[PDD] Add docs for the Parrot_PMC_push_* and Parrot_PMC_pop_* functions

[parrot.git] / docs / pdds / pdd28_strings.pod

# Copyright (C) 2008-2009, Parrot Foundation.
# $Id$

=head1 PDD 28: Strings

=head2 Abstract

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

=head2 Version

$Revision$

=head2 Definitions

=head3 Character

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.

=head3 Character Set

The Unicode Standard prefers the concepts of I<character repertoire> (a
collection of characters) and I<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.

=head3 Codepoint

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

=head3 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.

=head3 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.

=head3 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 I<grapheme cluster> (commonly simplified to
just I<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.

=head3 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 L<Unicode Normalization
Forms|http://www.unicode.org/reports/tr15/> for more details.

=head3 Grapheme Normalization Form

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

=head2 Description

=over 3

=item *

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.

=item *

Parrot provides an interface for interacting with strings and converting
between character sets and encodings.

=item *

Operations that require understanding the semantics of a string must respect
the character set of the string.

=item *

Operations that require understanding the layout of the string must respect
the encoding of the string.

=item *

In addition to common string formats, Parrot provides an additional string
format that is a sequence of 32-bit Unicode codepoints in NFG.

=back

=head2 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 C<*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.

=head3 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 C<i> without the dot but with two grave
(C<`>) accents (E<0x209>). Unicode can represent this letter as a composed
character C<0x209>, also known as C<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: C<LATIN SMALL LETTER I> (C<0x69>) followed by C<COMBINING
DOUBLE GRAVE ACCENT> (C<0x30F>). We use the term I<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 C<0x69 0x30F>, it needs to be replaced by C<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 C<LATIN SMALL LETTER
I WITH DOUBLE GRAVE>, so it is represented as a decomposed pair C<CYRILLIC
SMALL LETTER I> (C<0x438>) with C<COMBINING DOUBLE GRAVE ACCENT> (C<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 expression 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
C<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 C<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.

=head4 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.

=head4 Disadvantages

A 32-bit encoding is quite large, considering the fact that the Unicode
codespace only requires up to C<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.

=head4 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 C<0x438 0x30F>:

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

=head3 String API

Strings in the Parrot core should use the Parrot C<STRING> structure. Parrot
developers generally shouldn't deal with C<char *> or other string-like types
outside of this abstraction. It's also best not to access members of the
C<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 (C<STRING>s) 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;
        const struct _charset  *charset;
};

The fields are:

=over 4

=item _bufstart

A pointer to the buffer for the string data.

=item _buflen

The size of the buffer in bytes.

=item flags

Binary flags used for garbage collection, copy-on-write tracking, and other
metadata.

=item bufused

The amount of the buffer currently in use, in bytes.

=item strlen

The length of the string, in bytes. {{NOTE, not in characters, as characters
may be variably sized.}}

=item hashval

A cache of the hash value of the string, for rapid lookups when the string is
used as a hash key.

=item encoding

How the data is encoded (e.g. fixed 8-bit characters, UTF-8, or UTF-32).  Note
that this specifies encoding only -- it's valid to encode  EBCDIC characters
with the UTF-8 algorithm. Silly, but valid.

The encoding structure specifies the encoding (by index number and by name,
for ease of lookup), the maximum number of bytes that a single character will
occupy in that encoding, as well as functions for manipulating strings with
that encoding.

=item charset

What sort of string data is in the buffer, for example ASCII, EBCDIC, or
Unicode.

The charset structure specifies the character set (by index number and by
name) and provides functions for transcoding to and from that character set.

=back

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

{{DEPRECATION NOTE: the C<char *> pointer C<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.}}

=head4 Conversions between normalization form, encoding, and charset

Conversion will be done with a function called C<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 C<Parrot_str_grapheme_copy>. This function iterates through the
source string one grapheme at a time, using the character set function pointer
C<get_grapheme> (which may read ahead multiple characters with strings that
aren't in NFG). For each source grapheme, the function will call
C<set_grapheme> on the destination string (which may append multiple
characters in non-NFG strings). This conversion effectively uses an
intermediate NFG representation.


=head3 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.

=head4 Parrot_str_set (was string_set)

Set one string to a copy of the value of another string.

=head4 Parrot_str_new_COW (was Parrot_make_COW_reference)

Create a new copy-on-write string. Creating a new string header, clone the
struct members of the original string, and point to the same string buffer as
the original string.

=head4 Parrot_str_reuse_COW (was Parrot_reuse_COW_reference)

Create a new copy-on-write string. Clone the struct members of the original
string into a passed in string header, and point the reused string header to
the same string buffer as the original string.

=head4 Parrot_str_write_COW (was Parrot_unmake_COW)

If the specified Parrot string is copy-on-write, copy the string's contents
to a new string buffer and clear the copy-on-write flag.

=head4 Parrot_str_concat (was string_concat)

Concatenate two strings. Takes three arguments: two strings, and one integer
value of flags. If both string arguments are null, return a new string created
according to the integer flags.

=head4 Parrot_str_append (was string_append)

Append one string to another and return the result. In the default case, the
return value is the same as the first string argument (modifying that argument
in place). If the first argument is COW or read-only, then the return value is
a new string.

=head4 Parrot_str_new (was string_from_cstring)

Return a new string with the default encoding and character set. Accepts two
arguments, a C string (C<char *>) to initialize the value of the string, and
an integer length of the string (number of characters). If the integer length
isn't passed, the function will calculate the length.

{{NOTE: the integer length isn't really necessary, and is under consideration
for deprecation.}}

=head4 Parrot_str_new_noinit (was string_make_empty)

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

=head4 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
five arguments are a C string (C<char *>), an integer length of the string
argument in bytes, and struct pointers for encoding, character set, and
normalization form structs. If the C string (C<char *>) value is not passed,
returns an empty string. If the encoding, character set, or normalization form
are passed as null values, default values are used.

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

=head4 Parrot_str_new_constant (was const_string)

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

=head4 Parrot_str_resize (was string_grow)

Resize the string buffer of the given string adding the number of bytes passed
in the integer argument. If the argument is negative, remove the given number
of bytes. Throws an exception if shrinking the string buffer size will
truncate the string (if C<strlen> will be longer than C<buflen>).

=head4 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.

=head4 Parrot_str_grapheme_length

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

=head4 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
C<strlen> member of the C<STRING> struct directly.

=head4 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.

=head4 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.

=head4 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.

=head4 Parrot_str_copy (was string_copy)

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

=head4 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.

=head4 Parrot_str_repeat (was string_repeat)

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

=head4 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.

=head4 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.

=head4 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.

=head4 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.

=head4 Parrot_str_chopn (was string_chopn)

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

=head4 Parrot_str_chopn_inplace (was string_chopn_inplace).

Chop the requested number of characters off the end of a string, modifying the
original string.

=head4 Parrot_str_grapheme_chopn

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

=head4 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.)

=head4 Parrot_str_grapheme_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 NFG normalization to compare
the two strings.

=head4 Parrot_str_equal

Compare two strings, return 1 if they are equal, 0 if they are not equal.

=head4 Parrot_str_not_equal (was string_equal)

Compare two strings, return 0 if they are equal, 1 if they are not equal.

{{DEPRECATION NOTE: The return value of 'Parrot_str_equal' is reversed from
the old logic, but 'Parrot_str_not_equal' is provided as a drop-in
replacement for the old function.}}

=head4 Parrot_str_grapheme_equal

Compare two strings using NFG normalization, return 1 if they are equal, 0 if
they are not equal.

=head3 Internal String Functions

The following functions are used internally and are not part of the public
interface.

=head4 Parrot_str_init (was string_init)

Initialize Parrot's string subsystem, including string allocation and garbage
collection.

=head4 Parrot_str_finish (was string_deinit)

Terminate and clean up Parrot's string subsystem, including string allocation
and garbage collection.

=head4 string_max_bytes

Calculate the number of bytes needed to hold a given number of characters in a
particular encoding, multiplying the maximum possible width of a character in
the encoding by the number of characters requested.

{{NOTE: pretty primitive and not very useful. May be deprecated.}}

=head3 Deprecated String Functions

The following string functions are slated to be deprecated.

=head4 string_primary_encoding_for_representation

Not useful, it only ever returned ASCII.

=head4 string_rep_compatible

Only useful on a very narrow set of string encodings/character sets.

=head4 string_make

A crippled version of a string initializer, now replaced with the full version
C<Parrot_string_new_init>.

=head4 string_capacity

This was used to calculate the size of the buffer after the C<strstart>
pointer. Deprecated with C<strstart>.

=head4 string_ord

Replaced by C<Parrot_str_indexed>.

=head4 string_chr

This is handled just fine by C<Parrot_str_new>, we don't need a special
version for a single character.

=head4 make_writable

An archaic function that uses a method of describing strings that hasn't been
allowed for years.

=head4 string_to_cstring_nullable

Just the implementation of string_to_cstring, no need for a separate function
that specially allows returning a NULL string.

=head4 string_increment

Old Perl 5-style behavior where "aa" goes to "bb". Only useful for ASCII
strings, and not terribly useful even there.

=head4 Parrot_string_cstring

Unsafe, and behavior handled by Parrot_str_to_cstring.


=head4 Parrot_str_split

Splits the string C<str> at the delimiter C<delim>.

=head4 Parrot_str_free (was string_free)

Unsafe and unuseful, let the garbage collector take care.

=head3 String PMC API

The String PMC provides a high-level object interface to the string
functionality. It contains a standard Parrot string, holding the string data.

=head4 Vtable Functions

The String PMC implements the following vtable functions.

=over 4

=item init

Initialize a new String PMC.

=item clone

Clone a String PMC.

=item mark

Mark the string value of the String PMC as live.


=item get_integer

Return the integer representation of the string.

=item get_number

Return the floating-point representation of the string.

=item get_string

Return the string value of the String PMC.

=item get_bool

Return the boolean value of the string.

=item set_integer_native

Set the string to an integer value, transforming the integer to its string
equivalent.

=item set_bool

Set the string to a boolean (integer) value, transforming the boolean to its
string equivalent.

=item set_number_native

Set the string to a floating-point value by transforming the number to its
string equivalent.

=item set_string_native

Set the String PMC's stored string value to be the string argument. If the
passed in string is a constant, store a copy.

=item assign_string_native

Set the String PMC's stored string value to a copy of the string argument.

=item set_string_same

Set the String PMC's stored string value to the same as another String PMC's
stored string value. {{NOTE: uses direct access into the storage of the two
PMCs, very ugly.}}

=item set_pmc

Set the String PMC's stored string value to the same as another PMC's string
value, as returned by that PMC's C<get_string> vtable function.

=item *bitwise*

All the bitwise string vtable functions, for AND, OR, XOR, and NOT, both
inplace and standard return.

=item is_equal

Compares the string values of two PMCs and returns true if they match exactly.

=item is_equal_num

Compares the numeric values of two PMCs (first transforming any strings to
numbers) and returns true if they match exactly.

=item is_equal_string

Compares the string values of two PMCs and returns true if they match exactly.
{{ NOTE: the documentation for the PMC says that it returns FALSE if they
match.  This is not the desired behavior. }}

=item is_same

Compares two PMCs and returns true if they are the same PMC class and contain
the same string (not an equivalent string value, but aliases to the same
low-level string).

=item cmp

Compares two PMCs and returns 1 if SELF is shorter, 0 if they are equal length
strings, and -1 if the passed in string argument is shorter.

=item cmp_num

Compares the numeric values of two PMCs (first changing those values to
numbers) and returns 1 if SELF is smaller, 0 if they are equal, and -1 if the
passed in string argument is smaller.

=item cmp_string

Compares two PMCs and returns 1 if SELF is shorter, 0 if they are equal length
strings, and -1 if the passed in string argument is shorter.

=item substr

Extract a substring of a given length starting from a given offset (in
graphemes) and store the result in the string argument.

=item substr_str

Extract a substring of a given length starting from a given offset (in
graphemes) and return the string.

=item exists_keyed

Return true if the Nth grapheme in the string exists. Negative numbers count
from the end.

=item get_string_keyed

Return the Nth grapheme in the string. Negative numbers count from the end.

=item set_string_keyed

Insert a string at the Nth grapheme position in the string. {{NOTE: this is
different than the current implementation.}}

=item get_integer_keyed

Returns the integer value of the Nth C<char> in the string. {{DEPRECATE}}

=item set_integer_keyed

Replace the C<char> at the Nth character position in the string with the
C<char> that corresponds to the passed integer value key. {{DEPRECATE}}

=back

=head4 Methods

The String PMC provides the following methods.

=over 4

=item replace

Replace every occurrence of one string with another.

=item to_int

Return the integer equivalent of a string.

=item lower

Change the string to all lowercase.

=item trans

Translate an ASCII string with entries from a translation table.

{{NOTE: likely to be deprecated.}}

=item reverse

Reverse a string, one grapheme at a time. {{ NOTE: Currently only works for
ASCII strings, because it reverses one C<char> at a time. }}


=item is_integer

Checks if the string is just an integer. {{ NOTE: Currently only works for
ASCII strings, fix or deprecate. }}

=back


=head2 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"

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