PDD 28: Strings

Abstract

This PDD describes the conventions for strings in Parrot, including but not limited to support for multiple character sets, encodings, and languages.

A character is the abstract description of a symbol. It's the smallest chunk of text a computer knows how to deal with. Internally to the computer, a character (just like everything else) is a number, so a few further definitions are needed.

Character Set

The Unicode Standard prefers the concepts of character repertoire (a collection of characters) and character code (a mapping which tells you what number represents which character in the repertoire). Character set is commonly used to mean the standard which defines both a repertoire and a code.

Codepoint

A codepoint is the numeric representation of a character according to a given character set. So in ASCII, the character A has codepoint 0x41.

Encoding

An encoding determines how a codepoint is represented inside a computer. Simple encodings like ASCII define that the codepoints 0-127 simply live as their numeric equivalents inside an eight-bit bytes. Other fixed-width encodings like UCS-2 use more bytes to encode more codepoints. Variable-width encodings like UTF-8 use one byte for codepoints 0-127, two bytes for codepoints 127-2047, and so on.

Character sets and encodings are related but separate concepts. An encoding is the lower-level representation of a string's data, whereas the character set determines higher-level semantics. Typically, character set functions will ask a string's encoding functions to retrieve data from the string, and then process the retrieved data.

Combining Character

A combining character is a Unicode concept. It is a character which modifies the preceding character. For instance, accents, lines, circles, boxes, etc. which are not to be displayed on their own, but to be composed with the preceding character.

Grapheme

In linguistics, a grapheme is a single symbol in a writing system (letter, number, punctuation mark, kanji, hiragana, Arabic glyph, Devanagari symbol, etc), including any modifiers (diacritics, etc).

The Unicode Standard defines a grapheme cluster (commonly simplified to just grapheme) as one or more characters forming a visible whole when displayed, in other words, a bundle of a character and all of its combining characters. Because graphemes are the highest-level abstract idea of a "character", they're useful for converting between character sets.

Normalization Form

A normalization form standardizes the representation of a string by transforming a sequence of combining characters into a more complex character (composition), or by transforming a complex character into a sequence of composing characters (decomposition). The decomposition forms also define a standard order for the composing characters, to allow string comparisons. The Unicode Standard defines four normalization forms: NFC and NFKC are composition, NFD and NFKD are decomposition. See Unicode Normalization Forms for more details.

Grapheme Normalization Form

Grapheme normalization form (NFG) is a normalization which allocates exactly one codepoint to each grapheme.

Description

Parrot supports multiple string formats, and so users of Parrot strings must be aware at all times of string encoding issues and how these relate to the string interface.
Parrot provides an interface for interacting with strings and converting between character sets and encodings.
Operations that require understanding the semantics of a string must respect the character set of the string.
Operations that require understanding the layout of the string must respect the encoding of the string.
In addition to common string formats, Parrot provides an additional string format that is a sequence of 32-bit Unicode codepoints in NFG.

Implementation

Parrot was designed from the outset to support multiple string formats: multiple character sets and multiple encodings. We don't standardize on Unicode internally, converting all strings to Unicode strings, because for the majority of use cases it's still far more efficient to deal with whatever input data the user sends us.

Consumers of Parrot strings need to be aware that there is a plurality of string encodings inside Parrot. (Producers of Parrot strings can do whatever is most efficient for them.) To put it in simple terms: if you find yourself writing *s++ or any other C string idioms, you need to stop and think if that's what you really mean. Not everything is byte-based anymore.

Grapheme Normalization Form

Unicode characters can be expressed in a number of different ways according to the Unicode Standard. This is partly to do with maintaining compatibility with existing character encodings. For instance, in Serbo-Croatian and Slovenian, there's a letter which looks like an i without the dot but with two grave (`) accents (ȉ). Unicode can represent this letter as a composed character 0x209, also known as LATIN SMALL LETTER I WITH DOUBLE GRAVE, which does the job all in one go. It can also represent this letter as a decomposed sequence: LATIN SMALL LETTER I (0x69) followed by COMBINING DOUBLE GRAVE ACCENT (0x30F). We use the term grapheme to refer to a "letter" whether it's represented by a single codepoint or multiple codepoints.

String operations on this kind of variable-byte encoding can be complex and expensive. Operations like comparison and traversal require a series of computations and lookaheads, because any given grapheme may be a sequence of combining characters. The Unicode Standard defines several "normalization forms" that help with this problem. Normalization Form C (NFC), for example, decomposes everything, then re-composes as much as possible. So if you see the integer stream 0x69 0x30F, it needs to be replaced by 0x209. However, Unicode's normalization forms don't go quite far enough to completely solve the problem. For example, Serbo-Croat is sometimes also written with Cyrillic letters rather than Latin letters. Unicode doesn't have a single composed character for the Cyrillic equivalent of the Serbo-Croat LATIN SMALL LETTER I WITH DOUBLE GRAVE, so it is represented as a decomposed pair CYRILLIC SMALL LETTER I (0x438) with COMBINING DOUBLE GRAVE ACCENT (0x30F). This means that even in the most normalized Unicode form, string manipulation code must always assume a variable-byte encoding, and use expensive lookaheads. The cost is incurred on every operation, though the particular string operated on might not contain combining characters. It's particularly noticeable in parsing and regular expres699sion matches, where backtracking operations may re-traverse the characters of a simple string hundreds of times.

In order to reduce the cost of variable-byte operations and simplify some string manipulation tasks, Parrot defines an additional normalization: Normalization Form G (NFG). In NFG, every grapheme is guaranteed to be represented by a single codepoint. Graphemes that don't have a single codepoint representation in Unicode are given a dynamically generated codepoint unique to the NFG string.

An NFG string is a sequence of signed 32-bit Unicode codepoints. It's equivalent to UCS-4 except for the normalization form semantics. UCS-4 specifies an encoding for Unicode codepoints from 0 to 0x7FFFFFFF. In other words, any codepoints with the first bit set are undefined. NFG interprets the unused bit as a sign bit, and reserves all negative codepoints as dynamic codepoints. A negative codepoint acts as an index into a lookup table, which maps between a dynamic codepoint and its associated decomposition.

In practice, this goes as follows: When our Russified Serbo-Croat string is converted to NFG, it is normalized to a single character having the codepoint 0xFFFFFFFFF (in other words, -1 in 2's complement). At the same time, Parrot inserts an entry into the string's grapheme table at array index -1, containing the Unicode decomposition of the grapheme 0x00000438 0x000000030F.

Parrot will provide both grapheme-aware and codepoint-aware string operations, such as iterators for string traversal and calculations of string length. Individual language implementations can choose between the two types of operations depending on whether their string semantics are character-based or codepoint-based. For languages that don't currently have Unicode support, the grapheme operations will allow them to safely manipulate Unicode data without changing their string semantics.

Advantages

Applications that don't care about graphemes can handle a NFG codepoint in a string as if it's any other character. Only applications that care about the specific properties of Unicode characters need to take the load of peeking inside the grapheme table and reading the decomposition.

Using negative numbers for dynamic codepoints allows Parrot to check if a particular codepoint is dynamic using a single sign-comparison operation. It also means that NFG can be used without conflict on encodings from 7-bit (signed 8-bit integers) to 63-bit (using signed 64-bit integers) and beyond.

Because any grapheme from any character set can be represented by a single NFG codepoint, NFG strings are useful as an intermediate representation for converting between string types.

Disadvantages

A 32-bit encoding is quite large, considering the fact that the Unicode codespace only requires up to 0x10FFFF. The Unicode Consortium's FAQ notes that most Unicode interfaces use UTF-16 instead of UTF-32, out of memory considerations. This means that although Parrot will use 32-bit NFG strings for optimizations within operations, for the most part individual users should use the native character set and encoding of their data, rather than using NFG strings directly.

The conceptual cost of adding a normalization form beyond those defined in the Unicode Standard has to be considered. However, to fully support Unicode, Parrot already needs to keep track of what normalization form a given string is in, and provide functions to convert between normalization forms. The conceptual cost of one additional normalization form is relatively small.

The grapheme table

When constructing strings in NFG, graphemes not expressible as a single character in Unicode are represented by a dynamic codepoint index into the string's grapheme table. When Parrot comes across a multi-codepoint grapheme, it must first determine whether or not the grapheme already has an entry in the grapheme table. Therefore the table cannot strictly be an array, as that would make lookup inefficient. The grapheme table is represented, then, as both an array and a hash structure. The array interface provides forward-lookup and the hash interface reverse lookup. Converting a multi-codepoint grapheme into a dynamic codepoint can be demonstrated with the following Perl 5 pseudocode, for the grapheme 0x438 0x30F:

   $codepoint = ($grapheme_lookup->{0x438}{0x30F} ||= do {
                   push @grapheme_table, "\x{438}\x{30F}";
                   ~ $#grapheme_table;
                });
   push @string, $codepoint;

String API

Strings in the Parrot core should use the Parrot STRING structure. Parrot developers generally shouldn't deal with char * or other string-like types outside of this abstraction. It's also best not to access members of the STRING structure directly. The interpretation of the data inside the structure is determined by the data's encoding. Parrot's strings are encoding-aware so your functions don't need to be.

Parrot's internal strings (STRINGs) have the following structure:

    struct parrot_string_t {
        Parrot_UInt flags;
        void *     _bufstart;
        size_t     _buflen;
        char       *strstart;
        UINTVAL     bufused;
        UINTVAL     strlen;
        UINTVAL     hashval;
        const struct _encoding *encoding;
};

The fields are:

_bufstart
_buflen
flags
bufused
strlen
hashval 699 A cache of the hash value of the string, for rapid lookups when the string is used as a hash key.
encoding

{{DEPRECATION NOTE: the enum parrot_string_representation_t will be removed from the parrot string structure. It's been commented out for years.}}

{{DEPRECATION NOTE: the char * pointer strstart will be removed. It complicates the entire string subsystem for a tiny optimization on substring operations, and offset math is messy with encodings that aren't byte-based.}}

Conversions between normalization form, encoding, and charset

Conversion will be done with a function called Parrot_str_grapheme_copy:

    INTVAL Parrot_str_grapheme_copy(STRING *src, STRING *dst)

Converting a string from one format to another involves creating a new empty string with the required attributes, and passing the source string and the new string to Parrot_str_grapheme_copy. This function iterates through the source string one grapheme at a time, using the character set function pointer get_grapheme (which may read ahead multiple characters with strings that aren't in NFG). For each source grapheme, the function will call set_grapheme on the destination string (which may append multiple characters in non-NFG strings). This conversion effectively uses an intermediate NFG representation.

String Interface Functions

The current string functions will be maintained, with some modifications for the addition of the NFG string format. Many string functions that are part of Parrot's external API will be renamed for the standard "Parrot_*" naming conventions.

Parrot_str_concat (was string_concat)

Concatenate two strings. Takes two strings as arguments.

Parrot_str_new(interp, str, len)

Return a new string with the default encoding and character set. Accepts three arguments: A Parrot interpreter, a C string (char *) to initialize the value of the string, and an integer length of the string (number of characters). If a length of zero is passed, it will be calculated automatically.

Parrot_str_new_noinit (was string_make_empty)

Returns a new empty string with the default encoding and character set.

Parrot_str_new_init (was string_make_direct)

Returns a new string of the requested encoding, character set, and normalization form, initializing the string value to the value passed in. The three arguments are a C string (char *), an integer length of the string argument in bytes, and a struct pointer for the encoding struct. If the C string (char *) value is not passed, returns an empty string. If the encoding is passed as null value, a default value is used.

{{ NOTE: the crippled version of this function, string_make, used to accept a string name for the character set. This behavior is no longer supported, but Parrot_find_encoding and Parrot_find_charset can look up the encoding or character set structs. }}

Parrot_str_new_constant(interp, str)

Creates and returns a new Parrot constant string. Takes an interpreter and one C string (a char *) as an argument, the value of the constant string. The length of the C string is calculated internally.

Parrot_str_length (was string_compute_strlen)

Returns the number of characters in the string. Combining characters are each counted separately. Variable-width encodings may lookahead.

Parrot_str_grapheme_length

Returns the number of graphemes in the string. Groups of combining characters count as a single grapheme.

Parrot_str_byte_length (was string_length)

Returns the number of bytes in the string. The character width of variable-width encodings is ignored. Combining characters are not treated any differently than other characters. This is equivalent to accessing the strlen member of the STRING struct directly.

Parrot_str_indexed (was string_index)

Returns the character at the specified index (the Nth character from the start of the string). Combining characters are counted separately. Variable-width encodings will lookahead to capture full character values.

Parrot_str_chr (was string_chr)

Returns a single character string from a codepoint.

Parrot_str_grapheme_indexed

Returns the grapheme at the given index (the Nth grapheme from the string's start). Groups of combining characters count as a single grapheme, so this function may return multiple characters.

Parrot_str_find_index (was string_str_index)

Search for a given substring within a string. If it's found, return an integer index to the substring's location (the Nth character from the start of the string). Combining characters are counted separately. Variable-width encodings will lookahead to capture full character values. Returns -1 unless the substring is found.

Parrot_str_copy (was string_copy)

Make a COW copy a string (a new string header pointing to the same string buffer).

Parrot_str_grapheme_copy (new)

Accepts two string arguments: a destination and a source. Iterates through the source string one grapheme at a time and appends it to the destination string.

This function can be used to convert a string of one format to another format.

Parrot_str_repeat (was string_repeat)

Return a string containing the passed string argument, repeated the number of times in the integer argument.

Parrot_str_substr (was string_substr)

Return a substring starting at an integer offset with an integer length. The offset and length specify characters. Combining characters are counted separately. Variable-width encodings will lookahead to capture full character values.

Parrot_str_grapheme_substr

Return a substring starting at an integer offset with an integer length. The offset and length specify graphemes. Groups of combining characters count as a single grapheme.

Parrot_str_replace (was string_replace)

Replaces a substring within the first string argument with the second string argument. An integer offset and length, in characters, specify where the removed substring starts and how long it is.

Parrot_str_grapheme_replace

Replaces a substring within the first string argument with the second string argument. An integer offset and length in graphemes specify where the removed substring starts and how long it is.

Parrot_str_chopn (was string_chopn)

Chop the requested number of characters off the end of a string without modifying the original string.

Parrot_str_grapheme_chopn

Chop the requested number of graphemes off the end of a string without modifying the original string.

Parrot_str_compare (was string_compare)

Compare two strings to each other. Return 0 if they are equal, 1 if the first is greater and -1 if the second is greater. Uses character set collation order for the comparison. (Two strings that are logically equivalent in terms of display, but stored in different normalizations are not equal.)

init
clone
mark
get_integer
get_number
get_string
get_bool
set_integer_native
set_bool
set_number_native
set_string_native
assign_string_native
set_string_same
set_pmc
*bitwise*
is_equal
is_equal_num
is_equal_string
is_same
cmp
cmp_num
cmp_string
substr
exists_keyed
get_string_keyed
set_string_keyed
get_integer_keyed
set_integer_keyed

Methods

The String PMC provides the following methods.

replace
to_int
lower
trans
reverse
is_integer

References

http://sirviente.9grid.es/sources/plan9/sys/doc/utf.ps - Plan 9's Runes are not dissimilar to NFG strings, and this is a good introduction to the Unicode world.

http://www.unicode.org/reports/tr15/ - The Unicode Consortium's explanation of different normalization forms.

http://unicode.org/reports/tr29/ - "grapheme clusters" in the Unicode Standard Annex

"Unicode: A Primer", Tony Graham - Arguably the most readable book on how Unicode works.

"Advanced Perl Programming", Chapter 6, "Unicode"