# Module Netconversion

    module Netconversion: sig .. end

Conversion between character encodings

Contents

- Preliminaries
  - Unicode
  - Subsets of Unicode
  - Linking this module
  - Supported Encodings, Restrictions
  - Known Problems
- Interface
  - Direct Conversion
  - Reading Text Using Cursors
    - Byte Order Marks
  - Unicode String Functions

# Preliminaries

A character set is a set of characters where every character is identified by a code point. An encoding is a way of representing characters from a set in byte strings. For example, the Unicode character set has more than 96000 characters, and the code points have values from 0 to 0x10ffff (not all code points are assigned yet). The UTF-8 encoding represents the code points by sequences of 1 to 4 bytes. There are also encodings that represent code points from several sets, e.g EUC-JP covers four sets.

Encodings are enumerated by the type `encoding`, and names follow the convention ``Enc_*`, e.g. ``Enc_utf8`. Character sets are enumerated by the type `charset`, and names follow the convention ``Set_*`, e.g. ``Set_unicode`.

This module deals mainly with encodings. It is important to know that the same character set may have several encodings. For example, the Unicode character set can be encoded as UTF-8 or UTF-16. For the 8 bit character sets, however, there is usually only one encoding, e.g ``Set_iso88591` is always encoded as ``Enc_iso88591`.

In a single-byte encoding every code point is represented by one byte. This is what many programmers are accustomed at, and what the O'Caml language specially supports: A `string` is a sequence of `char`s, where `char` means an 8 bit quantity interpreted as character. For example, the following piece of code allocates a `string` of four `char`s, and assigns them individually:

     let s = String.create 4 in
     s.[0] <- 'G';
     s.[1] <- 'e';
     s.[2] <- 'r';
     s.[3] <- 'd';

In a multi-byte encoding there are code points that are represented by several bytes. As we still represent such text as `string`, the problem arises that a single `char`, actually a byte, often represents only a fraction of a full multi-byte character. There are two solutions:

- Give up the principle that text is represented by `string`. This is, for example, the approach chosen by `Camomile`, another O'Caml library dealing with Unicode. Instead, text is represented as `int array`. This way, the algorithms processing the text can remain the same.
- Give up the principle that individual characters can be directly accessed in a text. This is the primary way chosen by Ocamlnet. This means that there is not any longer the possibility to read or write the `n`th character of a text. One can, however, still compose texts by just concatenating the strings representing individual characters. Furthermore, it is possible to define a cursor for a text that moves sequentially along the text. The consequence is that programmers are restricted to sequential algorithms. Note that the majority of text processing falls into this class.

The corresponding piece of code for Ocamlnet's Unicode implementation is:

     let b = Buffer.create 80 in
     Buffer.add b (ustring_of_uchar `Enc_utf8 71);  (* 71 = code point of 'G' *)
     Buffer.add b (ustring_of_uchar `Enc_utf8 101); (* 101 = code point of 'e' *)
     Buffer.add b (ustring_of_uchar `Enc_utf8 114); (* 114 = code point of 'r' *)
     Buffer.add b (ustring_of_uchar `Enc_utf8 100); (* 100 = code point of 'd' *)
     let s = Buffer.contents b

It is important to always remember that a `char` is no longer a character but simply a byte. In many of the following explanations, we strictly distinguish between byte positions or byte counts, and character positions or character counts.

There a number of special effects that usually only occur in multi-byte encodings:

- Bad encodings: Not every byte sequence is legal. When scanning such text, the functions will raise the exception `Malformed_code` when they find illegal bytes.
- Unassigned code points: It may happen that a byte sequence is a correct representation for a code point, but that the code point is unassigned in the character set. When scanning, this is also covered by the exception `Malformed_code`. When converting from one encoding to another, it is also possible that the code point is only unassigned in the target character set. This case is usually handled by a substitution function `subst`, and if no such function is defined, by the exception `Cannot_represent`.
- Incomplete characters: The trailing bytes of a string may be the correct beginning of a byte sequence for a character, but not a complete sequence. Of course, if that string is the end of a text, this is just illegal, and also a case for `Malformed_code`. However, when text is processed chunk by chunk, this phenomenon may happen legally for all chunks but the last. For this reason, some of the functions below handle this case specially.
- Byte order marks: Some encodings have both big and little endian variants. A byte order mark at the beginning of the text declares which variant is actually used. This byte order mark is a declaration written like a character, but actually not a character.

There is a special class of encodings known as ASCII-compatible. They are important because there are lots of programs and protocols that only interpret bytes from 0 to 127, and treat the bytes from 128 to 255 as data. These programs can process texts as long as the bytes from 0 to 127 are used as in ASCII. Fortunately, many encodings are ASCII-compatible, including UTF-8.

## Unicode

`Netconversion` is centred around Unicode. The conversion from one encoding to another works by finding the Unicode code point of the character to convert, and by representing the code point in the target encoding, even if neither encodings have to do with Unicode. Of course, this approach requires that all character sets handled by `Netconversion` are subsets of Unicode.

The supported range of Unicode code points: 0 to 0xd7ff, 0xe000 to 0xfffd, 0x10000 to 0x10ffff. All these code points can be represented in UTF-8 and UTF-16. `Netconversion` does not know which of the code points are assigned and which not, and because of this, it simply allows all code points of the mentioned ranges (but for other character sets, the necessary lookup tables exist).

UTF-8: The UTF-8 representation can have one to four bytes. Malformed byte sequences are always rejected, even those that want to cheat the reader like "0xc0 0x80" for the code point 0. There is special support for the Java variant of UTF-8 (``Enc_java`). UTF-8 strings must not have a byte order mark (it would be interpreted as "zero-width space" character).

UTF-16: When reading from a string encoded as ``Enc_utf16`, a byte order mark is expected at the beginning. The detected variant (``Enc_utf16_le` or ``Enc_utf16_be`) is usually returned by the parsing function. The byte order mark is not included into the output string. - Some functions of this module cannot cope with ``Enc_utf16` (i.e. UTF-16 without endianess annotation), and will fail.

Once the endianess is determined, the code point 0xfeff is no longer interpreted as byte order mark, but as "zero-width non-breakable space".

Some code points are represented by pairs of 16 bit values, these are the so-called "surrogate pairs". They can only occur in UTF-16.

## Subsets of Unicode

The non-Unicode character sets are subsets of Unicode. Here, it may happen that a Unicode code point does not have a corresponding code point. In this case, certain rules are applied to handle this (see below). It is, however, ensured that every non-Unicode code point has a corresponding Unicode code point. (In other words, character sets cannot be supported for which this property does not hold.)

It is even possible to create further subsets artificially. The encoding ``Enc_subset(e,def)` means to derive a new encoding from the existing one `e`, but to only accept the code points for which the definition function `def` yields the value `true`. For example, the encoding

     `Enc_subset(`Enc_usascii,
                 fun i -> i <> 34 && i <> 38 && i <> 60 && i <> 62)

is ASCII without the bracket angles, the quotation mark, and the ampersand character, i.e. the subset of ASCII that can be included in HTML text without escaping.

If a code point is not defined by the encoding but found in a text, the reader will raise the exception `Malformed_code`. When text is output, however, the `subst` function will be called for undefined code points (which raises `Cannot_represent` by default). The `subst` function is an optional argument of many conversion functions that allows it to insert a substitution text for undefined code points. Note, however, that the substitution text is restricted to at most 50 characters (because unlimited length would lead to difficult problems we would like to avoid).

## Linking this module

Many encodings require lookup tables. The following encodings are built-in and always supported:

- Unicode: ``Enc_utf8`, ``Enc_java`, ``Enc_utf16`, ``Enc_utf16_le`, ``Enc_utf16_be`
- Other: ``Enc_usascii`, ``Enc_iso88591`, ``Enc_empty`

The lookup tables for the other encodings are usually loaded at runtime, but it is also possible to embed them in the generated binary executable. See the file `INSTALL` for details. The functions `available_input_encodings` and `available_output_encodings` can be invoked to find out which encodings can be loaded, or are available otherwise.

## Supported Encodings, Restrictions

I took the mappings from `www.unicode.org`, and the standard names of the character sets from IANA. Obviously, many character sets are missing that can be supported; especially ISO646 character sets, and many EBCDIC code pages. Stateful encodings like generic ISO-2022 have been omitted (stateless subsets of ISO-2022 like EUC can be supported, however; currently we support EUC-JP and EUC-KR).

Because of the copyright statement from Unicode, I cannot put the source tables that describe the mappings into the distribution. They are publicly available from `www.unicode.org`.

## Known Problems

- The following charsets do not have a bijective mapping to Unicode: adobe_standard_encoding, adobe_symbol_encoding, adobe_zapf_dingbats_encoding, cp1002 (0xFEBE). The current implementation simply removes one of the conflicting code point pairs - this might not what you want.
- Japanese encodings: JIS X 0208: The character 1/32 is mapped to 0xFF3C, and not to 0x005C.

# Interface

Naming conventions:

As it is possible to refer to substrings by either giving a byte offset or by counting whole characters, these naming conventions are helpful:

- Labels called `range_pos` and `range_len` refer to byte positions of characters, or substrings
- Labels called `count` refer to positions given as the number of characters relative to an origin

Furthermore:

- A `uchar` is a single Unicode code point represented as int
- A `ustring` is a string of encoded characters
- A `uarray` is an `array of int` representing a string

    exception Malformed_code

Raised when an illegal byte sequence is found

    exception Cannot_represent of int

Raised when a certain Unicode code point cannot be represented in the selected output encoding

    type encoding = [ `Enc_adobe_standard_encoding
           | `Enc_adobe_symbol_encoding
           | `Enc_adobe_zapf_dingbats_encoding
           | `Enc_cp037
           | `Enc_cp1006
           | `Enc_cp1026
           | `Enc_cp1047
           | `Enc_cp424
           | `Enc_cp437
           | `Enc_cp500
           | `Enc_cp737
           | `Enc_cp775
           | `Enc_cp850
           | `Enc_cp852
           | `Enc_cp855
           | `Enc_cp856
           | `Enc_cp857
           | `Enc_cp860
           | `Enc_cp861
           | `Enc_cp862
           | `Enc_cp863
           | `Enc_cp864
           | `Enc_cp865
           | `Enc_cp866
           | `Enc_cp869
           | `Enc_cp874
           | `Enc_cp875
           | `Enc_empty
           | `Enc_eucjp
           | `Enc_euckr
           | `Enc_iso88591
           | `Enc_iso885910
           | `Enc_iso885911
           | `Enc_iso885913
           | `Enc_iso885914
           | `Enc_iso885915
           | `Enc_iso885916
           | `Enc_iso88592
           | `Enc_iso88593
           | `Enc_iso88594
           | `Enc_iso88595
           | `Enc_iso88596
           | `Enc_iso88597
           | `Enc_iso88598
           | `Enc_iso88599
           | `Enc_java
           | `Enc_jis0201
           | `Enc_koi8r
           | `Enc_macroman
           | `Enc_subset of encoding * (int -> bool)
           | `Enc_usascii
           | `Enc_utf16
           | `Enc_utf16_be
           | `Enc_utf16_le
           | `Enc_utf8
           | `Enc_windows1250
           | `Enc_windows1251
           | `Enc_windows1252
           | `Enc_windows1253
           | `Enc_windows1254
           | `Enc_windows1255
           | `Enc_windows1256
           | `Enc_windows1257
           | `Enc_windows1258 ]

The polymorphic variant enumerating the supported encodings. We have:

- ``Enc_utf8`: UTF-8
- ``Enc_java`: The UTF-8 variant used by Java (the only difference is the representation of NUL)
- ``Enc_utf16`: UTF-16 with unspecified endianess (restricted)
- ``Enc_utf16_le`: UTF-16 little endian
- ``Enc_utf16_be`: UTF-16 big endian
- ``Enc_usascii`: US-ASCII (7 bits)
- ``Enc_iso8859` n: ISO-8859-n
- ``Enc_koi8r`: KOI8-R
- ``Enc_jis0201`: JIS-X-0201 (Roman and Katakana)
- ``Enc_eucjp`: EUC-JP (code points from US-ASCII, JIS-X-0202, -0208, and -0212)
- ``Enc_euckr`: EUC-KR (code points from US-ASCII, KS-X-1001)
- ``Enc_windows` n: WINDOWS-n
- ``Enc_cp` n: IBM code page n. Note that there are both ASCII- and EBCDIC-based code pages
- ``Enc_adobe_*`: Adobe-specific encodings, e.g. used in Adobe fonts
- ``Enc_mac*`: Macintosh-specific encodings
- ``Enc_subset(e,def)`: The subset of `e` by applying the definition function `def`
- ``Enc_empty`: The empty encoding (does not represent any character)

    type charset = [ `Set_adobe_standard_encoding
           | `Set_adobe_symbol_encoding
           | `Set_adobe_zapf_dingbats_encoding
           | `Set_cp037
           | `Set_cp1006
           | `Set_cp1026
           | `Set_cp1047
           | `Set_cp424
           | `Set_cp437
           | `Set_cp500
           | `Set_cp737
           | `Set_cp775
           | `Set_cp850
           | `Set_cp852
           | `Set_cp855
           | `Set_cp856
           | `Set_cp857
           | `Set_cp860
           | `Set_cp861
           | `Set_cp862
           | `Set_cp863
           | `Set_cp864
           | `Set_cp865
           | `Set_cp866
           | `Set_cp869
           | `Set_cp874
           | `Set_cp875
           | `Set_iso88591
           | `Set_iso885910
           | `Set_iso885911
           | `Set_iso885913
           | `Set_iso885914
           | `Set_iso885915
           | `Set_iso885916
           | `Set_iso88592
           | `Set_iso88593
           | `Set_iso88594
           | `Set_iso88595
           | `Set_iso88596
           | `Set_iso88597
           | `Set_iso88598
           | `Set_iso88599
           | `Set_jis0201
           | `Set_jis0208
           | `Set_jis0212
           | `Set_koi8r
           | `Set_ks1001
           | `Set_macroman
           | `Set_unicode
           | `Set_usascii
           | `Set_windows1250
           | `Set_windows1251
           | `Set_windows1252
           | `Set_windows1253
           | `Set_windows1254
           | `Set_windows1255
           | `Set_windows1256
           | `Set_windows1257
           | `Set_windows1258 ]

A `charset` is simply a set of code points. It does not say how the code points are encoded as bytes. Every encoding implies a certain charset (or several charsets) that can be encoded, but the reverse is not true.
Pre-evaluation of the encoding argument:

A number of the following functions can be made run faster if they are called several times for the same encoding. In this case, it is recommended to apply the function once partially with the encoding argument, and to call the resulting closure instead. For example, `ustring_of_uchar` supports this technique:

       let my_ustring_of_uchar = ustring_of_uchar my_enc in
       let s1 = my_ustring_of_uchar u1 ...
       let s2 = my_ustring_of_uchar u2 ...

This is much faster than

       let s1 = ustring_of_uchar my_enc u1 ...
       let s2 = ustring_of_uchar my_enc u2 ...

The availability of this optimization is indicated by the predicate PRE_EVAL(arg) where arg identifies the encoding argument.

Inlining

When a function can be inlined across module/library boundaries, this is indicated by the predicate INLINED. Of course, this works only for the ocamlopt compiler.

    val encoding_of_string : string -> encoding

Returns the encoding of the name of the encoding. Fails if the encoding is unknown. E.g. `encoding_of_string "iso-8859-1" = `Enc_iso88591`

Punctuation characters (e.g. "-") and year suffixes (e.g. ":1991") are ignored.

    val string_of_encoding : encoding -> string

Returns the name of the encoding.

    val is_ascii_compatible : encoding -> bool

"ASCII compatible" means: The bytes 1 to 127 represent the ASCII codes 1 to 127, and no other representation of a character contains the bytes 1 to 127.

For example, ISO-8859-1 is ASCII-compatible because the byte 1 to 127 mean the same as in ASCII, and all other characters use bytes greater than 127. UTF-8 is ASCII-compatible for the same reasons, it does not matter that there are multi-byte characters. EBCDIC is not ASCII-compatible because the bytes 1 to 127 do not mean the same as in ASCII. UTF-16 is not ASCII-compatible because the bytes 1 to 127 can occur in multi-byte representations of non-ASCII characters.

The byte 0 has been excluded from this definition because the C language uses it with a special meaning that has nothing to do with characters, so it is questionable to interpret the byte 0 anyway.

    val is_single_byte : encoding -> bool

Returns whether the encoding is a single-byte encoding

    val same_encoding : encoding -> encoding -> bool

Whether both encodings are the same. ``Enc_subset` encodings are only considered as equal when the definition functions are physically the same.

Warning: Don't use ( = ) to compare encodings because this may fail.

    val byte_order_mark : encoding -> string

Returns the byte order mark that must occur at the beginning of files to indicate whether "little endian" or "big endian" is used. If this does not apply to the encoding, an empty string is returned.

See also the section about "Byte Order Marks" below.

    val makechar : encoding -> int -> string

Deprecated.This function is deprecated since ocamlnet-0.96. Use `ustring_of_uchar` instead.
`makechar enc i:` Creates the string representing the Unicode code point `i` in encoding `enc`. Raises `Not_found` if the character is legal but cannot be represented in `enc`.

Possible encodings: everything but ``Enc_utf16`.

Evaluation hints:

- PRE_EVAL(encoding)

    val ustring_of_uchar : encoding -> int -> string

`ustring_of_uchar enc i`: Creates the string representing the Unicode code point `i` in encoding `enc`. Raises `Cannot_represent i` if the character is legal but cannot be represented in `enc`.

Possible encodings: everything but ``Enc_utf16`.

Evaluation hints:

- PRE_EVAL(encoding)

    val to_unicode : charset -> int -> int

Maps the code point of the charset to the corresponding Unicode code point, or raises `Malformed_code`, when the input number does not correspond to a code point.

Note ``Set_jis0208` and ``Set_jis0212`: Code points are usually given by a row and column number. The numeric code point returned by this function is computed by multiplying the row number (1..94) with 96, and by adding the column number (1..94), i.e. row*96+column.

Evaluation hints:

- PRE_EVAL(charset)

    val from_unicode : charset -> int -> int

Maps the Unicode code point to the corresponding code point of the charset, or raises `Cannot_represent` when there is no such corresponding code point.

Note ``Set_jis0208` and ``Set_jis0212`: Code points are usually given by a row and column number. The numeric code point returned by this function is computed by multiplying the row number (1..94) with 96, and by adding the column number (1..94), i.e. row*96+column.

Evaluation hints:

- PRE_EVAL(charset)

    val available_input_encodings : unit -> encoding list

Returns the list of all available encodings that can be used for input strings. The list reflects the set of loadable/linked `Netmapping` modules.

    val available_output_encodings : unit -> encoding list

Returns the list of all available encodings that can be used for output strings. The list reflects the set of loadable/linked `Netmapping` modules.

    val user_encoding : unit -> encoding option

Determines the preferred user encoding:

- Unix: This is the character set from the current locale
- Win32: This is derived from the current ANSI code page

If an error occurs while determining the result, the value `None` is returned.

    val win32_code_pages : (int * encoding) list

Mapping between Win32 code page numbers and Ocamlnet encodings. This is incomplete. The official list: http://msdn.microsoft.com/en-us/library/dd317756%28v=VS.85%29.aspx

## Direct Conversion

In order to convert a string from one encoding to another, call `convert` like in

     let s_utf8 =
        convert ~in_enc:`Enc_iso88591 ~out_enc:`Enc_utf8 s_latin1

which converts the ISO-8859-1 string `s_latin1` to the UTF-8 string `s_utf8`.

It is also possible to convert while reading from or writing to a file. This use case is effectively handled by the class `Netconversion.conversion_pipe`. See the explanations of this class for examples.

    val convert : ?subst:(int -> string) ->
           in_enc:encoding ->
           out_enc:encoding ->
           ?range_pos:int -> ?range_len:int -> string -> string

Converts the string from `in_enc` to `out_enc`, and returns it. The string must consist of a whole number of characters. If it ends with an incomplete multi-byte character, however, this is detected, and the exception `Malformed_code` will be raised. This exception is also raised for other encoding errors in the input string.

`subst` : This function is invoked for code points of `in_enc` that cannot be represented in `out_enc`, and the result of the function invocation is substituted (directly, without any further conversion). Restriction: The string returned by `subst` must not be longer than 50 bytes. If `subst` is missing, `Cannot_represent` is raised in this case.
`range_pos` : Selects a substring for conversion. `range_pos` is the byte position of the first character of the substring. (Default: 0)
`range_len` : Selects a substring for conversion. `range_len` is the length of the substring in bytes (Default: Length of the input string minus `range_pos`)

    val recode_string : in_enc:encoding ->
           out_enc:encoding -> ?subst:(int -> string) -> string -> string

Deprecated.This function is obsolete since ocamlnet-0.96. Use `convert` instead.
Recodes a complete string from `in_enc` to `out_enc`, and returns it. The function `subst` is invoked for code points of `in_enc` that cannot be represented in `out_enc`, and the result of the function invocation is substituted. Restriction: The string returned by `subst` must not be longer than 50 bytes. If `subst` is missing, `Not_found` is raised in this case.

    val recode : in_enc:encoding ->
           in_buf:string ->
           in_pos:int ->
           in_len:int ->
           out_enc:encoding ->
           out_buf:string ->
           out_pos:int ->
           out_len:int ->
           max_chars:int -> subst:(int -> string) -> int * int * encoding

Converts the character sequence contained in the at most `in_len` bytes of `in_buf` starting at byte position `in_pos`, and writes the result into at most `out_len` bytes of `out_buf` starting at byte position `out_pos`. At most `max_chars` characters are converted from `in_buf` to `out_buf`.

The characters in `in_buf` are assumed to be encoded as `in_enc`, and the characters in `out_buf` will be encoded as `out_enc`. The case `in_enc = out_enc` is not handled specially, and is carried out as fast as any other conversion.

If there is a code point which cannot be represented in `out_enc`, the function `subst` is called with the code point as argument, and the resulting string (which must already be encoded as `out_enc`) is inserted instead. It is possible that `subst` is called several times for the same character. Restriction: The string returned by subst must not be longer than 50 bytes.

It is allowed that the input buffer ends with an incomplete multi-byte character. This character is not converted, i.e. the conversion ends just before this character. This special condition is not indicated to the caller.

Returns The triple `(in_n, out_n, in_enc')` is returned:

- `in_n` is the actual number of bytes that have been converted from `in_buf`; `in_n` may be smaller than `in_len` because of incomplete multi-byte characters, or because the output buffer has less space for characters than the input buffer, or because of a change of the encoding variant.
- `out_n` is the actual number of bytes written into `out_buf`.
- `in_enc'` is normally identical to `in_enc`. However, there are cases where the encoding can be refined when looking at the byte sequence; for example whether a little endian or big endian variant of the encoding is used. `in_enc'` is the variant of `in_enc` that was used for the last converted character.

If there is at least one complete character in `in_buf`, and at least space for one complete character in `out_buf`, and `max_chars >= 1`, it is guaranteed that `in_n > 0 && out_n > 0`.

    class conversion_pipe : ?subst:int -> string -> in_enc:encoding -> out_enc:encoding -> unit -> Netchannels.io_obj_channel

This pipeline class (see `Netchannels` for more information) can be used to recode a netchannel while reading or writing.

    class recoding_pipe : ?subst:int -> string -> in_enc:encoding -> out_enc:encoding -> unit -> Netchannels.io_obj_channel

Recodes a channel like `conversion_pipe`.

## Reading Text Using Cursors

A cursor is a reference to a character in an encoded string. The properties of the current character can be obtained, and the cursor can be moved relative to its current position.

For example, the following loop outputs the Unicode code points of all characters of the UTF-8 input string `s`:

     let cs = create_cursor `Enc_utf8 s in
     while not (cursor_at_end cs) do
       let n = cursor_char_count cs in
       let ch = uchar_at cs in
       printf "At position %d: %d\n" n ch;
       move cs;
     done

For a more exact definition, cursors are modeled as follows: The reference to the encoded string is contained in the cursor. This can be a complete string, or an arbitrary substring (denoted by a range of valid byte positions). The cursor position can be initially set to an arbitrary byte position of the encoded string.

Cursor positions can be denoted by

- byte positions `p` in the encoded string, or by
- character counts `n` relative to the initial position.

Valid cursor positions are:

- `n=0`: This is always the initial cursor position
- `n>0`: Positive char counts refer to characters right to the initial character. The rightmost position is the position `n_max` past the rightmost character. The rightmost position does not have a code point.
- `n<0`: Negative char counts refer to characters left to the initial character. The leftmost position is the position `n_min` of the leftmost character.

For the empty string we have `n_min = n_max = 0`, complementing the above definition.

Cursors are moved to the left or right of their current position by a whole number of characters. When it is tried to move them past the leftmost or rightmost position, the cursor is placed to the leftmost or rightmost position, respectively, and the exception `Cursor_out_of_range` is raised.

There are two cases of illegal encodings:

- When the last byte sequence of the encoded string is an incomplete multi-byte character, this is detected, and the special exception `Partial_character` is raised when the code point of this character is read. Note that this can only happen at position `n_max-1`. It is allowed to move beyond this character to `n_max`.
- When an illegal byte sequence occurs in the encoded string (including an incomplete multi-byte character at the beginning of the string), it is not possible to move the cursor to this character, or across this character. When it is tried to do so, the cursor stops just before the bad sequence, and the exception `Malformed_code` is raised.

It is undefined what happens when the encoded string is modified while a cursor is in use referring to it.

    type cursor

A cursor denotes a character position in an encoded string

    exception End_of_string

Raised when it is tried to access the character after the end of the string (at position `n_max`)

    exception Cursor_out_of_range

Raised when it is tried to move the cursor beyond the beginning of the string or beyond the end of the string. In the latter case, it is legal to move the cursor to the position following the last character, but it is not possible to move it further.

    exception Partial_character

Raised when the last character of the string is an incomplete multi-byte character, and it is tried to get the code point (using `uchar_at`).

    exception Byte_order_mark

Raised when it is tried to get the code point of the BOM at the beginning of the string

    val create_cursor : ?range_pos:int ->
           ?range_len:int ->
           ?initial_rel_pos:int ->
           encoding -> string -> cursor

Creates a new cursor for the passed string and the passed encoding. By default, the allowed range of the cursor is the whole string, and the cursor is intially positioned at the beginning of the string. The range is the part of the string the cursor can move within.

Special behaviour for ``Enc_utf16`: UTF-16 with unspecified endianess is handled specially. First, this encoding is only accepted when `initial_rel_pos=0`. Second, the first two bytes must be a byte order mark (BOM) (if the string has a length of two bytes or more). The BOM counts as character without code point. The function `uchar_at` raises the exception `Byte_order_mark` when the BOM is accessed. Third, when the cursor is moved to the next character, the encoding as returned by `cursor_encoding` is changed to either ``Enc_utf16_le` or ``Enc_utf16_be` according to the BOM. The encoding changes back to ``Enc_utf16` when the cursor is moved back to the initial position.

`range_pos` : Restricts the range of the cursor to a substring. The argument `range_pos` is the byte position of the beginning of the range. (Defaults to 0)
`range_len` : Restricts the range of the cursor to a substring. The argument `range_len` is the length of the range. (Default: Length of the input string minus `range_pos`)
`initial_rel_pos` : The initial position of the cursor, given as bytes relative to `range_pos`. The character at this position is considered as the zeroth character of the string (as reported by `cursor_char_count`)

    val reinit_cursor : ?range_pos:int ->
           ?range_len:int ->
           ?initial_rel_pos:int ->
           ?enc:encoding -> string -> cursor -> unit

Reuses an existing cursor for a new purpose. The arguments are as in `create_cursor`.

    val copy_cursor : ?enc:encoding -> cursor -> cursor

Copies the cursor. The copy can be moved independently of the original cursor, but is applied to the same string. The copy starts at the byte position of the string where the original cursor is currently positioned.

`enc` : Optionally, the assumed encoding can be changed to a different one by passing `enc`.

    val cursor_target : cursor -> string

Returns the string of the cursor

Evaluation hints:

- INLINED

    val cursor_range : cursor -> int * int

Returns the valid range of the cursor as pair `(range_pos, range_len)`

Evaluation hints:

- INLINED

    val cursor_initial_rel_pos : cursor -> int

Returns the initial relative byte position of the cursor

Evaluation hints:

- INLINED

    val cursor_char_count : cursor -> int

Returns the character count of the cursor. The initial position (when `create_cursor` was called) has the number 0, positions to the right denote positive numbers, and positions to the left negative numbers.

Evaluation hints:

- INLINED

    val cursor_pos : cursor -> int

Returns the byte position of the cursor, i.e. the byte index of the string that corresponds to the cursor position. The function returns the absolute position (i.e. NOT relative to `cursor_range`).

Evaluation hints:

- INLINED

    val uchar_at : cursor -> int

Returns the Unicode code point of the character at the cursor. Raises `End_of_string` if the cursor is positioned past the last character. Raises `Partial_character` if the last character of the analysed string range is an incomplete multi-byte character. Raises `Byte_order_mark` if the first character of the string is a BOM (when the encoding has BOMs).

Evaluation hints:

- INLINED

    val cursor_byte_length : cursor -> int

Returns the byte length of the representation of the character at the cursor. This works also for incomplete multi-byte characters and BOMs. Raises `End_of_string` if the cursor is positioned past the last character.

Evaluation hints:

- INLINED

    val cursor_at_end : cursor -> bool

Returns whether the cursor is positioned past the last character.

Evaluation hints:

- INLINED

    val move : ?num:int -> cursor -> unit

Moves the cursor one character to the right, or if `num` is passed, this number of characters to the right. `num` can be negative in which case the cursor is moved to the left.

If the cursor were placed outside the valid range, the cursor would go into an illegal state, and because of this, this is handled as follows: the cursor moves to the leftmost or rightmost position (depending on the direction), and the exception `Cursor_out_of_range` is raised.

    val cursor_encoding : cursor -> encoding

Returns the encoding of the cursor. For some encodings, the returned encoding depends on the position of the cursor (see the note about UTF-8 in `create_cursor`)

Evaluation hints:

- INLINED

    val cursor_blit : cursor -> int array -> int -> int -> int

`cursor_blit cs ua pos len`: Copies at most `len` characters as code points from the cursor position and the following positions to the array `ua` at index `pos`. The number of copied characters is returned. If the cursor is already at the end of the string when this function is called, the exception `End_of_string` will be raised instead, and no characters are copied. The cursor positions containing byte order marks and partial characters are never copied; this is ensured by stopping the copying procedure just before these positions. This may even make the function return the number 0.

The function tries to copy as many characters as currently available in the already decoded part of the string the cursor is attached to. In the current implementation, this number is not higher than 250. You can call `cursor_blit_maxlen` to get an upper limit.

The function does not move the cursor.

    val cursor_blit_maxlen : cursor -> int

Returns the maximum number of characters `cursor_blit` can copy at the current cursor position. This is the number of characters `cursor_blit` would copy if the `len` argument were arbitrarily large.

Note that the value depends on the cursor position and on the contents of the cursor string.

This function raises `End_of_string` if the cursor is positioned at the end of the string.

    val cursor_blit_positions : cursor -> int array -> int -> int -> int

Works like `cursor_blit`, but copies the byte positions of the characters into `ua` instead of the code points.

When called directly after `cursor_blit` for the same cursor and with the same value of `len`, this function copies as many characters and thus returns the same number:

    let n1 = cursor_blit     cs ua ua_pos len in
     let n2 = cursor_blit_pos cs pa pa_pos len in
     assert (n1 = n2)

### Byte Order Marks

Because UTF-16 allows both little and big endian, files and other permanent representations of UTF-16 text are usually prepended by a byte order mark (BOM). There is confusion about the BOM among Unicode users, so the following explanations may be helpful.

Of course, the BOM is only used for external representations like files, as the endianess is always known for in-memory representations by the running program. This module has three encoding identifiers:

- ``Enc_utf16`: UTF-16 where the endianess is unknown
- ``Enc_utf16_le`: UTF-16 little endian
- ``Enc_utf16_be`: UTF-16 big endian

When a file is read, the endianess is unknown at the beginning. This is expressed by ``Enc_utf16`. When the BOM is read, the encoding is refined to either ``Enc_utf16_le` or ``Enc_utf16_be`, whatever the BOM says. This works as follows: The BOM is the representation of the code point 0xfeff as little or big endian, i.e. as byte sequences "0xfe 0xff" (big endian) or "0xff 0xfe" (little endian). As the "wrong" code point 0xfffe is intentionally unused, the reader can determine the endianess.

There is one problem, though. Unfortunately, the code point 0xfeff is also used for the "zero width non-breakable space" character. When this code point occurs later in the text, it is interpreted as this character. Of course, this means that one must know whether there is a BOM at the beginning, and if not, one must know the endianess. One cannot program in the style "well, let's see what is coming and guess".

Furthermore, the BOM is only used for encodings where one can specify the endianess. It must not be used for UTF-8, for example, as the byte order is fixed for this encoding. When a UTF-8 text begins with the code point 0xfeff, it is always the "zero width non-breakable space" character.

The functions of this module can all deal with BOMs when reading encoded text. In most cases, the BOM is hidden from the caller, and just handled automatically. Cursors, however, treat BOMs as special characters outside of the code set (exception `Byte_order_mark` is raised). The writing functions of this module do not generate BOMs, however, as there is no way to tell them that a BOM is needed. The function `byte_order_mark` can be used to output the BOM manually.

### Examples for Cursors

Create the cursor:

`let cs = create_cursor `Enc_utf8 "B\195\164r";;`

The cursor is now positioned at the 'B':

`uchar_at cs` returns `66` (i.e. B)

Move the cursor one character to the right. In UTF-8, this is a two-byte character consisting of the bytes 195 and 164:

`move cs ;;`

`uchar_at cs` returns `228` (i.e. a-Umlaut)

One can easily move the cursor to the end of the string:

`move ~num:max_int cs ;;`

This raises `Cursor_out_of_range`, but places the cursor at the end. This is the position past the last letter 'r':

`uchar_at cs` raises `End_of_string`

Go one character to the left:

`move ~num:(-1) cs ;;`

`uchar_at cs` returns `114` (i.e. r)

Cursors can only move relative to their current position. Of course, one can easily write a function that moves to an absolute position, like

     let move_abs n cs =
        let delta = n - cursor_pos cs in
        move ~num:delta cs

However, this operation is expensive (O(string length)), and should be avoided for efficient algorithms. Cursors are not arrays, and an algorithm should only be based on cursors when it is possible to iterate over the characters of the string one after another.

## Unicode String Functions

    val ustring_length : encoding -> ?range_pos:int -> ?range_len:int -> string -> int

Returns the length of the string in characters. The function fails when illegal byte sequences or incomplete characters are found in the string with `Malformed_code`.

Evaluation hints:

- PRE_EVAL(encoding)

`range_pos` : The byte position of the substring to measure (default: 0)
`range_len` : The byte length of the substring to measure (default: byte length of the input string minus `range_pos`)

    val ustring_iter : encoding ->
           (int -> unit) -> ?range_pos:int -> ?range_len:int -> string -> unit

Iterates over the characters of a string, and calls the passed function for every code point. The function raises `Malformed_code` when illegal byte sequences or incomplete characters are found.

`range_pos` : The byte position of the substring to iterate over (default: 0)
`range_len` : The byte length of the substring to iterate over (default: byte length of the input string minus `range_pos`)

    val ustring_map : encoding ->
           (int -> int list) -> ?range_pos:int -> ?range_len:int -> string -> string

Maps every character of a string to a list of characters, and returns the concatenated string. The `encoding` argument determines the encoding of both the argument and the result string. The map function gets every character as its Unicode code point, and must return the list of code points to map to.

The function raises `Malformed_code` when illegal byte sequences or incomplete characters are found.

`range_pos` : The byte position of the substring to map (default: 0)
`range_len` : The byte length of the substring to map (default: byte length of the input string minus `range_pos`)

    val ustring_sub : encoding ->
           int -> int -> ?range_pos:int -> ?range_len:int -> string -> string

`ustring_sub enc start length s`: Returns the substring of `s` starting at character count `start` and consisting of `length` characters. Note that `start` and `length` select the substring by multiples of (usually multibyte) characters, not bytes.

If the optional byte-based `range_pos` and `range_len` arguments are present, these arguments are taken to determine a first substring before `start` and `length` are applied to extract the final substring.

The function raises `Malformed_code` when illegal byte sequences or incomplete characters are found.

`range_pos` : The byte position of the substring to extract (default: 0)
`range_len` : The byte length of the substring to extract (default: byte length of the input string minus `range_pos`)

    val ustring_compare : encoding ->
           (int -> int -> int) ->
           ?range_pos:int ->
           ?range_len:int -> string -> ?range_pos:int -> ?range_len:int -> string -> int

Compares two strings lexicographically. The first argument is the encoding of both strings (which must be the same). The second argument is the function that compares two Unicode code points. It must return 0 if both characters are the same, a negative value if the first character is the smaller one, and a positive value if the second character is the smaller one.

The function raises `Malformed_code` when illegal byte sequences or incomplete characters are found.

`range_pos` : The byte position of the substring to compare (default: 0), referring to the following string argument
`range_len` : The byte length of the substring to compare (default: byte length of the input string minus `range_pos`), referring to the following string argument
`range_pos` : The byte position of the substring to compare (default: 0), referring to the following string argument
`range_len` : The byte length of the substring to compare (default: byte length of the input string minus `range_pos`), referring to the following string argument

    val uarray_of_ustring : encoding ->
           ?range_pos:int -> ?range_len:int -> string -> int array

Returns the characters of the string as array of Unicode code points.

`range_pos` : The byte position of the substring to extract (default: 0)
`range_len` : The byte length of the substring to extract (default: byte length of the input string minus `range_pos`)

    val ustring_of_uarray : ?subst:(int -> string) ->
           encoding -> ?pos:int -> ?len:int -> int array -> string

Returns the array of Unicode code points as encoded string.

`subst` : This function is called when a code point cannot be represented in the chosen character encoding. It must returns the (already encoded) string to substitute for this code point. By default (if ~subst is not passed), the exception `Cannot_represent` will be raised in this case.
`pos` : Selects a subarray: `pos` is the first array position to encode (default: 0)
`len` : Selects a subarray: `len` is the length of the subarray to encode (default: array length minus `pos`)

    exception Malformed_code_at of int

An illegal byte sequence is found at this byte position

    val verify : encoding -> ?range_pos:int -> ?range_len:int -> string -> unit

Checks whether the string is properly encoded. If so, () is returned. If not, the exception `Malformed_code_at` will be raised indicating the byte position where the problem occurs.

`range_pos` : The byte position of the substring to verify (default: 0)
`range_len` : The byte length of the substring to verify (default: byte length of the input string minus `range_pos`)