Day 3: Gear Ratios


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  • mykl@lemmy.world
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    11 months ago

    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;
    }