/**
 * Split after sentence punctuation and keep the punctuation on the left piece.
 *
 * Replaces lookbehind splits. Those are syntax errors on Safari before 16.4,
 * so this file must not grow one. A newline inside `punctuation`
 * is a kept boundary. `splitOnNewlines` instead discards `\n+` runs. Do not
 * combine that option with a newline in `punctuation`.
 */

export type SentenceSplitOptions = {
  /** Drop whitespace after the kept punctuation. Same as a trailing `\s*`. */
  consumeFollowingWhitespace?: boolean;
  /** Discard `\n+` even when the newline is not after punctuation. */
  splitOnNewlines?: boolean;
};

function isJsWhitespace(char: string): boolean {
  return char.length === 1 && /\s/u.test(char);
}

export function splitAfterSentencePunctuation(
  text: string,
  punctuation: string,
  options?: SentenceSplitOptions,
): string[] {
  const consumeFollowingWhitespace = options?.consumeFollowingWhitespace === true;
  const splitOnNewlines = options?.splitOnNewlines === true;
  const marks = new Set(punctuation);
  const parts: string[] = [];
  let start = 0;
  let index = 0;

  while (index < text.length) {
    const char = text.charAt(index);
    if (marks.has(char)) {
      const boundaryEnd = index + 1;
      let delimiterEnd = boundaryEnd;
      if (consumeFollowingWhitespace) {
        while (delimiterEnd < text.length && isJsWhitespace(text.charAt(delimiterEnd))) {
          delimiterEnd += 1;
        }
      }
      // A zero-width match at the end of the string does not add a trailing empty piece.
      if (delimiterEnd === text.length && delimiterEnd === boundaryEnd) break;
      parts.push(text.slice(start, boundaryEnd));
      start = delimiterEnd;
      index = delimiterEnd;
      continue;
    }
    if (splitOnNewlines && char === "\n") {
      let newlineEnd = index + 1;
      while (newlineEnd < text.length && text.charAt(newlineEnd) === "\n") newlineEnd += 1;
      parts.push(text.slice(start, index));
      start = newlineEnd;
      index = newlineEnd;
      continue;
    }
    index += 1;
  }

  parts.push(text.slice(start));
  return parts;
}