/**
 * Group heads and sort keys of a back-of-book index in Chinese and in
 * Arabic.
 *
 * Chinese (#182): the Latin initial of
 * the pinyin reading (A–Z) or the stroke count (一畫, 二畫 …) of an entry's
 * first character.
 *
 * No reading or stroke data ships with the engine. The collator the index
 * already sorts with knows both: CLDR's Chinese collations (`zh-u-co-pinyin`,
 * `zh-u-co-stroke`) order the characters by reading or by stroke count, and
 * place an index marker right before each group — U+FDD0 followed by the
 * letter, or by U+2800 + the stroke count (what ICU's AlphabeticIndex
 * reads). An entry's group is the last marker that sorts before its first
 * character, found by binary search. A collator without the markers (an
 * older ICU) is searched with the first character of each group instead,
 * generated from ICU 78's markers and checked by `indexChinese.test.ts`.
 * A collator that does not sort by reading or by strokes at all gives no
 * groups: the index then has no heads.
 *
 * Arabic (#372): entries sort and group by a key that drops what an Arabic
 * index ignores (see {@link arabicSortKey}), in the `ar` collation, whose
 * order is the hijāʾī one (ا ب ت ث … ن ه و ي); the head of a group is the
 * key's first letter.
 *
 * Japanese (#425): see `indexJapanese.ts`.
 */
/** How the entries of an index are grouped under heads (`index.groupBy`). */
export type IndexGrouping = 'letter' | 'pinyin' | 'stroke' | 'gojuon' | 'kana' | 'none';
/** The pinyin initials that open a group: no syllable starts with I, U or
 *  V. */
export declare const PINYIN_INITIALS: readonly string[];
/** The first character of each pinyin group of {@link PINYIN_INITIALS} in
 *  the `zh` pinyin collation (CLDR, ICU 78). */
export declare const PINYIN_BOUNDARIES: readonly string[];
/** The stroke counts that open a group in the `zh` stroke collation (CLDR,
 *  ICU 78): no character has 34, 37, 38 or 40–47 strokes. */
export declare const STROKE_COUNTS: readonly number[];
/** The first character of each stroke group of {@link STROKE_COUNTS}. */
export declare const STROKE_BOUNDARIES: readonly string[];
/** The collator's index marker before the pinyin group of `letter`. */
export declare const pinyinMarker: (letter: string) => string;
/** The collator's index marker before the group of `strokes` strokes. */
export declare const strokeMarker: (strokes: number) => string;
/** Whether `collator` can group by `grouping`. */
export declare function canGroupBy(collator: Intl.Collator, grouping: 'pinyin' | 'stroke'): boolean;
/** The pinyin initial (`'J'` for 贾) of the Han character `ch`, or
 *  undefined when `collator` has no pinyin groups. */
export declare function pinyinInitial(collator: Intl.Collator, ch: string): string | undefined;
/** The stroke count group (`13` for 賈) of the Han character `ch`, or
 *  undefined when `collator` has no stroke groups. */
export declare function strokeGroup(collator: Intl.Collator, ch: string): number | undefined;
/** The head of a stroke group: 一畫, 十三畫 (Traditional), 十三画
 *  (Simplified). */
export declare function strokeLabel(strokes: number, traditional: boolean): string;
/** The grouping `index.groupBy` asks for in `locale`: `auto` groups a
 *  Simplified Chinese index by pinyin, a Traditional one by strokes, a
 *  Japanese one by gojūon row (#425), any other by first letter. */
export declare function indexGrouping(groupBy: string | undefined, locale: string): IndexGrouping;
/** The locale an index grouped by `grouping` sorts in, so that its heads
 *  and its order agree: a Chinese locale with the collation named
 *  (`zh-Hant-u-co-pinyin`), Chinese otherwise (`zh-u-co-pinyin`,
 *  `zh-Hant-u-co-stroke`). */
export declare function sortLocaleFor(locale: string, grouping: IndexGrouping): string;
/**
 * The key an Arabic index entry sorts and groups by, from its text as
 * written:
 * 1. the vowel signs and tatweel go (ٱلْكِتَابُ reads as الكتاب);
 * 2. with `ignoreArticle`, a leading article goes (البصرة → بصرة, between
 *    بدر and بغداد); الله keeps it;
 * 3. presentation forms and ligatures are decomposed (ﻻ → لا), and the
 *    signs left on Arabic letters dropped, which takes the hamza off its
 *    seat (أحمد, إبراهيم, آدم → ا; مؤمن → مومن);
 * 4. ٱ, ء → ا; ى → ي; ة → ه.
 * Only Arabic letters change: a Latin word keeps its accents (the collator
 * weighs them as it always does).
 */
export declare function arabicSortKey(text: string, ignoreArticle: boolean): string;
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