Day six

6.input

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+................................#..#.............##.....#....#....................#.............................................#.
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+..#...#.........................................#....................#.....#........#....#......#......#................#.........
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+...............#....#.........#.............................................................................................#.....
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+......#..................#.....#..................#..#..................#.................#..#.....#...................#....#.....
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+...............................................................#................................................#..........#......
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+...#.....................#......................................................................#....................#............
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+......#......#........#....................................................................................#......................
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+.......##....................................................................................#.#...#..........................#...
+......#.............#.#.........................#....#......#......#...............#.........#..#.................................
+.........................#........#.........................#....#..................#.................#............#......#.#.....
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+..........#................#.............................#.............#.......#..........................................#.......
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+...................#.......................#......#.#..............#..###.....#......................#...........#.......#.#......

6.rb

@@ -0,0 +1,104 @@
+#!/usr/bin/env ruby
+
+problem = 6
+input = File.readlines("#{problem}.input").map(&:strip).map(&:chars)
+#input = DATA.read.split("\n").map(&:chars)
+
+DIRECTIONS = ["^", ">", "v", "<"]
+
+def find_start(input)
+  input.each_with_index do |row, x|
+    y = row.index { DIRECTIONS.include?(_1) }
+    return [x,y] unless y.nil?
+  end
+end
+
+def movement(direction)
+  [[-1, 0], [0, 1], [1, 0], [0, -1]][direction]
+end
+
+def out_of_bounds?(input, location)
+  location.first >= input.size ||
+    location.last >= input.first.size ||
+    location.first < 0 ||
+    location.last < 0
+end
+
+def obstructed?(input, location)
+  ["O", "#"].include?(input[location.first][location.last])
+end
+
+def traverse(input, starting_location, starting_direction)
+  location = starting_location.dup
+  direction = starting_direction.dup
+  visited = Hash.new { |h, k| h[k] = 0 }
+
+  while
+    visited[location] += 1
+
+    next_location = []
+    loop do
+      movement = movement(direction)
+      next_location = [
+        location.first + movement.first,
+        location.last + movement.last
+      ]
+      break if out_of_bounds?(input, next_location)
+      break unless obstructed?(input, next_location)
+      direction = (direction + 1) % 4
+    end
+
+    if out_of_bounds?(input, next_location)
+      return visited.keys
+    elsif visited[location] > 5
+      return :loop
+    else
+      location = next_location
+    end
+  end
+end
+
+def find_obstructions(input, nodes, start_location, start_direction)
+  last_value = "."
+  nodes.select.with_index do |node, i|
+    # undo last mark
+    if i > 0
+      last_location = nodes[i - 1]
+      input[last_location.first][last_location.last] = last_value
+    end
+    # mark node a barrier
+    last_value = input[node.first][node.last]
+    input[node.first][node.last] = "O"
+
+    # traverse
+    traverse(input, start_location, start_direction) == :loop
+  end
+end
+
+path = []
+location = find_start(input)
+direction = DIRECTIONS.index(input[location.first][location.last])
+
+visited = traverse(input, location, direction)
+
+start = Time.now
+result_2 = find_obstructions(input, visited, location, direction)
+finish = Time.now
+puts "Finished in #{finish - start}"
+
+result_1 = visited.uniq.size
+
+p result_1
+p result_2.uniq.size
+
+__END__
+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...