Day 8

8/input

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8/solution.mjs

@@ -0,0 +1,107 @@
+import { readFileSync } from 'node:fs';
+
+const input = readFileSync('input', 'utf-8');
+
+// Parse grid
+const grid = input
+  .split('\n')
+  .filter(line => line)
+  .map(line => line
+    .split('')
+    .filter(c => c)
+    .map(height => parseInt(height))
+  );
+
+const w = grid[0]?.length || 0;
+const h = grid.length;
+
+// Confirm grid is complete
+for (const row of grid) {
+  if (row.length !== w) {
+    throw new Error('Invalid grid row');
+  }
+}
+
+// Return an array with the remaining integers after start and up to end
+function rest(start, end) {
+  const steps = Math.abs(start - end);
+  const mult = start < end ? 1 : -1;
+  return new Array(steps)
+    .fill()
+    .map((_, i) => start + (i + 1) * mult)
+}
+
+
+// Calculate the maximum value returned by getHeight when called with a range
+// of values from start (non-inclusive) to end
+function maxHeight(start, end, getHeight) {
+  return rest(start, end).reduce(
+    (max, pos) => Math.max(max, getHeight(pos)),
+    0
+  );
+}
+
+// Create a map where each item represents the smallest of the next tallest
+// tree in all four directions
+const maxMap = grid.map((row, y) => row.map((height, x) => {
+  const u = maxHeight(y, 0,     (pos) => grid[pos][x]);
+  const d = maxHeight(y, h - 1, (pos) => grid[pos][x]);
+  const l = maxHeight(x, 0,     (pos) => grid[y][pos]);
+  const r = maxHeight(x, w - 1, (pos) => grid[y][pos]);
+
+  // Return the smallest tree of the tallest trees in each direction
+  return Math.min(u, r, d, l);
+}));
+
+// Create a map of visible trees
+const visibleTrees = grid.map((row, y) => row.map((height, x) => {
+  // Edge case
+  if (x === 0 || y === 0 || x === w - 1 || y === h - 1) {
+    return true;
+  }
+
+  const obscuredByHeight = maxMap[y][x];
+
+  return height > obscuredByHeight;
+}));
+
+// Sum visible trees
+const totalVisibleTrees = visibleTrees
+  .flat()
+  .reduce((total, visible) => total + visible, 0);
+
+console.log(`The total number of trees visible is: ${totalVisibleTrees}`);
+
+// Calculate how many trees are visible based on heights returned by getHeight
+// for a range of values from start (non-inclusive) to end
+function countVisibleTrees(start, end, maxHeight, getHeight) {
+  let count = 0;
+  for (const pos of rest(start, end)) {
+    count++;
+    if (getHeight(pos) >= maxHeight) {
+      break;
+    }
+  }
+  return count;
+}
+
+// Calculate the scenic score for a tree
+function scenicScore(x, y) {
+  const maxHeight = grid[y][x];
+  const u = countVisibleTrees(y, 0,     maxHeight, (pos) => grid[pos][x]);
+  const d = countVisibleTrees(y, h - 1, maxHeight, (pos) => grid[pos][x]);
+  const l = countVisibleTrees(x, 0,     maxHeight, (pos) => grid[y][pos]);
+  const r = countVisibleTrees(x, w - 1, maxHeight, (pos) => grid[y][pos]);
+
+  // Return the smallest tree of the tallest trees in each direction
+  return u * r * d * l;
+}
+
+
+// Create a map of scenic scores for all trees
+const scenicMap = grid.map((row, y) => row.map((height, x) =>
+  scenicScore(x, y)
+));
+
+const highestScenicScore = Math.max(...scenicMap.flat());
+console.log(`The highest possible scenic score is: ${highestScenicScore}`);