// Passages of text a note is tied to, kept as a W3C TextQuoteSelector keeps
// them: the exact text and a little of what comes before and after, so the
// passage is found again when the chat or the page draws it anew, and the
// right one of several alike.

/** How much text either side of a passage is kept to tell it from others like it. */
export const CONTEXT = 32;
/** The longest passage a note keeps. */
export const LONGEST = 2000;

export type Quote = { quote: string; prefix: string; suffix: string };

/** The passage between two offsets of a text, with its context. */
export function quoteAt(text: string, start: number, end: number): Quote {
  const stop = Math.min(end, start + LONGEST);
  return { quote: text.slice(start, stop), prefix: text.slice(Math.max(0, start - CONTEXT), start), suffix: text.slice(stop, stop + CONTEXT) };
}

function sharedEnd(a: string, b: string): number {
  let n = 0;
  while (n < a.length && n < b.length && a[a.length - 1 - n] === b[b.length - 1 - n]) n++;
  return n;
}

function sharedStart(a: string, b: string): number {
  let n = 0;
  while (n < a.length && n < b.length && a[n] === b[n]) n++;
  return n;
}

/**
 * Where a passage is in a text: of every place its words appear, the one
 * whose surroundings match the kept context best (the first on a tie), or
 * null when they appear nowhere.
 */
export function locate(text: string, { quote, prefix, suffix }: Quote): { start: number; end: number } | null {
  if (!quote) return null;
  let best = -1;
  let score = -1;
  for (let at = text.indexOf(quote), seen = 0; at >= 0 && seen < 1000; at = text.indexOf(quote, at + 1), seen++) {
    const around = sharedEnd(text.slice(Math.max(0, at - prefix.length), at), prefix) + sharedStart(text.slice(at + quote.length, at + quote.length + suffix.length), suffix);
    if (around > score) [best, score] = [at, around];
  }
  return best < 0 ? null : { start: best, end: best + quote.length };
}

/**
 * A passage as it reads, for a message or a label: blank lines and the
 * space around it dropped, each line's indent kept, so code stays code.
 */
export function tidy(quote: string): string {
  return quote
    .split('\n')
    .map((line) => line.trimEnd())
    .filter(Boolean)
    .join('\n')
    .trimStart();
}