Advent-of-Code-2022/8/solution.mjs

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}`);