Day 12

12/input

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12/solution.mjs

@@ -0,0 +1,98 @@
+import { readFileSync } from 'node:fs';
+
+const input = readFileSync('input', 'utf-8');
+
+function getMap() {
+  return input
+    .split('\n')
+    .filter(line => line)
+    .map((line, y) => line
+      .split('')
+      .map((elevation, x) => {
+        const isStart = elevation === 'S';
+        const isEnd = elevation === 'E';
+        elevation = elevation.replaceAll('S', 'a');
+        elevation = elevation.replaceAll('E', 'z');
+        const height = elevation.charCodeAt(0) - 97;
+        return {
+          x,
+          y,
+          height,
+          isStart,
+          isEnd,
+          path: null
+        };
+      })
+    );
+}
+
+const cardinals = [
+  [0, -1], // North
+  [1, 0],  // East
+  [0, 1],  // South
+  [-1, 0]  // West
+];
+
+// Recursively walk through map stopping whenever we encounter a square with a
+// shorter path than ours
+function solve(map, x, y, canMoveToSquare, path = []) {
+  const square = map[y][x];
+  path = [...path, [x, y]];
+
+  if (square.path && square.path.length <= path.length) {
+    return;
+  }
+
+  square.path = path;
+
+  cardinals
+    .map(c => map[y + c[1]]?.[x + c[0]])
+    // Filter undefined squares
+    .filter(s => s)
+    // Filter squares we can't reach
+    .filter(s => canMoveToSquare(square, s))
+    .forEach(s => solve(map, s.x, s.y, canMoveToSquare, [...path]));
+}
+
+function shortestPathStartToEnd() {
+  const map = getMap();
+  const start = map.flat().find(square => square.isStart);
+  solve(
+    map,
+    start.x,
+    start.y,
+    (from, to) => to.height <= from.height + 1
+  );
+  const end = map.flat().find(square => square.isEnd);
+  return end.path;
+}
+
+function shortestPathEndToA() {
+  const map = getMap();
+  const end = map.flat().find(square => square.isEnd);
+  solve(
+    map,
+    end.x,
+    end.y,
+    (from, to) => to.height >= from.height - 1
+  );
+
+  const squareWithShortestDistance = map
+    .flat()
+    .filter(square => square.path)
+    .filter(square => square.height === 0)
+    .sort((s1, s2) => s1.path.length - s2.path.length)[0];
+
+  return squareWithShortestDistance.path;
+}
+
+
+console.log('This will take a minute...');
+
+let path;
+
+path = shortestPathStartToEnd();
+console.log(`The least steps from start to end is: ${path.length - 1}`);
+
+path = shortestPathEndToA();
+console.log(`The least steps from 'a' to end is: ${path.length - 1}`);