented. The remaining modules from EEP 9 will therefore
appear in separate EEP's. This construction is made in agreement with
the original author of EEP 9.

The module ``binary`` is suggested to contain fast searching
algorithms together with some common operations on binaries already
present for lists (in the lists module).
    
Motivation
==========
    
While efficient searching is already present in the ``re`` library,
dedicated search functions can further speed up searching in
binaries, given an efficient implementation (i.e. Boyer-More
and Aho-Corassick algorithm). Another important advantage of
separate searching algorithms are ease of use to the programmer,
as the suggested interfaces do not require knowledge about regular
expression syntax and special characters in the binaries need not
be escaped. It's interesting to note how often regular expressions
are used for simple sub-string searching or replacement, which can
with this suggested module be done easily.

Decomposition of binaries are usually done by using bit-syntax.
However some common operations are useful to have as ordinary functions,
both for performance and to support a more traditiona functional
programming style.

Some operations for converting lists to binaries and v.v. are today in
the erlang module. BIFs concerning binaries now present have varied
view of zero vs. one-based positioning in binaries. I.e.
binay_to_list/3 uses one-based while split_binary/2 uses
zero-based. As the convention is to use zero-based, new functions for
convertion binaries to lists and v.v. are needed.

Binaries are in fact a shared data-type, with small binaries often
referencing parts of larger binaries in a way not controllable by
the programmer in a simple way. The bitstring data-type further
complicate things to the programmer in a way hard to easily
manage. I therefore also suggest some low level functions to
inspect binary representation and to clone binaries to ensure a
minimal representation.

As matching is not allowed in guard expressions, I furthermore suggest
that a function for extracting parts of binaries is added to the set
of guard BIFs. This would be consistent with the function element/2
being allowed in guards.

Rationale
=========

For the lists data type there is a help library providing functions for
common operations such as searching and splitting lists.  This EEP suggests
that a similar set of library functions should be created for binaries.
Many of the proposed functions are based on answers to questions regarding
binaries on the erlang-questions mailing list, e.g. "how do I convert a
number to a binary?". This EEP therefore suggests the addition of one
module in stdlib, namely a module ``binary`` which will implement the
requested functionality in an efficient way. Most of this module will
need to be implemented in native code (residing in the virtual
machine) why the proposed implementation will be delivered as "beta"
functionality in a forthcoming Erlang release.

The functionality suggested is the following:

- Functionality for searching, splitting and replacing in
  binaries. The functionality in some ways will overlap that of the
  regular expression library already present in Erlang, but will be
  even more efficient and will have a simpler interface. 

- Common operations on binaries that have their counterparts for lists
  already in the stdlib module ``lists``. While not all interfaces in
  the ``lists`` module are applicable to binaries, many are. This module
  also provides a good place for future operations on binaries,
  operations that are not applicable to lists or that we still don't
  know the need for.

- Functions for converting lists to binaries and v.v. These functions
  should have a consistent view of zero-based indexing in binaries.

- Operations on binaries concerning their internal
  representation. This functionality is sometimes necessary to avoid
  extensive use of memory due to the shared nature of the binaries. As
  operations on binaries do not involve copying when binaries are
  taken apart, programs can unknowingly (or at least unintentionally)
  keep references to large binaries by holding seemingly small amounts
  of data in the process. The O(1) nature of many operations on
  binaries makes the data sharing necessary, but the effects can
  sometimes be surprising. On the other hand, O(n) complexity and
  instant memory explosions when splitting a binary would be even more
  surprising, why the current behavior need to be retained. It is suggested
  that functions for both inspecting the nature of sharing of a binary
  and to clone a copy of a binary to avoid sharing effects is present
  in this suggested module.


All functionality is to be applied to byte oriented binaries, never
bitstrings that do not have a bitlength that is a multiple of
eight. All binaries supplied to and returned by these functions should
pass the is_binary/1 test, otherwise an error will be raised.

Suggested module reference
--------------------------

I suggest the following functionality (presented as an excerpt of an Erlang
manual pages). A discussion about the interface can be found below.

DATA TYPES
..........

**cp()**
  
Opaque data-type representing a compiled search-pattern.
guaranteed to be a tuple() to allow programs to distinguish it from
non precompiled search patterns.

**part() = {Pos,Length}**

- Start = int()
- Length = int()
  
A representaion of a part (or range) in a binary. ``Start`` is a
zero-based offset into a binary() and Length is the length of that
part. As input to functions in this module, a reverse part
specification is allowed, constructed with a negative ``Length``, so
that the part of the binary begins at ``Start`` + ``Length`` and is
-``Length`` long. This is useful for referencing the last N bytes
of a binary as ``{size(Binary), -N}``. The functions in this module
always return part()'s with positive ``Length``.

EXPORTS
.......

**compile_pattern(Pattern) -> cp()**

Types:

- Pattern = binary() | [ binary() ]

Builds an internal structure representing a compilation of a
search-pattern, later to be used in the find, split or replace
functions. The cp() returned is guaranteed to be a tuple() to allow
programs to distinguish it from non precompiled search patterns

When a list of binaries is given, it denotes a *set* of alternative
binaries to search for. I.e if ``[<<"functional">>, <<"programming">>]`` 
is given as ``Pattern``, this means ''either ``<<"functional">>`` *or*
``<<"programming">>``''. The pattern is a *set* of alternatives; when
only a single binary is given, the set has only one element. 

If pattern is not a binary or a flat proper list of binaries, a ``badarg``
exception will be raised.

**match(Subject, Pattern) -> Found |** ``no`` 

Types:

- Subject = binary()
- Pattern = binary() | [ binary() ] | cp()
- Found = part()

The same as match(Subject, Pattern, []).

**match(Subject,Pattern,Options) -> Found |** ``no`` 

Types:

- Subject = binary()
- Pattern = binary() | [ binary() ] | cp()
- Found = part()
- Options = [ Option ]
- Option = {scope, part()}

Searches for the first occurrence of Pattern in Subject and returns
the position and lengt.

The function will return
``{Pos,Length}`` for the binary in ``Pattern`` starting at
the lowest position in ``Subject``, Example::

    1> binary:find(<<"abcde">>, [<<"bcde">>,<<"cd">>],[]).
    {1,4}

Even though ``<<"cd">>`` ends before ``<<"bcde">>``, ``<<"bcde">>``
begins first and is therefore the first match. If two overlapping
matches begins at the same position, the longest is returned.

Summary of the options:

``{scope, {Start, Length}}``
        Only the given part is searched. Return values still have offsets
	from the beginning of ``Subject``. A negative ``Length`` is
        allowed as described in the **TYPES** section of this manual.

The found part() is returned, if none of the strings in ``Pattern`` is
found, the atom ``no`` is returned.

For a descrition of ``Pattern``, see ``compile_pattern/1``.

If ``{scope, {Start,Length}}`` is given in the options such that
``Start`` is larger than the size of ``Subject``, ``Start`` +
``Length`` is less than zero or ``Start`` + ``Length`` is larger than
the size of ``Subject``, a ``badarg`` exception is raised.

**matches(Subject, Pattern) -> Found**

Types:

- Subject = binary()
- Pattern = binary() | [ binary() ] | cp()
- Found = [ part() ] | []

The same as matches(Subject, Pattern, []).

**matches(Subject,Pattern,Options) -> Found**

Types:

- Subject = binary()
- Pattern = binary() | [ binary() ] | cp()
- Found = [ part() ] | []
- Options = [ Option ]
- Option = {scope, part()}

Works like match, but the ``Subject`` is search until exhausted and
a list of all non-overlapping parts present in Pattern are returned (in order).

The first and *longest* match is preferred
to a shorter, which is illustrated by the following example::

    1> binary:matches(<<"abcde">>, [<<"bcde">>,<<"bc">>>,<<"de">>],[]).
    [{1,4}]

\- the result shows that ``<<"bcde">>>`` is selected instead of the
shorter match ``<<"bc">>`` (which would have given raise to one more
match,``<<"de">>``). This corresponds to the behavior of
posix regular expressions (and programs like ``awk``), but is not
consistent with alternative matches in ``re`` (and Perl), where
instead lexical ordering in the search pattern selects which string
matches.

If none of the strings in pattern is found, an empty list is returned.

For a descrition of ``Pattern``, see ``compile_pattern/1`` and for a
desctioption of available options, see ``match/3``.

If ``{scope, {Start,Length}}`` is given in the options such that
``Start`` is larger than the size of ``Subject``, ``Start`` +
``Length`` is less than zero or ``Start`` + ``Length`` is larger than
the size of ``Subject``, a ``badarg`` exception is raised.

**split(Subject,Pattern) -> Parts**

Types:

- Subject = binary()
- Pattern = binary() | [ binary() ] | cp()
- Parts = [ binary() ]

The same as split(Subject, Pattern, []).

**split(Subject,Pattern,Options) -> Parts**

Types:

- Subject = binary()
- Pattern = binary() | [ binary() ] | cp()
- Parts = [ binary() ]
- Options = [ Option ]
- Option = {scope, part()} | trim | global

Splits Binary into a list of binaries based on ``Pattern``. If the
option ``global``is not given, only the first occurrence of
``Pattern`` in ``Subject`` will give rise to a split. 

The parts of ``Pattern`` actually found in ``Subject`` are not included
in the result.

Example::

	1> binary:split(<<1,255,4,0,0,0,2,3>>, [<<0,0,0>>,<<2>>],[]).
	[<<1,255,4>>, <<2,3>>] 
        2> binary:split(<<0,1,0,0,4,255,255,9>>, [<<0,0>>, <<255,255>>],[global]).
        [<<0,1>>,<<4>>,<<9>>] 


Summary of options:

``{scope, part()}``
	 Works as in ``binary:match/3`` and ``binary:matches/3``. Note
	 that this only defines the scope of the search for matching
	 strings, it does not cut the binary before splitting. The
	 bytes before and after the scope will be kept in the result. See example below.

``trim``
	Removes trailing empty parts of the result (as does ``trim``
	in ``re:split/3``)

``global`` 
	Repeats the split until the ``Subject`` is
	exhausted. Conceptually the ``global`` option makes ``split``
	work on the positions returned by ``binary:matches/3``, while
	it normally works on the position returned by
	``binary:match/3``.

Example of the difference between a ``scope`` and taking the binary apart 
before splitting::

	 1> binary:split(<<"banana">>,[<<"a">>],[{scope,{2,3}}]).
	 [<<"ban">>,<<"na">>]
	 2> binary:split(binary:part(<<"banana">>,{2,3}),[<<"a">>],[]).
	 [<<"n">>,<<"n">>]

The return type is always a list of binaries which are all referencing
``Subject``. This means that the data in ``Subject`` is not actually
copied to new binaries and that ``Subject`` cannot be garbage
collected until the results of the split are no longer referenced.

For a descrition of ``Pattern``, see ``compile_pattern/1``.

**replace(Subject,Pattern,Replacement) -> Result**

Types:

- Subject = binary()
- Pattern = binary() | [ binary() ] | cp()
- Replacement = binary()
- Result = binary()

The same as replace(Subject,Pattern,Replacement,[]).

**replace(Subject,Pattern,Replacement,Options) -> Result**

Types:

- Subject = binary()
- Pattern = binary() | [ binary() ] | cp()
- Replacement = binary()
- Result = binary()
- Options = [ Option ]
- Option = global | {scope, part()} | {insert_replaced, InsPos}
- InsPos = OnePos | [ OnePos ]
- OnePos = int() =< byte_size(Replacement)

Constructs a new binary by replacing the parts in ``Subject`` matching
``Pattern`` with the content of ``Replacement``.

If the matching sub-part of ``Subject`` giving raise to the
replacement is to be inserted in the result, the option
``{insert_replaced, InsPos}`` will insert the matching part into
``Replacement`` at the given position (or positions) before actually
inserting ``Replacement`` into the Subject. Example::

	 1> binary:replace(<<"abcde">>,<<"b">>,<<"[]">>,[{insert_replaced,1}]).
	 <<"a[b]cde">>
	 2> binary:replace(<<"abcde">>,[<<"b">>,<<"d">>],<<"[]">>,
	                  [global,{insert_replaced,1}]).
	 <<"a[b]c[d]e">>
	 3> binary:replace(<<"abcde">>,[<<"b">>,<<"d">>],<<"[]">>,
	                  [global,{insert_replaced,[1,1]}]).
	 <<"a[bb]c[dd]e">>
	 4> binary:replace(<<"abcde">>,[<<"b">>,<<"d">>],<<"[-]">>,
	                  [global,{insert_replaced,[1,2]}]).
	 <<"a[b-b]c[d-d]e">>

If any position given in InsPos is greater than the size of the replacement binary, a
``badarg`` exception is raised.

The options ``global`` and ``{scope, part()}`` works as for
``binary:split/3``. The return type is always a binary.

For a descrition of ``Pattern``, see ``compile_pattern/1``.

**longest_common_prefix(Binaries) -> int()**

Types:

- Binaries = [ binary() ]

Returns the length of the longest common prefix of the binaries in the
list ``Binaries``. Example::

     1> binary:longest_common_prefix([<<"erlang">>,<<"ergonomy">>]).
     2
     2> binary:longest_common_prefix([<<"erlang">>,<<"perl">>]).
     0

If ``Binaries`` is not a flat list of binaries, a ``badarg`` exception
is raised. 

**longest_common_suffix(Binaries) -> int()**

Types:

- Binaries = [ binary() ]

Returns the length of the longest common suffix of the binaries in the
list ``Binaries``. Example::

     1> binary:longest_common_suffix([<<"erlang">>,<<"fang">>]).
     3
     2> binary:longest_common_suffix([<<"erlang">>,<<"perl">>]).
     0

If ``Binaries`` is not a flat list of binaries, a ``badarg`` exception
is raised. 

**first(Subject) -> int()**

Types:

- Subject = binary()

Returns the first byte of the binary as an integer. If the binary
length is zero, a ``badarg`` exception is raised.

**last(Subject) -> int()**

Types:

- Subject = binary()

Returns the last byte of the binary as an integer. If the binary
length is zero, a ``badarg`` exception is raised.

**at(Subject, Pos) -> int()**

Types:
 
- Subject = binary()
- Pos = int() >= 0

Returns the byte at position ``Pos`` (zero-based) in the binary
``Subject`` as an integer. If ``Pos`` >= byte_size(Subject), a
``badarg`` exception is raised.

**part(Subject, PosLen) -> binary()**

Types:
 
- Subject = binary()
- PosLen = part()

Extracts the part of the binary described by ``PosLen``. 

Negative length can be used to extract bytes at the end of a binary::
	 
	 1> Bin = <<1,2,3,4,5,6,7,8,9,10>>.
	 2> binary:part(Bin,{byte_size(Bin), -5)).
	 <<6,7,8,9,10>>

If ``PosLen`` in any way references outside the binary, a ``badarg``
exception is raised.

**part(Subject, Pos, Len) -> binary()**

Types:
 
- Subject = binary()
- Pos = int()
- Len = int()

The same as part(Subject, {Pos, Len}). 

**bin_to_list(Subject) -> list()**

Types:
 
- Subject = binary()

The same as bin_to_list(Subject,{0,byte_size(Subject)}).

**bin_to_list(Subject, PosLen) -> list()**

- Subject = binary()
- PosLen = part()

Converts ``Subject`` to a list of int(), each int representing the
value of one byte. The ``part()`` denotes which part of the
``binary()`` to convert. Example::

	     1> binary:bin_to_list(<<"erlang">>,{1,3}).
	     "rla"
	     %% or [114,108,97] in list notation.

If ``PosLen`` in any way references outside the binary, a ``badarg``
exception is raised.

**bin_to_list(Subject, Pos, Len) -> list()**

Types:
 
- Subject = binary()
- Pos = int()
- Len = int()

The same as bin_to_list(Subject,{Pos,Len}).

**list_to_bin(ByteList) -> binary()**

Types:

- ByteList = iodata() (see module erlang)

Works exactly like erlang:list_to_binary/1, added for completeness.

**copy(Subject) -> binary()**

Types:

- Subject = binary()

The same as copy(Subject, 1).

**copy(Subject,N) -> binary()**

Types:

- Subject = binary()
- N = int() >= 0

Creates a binary with the content of ``Subject`` duplicated ``N``
times. 

This function will always create a new binary, even if ``N`` = 1. By
using ``copy/1`` on a binary referencing a larger binary, one might
free up the larger binary for garbage collection.  

NOTE! By deliberately copying a single binary to avoid referencing a
larger binary, one might, instead of freeing up the larger binary for
later garbage collection, create much more binary data than
needed. Sharing binary data is usually good. Only in special cases,
when small parts reference large binaries and the large binaries are
no longer used *in any process*, deliberate copying might be a good idea.

If ``N`` < 0, a ``badarg`` exception is raised.

**referenced_byte_size(binary()) -> int()**

If a binary references a larger binary (often described as being a
sub-binary), it can be useful to get the size of the actual referenced
binary. This function can be used in a program to trigger the
use of ``copy/1``. By copying a binary, one might dereference the
original, possibly large, binary which a smaller binary is a reference
to.

Example::

	store(Binary, GBSet) ->
	  NewBin = 
              case binary:referenced_byte_size(Binary) of
	          Large when Large > 2 * byte_size(Binary) ->
	      	     binary:copy(Binary);
                  _ ->
		     Binary
	      end,
          gb_sets:insert(NewBin,GBSet).

In this example, we chose to copy the binary content before inserting
it in the gb_set() if it references a binary more than twice the size
of the data we're going to keep. Of course different rules for when
copying will apply to different programs.

Binary sharing will occur whenever binaries are taken apart, this is
the fundamental reason why binaries are fast, decomposition can always
be done with O(1) complexity. In rare circumstances this data sharing
is however undesirable, why this function together with ``copy/1``
might be useful when optimizing for memory use.  

Example of binary sharing::

	1> A = binary:copy(<<1>>,100).
	<<1,1,1,1,1 ...
	2> byte_size(A).
	100
	3> binary:referenced_byte_size(A)
	100
	4> <<_:10/binary,B:10/binary,_/binary>> = A.
	<<1,1,1,1,1 ...
	5> byte_size(B).
        10
	6> binary:referenced_byte_size(B)
	100

NOTE! Binary data is shared among processes. If another process still
references the larger binary, copying the part this process uses only
consumes more memory and will not free up the larger binary for garbage
collection. Use this kind of intrusive functions with extreme care,
and only if a *real* problem is detected.
	
**encode_unsigned(Unsigned) -> binary()**

Types:

- Unsigned = int() >= 0

The same as encode_unsigned(Unsigned,big).

**encode_unsigned(Unsigned,Endianess) -> binary()**

Types:

- Unsigned = int() >= 0
- Endianess = big | little


Converts a positive integer the smallest possible representation
in in a binary digit representation, either big or little endian.

Example::

	1> binary:encode_unsigned(11111111,big). 
        <<169,138,199>>

**decode_unsigned(Subject) -> Unsigned**

Types:

- Subject = binary()
- Unsigned = int() >= 0

The same as encode_unsigned(Subject,big).

**decode_unsigned(Subject, Endianess) -> Unsigned**

Types:

- Subject = binary()
- Endianess = big | little
- Unsigned = int() >= 0

Converts the binary digit representation, in big or little endian, of
a positive integer in Subject to an Erlang int().

Example::

	1> binary:decode_unsigned(<<169,138,199>>,big). 
        11111111

Guard BIF
---------

I suggest adding the functions binary:part/2 and binary:part/3 to the
set of BIFs allowed in guard tests. As guard BIFs are traditionally
put in the erlang module, the following names for the guard BIFs are
suggested::

	erlang:binary_part/2
	erlang:binary_part/3

They should both work exactly as their counterparts in the binary module.

Interface design discussion
---------------------------

As with all modules, there are a lot of arguments about the actual
interface, sometimes more than about the functionality. In this case a
number of parameters has to be considered.

- Effectiveness - The interface should be constructed so that fast
  implementation is possible and so that code using the interface can
  be written in an effective way. To not create unnecessary garbage is
  one parameter, to allow for general code is another. 
  
- Parameter ordering - I've chosen to make the binary subject the
  first parameter in all applicable calls. Putting the subject first
  corresponds to the ``re`` interface. The ``lists`` module, however,
  usually has the subject as last parameter. We could go for that
  instead, but unfortunately the ``lists:sublist/{2,3}`` interface,
  which corresponds to the ``part`` function, has the subject
  first, why following the conventions of ``lists`` would not only
  break conformance with ``re``, it would also give us a generally
  non-stringent interface. The effect of not conforming to the
  ``lists`` interface is that using function names from that module
  would lead to confusion and therefore is avoided.

- Function naming - We have two related modules to take into account
  when naming functions here. The module ``re`` is related to the
  searching function (``match``, ``replace`` etc), while the ``lists``
  module is related to the decomposition functions (``first``,
  ``last`` etc). 

  I've basically retained the names from ``re`` when I find the
  functionality, both in concept and interface to be similar
  enough. The nature of regular expressions as small executable
  programs, which is to much to say for a collection of binaries as
  the patterns are in this module, prohibits the use of the function
  name ``run`` for actually doing the searching. We use ``match`` and 
  ``matches`` instead of ``run``. 

  As this module is more general than ``re``, a function name like
  ``compile`` is not really good. ``re:compile`` means "compile a
  regular expression", but what would ``binary:compile`` mean?
  Therefore the pre-processing function is instead called
  ``compile_pattern``.

  When it comes to the ``lists`` module, the parameter ordering has
  prevented me from reusing any function names but ``last``, which
  only takes one parameter in ``lists`` and there is no real
  alternative there.

- Options or multiple functions - I believe a good rule of thumb is to
  not have options that change the return type of the function, which
  would have been the case if we i.e. had a ``global`` option to
  ``match/3`` instead of a separate ``matches/3`` function.

  The fact that there are a manageable set of possible return types
  for the searching and decomposition functiona allows us to follow
  that rule of thumb.

  (Unfortunately that rule could not be easilly followed in ``re``, as the
  rich assortment of options would have given rise to a non-manageable
  amount of function names). 

Performance
===========

Although the decomposition functions are not really faster than using
bit-syntax for decomposition, they create slightly less garbage than
the bit syntax. As they are not slower than bit-syntax, they also have
a purpose in allowing for a different programming style.

The match/replace/split functionality should be compared to similar
functionality in the ``re`` module. Implementation methods has to be
chosen so that this modules search functions are faster, or possibly
even significantly faster, than ``re``.

Reference implementation
========================

A reference implementation will be included as beta functionality in
R13B04. Prereleases may be available via github.

References
==========

    [1] EEP 9, the original work from which this EEP is derived.


Copyright
=========

    This document is licensed under the Creative Commons license.