// Copyright (c) Meta Platforms, Inc. and affiliates.

/**
 * @file characters.ts
 * @input Strings that may contain characters made of several code units
 *        (emoji, flags, joined emoji sequences, accented letters)
 * @output Exports characterCount, firstCharacter, truncateCharacters —
 *         replacements for .length / .charAt(0) / .slice(0, n) that measure
 *         and cut user-visible strings the way a person reads them
 * @position Shared utility; consumed by Avatar, TextArea, PowerSearch, Table
 *
 * SYNC: When modified, update:
 * - /packages/core/src/utils/characters.test.ts
 * - /packages/core/src/utils/index.ts
 */

/**
 * Reuse a single segmenter when the runtime supports Intl.Segmenter.
 * `grapheme` is the Intl granularity name for a user-perceived character.
 */
const characterSegmenter =
  typeof Intl.Segmenter === 'function'
    ? new Intl.Segmenter(undefined, {granularity: 'grapheme'})
    : null;

/**
 * Split a string into user-perceived characters. The code-point fallback
 * keeps surrogate pairs intact but may split joined emoji sequences and flag
 * pairs on runtimes without Intl.Segmenter.
 */
function splitCharacters(str: string): string[] {
  if (characterSegmenter) {
    return [...characterSegmenter.segment(str)].map(s => s.segment);
  }
  return [...str];
}

/**
 * Number of characters in a string — the count a person would give, where one
 * emoji, flag, or accented letter is one character. Replacement for `.length`
 * on user-visible strings, which counts internal storage units instead.
 */
export function characterCount(str: string): number {
  if (str === '') {
    return 0;
  }
  return splitCharacters(str).length;
}

/**
 * The first character of a string, or '' when empty. Replacement for
 * `.charAt(0)`, which can return half of an emoji. Reads only the first
 * character, so cost does not scale with the length of the string.
 */
export function firstCharacter(str: string): string {
  if (characterSegmenter) {
    const first = characterSegmenter.segment(str)[Symbol.iterator]().next();
    return first.done ? '' : first.value.segment;
  }
  const codePoint = str.codePointAt(0);
  return codePoint == null ? '' : String.fromCodePoint(codePoint);
}

/**
 * Truncate to at most `max` characters, ellipsis included: strings within
 * `max` characters pass through unchanged; longer strings are cut so the
 * result — content plus `ellipsis` — is exactly `max` characters (or just the
 * ellipsis, when `max` is smaller than the ellipsis itself). Replacement for
 * `str.slice(0, n) + '…'`, which can cut an emoji in half.
 *
 * @example
 * ```
 * truncateCharacters('abcdefghij', 5) // 'abcd…'
 * truncateCharacters('abcdefghij', 8, '...') // 'abcde...'
 * ```
 */
export function truncateCharacters(
  str: string,
  max: number,
  ellipsis = '…',
): string {
  const characters = splitCharacters(str);
  if (characters.length <= max) {
    return str;
  }
  const keep = Math.max(max - characterCount(ellipsis), 0);
  return characters.slice(0, keep).join('') + ellipsis;
}