2024 day 12 part 2 in js

2024/12/example4.txt

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+EEEEE
+EXXXX
+EEEEE
+EXXXX
+EEEEE

2024/12/example5.txt

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+AAAAAA
+AAABBA
+AAABBA
+ABBAAA
+ABBAAA
+AAAAAA

2024/12/solution2.js

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+
+const input = (await Bun.file(Bun.argv[2]).text()).trim()
+
+let map = input.split("\n")
+
+const dirs = [[-1,0], [0,-1], [1,0], [0, 1]];
+const height = map.length;
+const width = height;
+
+const key = (x,y) => x + y*width;
+const vkey = (x,y) => y + x*height;
+const visited = new Set();
+
+function search(x,y) {
+  if (visited.has(key(x,y))) {
+    return {area: 0, hsides: new Set()}
+  }
+  visited.add(key(x,y))
+  const letter = map[y][x]
+  let area = 1
+  let hsides = new Set()
+  for (const [dx, dy] of dirs) {
+    const nletter = map[y + dy]?.[x + dx];
+    if (letter !== nletter) {
+      if (dx === 0) {
+        hsides.add({y: y+dy/2, key: key(x,y)})
+      }
+      continue
+    }
+    const n = search(x + dx, y + dy)
+    area += n.area
+    hsides = hsides.union(n.hsides)
+  }
+
+  return {area, hsides}
+}
+
+const regions = [];
+
+for (let y = 0; y < height; y++) {
+  for (let x = 0; x < width; x++) {
+    if (visited.has(key(x,y))) continue
+    regions.push(search(x,y))
+  }
+}
+
+
+let cost = 0;
+for (const {area, hsides} of regions) {
+  // hacky filter of all perimeter nodes to extract adjacent
+  const horizontal = Array.from(hsides)
+    .sort((a, b) => a.key - b.key)
+    .sort((a, b) => a.y - b.y)
+    .filter(({y, key}, i, a) => a[i-1]?.key + 1 !== key || a[i-1]?.y !== y)
+    .length
+
+  // there's always an equal amount of horizontal and vertical sides
+  cost += area * (horizontal * 2)
+}
+
+console.log(cost)
+
+