Day eighteen

18/main.rb

@@ -0,0 +1,116 @@
+#!/usr/bin/env ruby
+
+require "debug"
+
+input = (ARGV.first.nil? ? DATA : ARGF)
+  .readlines(chomp: true)
+  .map { _1.split(",").map(&:to_i) }
+
+GRID_SIZE = 71
+SIMULATED = 1024
+
+grid = Array.new(GRID_SIZE) { Array.new(GRID_SIZE) { "." } }
+
+class PriorityQueue < Array
+  alias_method :extract_min, :shift
+  alias_method :peek, :first
+
+  def <<(v)
+    super
+
+    sort_by!(&:last)
+  end
+end
+
+def print_grid(grid)
+  grid.each_with_index do |row, y|
+    row.each_with_index do |val, x|
+      print val
+    end
+    puts
+  end
+  puts
+  puts
+end
+
+def dijkstra(grid, corrupted, start, target)
+  corrupted = corrupted.to_h { [_1, true] }
+  queue = PriorityQueue.new
+  dist = Hash.new
+  prev = Hash.new
+
+  queue << [start, 0]
+  dist[start] = 0
+  prev[start] = nil
+
+  until queue.empty?
+    node, steps = queue.extract_min
+
+    [[-1, 0], [0, 1], [1, 0], [0, -1]]
+      .map { |dx, dy| [node.first + dx, node.last + dy] }
+      .reject do |neighbor|
+        neighbor.first < 0 ||
+          neighbor.last < 0 ||
+          neighbor.first >= grid.size ||
+          neighbor.last >= grid.size ||
+          corrupted[neighbor]
+      end.each do |neighbor|
+        if dist[neighbor].nil? || steps + 1 < dist[neighbor]
+          dist[neighbor] = steps + 1
+          prev[neighbor] = node
+          queue << [neighbor, steps + 1]
+        end
+      end
+  end
+
+  path = Array.new
+  u = target
+  while prev[u]
+    path.unshift(u)
+    u = prev[u]
+  end
+
+  return dist, path
+end
+
+dist, _ = dijkstra(grid, input[...1024], [0, 0], [GRID_SIZE - 1, GRID_SIZE - 1])
+
+part_1 = dist[[GRID_SIZE - 1, GRID_SIZE - 1]]
+p part_1
+
+part_2 = input[1024..].bsearch do
+  dijkstra(
+    grid,
+    input[...1024] + input[1024..input.index(_1)],
+    [0, 0],
+    [GRID_SIZE - 1, GRID_SIZE - 1]
+  ).last.empty?
+end
+p part_2
+
+__END__
+5,4
+4,2
+4,5
+3,0
+2,1
+6,3
+2,4
+1,5
+0,6
+3,3
+2,6
+5,1
+1,2
+5,5
+2,5
+6,5
+1,4
+0,4
+6,4
+1,1
+6,1
+1,0
+0,5
+1,6
+2,0