longest-path-prefix.ts

/**
 * Finds the common directory path prefix for a list of file paths.
 * Returns a path ending with '/' or empty string if no common prefix exists.
 *
 * @param files - An iterable of file paths to process
 * @returns The common directory prefix ending with '/' or empty string
 */
export function findCommonPathPrefix(files: Iterable<string>): string {
  const iter = Iterator.from(files);

  // Check for empty iterable
  const first = iter.next();
  if (first.done) return '';

  // Check for single element iterable
  const second = iter.next();
  if (second.done) {
    const lastSlash = first.value.lastIndexOf('/');
    return lastSlash >= 0 ? first.value.substring(0, lastSlash + 1) : '';
  }

  // Find common prefix between first two paths
  let commonPrefix = '';
  let lastSlash = -1;
  const secondIterator = Iterator.from(second.value);
  for (const char of Iterator.from(first.value)) {
    if (char === '/') lastSlash = commonPrefix.length;
    const secondChar = secondIterator.next();
    if (secondChar.done) break;
    if (char.localeCompare(secondChar.value, undefined, { sensitivity: 'accent' }) !== 0) {
      break;
    }
    commonPrefix += char;
  }
  commonPrefix = lastSlash >= 0 ? commonPrefix.substring(0, lastSlash + 1) : '';

  // Return empty if no common directory or just root directory
  if (!commonPrefix || commonPrefix === '/') return commonPrefix;

  // If no more elements, return the common prefix
  const next = iter.next();
  if (next.done) return commonPrefix;

  // Continue recursively with the current common prefix and remaining elements
  return findCommonPathPrefix(
    (function* (): Iterable<string> {
      yield commonPrefix;
      yield next.value;
      yield* iter;
    })(),
  );
}