Day 18

18/solution.mjs

@@ -0,0 +1,121 @@
+import { readFileSync } from 'node:fs';
+
+const input = readFileSync('input', 'utf-8');
+
+const cubes = input
+  .split('\n')
+  .filter(line => !!line.trim())
+  .map(coords => {
+    const [x, y, z] = coords.split(',').map(n => parseInt(n));
+    return {x, y, z};
+  });
+
+const surfaceArea = cubes.map(cubeA => {
+  const touchingCubes = cubes.filter(cubeB => {
+    const xDiff = Math.abs(cubeA.x - cubeB.x);
+    const yDiff = Math.abs(cubeA.y - cubeB.y);
+    const zDiff = Math.abs(cubeA.z - cubeB.z);
+
+    return (
+      (xDiff === 1 && yDiff === 0 && zDiff === 0) ||
+      (yDiff === 1 && zDiff === 0 && xDiff === 0) ||
+      (zDiff === 1 && xDiff === 0 && yDiff === 0)
+    );
+  });
+
+  const nonConnectedSides = 6 - touchingCubes.length;
+  return nonConnectedSides;
+}).reduce((total, sides) => total + sides);
+
+console.log(`The surface area of the droplet is: ${surfaceArea}`);
+
+function minMax(dimension) {
+  return [
+    cubes.reduce((min, coords) => Math.min(min, coords[dimension]), Infinity),
+    cubes.reduce((max, coords) => Math.max(max, coords[dimension]), -Infinity),
+  ];
+}
+
+function isCoordEqual(coordA, coordB) {
+  return coordA.every((v, i) => coordB[i] === v);
+}
+
+function uniqueCoordsFilter(coordA, i, coords) {
+  return coords.findLastIndex(coordB => isCoordEqual(coordA, coordB)) === i;
+}
+
+function getAllConnected(startX, startY, startZ, isConnected) {
+  const connected = [];
+  let toVisit = [[startX, startY, startZ]];
+
+  while (true) {
+    connected.push(...toVisit);
+    toVisit = toVisit.flatMap(([x, y, z]) => [
+      [x + 1, y,     z    ],
+      [x - 1, y,     z    ],
+      [x,     y + 1, z    ],
+      [x,     y - 1, z    ],
+      [x,     y,     z + 1],
+      [x,     y,     z - 1]
+    ])
+      .filter(coordA =>
+        !connected.find(coordB => isCoordEqual(coordA, coordB))
+      )
+      .filter(uniqueCoordsFilter)
+      .filter(([x, y, z]) => isConnected(x, y, z));
+
+    if (!toVisit.length) {
+      break;
+    }
+  }
+
+  return connected;
+}
+
+const [minX, maxX] = minMax('x');
+const [minY, maxY] = minMax('y');
+const [minZ, maxZ] = minMax('z');
+
+// Get all coordinates external to droplet with a border of 2
+const connected = getAllConnected(minX - 2, minY - 2, minZ - 2, (x, y, z) => {
+  return (
+    x >= minX - 2 &&
+    x <= maxX + 2 &&
+    y >= minY - 2 &&
+    y <= maxY + 2 &&
+    z >= minZ - 2 &&
+    z <= maxZ + 2 &&
+    // Not part of the droplet
+    !cubes.find(coord => coord.x === x && coord.y === y && coord.z === z)
+  );
+});
+
+// Calculate internal surface area of inverted droplet
+const externalSurfaceArea = connected.map(([ax, ay, az]) => {
+  // Skip coordinates right on the edge
+  if (
+    ax < minX - 1 || ax > maxX + 1 ||
+    ay < minY - 1 || ay > maxY + 1 ||
+    az < minZ - 1 || az > maxZ + 1
+  ) {
+    return 0;
+  }
+
+  const touchingCubes = connected.filter(([bx, by, bz]) => {
+    const xDiff = Math.abs(ax - bx);
+    const yDiff = Math.abs(ay - by);
+    const zDiff = Math.abs(az - bz);
+
+    return (
+      (xDiff === 1 && yDiff === 0 && zDiff === 0) ||
+      (yDiff === 1 && zDiff === 0 && xDiff === 0) ||
+      (zDiff === 1 && xDiff === 0 && yDiff === 0)
+    );
+  });
+  const nonConnectedSides = 6 - touchingCubes.length;
+  return nonConnectedSides;
+}).reduce((total, sides) => total + sides);
+
+console.log(
+  `The EXTERNAL surface area of the droplet is: ${externalSurfaceArea}`
+);