Day 20b Part Two - Code
Strangely, the exit isn't open when you reach it. Then, you remember: the ancient Plutonians were famous for building recursive spaces.
The marked connections in the maze aren't portals: they physically connect to a larger or smaller copy of the maze. Specifically, the labeled tiles around the inside edge actually connect to a smaller copy of the same maze, and the smaller copy's inner labeled tiles connect to yet a smaller copy, and so on.
When you enter the maze, you are at the outermost level; when at the outermost level, only the outer labels AA and ZZ function (as the start and end, respectively); all other outer labeled tiles are effectively walls. At any other level, AA and ZZ count as walls, but the other outer labeled tiles bring you one level outward.
Your goal is to find a path through the maze that brings you back to ZZ at the outermost level of the maze.
In the first example (in part 1), the shortest path is now the loop around the right side. If the starting level is 0, then taking the previously-shortest path would pass through BC (to level 1), DE (to level 2), and FG (back to level 1). Because this is not the outermost level, ZZ is a wall, and the only option is to go back around to BC, which would only send you even deeper into the recursive maze.
In the second example above, there is no path that brings you to ZZ at the outermost level.
Here is a more interesting example:
Z L X W C
Z P Q B K
###########.#.#.#.#######.###############
#...#.......#.#.......#.#.......#.#.#...#
###.#.#.#.#.#.#.#.###.#.#.#######.#.#.###
#.#...#.#.#...#.#.#...#...#...#.#.......#
#.###.#######.###.###.#.###.###.#.#######
#...#.......#.#...#...#.............#...#
#.#########.#######.#.#######.#######.###
#...#.# F R I Z #.#.#.#
#.###.# D E C H #.#.#.#
#.#...# #...#.#
#.###.# #.###.#
#.#....OA WB..#.#..ZH
#.###.# #.#.#.#
CJ......# #.....#
####### #######
#.#....CK #......IC
#.###.# #.###.#
#.....# #...#.#
###.### #.#.#.#
XF....#.# RF..#.#.#
#####.# #######
#......CJ NM..#...#
###.#.# #.###.#
RE....#.# #......RF
###.### X X L #.#.#.#
#.....# F Q P #.#.#.#
###.###########.###.#######.#########.###
#.....#...#.....#.......#...#.....#.#...#
#####.#.###.#######.#######.###.###.#.#.#
#.......#.......#.#.#.#.#...#...#...#.#.#
#####.###.#####.#.#.#.#.###.###.#.###.###
#.......#.....#.#...#...............#...#
#############.#.#.###.###################
A O F N
A A D M
- Walk from AA to XF (16 steps)
- Recurse into level 1 through XF (1 step)
- Walk from XF to CK (10 steps)
- Recurse into level 2 through CK (1 step)
- Walk from CK to ZH (14 steps)
- Recurse into level 3 through ZH (1 step)
- Walk from ZH to WB (10 steps)
- Recurse into level 4 through WB (1 step)
- Walk from WB to IC (10 steps)
- Recurse into level 5 through IC (1 step)
- Walk from IC to RF (10 steps)
- Recurse into level 6 through RF (1 step)
- Walk from RF to NM (8 steps)
- Recurse into level 7 through NM (1 step)
- Walk from NM to LP (12 steps)
- Recurse into level 8 through LP (1 step)
- Walk from LP to FD (24 steps)
- Recurse into level 9 through FD (1 step)
- Walk from FD to XQ (8 steps)
- Recurse into level 10 through XQ (1 step)
- Walk from XQ to WB (4 steps)
- Return to level 9 through WB (1 step)
- Walk from WB to ZH (10 steps)
- Return to level 8 through ZH (1 step)
- Walk from ZH to CK (14 steps)
- Return to level 7 through CK (1 step)
- Walk from CK to XF (10 steps)
- Return to level 6 through XF (1 step)
- Walk from XF to OA (14 steps)
- Return to level 5 through OA (1 step)
- Walk from OA to CJ (8 steps)
- Return to level 4 through CJ (1 step)
- Walk from CJ to RE (8 steps)
- Return to level 3 through RE (1 step)
- Walk from RE to IC (4 steps)
- Recurse into level 4 through IC (1 step)
- Walk from IC to RF (10 steps)
- Recurse into level 5 through RF (1 step)
- Walk from RF to NM (8 steps)
- Recurse into level 6 through NM (1 step)
- Walk from NM to LP (12 steps)
- Recurse into level 7 through LP (1 step)
- Walk from LP to FD (24 steps)
- Recurse into level 8 through FD (1 step)
- Walk from FD to XQ (8 steps)
- Recurse into level 9 through XQ (1 step)
- Walk from XQ to WB (4 steps)
- Return to level 8 through WB (1 step)
- Walk from WB to ZH (10 steps)
- Return to level 7 through ZH (1 step)
- Walk from ZH to CK (14 steps)
- Return to level 6 through CK (1 step)
- Walk from CK to XF (10 steps)
- Return to level 5 through XF (1 step)
- Walk from XF to OA (14 steps)
- Return to level 4 through OA (1 step)
- Walk from OA to CJ (8 steps)
- Return to level 3 through CJ (1 step)
- Walk from CJ to RE (8 steps)
- Return to level 2 through RE (1 step)
- Walk from RE to XQ (14 steps)
- Return to level 1 through XQ (1 step)
- Walk from XQ to FD (8 steps)
- Return to level 0 through FD (1 step)
- Walk from FD to ZZ (18 steps)
This path takes a total of 396 steps to move from AA at the outermost layer to ZZ at the outermost layer.
In your maze, when accounting for recursion, how many steps does it take to get from the open tile marked AA to the open tile marked ZZ, both at the outermost layer?
The code builds off of the code in part 1:
// same as part 1 - only added to improve readability for this markdown.
interface IMaze {
name: string;
grid: string[][];
entrance: IPoint;
exit: IPoint;
portals: Map<string, [IPoint, IPoint] | [IPoint]>;
portalLocations: Map<string, {key: string; isInner: boolean}>; // key is IPoint in toString()
}
const toGraph = ({entrance, exit, grid, portals, portalLocations}: IMaze) => {
const graph = new WGraph<string, number>(); // weight is steps between portals
const queue = new PriorityQueue<{at: IPoint; lastPortal: string; steps: number; level: number}>(p => -1 * p.steps);
const visited = new Map<string, number>(); // IPoint,level -> distance from entrance
const isValid = makeIsValid(grid, entrance);
queue.enqueue({at: entrance, steps: 0, lastPortal: entranceKey, level: 0});
visited.set(toKey(entrance) + '|0', 0); // visited is now key|level
while (!queue.isEmpty()) {
const { at, steps, lastPortal, level } = queue.dequeue()!;
if (equals(at, exit) && level === -1) // Z is at level 0, and it'll do level - 1 = -1
break; // done
// we don't check visited here because we may want to revisit a visited node if the steps are shorter
// neighbors are reachable dots (.), some of which may be portals
const neighbors = getNeighbors(at).filter(isValid);
for (const neighbor of neighbors) {
const neighborStr = toKey(neighbor);
const newSteps = steps + 1;
const visitedKey = neighborStr + '|' + level;
if (!visited.has(visitedKey) || visited.get(visitedKey)! > newSteps) {
const neighborAtPortal = portalLocations.get(neighborStr);
if (neighborAtPortal != null) {
const neighborPortalName = neighborAtPortal.key;
if (neighborPortalName === lastPortal)
continue;
const portalTo = first(portals.get(neighborPortalName)!.filter(p => !equals(p, neighbor)));
const newLevel = neighborAtPortal.isInner ? level + 1 : level - 1;
if ((neighborPortalName === entranceKey || neighborPortalName === exitKey) && level !== 0) {
visited.set(visitedKey, newSteps);
continue;
}
if (!neighborAtPortal.isInner && level === 0 && neighborPortalName !== exitKey) {
visited.set(visitedKey, newSteps);
continue;
}
queue.enqueue({
at: portalTo ?? neighbor, // possible if portal === 'ZZ'
lastPortal: neighborPortalName,
steps: newSteps + 1,
level: newLevel
});
graph.addDirectedEdge(lastPortal, neighborPortalName, newSteps);
visited.set(toKey(portalTo) + '|' + newLevel, newSteps);
}
else {
queue.enqueue({
at: neighbor,
lastPortal: lastPortal,
steps: newSteps,
level
});
}
visited.set(visitedKey, newSteps);
}
}
}
return graph;
};And the inner part isn't anything fancy. It just guestimates what the borders are for outer/inner:
const makeIsInner = (grid: string[][]) => (row: number, col: number, verticalChange: boolean) => {
if (verticalChange) {
if (row < 5 || row > grid.length - 5)
return false;
return true;
}
if (col < 5 || col > grid[row].length - 5)
return false;
return true;
};