Year 2024 Day 8

2024/day08.rb

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+lines = STDIN.read.split("\n")
+grid = lines.map { |line| line.chars }
+
+antennas = {}
+
+0.upto(grid.size - 1).each do |i|
+  0.upto(grid.first.size - 1).each do |j|
+    (antennas[grid[i][j]] ||= []) << [i, j] if grid[i][j] != '.'
+  end
+end
+
+# Part 1
+unique_locations = Set.new
+antennas.each do |k, points|
+  points.combination(2).each do |((ai, aj), (bi, bj))|
+    antinodes = [
+      [(ai - bi) + ai, (aj - bj) + aj],
+      [(bi - ai) + bi, (bj - aj) + bj]
+    ].filter { |(i, j)| i >= 0 && j >= 0 && i <= (grid.size - 1) && j <= (grid.first.size - 1) }
+
+    antinodes.each { |a| unique_locations.add(a) }
+  end
+end
+puts unique_locations.size
+
+# Part 2
+antinodes = Set.new
+antennas.each do |k, points|
+  points.combination(2).each do |((ai, aj), (bi, bj))|
+    n = 0
+    while
+      i = n * (ai - bi) + ai
+      j = n * (aj - bj) + aj
+      break if i < 0 || j < 0 || i > (grid.size - 1) || j > (grid.first.size - 1)
+      antinodes.add([i, j])
+      n += 1
+    end
+    n = 0
+    while
+      i = n * (bi - ai) + bi
+      j = n * (bj - aj) + bj
+      break if i < 0 || j < 0 || i > (grid.size - 1) || j > (grid.first.size - 1)
+      antinodes.add([i, j])
+      n += 1
+    end
+  end
+end
+puts antinodes.size

2024/examples/day08.txt

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+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............