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NAME

       unicode - Functions for converting Unicode characters.

DESCRIPTION

       This module contains functions for converting between different character representations.
       It converts between ISO Latin-1 characters and Unicode characters, but it can also convert
       between different Unicode encodings (like UTF-8, UTF-16, and UTF-32).

       The  default  Unicode encoding in Erlang is in binaries UTF-8, which is also the format in
       which built-in functions and libraries in OTP expect  to  find  binary  Unicode  data.  In
       lists,  Unicode  data  is encoded as integers, each integer representing one character and
       encoded simply as the Unicode code point for the character.

       Other Unicode encodings than integers representing code points or UTF-8  in  binaries  are
       referred  to  as  "external  encodings". The ISO Latin-1 encoding is in binaries and lists
       referred to as latin1-encoding.

       It is recommended to only use external encodings for communication with external  entities
       where  this is required. When working inside the Erlang/OTP environment, it is recommended
       to keep binaries in UTF-8 when representing Unicode characters. ISO  Latin-1  encoding  is
       supported both for backward compatibility and for communication with external entities not
       supporting Unicode character sets.

       Programs should always operate on  a  normalized  form  and  compare  canonical-equivalent
       Unicode  characters as equal. All characters should thus be normalized to one form once on
       the system borders. One of  the  following  functions  can  convert  characters  to  their
       normalized        forms        characters_to_nfc_list/1,       characters_to_nfc_binary/1,
       characters_to_nfd_list/1    or    characters_to_nfd_binary/1.     For     general     text
       characters_to_nfc_list/1  or  characters_to_nfc_binary/1 is preferred, and for identifiers
       one of the compatibility normalization functions, such  as  characters_to_nfkc_list/1,  is
       preferred  for  security  reasons. The normalization functions where introduced in OTP 20.
       Additional information on normalization can be found in the Unicode FAQ.

DATA TYPES

       encoding() =
           latin1 | unicode | utf8 | utf16 |
           {utf16, endian()} |
           utf32 |
           {utf32, endian()}

       endian() = big | little

       unicode_binary() = binary()

              A binary() with characters encoded in the UTF-8 coding standard.

       chardata() = charlist() | unicode_binary()

       charlist() =
           maybe_improper_list(char() | unicode_binary() | charlist(),
                               unicode_binary() | [])

       external_unicode_binary() = binary()

              A binary() with characters coded in a user-specified Unicode  encoding  other  than
              UTF-8 (that is, UTF-16 or UTF-32).

       external_chardata() =
           external_charlist() | external_unicode_binary()

       external_charlist() =
           maybe_improper_list(char() |
                               external_unicode_binary() |
                               external_charlist(),
                               external_unicode_binary() | [])

       latin1_binary() = binary()

              A binary() with characters coded in ISO Latin-1.

       latin1_char() = byte()

              An integer() representing a valid ISO Latin-1 character (0-255).

       latin1_chardata() = latin1_charlist() | latin1_binary()

              Same as iodata().

       latin1_charlist() =
           maybe_improper_list(latin1_char() |
                               latin1_binary() |
                               latin1_charlist(),
                               latin1_binary() | [])

              Same as iolist().

EXPORTS

       bom_to_encoding(Bin) -> {Encoding, Length}

              Types:

                 Bin = binary()
                    A binary() such that byte_size(Bin) >= 4.
                 Encoding =
                     latin1 | utf8 | {utf16, endian()} | {utf32, endian()}
                 Length = integer() >= 0
                 endian() = big | little

              Checks  for  a  UTF  Byte  Order  Mark  (BOM)  in the beginning of a binary. If the
              supplied binary Bin begins with a valid BOM for either UTF-8,  UTF-16,  or  UTF-32,
              the function returns the encoding identified along with the BOM length in bytes.

              If no BOM is found, the function returns {latin1,0}.

       characters_to_binary(Data) -> Result

              Types:

                 Data = latin1_chardata() | chardata() | external_chardata()
                 Result =
                     binary() |
                     {error, binary(), RestData} |
                     {incomplete, binary(), binary()}
                 RestData = latin1_chardata() | chardata() | external_chardata()

              Same as characters_to_binary(Data, unicode, unicode).

       characters_to_binary(Data, InEncoding) -> Result

              Types:

                 Data = latin1_chardata() | chardata() | external_chardata()
                 InEncoding = encoding()
                 Result =
                     binary() |
                     {error, binary(), RestData} |
                     {incomplete, binary(), binary()}
                 RestData = latin1_chardata() | chardata() | external_chardata()

              Same as characters_to_binary(Data, InEncoding, unicode).

       characters_to_binary(Data, InEncoding, OutEncoding) -> Result

              Types:

                 Data = latin1_chardata() | chardata() | external_chardata()
                 InEncoding = OutEncoding = encoding()
                 Result =
                     binary() |
                     {error, binary(), RestData} |
                     {incomplete, binary(), binary()}
                 RestData = latin1_chardata() | chardata() | external_chardata()

              Behaves as characters_to_list/2, but produces a binary instead of a Unicode list.

              InEncoding defines how input is to be interpreted if binaries are present in Data

              OutEncoding defines in what format output is to be generated.

              Options:

                unicode:
                  An  alias for utf8, as this is the preferred encoding for Unicode characters in
                  binaries.

                utf16:
                  An alias for {utf16,big}.

                utf32:
                  An alias for {utf32,big}.

              The atoms big and little denote big- or little-endian encoding.

              Errors and exceptions occur as in characters_to_list/2, but the second  element  in
              tuple error or incomplete is a binary() and not a list().

       characters_to_list(Data) -> Result

              Types:

                 Data = latin1_chardata() | chardata() | external_chardata()
                 Result =
                     list() |
                     {error, list(), RestData} |
                     {incomplete, list(), binary()}
                 RestData = latin1_chardata() | chardata() | external_chardata()

              Same as characters_to_list(Data, unicode).

       characters_to_list(Data, InEncoding) -> Result

              Types:

                 Data = latin1_chardata() | chardata() | external_chardata()
                 InEncoding = encoding()
                 Result =
                     list() |
                     {error, list(), RestData} |
                     {incomplete, list(), binary()}
                 RestData = latin1_chardata() | chardata() | external_chardata()

              Converts  a  possibly  deep  list  of integers and binaries into a list of integers
              representing Unicode characters. The binaries in  the  input  can  have  characters
              encoded as one of the following:

                * ISO Latin-1 (0-255, one character per byte). Here, case parameter InEncoding is
                  to be specified as latin1.

                * One of the UTF-encodings, which is specified as parameter InEncoding.

              Note that  integers  in  the  list  always  represent  code  points  regardless  of
              InEncoding  passed.  If  InEncoding  latin1  is  passed, only code points < 256 are
              allowed; otherwise, all valid unicode code points are allowed.

              If InEncoding is latin1, parameter Data corresponds to the iodata() type,  but  for
              unicode,  parameter  Data can contain integers > 255 (Unicode characters beyond the
              ISO Latin-1 range), which makes it invalid as iodata().

              The purpose of the function is mainly to convert combinations of Unicode characters
              into  a  pure  Unicode  string  in  list representation for further processing. For
              writing the data to an external entity, the reverse function characters_to_binary/3
              comes in handy.

              Option  unicode is an alias for utf8, as this is the preferred encoding for Unicode
              characters in binaries. utf16 is an alias for {utf16,big} and utf32 is an alias for
              {utf32,big}. The atoms big and little denote big- or little-endian encoding.

              If  the  data  cannot  be  converted, either because of illegal Unicode/ISO Latin-1
              characters in the list, or because of invalid UTF  encoding  in  any  binaries,  an
              error  tuple  is  returned.  The  error  tuple  contains  the  tag  error,  a  list
              representing the characters that could be converted before the error occurred and a
              representation  of  the characters including and after the offending integer/bytes.
              The last part is mostly for debugging, as it still constitutes a possibly  deep  or
              mixed  list,  or  both, not necessarily of the same depth as the original data. The
              error occurs when traversing the list and whatever is left to  decode  is  returned
              "as is".

              However,  if  the input Data is a pure binary, the third part of the error tuple is
              guaranteed to be a binary as well.

              Errors occur for the following reasons:

                * Integers out of range.

                  If InEncoding is latin1, an error occurs whenever an integer > 255 is found  in
                  the lists.

                  If  InEncoding  is  of  a  Unicode type, an error occurs whenever either of the
                  following is found:

                  * An integer > 16#10FFFF (the maximum Unicode character)

                  * An integer in the range 16#D800 to 16#DFFF (invalid range reserved for UTF-16
                    surrogate pairs)

                * Incorrect UTF encoding.

                  If  InEncoding is one of the UTF types, the bytes in any binaries must be valid
                  in that encoding.

                  Errors can occur for various reasons, including the following:

                  * "Pure" decoding errors (like the upper bits of the bytes being wrong).

                  * The bytes are decoded to a too large number.

                  * The bytes are decoded to a code point in the invalid Unicode range.

                  * Encoding is "overlong", meaning that a number should  have  been  encoded  in
                    fewer bytes.

                  The  case  of  a  truncated  UTF  is handled specially, see the paragraph about
                  incomplete binaries below.

                  If InEncoding is latin1, binaries are always valid  as  long  as  they  contain
                  whole bytes, as each byte falls into the valid ISO Latin-1 range.

              A  special type of error is when no actual invalid integers or bytes are found, but
              a trailing binary() consists of too few bytes to decode the  last  character.  This
              error  can  occur  if  bytes are read from a file in chunks or if binaries in other
              ways are split on  non-UTF  character  boundaries.  An  incomplete  tuple  is  then
              returned  instead  of  the  error tuple. It consists of the same parts as the error
              tuple, but the tag is incomplete instead of error and the last  element  is  always
              guaranteed  to  be  a  binary  consisting of the first part of a (so far) valid UTF
              character.

              If one UTF character is split over  two  consecutive  binaries  in  the  Data,  the
              conversion  succeeds.  This  means  that a character can be decoded from a range of
              binaries as long as the whole range is specified as input without errors occurring.

              Example:

              decode_data(Data) ->
                 case unicode:characters_to_list(Data,unicode) of
                    {incomplete,Encoded, Rest} ->
                          More = get_some_more_data(),
                          Encoded ++ decode_data([Rest, More]);
                    {error,Encoded,Rest} ->
                          handle_error(Encoded,Rest);
                    List ->
                          List
                 end.

              However, bit strings that are not whole bytes are not allowed, so a  UTF  character
              must be split along 8-bit boundaries to ever be decoded.

              A badarg exception is thrown for the following cases:

                * Any parameters are of the wrong type.

                * The list structure is invalid (a number as tail).

                * The binaries do not contain whole bytes (bit strings).

       characters_to_nfc_list(CD :: chardata()) ->
                                 [char()] | {error, [char()], chardata()}

              Converts  a possibly deep list of characters and binaries into a Normalized Form of
              canonical equivalent Composed characters according to the Unicode standard.

              Any binaries in the input must be encoded with utf8 encoding.

              The result is a list of characters.

              3> unicode:characters_to_nfc_list([<<"abc..a">>,[778],$a,[776],$o,[776]]).
              "abc..åäö"

       characters_to_nfc_binary(CD :: chardata()) ->
                                   unicode_binary() |
                                   {error, unicode_binary(), chardata()}

              Converts a possibly deep list of characters and binaries into a Normalized Form  of
              canonical equivalent Composed characters according to the Unicode standard.

              Any binaries in the input must be encoded with utf8 encoding.

              The result is an utf8 encoded binary.

              4> unicode:characters_to_nfc_binary([<<"abc..a">>,[778],$a,[776],$o,[776]]).
              <<"abc..åäö"/utf8>>

       characters_to_nfd_list(CD :: chardata()) ->
                                 [char()] | {error, [char()], chardata()}

              Converts  a possibly deep list of characters and binaries into a Normalized Form of
              canonical equivalent Decomposed characters according to the Unicode standard.

              Any binaries in the input must be encoded with utf8 encoding.

              The result is a list of characters.

              1> unicode:characters_to_nfd_list("abc..åäö").
              [97,98,99,46,46,97,778,97,776,111,776]

       characters_to_nfd_binary(CD :: chardata()) ->
                                   unicode_binary() |
                                   {error, unicode_binary(), chardata()}

              Converts a possibly deep list of characters and binaries into a Normalized Form  of
              canonical equivalent Decomposed characters according to the Unicode standard.

              Any binaries in the input must be encoded with utf8 encoding.

              The result is an utf8 encoded binary.

              2> unicode:characters_to_nfd_binary("abc..åäö").
              <<97,98,99,46,46,97,204,138,97,204,136,111,204,136>>

       characters_to_nfkc_list(CD :: chardata()) ->
                                  [char()] |
                                  {error, [char()], chardata()}

              Converts  a possibly deep list of characters and binaries into a Normalized Form of
              compatibly equivalent Composed characters according to the Unicode standard.

              Any binaries in the input must be encoded with utf8 encoding.

              The result is a list of characters.

              3> unicode:characters_to_nfkc_list([<<"abc..a">>,[778],$a,[776],$o,[776],[65299,65298]]).
              "abc..åäö32"

       characters_to_nfkc_binary(CD :: chardata()) ->
                                    unicode_binary() |
                                    {error, unicode_binary(), chardata()}

              Converts a possibly deep list of characters and binaries into a Normalized Form  of
              compatibly equivalent Composed characters according to the Unicode standard.

              Any binaries in the input must be encoded with utf8 encoding.

              The result is an utf8 encoded binary.

              4> unicode:characters_to_nfkc_binary([<<"abc..a">>,[778],$a,[776],$o,[776],[65299,65298]]).
              <<"abc..åäö32"/utf8>>

       characters_to_nfkd_list(CD :: chardata()) ->
                                  [char()] |
                                  {error, [char()], chardata()}

              Converts  a possibly deep list of characters and binaries into a Normalized Form of
              compatibly equivalent Decomposed characters according to the Unicode standard.

              Any binaries in the input must be encoded with utf8 encoding.

              The result is a list of characters.

              1> unicode:characters_to_nfkd_list(["abc..åäö",[65299,65298]]).
              [97,98,99,46,46,97,778,97,776,111,776,51,50]

       characters_to_nfkd_binary(CD :: chardata()) ->
                                    unicode_binary() |
                                    {error, unicode_binary(), chardata()}

              Converts a possibly deep list of characters and binaries into a Normalized Form  of
              compatibly equivalent Decomposed characters according to the Unicode standard.

              Any binaries in the input must be encoded with utf8 encoding.

              The result is an utf8 encoded binary.

              2> unicode:characters_to_nfkd_binary(["abc..åäö",[65299,65298]]).
              <<97,98,99,46,46,97,204,138,97,204,136,111,204,136,51,50>>

       encoding_to_bom(InEncoding) -> Bin

              Types:

                 Bin = binary()
                    A binary() such that byte_size(Bin) >= 4.
                 InEncoding = encoding()

              Creates  a  UTF Byte Order Mark (BOM) as a binary from the supplied InEncoding. The
              BOM is, if supported at all, expected to be placed first in UTF  encoded  files  or
              messages.

              The function returns <<>> for latin1 encoding, as there is no BOM for ISO Latin-1.

              Notice  that  the  BOM  for UTF-8 is seldom used, and it is really not a byte order
              mark. There are obviously no byte order issues with UTF-8, so the BOM is only there
              to differentiate UTF-8 encoding from other UTF formats.