export type TextSegment = {
  text: string;
  start: number;
  end: number;
};

const graphemeSegmenter = new Intl.Segmenter(undefined, {
  granularity: "grapheme",
});
const pasteMarkerPattern = /\[paste #(\d+)( (\+\d+ lines|\d+ chars))?\]/g;

function segmentGraphemes(text: string, offset: number): TextSegment[] {
  return [...graphemeSegmenter.segment(text)].map((segment) => ({
    text: segment.segment,
    start: offset + segment.index,
    end: offset + segment.index + segment.segment.length,
  }));
}

/**
 * Segment prompt text into cursor-safe units. Known Pi large-paste markers stay
 * atomic so vi motions and operators cannot split their rendered placeholder.
 */
export function segmentPromptText(
  text: string,
  pasteMarkerIds: ReadonlySet<number> = new Set(),
): TextSegment[] {
  if (pasteMarkerIds.size === 0 || !text.includes("[paste #")) {
    return segmentGraphemes(text, 0);
  }

  const segments: TextSegment[] = [];
  let offset = 0;

  for (const match of text.matchAll(pasteMarkerPattern)) {
    const index = match.index;
    const marker = match[0];
    const id = Number(match[1]);
    if (index === undefined || marker === undefined || !pasteMarkerIds.has(id))
      continue;

    segments.push(...segmentGraphemes(text.slice(offset, index), offset));
    segments.push({ text: marker, start: index, end: index + marker.length });
    offset = index + marker.length;
  }

  segments.push(...segmentGraphemes(text.slice(offset), offset));
  return segments;
}

/** Return whether an offset lies between complete cursor-safe segments. */
export function isSegmentBoundary(
  segments: readonly TextSegment[],
  offset: number,
): boolean {
  if (!Number.isInteger(offset) || offset < 0) return false;
  if (segments.length === 0) return offset === 0;

  return offset === 0 || segments.some((segment) => segment.end === offset);
}