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# std.uni

The std.`uni` module provides an implementation of fundamental Unicode algorithms and data structures. This doesn't include UTF encoding and decoding primitives, see std.utf.decode and std.utf.encode in std.utf for this functionality.

All primitives listed operate on Unicode characters and sets of characters. For functions which operate on ASCII characters and ignore Unicode characters, see std.ascii. For definitions of Unicode character, code point and other terms used throughout this module see the terminology section below.

The focus of this module is the core needs of developing Unicode-aware applications. To that effect it provides the following optimized primitives:

- Character classification by category and common properties: isAlpha, isWhite and others.
- Case-insensitive string comparison (sicmp, icmp).
- Converting text to any of the four normalization forms via normalize.
- Decoding (decodeGrapheme) and iteration (byGrapheme, graphemeStride) by user-perceived characters, that is by Grapheme clusters.
- Decomposing and composing of individual character(s) according to canonical or compatibility rules, see compose and decompose, including the specific version for Hangul syllables composeJamo and decomposeHangul.

It's recognized that an application may need further enhancements and extensions, such as less commonly known algorithms, or tailoring existing ones for region specific needs. To help users with building any extra functionality beyond the core primitives, the module provides:

- CodepointSet, a type for easy manipulation of sets of characters. Besides the typical set algebra it provides an unusual feature: a D source code generator for detection of code points in this set. This is a boon for meta-programming parser frameworks, and is used internally to power classification in small sets like isWhite.
- A way to construct optimal packed multi-stage tables also known as a special case of Trie. The functions codepointTrie, codepointSetTrie construct custom tries that map dchar to value. The end result is a fast and predictable Ο(1) lookup that powers functions like isAlpha and combiningClass, but for user-defined data sets.
- A useful technique for Unicode-aware parsers that perform character classification of encoded code points is to avoid unnecassary decoding at all costs. utfMatcher provides an improvement over the usual workflow of decode-classify-process, combining the decoding and classification steps. By extracting necessary bits directly from encoded code units matchers achieve significant performance improvements. See MatcherConcept for the common interface of UTF matchers.
- Generally useful building blocks for customized normalization: combiningClass for querying combining class and allowedIn for testing the Quick_Check property of a given normalization form.
- Access to a large selection of commonly used sets of code points. Supported sets include Script, Block and General Category. The exact contents of a set can be observed in the CLDR utility, on the property index page of the Unicode website. See unicode for easy and (optionally) compile-time checked set queries.

### Synopsis

    import std.uni;
    void main()
    {
        // initialize code point sets using script/block or property name
        // now 'set' contains code points from both scripts.
        auto set = unicode("Cyrillic") | unicode("Armenian");
        // same thing but simpler and checked at compile-time
        auto ascii = unicode.ASCII;
        auto currency = unicode.Currency_Symbol;

        // easy set ops
        auto a = set & ascii;
        assert(a.empty); // as it has no intersection with ascii
        a = set | ascii;
        auto b = currency - a; // subtract all ASCII, Cyrillic and Armenian

        // some properties of code point sets
        assert(b.length > 45); // 46 items in Unicode 6.1, even more in 6.2
        // testing presence of a code point in a set
        // is just fine, it is O(logN)
        assert(!b['$']);
        assert(!b['\u058F']); // Armenian dram sign
        assert(b['¥']);

        // building fast lookup tables, these guarantee O(1) complexity
        // 1-level Trie lookup table essentially a huge bit-set ~262Kb
        auto oneTrie = toTrie!1(b);
        // 2-level far more compact but typically slightly slower
        auto twoTrie = toTrie!2(b);
        // 3-level even smaller, and a bit slower yet
        auto threeTrie = toTrie!3(b);
        assert(oneTrie['£']);
        assert(twoTrie['£']);
        assert(threeTrie['£']);

        // build the trie with the most sensible trie level
        // and bind it as a functor
        auto cyrillicOrArmenian = toDelegate(set);
        auto balance = find!(cyrillicOrArmenian)("Hello ընկեր!");
        assert(balance == "ընկեր!");
        // compatible with bool delegate(dchar)
        bool delegate(dchar) bindIt = cyrillicOrArmenian;

        // Normalization
        string s = "Plain ascii (and not only), is always normalized!";
        assert(s is normalize(s));// is the same string

        string nonS = "A\u0308ffin"; // A ligature
        auto nS = normalize(nonS); // to NFC, the W3C endorsed standard
        assert(nS == "Äffin");
        assert(nS != nonS);
        string composed = "Äffin";

        assert(normalize!NFD(composed) == "A\u0308ffin");
        // to NFKD, compatibility decomposition useful for fuzzy matching/searching
        assert(normalize!NFKD("2¹⁰") == "210");
    }

### Terminology

The following is a list of important Unicode notions and definitions. Any conventions used specifically in this module alone are marked as such. The descriptions are based on the formal definition as found in chapter three of The Unicode Standard Core Specification.

Abstract character
A unit of information used for the organization, control, or representation of textual data. Note that:

- When representing data, the nature of that data is generally symbolic as opposed to some other kind of data (for example, visual).
- An abstract character has no concrete form and should not be confused with a glyph.
- An abstract character does not necessarily correspond to what a user thinks of as a “character” and should not be confused with a Grapheme.
- The abstract characters encoded (see Encoded character) are known as Unicode abstract characters.
- Abstract characters not directly encoded by the Unicode Standard can often be represented by the use of combining character sequences.

Canonical decomposition
The decomposition of a character or character sequence that results from recursively applying the canonical mappings found in the Unicode Character Database and these described in Conjoining Jamo Behavior (section 12 of Unicode Conformance).

Canonical composition
The precise definition of the Canonical composition is the algorithm as specified in Unicode Conformance section 11. Informally it's the process that does the reverse of the canonical decomposition with the addition of certain rules that e.g. prevent legacy characters from appearing in the composed result.

Canonical equivalent
Two character sequences are said to be canonical equivalents if their full canonical decompositions are identical.

Character
Typically differs by context. For the purpose of this documentation the term character implies encoded character, that is, a code point having an assigned abstract character (a symbolic meaning).

Code point
Any value in the Unicode codespace; that is, the range of integers from 0 to 10FFFF (hex). Not all code points are assigned to encoded characters.

Code unit
The minimal bit combination that can represent a unit of encoded text for processing or interchange. Depending on the encoding this could be: 8-bit code units in the UTF-8 (char), 16-bit code units in the UTF-16 (wchar), and 32-bit code units in the UTF-32 (dchar). Note that in UTF-32, a code unit is a code point and is represented by the D dchar type.

Combining character
A character with the General Category of Combining Mark(M).

- All characters with non-zero canonical combining class are combining characters, but the reverse is not the case: there are combining characters with a zero combining class.
- These characters are not normally used in isolation unless they are being described. They include such characters as accents, diacritics, Hebrew points, Arabic vowel signs, and Indic matras.

Combining class
A numerical value used by the Unicode Canonical Ordering Algorithm to determine which sequences of combining marks are to be considered canonically equivalent and which are not.

Compatibility decomposition
The decomposition of a character or character sequence that results from recursively applying both the compatibility mappings and the canonical mappings found in the Unicode Character Database, and those described in Conjoining Jamo Behavior no characters can be further decomposed.

Compatibility equivalent
Two character sequences are said to be compatibility equivalents if their full compatibility decompositions are identical.

Encoded character
An association (or mapping) between an abstract character and a code point.

Glyph
The actual, concrete image of a glyph representation having been rasterized or otherwise imaged onto some display surface.

Grapheme base
A character with the property Grapheme_Base, or any standard Korean syllable block.

Grapheme cluster
Defined as the text between grapheme boundaries as specified by Unicode Standard Annex #29, Unicode text segmentation. Important general properties of a grapheme:

- The grapheme cluster represents a horizontally segmentable unit of text, consisting of some grapheme base (which may consist of a Korean syllable) together with any number of nonspacing marks applied to it.
- A grapheme cluster typically starts with a grapheme base and then extends across any subsequent sequence of nonspacing marks. A grapheme cluster is most directly relevant to text rendering and processes such as cursor placement and text selection in editing, but may also be relevant to comparison and searching.
- For many processes, a grapheme cluster behaves as if it was a single character with the same properties as its grapheme base. Effectively, nonspacing marks apply graphically to the base, but do not change its properties.

This module defines a number of primitives that work with graphemes: Grapheme, decodeGrapheme and graphemeStride. All of them are using extended grapheme boundaries as defined in the aforementioned standard annex.

Nonspacing mark
A combining character with the General Category of Nonspacing Mark (Mn) or Enclosing Mark (Me).

Spacing mark
A combining character that is not a nonspacing mark.

### Normalization

The concepts of canonical equivalent or compatibility equivalent characters in the Unicode Standard make it necessary to have a full, formal definition of equivalence for Unicode strings. String equivalence is determined by a process called normalization, whereby strings are converted into forms which are compared directly for identity. This is the primary goal of the normalization process, see the function normalize to convert into any of the four defined forms.

A very important attribute of the Unicode Normalization Forms is that they must remain stable between versions of the Unicode Standard. A Unicode string normalized to a particular Unicode Normalization Form in one version of the standard is guaranteed to remain in that Normalization Form for implementations of future versions of the standard.

The Unicode Standard specifies four normalization forms. Informally, two of these forms are defined by maximal decomposition of equivalent sequences, and two of these forms are defined by maximal composition of equivalent sequences.

- Normalization Form D (NFD): The canonical decomposition of a character sequence.
- Normalization Form KD (NFKD): The compatibility decomposition of a character sequence.
- Normalization Form C (NFC): The canonical composition of the canonical decomposition of a coded character sequence.
- Normalization Form KC (NFKC): The canonical composition of the compatibility decomposition of a character sequence

The choice of the normalization form depends on the particular use case. NFC is the best form for general text, since it's more compatible with strings converted from legacy encodings. NFKC is the preferred form for identifiers, especially where there are security concerns. NFD and NFKD are the most useful for internal processing.

### Construction of lookup tables

The Unicode standard describes a set of algorithms that depend on having the ability to quickly look up various properties of a code point. Given the the codespace of about 1 million code points, it is not a trivial task to provide a space-efficient solution for the multitude of properties.

Common approaches such as hash-tables or binary search over sorted code point intervals (as in InversionList) are insufficient. Hash-tables have enormous memory footprint and binary search over intervals is not fast enough for some heavy-duty algorithms.

The recommended solution (see Unicode Implementation Guidelines) is using multi-stage tables that are an implementation of the Trie data structure with integer keys and a fixed number of stages. For the remainder of the section this will be called a fixed trie. The following describes a particular implementation that is aimed for the speed of access at the expense of ideal size savings.

Taking a 2-level Trie as an example the principle of operation is as follows. Split the number of bits in a key (code point, 21 bits) into 2 components (e.g. 15 and 8). The first is the number of bits in the index of the trie and the other is number of bits in each page of the trie. The layout of the trie is then an array of size 2^^bits-of-index followed an array of memory chunks of size 2^^bits-of-page/bits-per-element.

The number of pages is variable (but not less then 1) unlike the number of entries in the index. The slots of the index all have to contain a number of a page that is present. The lookup is then just a couple of operations - slice the upper bits, lookup an index for these, take a page at this index and use the lower bits as an offset within this page.

Assuming that pages are laid out consequently in one array at pages, the pseudo-code is:

    auto elemsPerPage = (2 ^^ bits_per_page) / Value.sizeOfInBits;
    pages[index[n >> bits_per_page]][n & (elemsPerPage - 1)];

Where if elemsPerPage is a power of 2 the whole process is a handful of simple instructions and 2 array reads. Subsequent levels of the trie are introduced by recursing on this notion - the index array is treated as values. The number of bits in index is then again split into 2 parts, with pages over 'current-index' and the new 'upper-index'.

For completeness a level 1 trie is simply an array. The current implementation takes advantage of bit-packing values when the range is known to be limited in advance (such as bool). See also BitPacked for enforcing it manually. The major size advantage however comes from the fact that multiple identical pages on every level are merged by construction.

The process of constructing a trie is more involved and is hidden from the user in a form of the convenience functions codepointTrie, codepointSetTrie and the even more convenient toTrie. In general a set or built-in AA with dchar type can be turned into a trie. The trie object in this module is read-only (immutable); it's effectively frozen after construction.

### Unicode properties

This is a full list of Unicode properties accessible through unicode with specific helpers per category nested within. Consult the CLDR utility when in doubt about the contents of a particular set.

General category sets listed below are only accessible with the unicode shorthand accessor.

General category
Abb. | Long form | Abb. | Long form | Abb. | Long form
--- | --- | --- | --- | --- | ---
L | Letter | Cn | Unassigned | Po | Other_Punctuation
Ll | Lowercase_Letter | Co | Private_Use | Ps | Open_Punctuation
Lm | Modifier_Letter | Cs | Surrogate | S | Symbol
Lo | Other_Letter | N | Number | Sc | Currency_Symbol
Lt | Titlecase_Letter | Nd | Decimal_Number | Sk | Modifier_Symbol
Lu | Uppercase_Letter | Nl | Letter_Number | Sm | Math_Symbol
M | Mark | No | Other_Number | So | Other_Symbol
Mc | Spacing_Mark | P | Punctuation | Z | Separator
Me | Enclosing_Mark | Pc | Connector_Punctuation | Zl | Line_Separator
Mn | Nonspacing_Mark | Pd | Dash_Punctuation | Zp | Paragraph_Separator
C | Other | Pe | Close_Punctuation | Zs | Space_Separator
Cc | Control | Pf | Final_Punctuation | - | Any
Cf | Format | Pi | Initial_Punctuation | - | ASCII

Sets for other commonly useful properties that are accessible with unicode:

Common binary properties
Name | Name | Name
--- | --- | ---
Alphabetic | Ideographic | Other_Uppercase
ASCII_Hex_Digit | IDS_Binary_Operator | Pattern_Syntax
Bidi_Control | ID_Start | Pattern_White_Space
Cased | IDS_Trinary_Operator | Quotation_Mark
Case_Ignorable | Join_Control | Radical
Dash | Logical_Order_Exception | Soft_Dotted
Default_Ignorable_Code_Point | Lowercase | STerm
Deprecated | Math | Terminal_Punctuation
Diacritic | Noncharacter_Code_Point | Unified_Ideograph
Extender | Other_Alphabetic | Uppercase
Grapheme_Base | Other_Default_Ignorable_Code_Point | Variation_Selector
Grapheme_Extend | Other_Grapheme_Extend | White_Space
Grapheme_Link | Other_ID_Continue | XID_Continue
Hex_Digit | Other_ID_Start | XID_Start
Hyphen | Other_Lowercase
ID_Continue | Other_Math

Below is the table with block names accepted by unicode.block. Note that the shorthand version unicode requires "In" to be prepended to the names of blocks so as to disambiguate scripts and blocks.

Blocks
Aegean Numbers | Ethiopic Extended | Mongolian
Alchemical Symbols | Ethiopic Extended-A | Musical Symbols
Alphabetic Presentation Forms | Ethiopic Supplement | Myanmar
Ancient Greek Musical Notation | General Punctuation | Myanmar Extended-A
Ancient Greek Numbers | Geometric Shapes | New Tai Lue
Ancient Symbols | Georgian | NKo
Arabic | Georgian Supplement | Number Forms
Arabic Extended-A | Glagolitic | Ogham
Arabic Mathematical Alphabetic Symbols | Gothic | Ol Chiki
Arabic Presentation Forms-A | Greek and Coptic | Old Italic
Arabic Presentation Forms-B | Greek Extended | Old Persian
Arabic Supplement | Gujarati | Old South Arabian
Armenian | Gurmukhi | Old Turkic
Arrows | Halfwidth and Fullwidth Forms | Optical Character Recognition
Avestan | Hangul Compatibility Jamo | Oriya
Balinese | Hangul Jamo | Osmanya
Bamum | Hangul Jamo Extended-A | Phags-pa
Bamum Supplement | Hangul Jamo Extended-B | Phaistos Disc
Basic Latin | Hangul Syllables | Phoenician
Batak | Hanunoo | Phonetic Extensions
Bengali | Hebrew | Phonetic Extensions Supplement
Block Elements | High Private Use Surrogates | Playing Cards
Bopomofo | High Surrogates | Private Use Area
Bopomofo Extended | Hiragana | Rejang
Box Drawing | Ideographic Description Characters | Rumi Numeral Symbols
Brahmi | Imperial Aramaic | Runic
Braille Patterns | Inscriptional Pahlavi | Samaritan
Buginese | Inscriptional Parthian | Saurashtra
Buhid | IPA Extensions | Sharada
Byzantine Musical Symbols | Javanese | Shavian
Carian | Kaithi | Sinhala
Chakma | Kana Supplement | Small Form Variants
Cham | Kanbun | Sora Sompeng
Cherokee | Kangxi Radicals | Spacing Modifier Letters
CJK Compatibility | Kannada | Specials
CJK Compatibility Forms | Katakana | Sundanese
CJK Compatibility Ideographs | Katakana Phonetic Extensions | Sundanese Supplement
CJK Compatibility Ideographs Supplement | Kayah Li | Superscripts and Subscripts
CJK Radicals Supplement | Kharoshthi | Supplemental Arrows-A
CJK Strokes | Khmer | Supplemental Arrows-B
CJK Symbols and Punctuation | Khmer Symbols | Supplemental Mathematical Operators
CJK Unified Ideographs | Lao | Supplemental Punctuation
CJK Unified Ideographs Extension A | Latin-1 Supplement | Supplementary Private Use Area-A
CJK Unified Ideographs Extension B | Latin Extended-A | Supplementary Private Use Area-B
CJK Unified Ideographs Extension C | Latin Extended Additional | Syloti Nagri
CJK Unified Ideographs Extension D | Latin Extended-B | Syriac
Combining Diacritical Marks | Latin Extended-C | Tagalog
Combining Diacritical Marks for Symbols | Latin Extended-D | Tagbanwa
Combining Diacritical Marks Supplement | Lepcha | Tags
Combining Half Marks | Letterlike Symbols | Tai Le
Common Indic Number Forms | Limbu | Tai Tham
Control Pictures | Linear B Ideograms | Tai Viet
Coptic | Linear B Syllabary | Tai Xuan Jing Symbols
Counting Rod Numerals | Lisu | Takri
Cuneiform | Low Surrogates | Tamil
Cuneiform Numbers and Punctuation | Lycian | Telugu
Currency Symbols | Lydian | Thaana
Cypriot Syllabary | Mahjong Tiles | Thai
Cyrillic | Malayalam | Tibetan
Cyrillic Extended-A | Mandaic | Tifinagh
Cyrillic Extended-B | Mathematical Alphanumeric Symbols | Transport And Map Symbols
Cyrillic Supplement | Mathematical Operators | Ugaritic
Deseret | Meetei Mayek | Unified Canadian Aboriginal Syllabics
Devanagari | Meetei Mayek Extensions | Unified Canadian Aboriginal Syllabics Extended
Devanagari Extended | Meroitic Cursive | Vai
Dingbats | Meroitic Hieroglyphs | Variation Selectors
Domino Tiles | Miao | Variation Selectors Supplement
Egyptian Hieroglyphs | Miscellaneous Mathematical Symbols-A | Vedic Extensions
Emoticons | Miscellaneous Mathematical Symbols-B | Vertical Forms
Enclosed Alphanumerics | Miscellaneous Symbols | Yijing Hexagram Symbols
Enclosed Alphanumeric Supplement | Miscellaneous Symbols and Arrows | Yi Radicals
Enclosed CJK Letters and Months | Miscellaneous Symbols And Pictographs | Yi Syllables
Enclosed Ideographic Supplement | Miscellaneous Technical
Ethiopic | Modifier Tone Letters

Below is the table with script names accepted by unicode.script and by the shorthand version unicode:

Scripts
Arabic | Hanunoo | Old_Italic
Armenian | Hebrew | Old_Persian
Avestan | Hiragana | Old_South_Arabian
Balinese | Imperial_Aramaic | Old_Turkic
Bamum | Inherited | Oriya
Batak | Inscriptional_Pahlavi | Osmanya
Bengali | Inscriptional_Parthian | Phags_Pa
Bopomofo | Javanese | Phoenician
Brahmi | Kaithi | Rejang
Braille | Kannada | Runic
Buginese | Katakana | Samaritan
Buhid | Kayah_Li | Saurashtra
Canadian_Aboriginal | Kharoshthi | Sharada
Carian | Khmer | Shavian
Chakma | Lao | Sinhala
Cham | Latin | Sora_Sompeng
Cherokee | Lepcha | Sundanese
Common | Limbu | Syloti_Nagri
Coptic | Linear_B | Syriac
Cuneiform | Lisu | Tagalog
Cypriot | Lycian | Tagbanwa
Cyrillic | Lydian | Tai_Le
Deseret | Malayalam | Tai_Tham
Devanagari | Mandaic | Tai_Viet
Egyptian_Hieroglyphs | Meetei_Mayek | Takri
Ethiopic | Meroitic_Cursive | Tamil
Georgian | Meroitic_Hieroglyphs | Telugu
Glagolitic | Miao | Thaana
Gothic | Mongolian | Thai
Greek | Myanmar | Tibetan
Gujarati | New_Tai_Lue | Tifinagh
Gurmukhi | Nko | Ugaritic
Han | Ogham | Vai
Hangul | Ol_Chiki | Yi

Below is the table of names accepted by unicode.hangulSyllableType.

Hangul syllable type
Abb. | Long form
--- | ---
L | Leading_Jamo
LV | LV_Syllable
LVT | LVT_Syllable
T | Trailing_Jamo
V | Vowel_Jamo

References: ASCII Table, Wikipedia, The Unicode Consortium, Unicode normalization forms, Unicode text segmentation Unicode Implementation Guidelines Unicode Conformance

Trademarks: Unicode(tm) is a trademark of Unicode, Inc.

License:
Boost License 1.0.
Authors:
Dmitry Olshansky

Source: std/uni.d

Standards:
Unicode v6.2

enum dchar `lineSep`;
  Constant code point (0x2028) - line separator.
enum dchar `paraSep`;
  Constant code point (0x2029) - paragraph separator.
enum dchar `nelSep`;
  Constant code point (0x0085) - next line.
template `isCodepointSet`(T)
  Tests if T is some kind a set of code points. Intended for template constraints.
enum auto `isIntegralPair`(T, V = uint);
  Tests if T is a pair of integers that implicitly convert to V. The following code must compile for any pair T:

      (T x){ V a = x[0]; V b = x[1];}

  The following must not compile:

      (T x){ V c = x[2];}

alias `CodepointSet` = InversionList!(GcPolicy).InversionList;
  The recommended default type for set of code points. For details, see the current implementation: InversionList.
struct `CodepointInterval`;
  The recommended type of std.typecons.Tuple to represent [a, b) intervals of code points. As used in InversionList. Any interval type should pass isIntegralPair trait.
struct `InversionList`(SP = GcPolicy);

  `InversionList` is a set of code points represented as an array of open-right [a, b) intervals (see CodepointInterval above). The name comes from the way the representation reads left to right. For instance a set of all values [10, 50), [80, 90), plus a singular value 60 looks like this:

      10, 50, 60, 61, 80, 90

  The way to read this is: start with negative meaning that all numbers smaller then the next one are not present in this set (and positive - the contrary). Then switch positive/negative after each number passed from left to right.

  This way negative spans until 10, then positive until 50, then negative until 60, then positive until 61, and so on. As seen this provides a space-efficient storage of highly redundant data that comes in long runs. A description which Unicode character properties fit nicely. The technique itself could be seen as a variation on RLE encoding.

  Sets are value types (just like int is) thus they are never aliased.

  Example:

      auto a = CodepointSet('a', 'z'+1);
      auto b = CodepointSet('A', 'Z'+1);
      auto c = a;
      a = a | b;
      assert(a == CodepointSet('A', 'Z'+1, 'a', 'z'+1));
      assert(a != c);

  See also unicode for simpler construction of sets from predefined ones.

  Memory usage is 8 bytes per each contiguous interval in a set. The value semantics are achieved by using the COW technique and thus it's not safe to cast this type to shared.

  Note:

  It's not recommended to rely on the template parameters or the exact type of a current code point set in std.uni. The type and parameters may change when the standard allocators design is finalized. Use isCodepointSet with templates or just stick with the default alias CodepointSet throughout the whole code base.

  pure this(Set)(Set `set`)
  if (isCodepointSet!Set);
    Construct from another code point `set` of any type.
  pure this(Range)(Range `intervals`)
  if (isForwardRange!Range && isIntegralPair!(ElementType!Range));
    Construct a set from a forward range of code point `intervals`.
  this()(uint[] `intervals`...);
    Construct a set from plain values of code point `intervals`.
    Examples:

        import std.algorithm.comparison : equal;

        auto set = CodepointSet('a', 'z'+1, 'а', 'я'+1);
        foreach (v; 'a'..'z'+1)
            assert(set[v]);
        // Cyrillic lowercase interval
        foreach (v; 'а'..'я'+1)
            assert(set[v]);
        //specific order is not required, intervals may interesect
        auto set2 = CodepointSet('а', 'я'+1, 'a', 'd', 'b', 'z'+1);
        //the same end result
        assert(set2.byInterval.equal(set.byInterval));

  @property auto `byInterval`();
    Get range that spans all of the code point intervals in this InversionList.

    Example:

        import std.algorithm.comparison : equal;
        import std.typecons : tuple;

        auto set = CodepointSet('A', 'D'+1, 'a', 'd'+1);

        assert(set.byInterval.equal([tuple('A','E'), tuple('a','e')]));

  const bool `opIndex`(uint `val`);
    Tests the presence of code point `val` in this set.
    Examples:

        auto gothic = unicode.Gothic;
        // Gothic letter ahsa
        assert(gothic['\U00010330']);
        // no ascii in Gothic obviously
        assert(!gothic['$']);

  @property size_t `length`();
    Number of code points in this set
  This `opBinary`(string op, U)(U `rhs`)
  if (isCodepointSet!U || is(U : dchar));

    Sets support natural syntax for set algebra, namely:

    Operator | Math notation | Description
    --- | --- | ---
    & | a ∩ b | intersection
    | | a ∪ b | union
    - | a ∖ b | subtraction
    ~ | a ~ b | symmetric set difference i.e. (a ∪ b) \ (a ∩ b)

    Examples:

        import std.algorithm.comparison : equal;
        import std.range : iota;

        auto lower = unicode.LowerCase;
        auto upper = unicode.UpperCase;
        auto ascii = unicode.ASCII;

        assert((lower & upper).empty); // no intersection
        auto lowerASCII = lower & ascii;
        assert(lowerASCII.byCodepoint.equal(iota('a', 'z'+1)));
        // throw away all of the lowercase ASCII
        assert((ascii - lower).length == 128 - 26);

        auto onlyOneOf = lower ~ ascii;
        assert(!onlyOneOf['Δ']); // not ASCII and not lowercase
        assert(onlyOneOf['$']); // ASCII and not lowercase
        assert(!onlyOneOf['a']); // ASCII and lowercase
        assert(onlyOneOf['я']); // not ASCII but lowercase

        // throw away all cased letters from ASCII
        auto noLetters = ascii - (lower | upper);
        assert(noLetters.length == 128 - 26*2);

  ref This `opOpAssign`(string op, U)(U `rhs`)
  if (isCodepointSet!U || is(U : dchar));
    The 'op=' versions of the above overloaded operators.
  const bool `opBinaryRight`(string op : "in", U)(U `ch`)
  if (is(U : dchar));
    Tests the presence of codepoint `ch` in this set, the same as opIndex.
    Examples:

        assert('я' in unicode.Cyrillic);
        assert(!('z' in unicode.Cyrillic));

  auto `opUnary`(string op : "!")();
    Obtains a set that is the inversion of this set.
    See Also:
    inverted
  @property auto `byCodepoint`();
    A range that spans each code point in this set.
    Examples:

        import std.algorithm.comparison : equal;
        import std.range : iota;

        auto set = unicode.ASCII;
        set.byCodepoint.equal(iota(0, 0x80));

  void `toString`(Writer)(scope Writer `sink`, FormatSpec!char `fmt`);
    Obtain a textual representation of this InversionList in form of open-right intervals.
    The formatting flag is applied individually to each value, for example:
    - %s and %d format the intervals as a [low .. high) range of integrals
    - %x formats the intervals as a [low .. high) range of lowercase hex characters
    - %X formats the intervals as a [low .. high) range of uppercase hex characters
    Examples:

        import std.conv : to;
        import std.format : format;
        import std.uni : unicode;

        assert(unicode.Cyrillic.to!string ==
            "[1024..1157) [1159..1320) [7467..7468) [7544..7545) [11744..11776) [42560..42648) [42655..42656)");

        // The specs '%s' and '%d' are equivalent to the to!string call above.
        assert(format("%d", unicode.Cyrillic) == unicode.Cyrillic.to!string);

        assert(format("%#x", unicode.Cyrillic) ==
            "[0x400..0x485) [0x487..0x528) [0x1d2b..0x1d2c) [0x1d78..0x1d79) [0x2de0..0x2e00) "
            ~"[0xa640..0xa698) [0xa69f..0xa6a0)");

        assert(format("%#X", unicode.Cyrillic) ==
            "[0X400..0X485) [0X487..0X528) [0X1D2B..0X1D2C) [0X1D78..0X1D79) [0X2DE0..0X2E00) "
            ~"[0XA640..0XA698) [0XA69F..0XA6A0)");

  ref auto `add`()(uint `a`, uint `b`);
    Add an interval [`a`, `b`) to this set.
    Examples:

        CodepointSet someSet;
        someSet.add('0', '5').add('A','Z'+1);
        someSet.add('5', '9'+1);
        assert(someSet['0']);
        assert(someSet['5']);
        assert(someSet['9']);
        assert(someSet['Z']);

  @property auto `inverted`();
    Obtains a set that is the inversion of this set.
    See the '!' opUnary for the same but using operators.
    Examples:

        auto set = unicode.ASCII;
        // union with the inverse gets all of the code points in the Unicode
        assert((set | set.inverted).length == 0x110000);
        // no intersection with the inverse
        assert((set & set.inverted).empty);

  string `toSourceCode`(string `funcName` = "");
    Generates string with D source code of unary function with name of `funcName` taking a single dchar argument. If `funcName` is empty the code is adjusted to be a lambda function.
    The function generated tests if the code point passed belongs to this set or not. The result is to be used with string mixin. The intended usage area is aggressive optimization via meta programming in parser generators and the like.

    Note: Use with care for relatively small or regular sets. It could end up being slower then just using multi-staged tables.

    Example:

        import std.stdio;

        // construct set directly from [a, b$RPAREN intervals
        auto set = CodepointSet(10, 12, 45, 65, 100, 200);
        writeln(set);
        writeln(set.toSourceCode("func"));

    The above outputs something along the lines of:

        bool func(dchar ch)  @safe pure nothrow @nogc
        {
            if (ch < 45)
            {
                if (ch == 10 || ch == 11) return true;
                return false;
            }
            else if (ch < 65) return true;
            else
            {
                if (ch < 100) return false;
                if (ch < 200) return true;
                return false;
            }
        }

  const @property bool `empty`();
    True if this set doesn't contain any code points.
    Examples:

        CodepointSet emptySet;
        assert(emptySet.length == 0);
        assert(emptySet.empty);

template `codepointSetTrie`(sizes...) if (sumOfIntegerTuple!sizes == 21)
  A shorthand for creating a custom multi-level fixed Trie from a CodepointSet. sizes are numbers of bits per level, with the most significant bits used first.

  Note: The sum of sizes must be equal 21.

  See Also:
  toTrie, which is even simpler.

  Example:

      {
          import std.stdio;
          auto set = unicode("Number");
          auto trie = codepointSetTrie!(8, 5, 8)(set);
          writeln("Input code points to test:");
          foreach (line; stdin.byLine)
          {
              int count=0;
              foreach (dchar ch; line)
                  if (trie[ch])// is number
                      count++;
              writefln("Contains %d number code points.", count);
          }
      }

template `CodepointSetTrie`(sizes...) if (sumOfIntegerTuple!sizes == 21)
  Type of Trie generated by codepointSetTrie function.
template `codepointTrie`(T, sizes...) if (sumOfIntegerTuple!sizes == 21)
  A slightly more general tool for building fixed Trie for the Unicode data.
  Specifically unlike codepointSetTrie it's allows creating mappings of dchar to an arbitrary type T.

  Note: Overload taking CodepointSets will naturally convert only to bool mapping Tries.

template `CodepointTrie`(T, sizes...) if (sumOfIntegerTuple!sizes == 21)
  Type of Trie as generated by codepointTrie function.
struct `MatcherConcept`;
  Conceptual type that outlines the common properties of all UTF Matchers.

  Note: For illustration purposes only, every method call results in assertion failure. Use utfMatcher to obtain a concrete matcher for UTF-8 or UTF-16 encodings.

  bool `match`(Range)(ref Range `inp`)
  if (isRandomAccessRange!Range && is(ElementType!Range : char));
  bool `skip`(Range)(ref Range `inp`)
  if (isRandomAccessRange!Range && is(ElementType!Range : char));
  bool `test`(Range)(ref Range `inp`)
  if (isRandomAccessRange!Range && is(ElementType!Range : char));

    Perform a semantic equivalent 2 operations: decoding a code point at front of `inp` and testing if it belongs to the set of code points of this matcher.

    The effect on `inp` depends on the kind of function called:

    Match. If the codepoint is found in the set then range `inp` is advanced by its size in code units, otherwise the range is not modifed.

    Skip. The range is always advanced by the size of the tested code point regardless of the result of `test`.

    Test. The range is left unaffected regardless of the result of `test`.

    Examples:

        string truth = "2² = 4";
        auto m = utfMatcher!char(unicode.Number);
        assert(m.match(truth)); // '2' is a number all right
        assert(truth == "² = 4"); // skips on match
        assert(m.match(truth)); // so is the superscript '2'
        assert(!m.match(truth)); // space is not a number
        assert(truth == " = 4"); // unaffected on no match
        assert(!m.skip(truth)); // same test ...
        assert(truth == "= 4"); // but skips a codepoint regardless
        assert(!m.test(truth)); // '=' is not a number
        assert(truth == "= 4"); // test never affects argument

  @property auto `subMatcher`(Lengths...)();
    Advanced feature - provide direct access to a subset of matcher based a set of known encoding lengths. Lengths are provided in code units. The sub-matcher then may do less operations per any test/match.
    Use with care as the sub-matcher won't match any code points that have encoded length that doesn't belong to the selected set of lengths. Also the sub-matcher object references the parent matcher and must not be used past the liftetime of the latter.
    Another caveat of using sub-matcher is that skip is not available preciesly because sub-matcher doesn't detect all lengths.
    Examples:

        auto m = utfMatcher!char(unicode.Number);
        string square = "2²";
        // about sub-matchers
        assert(!m.subMatcher!(2,3,4).test(square)); // ASCII no covered
        assert(m.subMatcher!1.match(square)); // ASCII-only, works
        assert(!m.subMatcher!1.test(square)); // unicode '²'
        assert(m.subMatcher!(2,3,4).match(square));  //
        assert(square == "");
        wstring wsquare = "2²";
        auto m16 = utfMatcher!wchar(unicode.Number);
        // may keep ref, but the orignal (m16) must be kept alive
        auto bmp = m16.subMatcher!1;
        assert(bmp.match(wsquare)); // Okay, in basic multilingual plan
        assert(bmp.match(wsquare)); // And '²' too

enum auto `isUtfMatcher`(M, C);
  Test if M is an UTF Matcher for ranges of Char.
@trusted auto `utfMatcher`(Char, Set)(Set `set`)
if (isCodepointSet!Set);
  Constructs a matcher object to classify code points from the `set` for encoding that has Char as code unit.
  See MatcherConcept for API outline.
auto `toTrie`(size_t level, Set)(Set `set`)
if (isCodepointSet!Set);
  Convenience function to construct optimal configurations for packed Trie from any `set` of code points.
  The parameter level indicates the number of trie levels to use, allowed values are: 1, 2, 3 or 4. Levels represent different trade-offs speed-size wise.

  Level 1 is fastest and the most memory hungry (a bit array).

  Level 4 is the slowest and has the smallest footprint.

  See the Synopsis section for example.

  Note: Level 4 stays very practical (being faster and more predictable) compared to using direct lookup on the `set` itself.

auto `toDelegate`(Set)(Set `set`)
if (isCodepointSet!Set);

  Builds a Trie with typically optimal speed-size trade-off and wraps it into a delegate of the following type: bool delegate(dchar ch).

  Effectively this creates a 'tester' lambda suitable for algorithms like std.algorithm.find that take unary predicates.

  See the Synopsis section for example.
struct `unicode`;
  A single entry point to lookup Unicode code point sets by name or alias of a block, script or general category.
  It uses well defined standard rules of property name lookup. This includes fuzzy matching of names, so that 'White_Space', 'white-SpAce' and 'whitespace' are all considered equal and yield the same set of white space characters.
  pure @property auto `opDispatch`(string name)();
    Performs the lookup of set of code points with compile-time correctness checking. This short-cut version combines 3 searches: across blocks, scripts, and common binary properties.
    Note that since scripts and blocks overlap the usual trick to disambiguate is used - to get a block use unicode.InBlockName, to search a script use unicode.ScriptName.
    See Also:
    block, script and (not included in this search) hangulSyllableType.
  auto `opCall`(C)(in C[] `name`)
  if (is(C : dchar));
    The same lookup across blocks, scripts, or binary properties, but performed at run-time. This version is provided for cases where `name` is not known beforehand; otherwise compile-time checked opDispatch is typically a better choice.
    See the table of properties for available sets.
  struct `block`;
    Narrows down the search for sets of code points to all Unicode blocks.

    Note: Here `block` names are unambiguous as no scripts are searched and thus to search use simply unicode.`block`.BlockName notation.

    See table of properties for available sets.
    See Also:
    table of properties.
    Examples:

        // use .block for explicitness
        assert(unicode.block.Greek_and_Coptic == unicode.InGreek_and_Coptic);

  struct `script`;
    Narrows down the search for sets of code points to all Unicode scripts.
    See the table of properties for available sets.
    Examples:

        auto arabicScript = unicode.script.arabic;
        auto arabicBlock = unicode.block.arabic;
        // there is an intersection between script and block
        assert(arabicBlock['؁']);
        assert(arabicScript['؁']);
        // but they are different
        assert(arabicBlock != arabicScript);
        assert(arabicBlock == unicode.inArabic);
        assert(arabicScript == unicode.arabic);

  struct `hangulSyllableType`;
    Fetch a set of code points that have the given hangul syllable type.
    Other non-binary properties (once supported) follow the same notation - unicode.propertyName.propertyValue for compile-time checked access and unicode.propertyName(propertyValue) for run-time checked one.
    See the table of properties for available sets.
    Examples:

        // L here is syllable type not Letter as in unicode.L short-cut
        auto leadingVowel = unicode.hangulSyllableType("L");
        // check that some leading vowels are present
        foreach (vowel; '\u1110'..'\u115F')
            assert(leadingVowel[vowel]);
        assert(leadingVowel == unicode.hangulSyllableType.L);

size_t `graphemeStride`(C)(in C[] `input`, size_t `index`)
if (is(C : dchar));
  Computes the length of grapheme cluster starting at `index`. Both the resulting length and the `index` are measured in code units.
  Parameters:

  C | type that is implicitly convertible to dchars
  C[] `input` | array of grapheme clusters
  size_t `index` | starting `index` into `input`[]

  Returns:
  length of grapheme cluster
  Examples:

      assert(graphemeStride("  ", 1) == 1);
      // A + combing ring above
      string city = "A\u030Arhus";
      size_t first = graphemeStride(city, 0);
      assert(first == 3); //\u030A has 2 UTF-8 code units
      assert(city[0 .. first] == "A\u030A");
      assert(city[first..$] == "rhus");

Grapheme `decodeGrapheme`(Input)(ref Input `inp`)
if (isInputRange!Input && is(Unqual!(ElementType!Input) == dchar));
  Reads one full grapheme cluster from an input range of dchar `inp`.
  For examples see the Grapheme below.

  Note: This function modifies `inp` and thus `inp` must be an L-value.

auto `byGrapheme`(Range)(Range `range`)
if (isInputRange!Range && is(Unqual!(ElementType!Range) == dchar));

  Iterate a string by grapheme.

  Useful for doing string manipulation that needs to be aware of graphemes.

  See Also:
  byCodePoint
  Examples:

      import std.algorithm.comparison : equal;
      import std.range : take, drop;
      import std.range.primitives : walkLength;
      auto text = "noe\u0308l"; // noël using e + combining diaeresis
      assert(text.walkLength == 5); // 5 code points

      auto gText = text.byGrapheme;
      assert(gText.walkLength == 4); // 4 graphemes

      assert(gText.take(3).equal("noe\u0308".byGrapheme));
      assert(gText.drop(3).equal("l".byGrapheme));

auto `byCodePoint`(Range)(Range `range`)
if (isInputRange!Range && is(Unqual!(ElementType!Range) == Grapheme));
Range `byCodePoint`(Range)(Range `range`)
if (isInputRange!Range && is(Unqual!(ElementType!Range) == dchar));

  Lazily transform a `range` of Graphemes to a `range` of code points.

  Useful for converting the result to a string after doing operations on graphemes.

  Acts as the identity function when given a `range` of code points.

  Examples:

      import std.array : array;
      import std.conv : text;
      import std.range : retro;

      string s = "noe\u0308l"; // noël

      // reverse it and convert the result to a string
      string reverse = s.byGrapheme
          .array
          .retro
          .byCodePoint
          .text;

      assert(reverse == "le\u0308on"); // lëon

struct `Grapheme`;

  A structure designed to effectively pack characters of a grapheme cluster.

  `Grapheme` has value semantics so 2 copies of a `Grapheme` always refer to distinct objects. In most actual scenarios a `Grapheme` fits on the stack and avoids memory allocation overhead for all but quite long clusters.

  See Also:
  decodeGrapheme, graphemeStride
  this(C)(in C[] `chars`...)
  if (is(C : dchar));
  this(Input)(Input `seq`)
  if (!isDynamicArray!Input && isInputRange!Input && is(ElementType!Input : dchar));
    Ctor
  const pure nothrow @nogc @trusted dchar `opIndex`(size_t `index`);
    Gets a code point at the given `index` in this cluster.
  pure nothrow @nogc @trusted void `opIndexAssign`(dchar `ch`, size_t `index`);
    Writes a code point `ch` at given `index` in this cluster.

    Warning: Use of this facility may invalidate grapheme cluster, see also Grapheme.valid.

    Examples:

        auto g = Grapheme("A\u0302");
        assert(g[0] == 'A');
        assert(g.valid);
        g[1] = '~'; // ASCII tilda is not a combining mark
        assert(g[1] == '~');
        assert(!g.valid);

  pure nothrow @nogc @system SliceOverIndexed!Grapheme `opSlice`(size_t `a`, size_t `b`);
  pure nothrow @nogc @system SliceOverIndexed!Grapheme `opSlice`();
    Random-access range over Grapheme's characters.

    Warning: Invalidates when this Grapheme leaves the scope, attempts to use it then would lead to memory corruption.

  const pure nothrow @nogc @property @trusted size_t `length`();
    Grapheme cluster `length` in code points.
  ref auto `opOpAssign`(string op)(dchar `ch`);
    Append character `ch` to this grapheme.

    Warning: Use of this facility may invalidate grapheme cluster, see also valid.

    See Also:
    Grapheme.valid
    Examples:

        import std.algorithm.comparison : equal;
        auto g = Grapheme("A");
        assert(g.valid);
        g ~= '\u0301';
        assert(g[].equal("A\u0301"));
        assert(g.valid);
        g ~= "B";
        // not a valid grapheme cluster anymore
        assert(!g.valid);
        // still could be useful though
        assert(g[].equal("A\u0301B"));

  ref auto `opOpAssign`(string op, Input)(Input `inp`)
  if (isInputRange!Input && is(ElementType!Input : dchar));
    Append all characters from the input range `inp` to this Grapheme.
  @property bool `valid`()();
    True if this object contains `valid` extended grapheme cluster. Decoding primitives of this module always return a `valid` Grapheme.
    Appending to and direct manipulation of grapheme's characters may render it no longer `valid`. Certain applications may chose to use Grapheme as a "small string" of any code points and ignore this property entirely.
int `sicmp`(S1, S2)(S1 `r1`, S2 `r2`)
if (isInputRange!S1 && isSomeChar!(ElementEncodingType!S1) && isInputRange!S2 && isSomeChar!(ElementEncodingType!S2));

  Does basic case-insensitive comparison of `r1` and `r2`. This function uses simpler comparison rule thus achieving better performance than icmp. However keep in mind the warning below.

  Parameters:

  S1 `r1` | an input range of characters
  S2 `r2` | an input range of characters

  Returns:
  An int that is 0 if the strings match, <0 if `r1` is lexicographically "less" than `r2`, >0 if `r1` is lexicographically "greater" than `r2`

  Warning: This function only handles 1:1 code point mapping and thus is not sufficient for certain alphabets like German, Greek and few others.

  See Also:
  icmp std.algorithm.comparison.cmp
  Examples:

      assert(sicmp("Август", "авгусТ") == 0);
      // Greek also works as long as there is no 1:M mapping in sight
      assert(sicmp("ΌΎ", "όύ") == 0);
      // things like the following won't get matched as equal
      // Greek small letter iota with dialytika and tonos
      assert(sicmp("ΐ", "\u03B9\u0308\u0301") != 0);

      // while icmp has no problem with that
      assert(icmp("ΐ", "\u03B9\u0308\u0301") == 0);
      assert(icmp("ΌΎ", "όύ") == 0);

int `icmp`(S1, S2)(S1 `r1`, S2 `r2`)
if (isForwardRange!S1 && isSomeChar!(ElementEncodingType!S1) && isForwardRange!S2 && isSomeChar!(ElementEncodingType!S2));
  Does case insensitive comparison of `r1` and `r2`. Follows the rules of full case-folding mapping. This includes matching as equal german ß with "ss" and other 1:M code point mappings unlike sicmp. The cost of `icmp` being pedantically correct is slightly worse performance.
  Parameters:

  S1 `r1` | a forward range of characters
  S2 `r2` | a forward range of characters

  Returns:
  An int that is 0 if the strings match, <0 if str1 is lexicographically "less" than str2, >0 if str1 is lexicographically "greater" than str2
  See Also:
  sicmp std.algorithm.comparison.cmp
  Examples:

      assert(icmp("Rußland", "Russland") == 0);
      assert(icmp("ᾩ -> \u1F70\u03B9", "\u1F61\u03B9 -> ᾲ") == 0);

  Examples:
  By using std.utf.byUTF and its aliases, GC allocations via auto-decoding and thrown exceptions can be avoided, making `icmp` @safe @nogc nothrow pure.

      import std.utf : byDchar;

      assert(icmp("Rußland".byDchar, "Russland".byDchar) == 0);
      assert(icmp("ᾩ -> \u1F70\u03B9".byDchar, "\u1F61\u03B9 -> ᾲ".byDchar) == 0);

pure nothrow @nogc @safe ubyte `combiningClass`(dchar `ch`);

  Returns the combining class of `ch`.

  Examples:

      // shorten the code
      alias CC = combiningClass;

      // combining tilda
      assert(CC('\u0303') == 230);
      // combining ring below
      assert(CC('\u0325') == 220);
      // the simple consequence is that  "tilda" should be
      // placed after a "ring below" in a sequence

enum `UnicodeDecomposition`: int;
  Unicode character decomposition type.
  `Canonical`
    `Canonical` decomposition. The result is canonically equivalent sequence.
  `Compatibility`
    `Compatibility` decomposition. The result is compatibility equivalent sequence.

    Note: `Compatibility` decomposition is a lossy conversion, typically suitable only for fuzzy matching and internal processing.

pure nothrow @safe dchar `compose`(dchar `first`, dchar `second`);
  Try to canonically `compose` 2 characters. Returns the composed character if they do `compose` and dchar.init otherwise.
  The assumption is that `first` comes before `second` in the original text, usually meaning that the `first` is a starter.

  Note: Hangul syllables are not covered by this function. See composeJamo below.

  Examples:

      assert(compose('A','\u0308') == '\u00C4');
      assert(compose('A', 'B') == dchar.init);
      assert(compose('C', '\u0301') == '\u0106');
      // note that the starter is the first one
      // thus the following doesn't compose
      assert(compose('\u0308', 'A') == dchar.init);

@safe Grapheme `decompose`(UnicodeDecomposition decompType = Canonical)(dchar `ch`);
  Returns a full Canonical (by default) or Compatibility decomposition of character `ch`. If no decomposition is available returns a Grapheme with the `ch` itself.

  Note: This function also decomposes hangul syllables as prescribed by the standard.

  See Also:
  decomposeHangul for a restricted version that takes into account only hangul syllables but no other decompositions.
  Examples:

      import std.algorithm.comparison : equal;

      assert(compose('A','\u0308') == '\u00C4');
      assert(compose('A', 'B') == dchar.init);
      assert(compose('C', '\u0301') == '\u0106');
      // note that the starter is the first one
      // thus the following doesn't compose
      assert(compose('\u0308', 'A') == dchar.init);

      assert(decompose('Ĉ')[].equal("C\u0302"));
      assert(decompose('D')[].equal("D"));
      assert(decompose('\uD4DC')[].equal("\u1111\u1171\u11B7"));
      assert(decompose!Compatibility('¹')[].equal("1"));

@safe Grapheme `decomposeHangul`(dchar `ch`);
  Decomposes a Hangul syllable. If `ch` is not a composed syllable then this function returns Grapheme containing only `ch` as is.
  Examples:

      import std.algorithm.comparison : equal;
      assert(decomposeHangul('\uD4DB')[].equal("\u1111\u1171\u11B6"));

pure nothrow @nogc @safe dchar `composeJamo`(dchar `lead`, dchar `vowel`, dchar `trailing` = (dchar).init);
  Try to compose hangul syllable out of a leading consonant (`lead`), a `vowel` and optional `trailing` consonant jamos.
  On success returns the composed LV or LVT hangul syllable.
  If any of `lead` and `vowel` are not a valid hangul jamo of the respective character class returns dchar.init.
  Examples:

      assert(composeJamo('\u1111', '\u1171', '\u11B6') == '\uD4DB');
      // leaving out T-vowel, or passing any codepoint
      // that is not trailing consonant composes an LV-syllable
      assert(composeJamo('\u1111', '\u1171') == '\uD4CC');
      assert(composeJamo('\u1111', '\u1171', ' ') == '\uD4CC');
      assert(composeJamo('\u1111', 'A') == dchar.init);
      assert(composeJamo('A', '\u1171') == dchar.init);

enum `NormalizationForm`: int;
  Enumeration type for normalization forms, passed as template parameter for functions like normalize.
`NFC`
`NFD`
`NFKC`
`NFKD`
  Shorthand aliases from values indicating normalization forms.
inout(C)[] `normalize`(NormalizationForm norm = NFC, C)(inout(C)[] `input`);
  Returns `input` string normalized to the chosen form. Form C is used by default.
  For more information on normalization forms see the normalization section.

  Note: In cases where the string in question is already normalized, it is returned unmodified and no memory allocation happens.

  Examples:

      // any encoding works
      wstring greet = "Hello world";
      assert(normalize(greet) is greet); // the same exact slice

      // An example of a character with all 4 forms being different:
      // Greek upsilon with acute and hook symbol (code point 0x03D3)
      assert(normalize!NFC("ϓ") == "\u03D3");
      assert(normalize!NFD("ϓ") == "\u03D2\u0301");
      assert(normalize!NFKC("ϓ") == "\u038E");
      assert(normalize!NFKD("ϓ") == "\u03A5\u0301");

bool `allowedIn`(NormalizationForm norm)(dchar `ch`);
  Tests if dchar `ch` is always allowed (Quick_Check=YES) in normalization form norm.
  Examples:

      // e.g. Cyrillic is always allowed, so is ASCII
      assert(allowedIn!NFC('я'));
      assert(allowedIn!NFD('я'));
      assert(allowedIn!NFKC('я'));
      assert(allowedIn!NFKD('я'));
      assert(allowedIn!NFC('Z'));

pure nothrow @nogc @safe bool `isWhite`(dchar `c`);
  Whether or not `c` is a Unicode whitespace character. (general Unicode category: Part of C0(tab, vertical tab, form feed, carriage return, and linefeed characters), Zs, Zl, Zp, and NEL(U+0085))
pure nothrow @nogc @safe bool `isLower`(dchar `c`);
  Return whether `c` is a Unicode lowercase character.
pure nothrow @nogc @safe bool `isUpper`(dchar `c`);
  Return whether `c` is a Unicode uppercase character.
auto `asLowerCase`(Range)(Range `str`)
if (isInputRange!Range && isSomeChar!(ElementEncodingType!Range) && !isConvertibleToString!Range);
auto `asUpperCase`(Range)(Range `str`)
if (isInputRange!Range && isSomeChar!(ElementEncodingType!Range) && !isConvertibleToString!Range);
  Convert input range or string to upper or lower case.
  Does not allocate memory. Characters in UTF-8 or UTF-16 format that cannot be decoded are treated as std.utf.replacementDchar.
  Parameters:

  Range `str` | string or range of characters

  Returns:
  an InputRange of dchars
  See Also:
  toUpper, toLower
  Examples:

      import std.algorithm.comparison : equal;

      assert("hEllo".asUpperCase.equal("HELLO"));

auto `asCapitalized`(Range)(Range `str`)
if (isInputRange!Range && isSomeChar!(ElementEncodingType!Range) && !isConvertibleToString!Range);
  Capitalize input range or string, meaning convert the first character to upper case and subsequent characters to lower case.
  Does not allocate memory. Characters in UTF-8 or UTF-16 format that cannot be decoded are treated as std.utf.replacementDchar.
  Parameters:

  Range `str` | string or range of characters

  Returns:
  an InputRange of dchars
  See Also:
  toUpper, toLower asUpperCase, asLowerCase
  Examples:

      import std.algorithm.comparison : equal;

      assert("hEllo".asCapitalized.equal("Hello"));

pure @trusted void `toLowerInPlace`(C)(ref C[] `s`)
if (is(C == char) || is(C == wchar) || is(C == dchar));
  Converts `s` to lowercase (by performing Unicode lowercase mapping) in place. For a few characters string length may increase after the transformation, in such a case the function reallocates exactly once. If `s` does not have any uppercase characters, then `s` is unaltered.
pure @trusted void `toUpperInPlace`(C)(ref C[] `s`)
if (is(C == char) || is(C == wchar) || is(C == dchar));
  Converts `s` to uppercase (by performing Unicode uppercase mapping) in place. For a few characters string length may increase after the transformation, in such a case the function reallocates exactly once. If `s` does not have any lowercase characters, then `s` is unaltered.
pure nothrow @nogc @safe dchar `toLower`(dchar `c`);
  If `c` is a Unicode uppercase character, then its lowercase equivalent is returned. Otherwise `c` is returned.

  Warning: certain alphabets like German and Greek have no 1:1 upper-lower mapping. Use overload of `toLower` which takes full string instead.

pure @trusted S `toLower`(S)(S `s`)
if (isSomeString!S);
  Returns a string which is identical to `s` except that all of its characters are converted to lowercase (by preforming Unicode lowercase mapping). If none of `s` characters were affected, then `s` itself is returned.
pure nothrow @nogc @safe dchar `toUpper`(dchar `c`);
  If `c` is a Unicode lowercase character, then its uppercase equivalent is returned. Otherwise `c` is returned.

  Warning: Certain alphabets like German and Greek have no 1:1 upper-lower mapping. Use overload of `toUpper` which takes full string instead.

  `toUpper` can be used as an argument to std.algorithm.iteration.map to produce an algorithm that can convert a range of characters to upper case without allocating memory. A string can then be produced by using std.algorithm.mutation.copy to send it to an std.array.appender.
  Examples:

      import std.algorithm.iteration : map;
      import std.algorithm.mutation : copy;
      import std.array : appender;

      auto abuf = appender!(char[])();
      "hello".map!toUpper.copy(&abuf);
      assert(abuf.data == "HELLO");

pure @trusted S `toUpper`(S)(S `s`)
if (isSomeString!S);
  Returns a string which is identical to `s` except that all of its characters are converted to uppercase (by preforming Unicode uppercase mapping). If none of `s` characters were affected, then `s` itself is returned.
pure nothrow @nogc @safe bool `isAlpha`(dchar `c`);
  Returns whether `c` is a Unicode alphabetic character (general Unicode category: Alphabetic).
pure nothrow @nogc @safe bool `isMark`(dchar `c`);
  Returns whether `c` is a Unicode mark (general Unicode category: Mn, Me, Mc).
pure nothrow @nogc @safe bool `isNumber`(dchar `c`);
  Returns whether `c` is a Unicode numerical character (general Unicode category: Nd, Nl, No).
pure nothrow @nogc @safe bool `isAlphaNum`(dchar `c`);
  Returns whether `c` is a Unicode alphabetic character or number. (general Unicode category: Alphabetic, Nd, Nl, No).
  Parameters:

  dchar `c` | any Unicode character

  Returns:
  true if the character is in the Alphabetic, Nd, Nl, or No Unicode categories
pure nothrow @nogc @safe bool `isPunctuation`(dchar `c`);
  Returns whether `c` is a Unicode punctuation character (general Unicode category: Pd, Ps, Pe, Pc, Po, Pi, Pf).
pure nothrow @nogc @safe bool `isSymbol`(dchar `c`);
  Returns whether `c` is a Unicode symbol character (general Unicode category: Sm, Sc, Sk, So).
pure nothrow @nogc @safe bool `isSpace`(dchar `c`);
  Returns whether `c` is a Unicode space character (general Unicode category: Zs)

  Note: This doesn't include '\n', '\r', \t' and other non-space character. For commonly used less strict semantics see isWhite.

pure nothrow @nogc @safe bool `isGraphical`(dchar `c`);
  Returns whether `c` is a Unicode graphical character (general Unicode category: L, M, N, P, S, Zs).
pure nothrow @nogc @safe bool `isControl`(dchar `c`);
  Returns whether `c` is a Unicode control character (general Unicode category: Cc).
pure nothrow @nogc @safe bool `isFormat`(dchar `c`);
  Returns whether `c` is a Unicode formatting character (general Unicode category: Cf).
pure nothrow @nogc @safe bool `isPrivateUse`(dchar `c`);
  Returns whether `c` is a Unicode Private Use code point (general Unicode category: Co).
pure nothrow @nogc @safe bool `isSurrogate`(dchar `c`);
  Returns whether `c` is a Unicode surrogate code point (general Unicode category: Cs).
pure nothrow @nogc @safe bool `isSurrogateHi`(dchar `c`);
  Returns whether `c` is a Unicode high surrogate (lead surrogate).
pure nothrow @nogc @safe bool `isSurrogateLo`(dchar `c`);
  Returns whether `c` is a Unicode low surrogate (trail surrogate).
pure nothrow @nogc @safe bool `isNonCharacter`(dchar `c`);
  Returns whether `c` is a Unicode non-character i.e. a code point with no assigned abstract character. (general Unicode category: Cn)

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