GNU libunistring: 13. Normalization forms (composition and decomposition) <uninorm.h>

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# 13. Normalization forms (composition and decomposition) `<uninorm.h>`

This include file defines functions for transforming Unicode strings to one of the four normal forms, known as NFC, NFD, NKFC, NFKD. These transformations involve decomposition and — for NFC and NFKC — composition of Unicode characters.

## 13.1 Decomposition of Unicode characters

The following enumerated values are the possible types of decomposition of a Unicode character.

Constant: int UC_DECOMP_CANONICAL

  Denotes canonical decomposition.

Constant: int UC_DECOMP_FONT

  UCD marker: `<font>`. Denotes a font variant (e.g. a blackletter form).

Constant: int UC_DECOMP_NOBREAK

  UCD marker: `<noBreak>`. Denotes a no-break version of a space or hyphen.

Constant: int UC_DECOMP_INITIAL

  UCD marker: `<initial>`. Denotes an initial presentation form (Arabic).

Constant: int UC_DECOMP_MEDIAL

  UCD marker: `<medial>`. Denotes a medial presentation form (Arabic).

Constant: int UC_DECOMP_FINAL

  UCD marker: `<final>`. Denotes a final presentation form (Arabic).

Constant: int UC_DECOMP_ISOLATED

  UCD marker: `<isolated>`. Denotes an isolated presentation form (Arabic).

Constant: int UC_DECOMP_CIRCLE

  UCD marker: `<circle>`. Denotes an encircled form.

Constant: int UC_DECOMP_SUPER

  UCD marker: `<super>`. Denotes a superscript form.

Constant: int UC_DECOMP_SUB

  UCD marker: `<sub>`. Denotes a subscript form.

Constant: int UC_DECOMP_VERTICAL

  UCD marker: `<vertical>`. Denotes a vertical layout presentation form.

Constant: int UC_DECOMP_WIDE

  UCD marker: `<wide>`. Denotes a wide (or zenkaku) compatibility character.

Constant: int UC_DECOMP_NARROW

  UCD marker: `<narrow>`. Denotes a narrow (or hankaku) compatibility character.

Constant: int UC_DECOMP_SMALL

  UCD marker: `<small>`. Denotes a small variant form (CNS compatibility).

Constant: int UC_DECOMP_SQUARE

  UCD marker: `<square>`. Denotes a CJK squared font variant.

Constant: int UC_DECOMP_FRACTION

  UCD marker: `<fraction>`. Denotes a vulgar fraction form.

Constant: int UC_DECOMP_COMPAT

  UCD marker: `<compat>`. Denotes an otherwise unspecified compatibility character.

The following constant denotes the maximum size of decomposition of a single Unicode character.

Macro: unsigned int UC_DECOMPOSITION_MAX_LENGTH

  This macro expands to a constant that is the required size of buffer passed to the `uc_decomposition` and `uc_canonical_decomposition` functions.

The following functions decompose a Unicode character.

Function: int uc_decomposition (ucs4_t uc, int *decomp_tag, ucs4_t *decomposition)

  Returns the character decomposition mapping of the Unicode character uc. decomposition must point to an array of at least `UC_DECOMPOSITION_MAX_LENGTH` `ucs_t` elements.

  When a decomposition exists, `decomposition[0..n-1]` and `*decomp_tag` are filled and n is returned. Otherwise -1 is returned.

Function: int uc_canonical_decomposition (ucs4_t uc, ucs4_t *decomposition)

  Returns the canonical character decomposition mapping of the Unicode character uc. decomposition must point to an array of at least `UC_DECOMPOSITION_MAX_LENGTH` `ucs_t` elements.

  When a decomposition exists, `decomposition[0..n-1]` is filled and n is returned. Otherwise -1 is returned.

## 13.2 Composition of Unicode characters

The following function composes a Unicode character from two Unicode characters.

Function: ucs4_t uc_composition (ucs4_t uc1, ucs4_t uc2)

  Attempts to combine the Unicode characters uc1, uc2. uc1 is known to have canonical combining class 0.

  Returns the combination of uc1 and uc2, if it exists. Returns 0 otherwise.

  Not all decompositions can be recombined using this function. See the Unicode file ‘`CompositionExclusions.txt`’ for details.

## 13.3 Normalization of strings

The Unicode standard defines four normalization forms for Unicode strings. The following type is used to denote a normalization form.

Type: uninorm_t

  An object of type `uninorm_t` denotes a Unicode normalization form. This is a scalar type; its values can be compared with `==`.

The following constants denote the four normalization forms.

Macro: uninorm_t UNINORM_NFD

  Denotes Normalization form D: canonical decomposition.

Macro: uninorm_t UNINORM_NFC

  Normalization form C: canonical decomposition, then canonical composition.

Macro: uninorm_t UNINORM_NFKD

  Normalization form KD: compatibility decomposition.

Macro: uninorm_t UNINORM_NFKC

  Normalization form KC: compatibility decomposition, then canonical composition.

The following functions operate on `uninorm_t` objects.

Function: bool uninorm_is_compat_decomposing (uninorm_t nf)

  Tests whether the normalization form nf does compatibility decomposition.

Function: bool uninorm_is_composing (uninorm_t nf)

  Tests whether the normalization form nf includes canonical composition.

Function: uninorm_t uninorm_decomposing_form (uninorm_t nf)

  Returns the decomposing variant of the normalization form nf. This maps NFC,NFD → NFD and NFKC,NFKD → NFKD.

The following functions apply a Unicode normalization form to a Unicode string.

Function: uint8_t * u8_normalize (uninorm_t nf, const uint8_t *s, size_t n, uint8_t *resultbuf, size_t *lengthp)
Function: uint16_t * u16_normalize (uninorm_t nf, const uint16_t *s, size_t n, uint16_t *resultbuf, size_t *lengthp)
Function: uint32_t * u32_normalize (uninorm_t nf, const uint32_t *s, size_t n, uint32_t *resultbuf, size_t *lengthp)

  Returns the specified normalization form of a string.

## 13.4 Normalizing comparisons

The following functions compare Unicode string, ignoring differences in normalization.

Function: int u8_normcmp (const uint8_t *s1, size_t n1, const uint8_t *s2, size_t n2, uninorm_t nf, int *resultp)
Function: int u16_normcmp (const uint16_t *s1, size_t n1, const uint16_t *s2, size_t n2, uninorm_t nf, int *resultp)
Function: int u32_normcmp (const uint32_t *s1, size_t n1, const uint32_t *s2, size_t n2, uninorm_t nf, int *resultp)

  Compares s1 and s2, ignoring differences in normalization.

  nf must be either `UNINORM_NFD` or `UNINORM_NFKD`.

  If successful, sets `*resultp` to -1 if s1 < s2, 0 if s1 = s2, 1 if s1 > s2, and returns 0. Upon failure, returns -1 with `errno` set.

Function: char * u8_normxfrm (const uint8_t *s, size_t n, uninorm_t nf, char *resultbuf, size_t *lengthp)
Function: char * u16_normxfrm (const uint16_t *s, size_t n, uninorm_t nf, char *resultbuf, size_t *lengthp)
Function: char * u32_normxfrm (const uint32_t *s, size_t n, uninorm_t nf, char *resultbuf, size_t *lengthp)

  Converts the string s of length n to a NUL-terminated byte sequence, in such a way that comparing `u8_normxfrm (s1)` and `u8_normxfrm (s2)` with the `u8_cmp2` function is equivalent to comparing s1 and s2 with the `u8_normcoll` function.

  nf must be either `UNINORM_NFC` or `UNINORM_NFKC`.

Function: int u8_normcoll (const uint8_t *s1, size_t n1, const uint8_t *s2, size_t n2, uninorm_t nf, int *resultp)
Function: int u16_normcoll (const uint16_t *s1, size_t n1, const uint16_t *s2, size_t n2, uninorm_t nf, int *resultp)
Function: int u32_normcoll (const uint32_t *s1, size_t n1, const uint32_t *s2, size_t n2, uninorm_t nf, int *resultp)

  Compares s1 and s2, ignoring differences in normalization, using the collation rules of the current locale.

  nf must be either `UNINORM_NFC` or `UNINORM_NFKC`.

  If successful, sets `*resultp` to -1 if s1 < s2, 0 if s1 = s2, 1 if s1 > s2, and returns 0. Upon failure, returns -1 with `errno` set.

## 13.5 Normalization of streams of Unicode characters

A “stream of Unicode characters” is essentially a function that accepts an `ucs4_t` argument repeatedly, optionally combined with a function that “flushes” the stream.

Type: struct uninorm_filter

  This is the data type of a stream of Unicode characters that normalizes its input according to a given normalization form and passes the normalized character sequence to the encapsulated stream of Unicode characters.

Function: struct uninorm_filter * uninorm_filter_create (uninorm_t nf, int (*stream_func) (void *stream_data, ucs4_t uc), void *stream_data)

  Creates and returns a normalization filter for Unicode characters.

  The pair (stream_func, stream_data) is the encapsulated stream. `stream_func (stream_data, uc)` receives the Unicode character uc and returns 0 if successful, or -1 with `errno` set upon failure.

  Returns the new filter, or NULL with `errno` set upon failure.

Function: int uninorm_filter_write (struct uninorm_filter *filter, ucs4_t uc)

  Stuffs a Unicode character into a normalizing filter. Returns 0 if successful, or -1 with `errno` set upon failure.

Function: int uninorm_filter_flush (struct uninorm_filter *filter)

  Brings data buffered in the filter to its destination, the encapsulated stream.

  Returns 0 if successful, or -1 with `errno` set upon failure.

  Note! If after calling this function, additional characters are written into the filter, the resulting character sequence in the encapsulated stream will not necessarily be normalized.

Function: int uninorm_filter_free (struct uninorm_filter *filter)

  Brings data buffered in the filter to its destination, the encapsulated stream, then closes and frees the filter.

  Returns 0 if successful, or -1 with `errno` set upon failure.

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