DavidEGrayson · December 22, 2022 07:00 · Dec 22, 2022 · Dec 22, 2022
diff --git a/advent.rb b/advent.rb
@@ -0,0 +1,151 @@
+map = ['']
+instructions = []
+File.foreach('input.txt') do |line|
+  md = line.match(/\d/)
+  if md
+    line.scan(/(\d+|R|L)/) do |m|
+      if 'LR'.include?(m[0])
+        instructions << m[0].to_sym
+      else
+        instructions << m[0].to_i
+      end
+    end
+  else
+    map << ' ' + line.chomp + ' '
+  end
+end
+map << ''
+width = map.map(&:size).max
+map = map.map { |line| line.ljust(width) }
+$map = map
+
+def transform(in_a, in_b, out_a, out_b, input)
+  mult = (out_b - out_a) / (in_b - in_a)
+  raise if ![-1, 1].include?(mult)
+  out_a + (input - in_a) * mult
+end
+
+def move1(x, y, face, sanity=false)
+  new_x = x
+  new_y = y
+  new_face = face
+
+  while true
+    tmp_x, tmp_y, tmp_face = new_x, new_y, new_face
+
+    dx = [1, 0, -1, 0].fetch(new_face)
+    dy = [0, 1, 0, -1].fetch(new_face)
+    new_y = (new_y + dy) % $map.size
+    new_x = (new_x + dx) % $map[new_y].size
+
+    if $map[new_y][new_x] == ' '
+      if new_x == 151 && (1..50).include?(new_y)       # B to D
+        new_x, new_y = 100, transform(1, 50, 150, 101, new_y)
+        new_face = 2
+
+      elsif new_x == 101 && (101..150).include?(new_y) # D to B
+        new_x, new_y = 150, transform(150, 101, 1, 50, new_y)
+        new_face = 2
+
+      elsif (51..100).include?(new_x) && new_y == 0    # A to F
+        new_x, new_y = 1, transform(51, 100, 151, 200, new_x)
+        new_face = 0
+
+      elsif new_x == 0 && (151..200).include?(new_y)   # F to A
+        new_x, new_y = transform(151, 200, 51, 100, new_y), 1
+        new_face = 1
+
+      elsif (1..50).include?(new_x) && new_y == 201    # F to B
+        new_x, new_y = new_x + 100, 1
+        new_face = 1  # redundant
+
+      elsif (101..150).include?(new_x) && new_y == 0   # B to F
+        new_x, new_y = new_x - 100, 200
+        new_face = 3  # redundant
+
+      elsif new_x == 51 && (151..200).include?(new_y)  # F to D
+        new_x, new_y = transform(151, 200, 51, 100, new_y), 150
+        new_face = 3
+
+      elsif (51..100).include?(new_x) && new_y == 151  # D to F
+        new_x, new_y = 50, transform(51, 100, 151, 200, new_x)
+        new_face = 2
+
+      elsif new_x == 50 && (1..50).include?(new_y)     # A to E
+        new_x, new_y = 1, transform(1, 50, 150, 101, new_y)
+        new_face = 0
+
+      elsif new_x == 0 && (101..150).include?(new_y)   # E to A
+        new_x, new_y = 51, transform(150, 101, 1, 50, new_y)
+        new_face = 0
+
+      elsif (1..50).include?(new_x) && new_y == 100    # E to C
+        new_x, new_y = 51, transform(1, 50, 51, 100, new_x)
+        new_face = 0
+
+      elsif new_x == 50 && (51..100).include?(new_y)   # C to E
+        new_x, new_y = transform(51, 100, 1, 50, new_y), 101
+        new_face = 1
+
+      elsif new_x == 101 && (51..100).include?(new_y)  # C to B
+        new_x, new_y = transform(51, 100, 101, 150, new_y), 50
+        new_face = 3
+
+      elsif (101..150).include?(new_x) && new_y == 51  # B to C
+        new_x, new_y = 100, transform(101, 150, 51, 100, new_x)
+        new_face = 2
+
+      else
+        $stderr.puts "Unknown space: #{new_x},#{new_y}, #{new_face}"
+        exit 2
+      end
+
+      # Sanity check: if we moved backwards do we get to where we were before?
+      if !sanity
+        s_x, s_y, s_face = move1(new_x, new_y, ((new_face + 2) % 4), true)
+        if s_x != tmp_x || s_y != tmp_y || s_face != ((tmp_face + 2) % 4)
+          $stderr.puts "Sanity check failed."
+          $stderr.puts "tmp:  #{tmp_x},#{tmp_y}, #{tmp_face}"
+          $stderr.puts "new:  #{new_x},#{new_y}, #{new_face}"
+          $stderr.puts "sane: #{s_x},#{s_y}, #{s_face}"
+          exit 3
+        end
+      end
+    end
+    if $map[new_y][new_x] == ' '
+      $stderr.puts "Still in a space: #{new_x},#{new_y}, #{new_face}"
+      exit 3
+    end
+    break
+  end
+
+  [new_x, new_y, new_face]
+end
+
+y = 1
+x = map[y].index('.')
+face = 0   # 0 = right, 1 = down, etc.
+
+instructions.each do |inst|
+  puts "Executing #{inst}"
+  case inst
+  when :L
+    face = (face - 1) % 4
+  when :R
+    face = (face + 1) % 4
+  when Integer
+    inst.times do
+      new_x, new_y, new_face = move1(x, y, face)
+      if map[new_y][new_x] == '.'
+        x = new_x
+        y = new_y
+        face = new_face
+      elsif map[new_y][new_x] == '#'
+        break  # Blocked by a wall.
+      end
+    end
+  end
+end
+map[y][x] = '*'
+
+puts 1000 * y + 4 * x + face