Dart Solution

Holy moley, if this year is intended to be impervious to AI solution, it’s also working pretty well as a filter to this paltry Human Intelligence.

Find interesting symbols, look around for digits, and expand these into numbers. A dirty hacky solution leaning on a re-used Grid class. Not recommended.

late ListGrid grid;

Index look(Index ix, Index dir) {
  var here = ix;
  var next = here + dir;
  while (grid.isInBounds(next) && '1234567890'.contains(grid.at(next))) {
    here = next;
    next = here + dir;
  }
  return here;
}

/// Return start and end indices of a number at a point.
(Index, Index) expandNumber(Index ix) =>
    (look(ix, Grid.dirs['L']!), look(ix, Grid.dirs['R']!));

int parseNumber((Index, Index) e) => int.parse([
      for (var i = e.$1; i != e.$2 + Grid.dirs['R']!; i += Grid.dirs['R']!)
        grid.at(i)
    ].join());

/// Return de-duplicated positions of all numbers near the given symbols.
nearSymbols(Set syms) => [
      for (var ix in grid.indexes.where((i) => syms.contains(grid.at(i))))
        {
          for (var n in grid
              .near8(ix)
              .where((n) => ('1234567890'.contains(grid.at(n)))))
            expandNumber(n)
        }.toList()
    ];

part1(List lines) {
  grid = ListGrid([for (var e in lines) e.split('')]);
  var syms = lines
      .join('')
      .split('')
      .toSet()
      .difference('0123456789.'.split('').toSet());
  // Find distinct number locations near symbols and sum them.
  return {
    for (var ns in nearSymbols(syms))
      for (var n in ns) n
  }.map(parseNumber).sum;
}

part2(List lines) {
  grid = ListGrid([for (var e in lines) e.split('')]);
  // Look for _pairs_ of numbers near '*' and multiply them.
  var products = [
    for (var ns in nearSymbols({'*'}).where((e) => e.length == 2))
      ns.map(parseNumber).reduce((s, t) => s * t)
  ];
  return products.sum;
}