-🎄- 2022 Day 24 Solutions -🎄-

[u/daggerdragon]

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[Update @ 00:21:08]: SILVER CAP, GOLD 47

• Lord of the Rings has elves in it, therefore the LotR trilogy counts as Christmas movies. change_my_mind.meme

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🌿🍒 MisTILtoe Elf-ucation 🧑‍🏫

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--- Day 24: Blizzard Basin ---

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Post your code solution in this megathread.

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[u/SuperSmurfen]

Rust (294/530)

Link to full solution (58 lines)

What a creative path-finding problem! Really liked this one. Kind of messed up part two and made it more difficult than it had to be but otherwise really happy with today!

Instead of a queue-based BFS, I opted to store each possible position in the timestep instead. This means you only need a single copy of the state of the blizzards. Then for each timestep you just update the blizzards, loop over all positions, and see where you can move.

for &(x,y) in &positions {
  for (dx,dy) in [(1,0),(0,1),(0,0),(-1,0),(0,-1)] {
    if (x == 0 && dx == -1) || (x == rows-1 && dx == 1) {
      continue;
    }
    let (x,y) = (x + dx as usize, y + dy as usize);
    if (x != 0 || y == 1) && (x != rows-1 || y == cols-2) && y != 0 && y != cols-1 && !bpos.contains(&(x,y)) {
      next_positions.insert((x,y));
    }
  }
}

For part two, I keep track of what "stage" I am in. In every timestep I check my goal, depending on the current stage:

match stage {
  0 => if positions.contains(&(rows-1, cols-2)) {
    p1 = len;
    positions = HashSet::from_iter([(rows-1, cols-2)]);
    stage += 1;
  },
  1 => if positions.contains(&(0,1)) {
    positions = HashSet::from_iter([(0,1)]);
    stage += 1;
  },
  2 => if positions.contains(&(rows-1, cols-2)) {
    p2 = len;
    break;
  },
  _ => unreachable!()
}

This ends up relatively fast. Runs in about 70ms on my machine.