1 files changed, 80 insertions, 0 deletions
diff --git a/day09/part1.rb b/day09/part1.rb
new file mode 100644
index 0000000..58debcd
--- /dev/null
+++ b/day09/part1.rb
@@ -0,0 +1,80 @@
+class Pos
+  def initialize(x = 0, y = 0)
+    @x = x
+    @y = y
+  end
+
+  attr_reader :x, :y
+
+  def move(dir)
+    case dir
+    when "U"
+      @y += 1
+    when "D"
+      @y -= 1
+    when "R"
+      @x += 1
+    when "L"
+      @x -= 1
+    end
+  end
+
+  def follow(head)
+    if    head.x == @x - 1 && head.y == @y + 2
+      @x -= 1
+      @y += 1
+    elsif head.x == @x     && head.y == @y + 2
+      @y += 1
+    elsif head.x == @x + 1 && head.y == @y + 2
+      @x += 1
+      @y += 1
+
+    elsif head.x == @x + 2 && head.y == @y + 1
+      @x += 1
+      @y += 1
+    elsif head.x == @x + 2 && head.y == @y
+      @x += 1
+    elsif head.x == @x + 2 && head.y == @y - 1
+      @x += 1
+      @y -= 1
+
+    elsif head.x == @x + 1 && head.y == @y - 2
+      @x += 1
+      @y -= 1
+    elsif head.x == @x     && head.y == @y - 2
+      @y -= 1
+    elsif head.x == @x - 1 && head.y == @y - 2
+      @x -= 1
+      @y -= 1
+
+    elsif head.x == @x - 2 && head.y == @y - 1
+      @x -= 1
+      @y -= 1
+    elsif head.x == @x - 2 && head.y == @y
+      @x -= 1
+    elsif head.x == @x - 2 && head.y == @y + 1
+      @x -= 1
+      @y += 1
+    end
+  end
+
+  def coord
+    [@x, @y]
+  end
+end
+
+moves = $stdin.readlines.map(&:strip).map(&:split)
+
+head = Pos.new
+tail = Pos.new
+visited = [tail.coord]
+
+moves.each do |dir, dist|
+  dist.to_i.times do
+    head.move(dir)
+    tail.follow(head)
+    visited << tail.coord
+  end
+end
+
+puts visited.uniq.count