styledTextRangeIII

/**
 * Replace text in [start, end) and keep style ranges in sync.
 *
 * Idea:
 * - Keep ranges before start unchanged.
 * - Drop removed middle [start, end).
 * - Insert replacement range if replacement text is non-empty.
 * - Shift ranges after end by length delta.
 * - Merge adjacent ranges with same style.
 *
 * @param {{
 *   text: string,
 *   ranges: { start: number, end: number, style: string }[],
 * }} node
 * @param {number} start
 * @param {number} end
 * @param {{ text: string, style: string }} replacement
 * @returns {{
 *   text: string,
 *   ranges: { start: number, end: number, style: string }[],
 * }}
 */
export default function replaceTextRange(node, start, end, replacement) {
  const newText =
    node.text.slice(0, start) +
    replacement.text +
    node.text.slice(end);

  const insertLength = replacement.text.length;
  const removedLength = end - start;
  const delta = insertLength - removedLength;

  const ranges = [];

  for (const range of node.ranges) {
    // Part before replacement.
    if (range.start < start) {
      ranges.push({
        start: range.start,
        end: Math.min(range.end, start),
        style: range.style,
      });
    }

    // Part after replacement.
    if (range.end > end) {
      ranges.push({
        start: Math.max(range.start, end) + delta,
        end: range.end + delta,
        style: range.style,
      });
    }
  }

  // Insert replacement style range.
  if (insertLength > 0) {
    ranges.push({
      start,
      end: start + insertLength,
      style: replacement.style,
    });
  }

  ranges.sort((a, b) => a.start - b.start || a.end - b.end);

  return {
    text: newText,
    ranges: mergeAdjacentRanges(ranges),
  };
}

function mergeAdjacentRanges(ranges) {
  const merged = [];

  for (const range of ranges) {
    if (range.start === range.end) continue;

    const last = merged[merged.length - 1];

    if (last && last.end === range.start && last.style === range.style) {
      last.end = range.end;
    } else {
      merged.push({ ...range });
    }
  }

  return merged;
}