type Insert = { kind: 'insert'; index: number; text: string };
type Replace = { kind: 'replace'; index: number; length: number; text: string };
type Delete = { kind: 'delete'; index: number; length: number };

export type SourceTextUpdate = Insert | Replace | Delete;

export default function updateSourceText(sourceText: string, updates: SourceTextUpdate[]): string {
  if (updates.length === 0) {
    return sourceText;
  }

  const sortedUpdates = [...updates].sort((update1, update2) => update1.index - update2.index);

  verifyUpdates(sourceText, sortedUpdates);

  let out = '';
  let index = 0;
  sortedUpdates.forEach((update) => {
    out += sourceText.slice(index, update.index);

    if (update.kind === 'insert') {
      out += update.text;
      index = update.index;
    } else if (update.kind === 'replace') {
      out += update.text;
      index = update.index + update.length;
    } else if (update.kind === 'delete') {
      index = update.index + update.length;
    } else {
      unreachable(update);
    }
  });
  out += sourceText.slice(index);

  return out;
}

function verifyUpdates(_sourceText: string, updates: SourceTextUpdate[]) {
  updates.forEach((update) => {
    if (update.index < 0) {
      throw new Error('Update has negative index');
    }
    if ((update.kind === 'replace' || update.kind === 'delete') && update.length < 0) {
      throw new Error('Update has negative length');
    }
    // TODO(brie): throw if out of sourceText bounds
  });
  // TODO(brie): throw if overlapping updates
}

function unreachable(_arg: never): never {
  throw new Error('');
}