2025aoc5.js

input = document.querySelector('pre').innerText;
[ranges, ids] = input.split('\n\n').map(x => x.split('\n'));
ranges = ranges.map(r => r.split('-').map(x => parseInt(x)));

console.log(ids.filter(id => ranges.some(r => id >= r[0] && id <= r[1])).length)

function mergeRanges() {
  const N = ranges.length
  for (let i = 0; i < N; ++i)
  for (let j = 0; j < N; ++j) {
    const [a, b] = [ranges[i], ranges[j]]
    if (i == j || !a || !b) continue
    if (b[0] <= a[1] && a[0] <= b[0]) {
      ranges[i] = [Math.min(a[0], b[0]), Math.max(a[1], b[1])]
      ranges[j] = undefined
    }
  }
  return N - (ranges = ranges.filter(r => !!r)).length
}
while (mergeRanges() > 0);

console.log(ranges.reduce((a, [from, to]) => a + (to - from + 1), 0))