Author: Patrik Nyblom , Fredrik Svahn Status: Final/R14A Implemented in OTP release R14A Type: Standards Track Created: 28-Nov-2009 Erlang-Version: OTP_R13B03 Post-History: **** EEP 31: Binary manipulation and searching module ---- 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 [EEP 9][] suggests several modules and is partially superseded by later EEP's (i.e. [EEP 11][]), while still containing valuable suggestions not yet implemented.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 traditional 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. ``binary_to_list/3`` uses one-based while ``split_binary/2`` uses zero-based. As the convention is to use zero-based, new functions for converting 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 representation 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 length. 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 description 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 description 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 description 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 description 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 to 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 functions allows us to follow that rule of thumb. (Unfortunately that rule could not be easily 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 was available on GitHub development branch before the final inclusion in R14A. [EEP 9]: eep-0009.md "EEP 9, the original work from which this EEP is derived" [EEP 11]: eep-0011.md "EEP 11, interesting extensions to EEP 9" [CCA3.0]: http://creativecommons.org/licenses/by/3.0/ "Creative Commons Attribution 3.0 License" Copyright ========= This document is licensed under the [Creative Commons license][CCA3.0]. 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