Dart

A bit easier than I first thought it was going to be. I might try it in Uiua later now I’ve got my head around it.

import 'package:collection/collection.dart';
import 'package:more/more.dart';

(int, int) solve(List<String> lines) {
  var parts = lines.splitAfter((e) => e == '');
  var pred = SetMultimap.fromEntries(parts.first.skipLast(1).map((e) {
    var ps = e.split('|').map(int.parse);
    return MapEntry(ps.last, ps.first);
  }));
  ordering(a, b) => pred[a].contains(b) ? 1 : 0;

  var pageSets = parts.last.map((e) => e.split(',').map(int.parse).toList());
  var ret1 = 0, ret2 = 0;
  for (var pages in pageSets) {
    if (pages.isSorted(ordering)) {
      ret1 += pages[pages.length ~/ 2];
    } else {
      pages.sort(ordering);
      ret2 += pages[pages.length ~/ 2];
    }
  }
  return (ret1, ret2);
}

part1(List<String> lines) => solve(lines).$1;
part2(List<String> lines) => solve(lines).$2;