Pancake Flipper

pancake-flipper.test.js

// pancakes are sorted if they are ordered from 1 to N
function isSorted(pancakes) {
  return pancakes.every((p, i) => p === i + 1);
}

function flip(pancakes) {
  const instructions = [''];
  while (!isSorted(pancakes)) {
    const maxUnsorted = pancakes.findLastIndex((p, i) => p !== i + 1) + 1;
    const flipCount = pancakes.indexOf(maxUnsorted) + 1;
    if (flipCount > 1) {
      pancakes.splice(0, flipCount, ...pancakes.slice(0, flipCount).reverse());
      instructions.push(`Flip ${flipCount} pancakes: ${pancakes.join(', ')}`);
    }
    pancakes.splice(
      0,
      maxUnsorted,
      ...pancakes.slice(0, maxUnsorted).reverse()
    );
    instructions.push(`Flip ${maxUnsorted} pancakes: ${pancakes.join(', ')}`);
  }
  return instructions.map((instr, i) => instr && `${i}. ${instr}`).join('\n');
}

test('flip', () => {
  expect(flip([3, 1, 4, 5, 9, 2, 6, 8, 7])).toBe(`
1. Flip 5 pancakes: 9, 5, 4, 1, 3, 2, 6, 8, 7
2. Flip 9 pancakes: 7, 8, 6, 2, 3, 1, 4, 5, 9
3. Flip 2 pancakes: 8, 7, 6, 2, 3, 1, 4, 5, 9
4. Flip 8 pancakes: 5, 4, 1, 3, 2, 6, 7, 8, 9
5. Flip 5 pancakes: 2, 3, 1, 4, 5, 6, 7, 8, 9
6. Flip 2 pancakes: 3, 2, 1, 4, 5, 6, 7, 8, 9
7. Flip 3 pancakes: 1, 2, 3, 4, 5, 6, 7, 8, 9`);
});