2022 Day 9 Part 1

2022/day9.rb

@@ -0,0 +1,62 @@
+require "set"
+
+$headX = 0
+$headY = 0
+$tailX = 0
+$tailY = 0
+
+$tailPositions = Set[[$tailX, $tailY]]
+
+def moveStep(dx, dy)
+  $headX += dx
+  $headY += dy
+  
+  dx = $headX - $tailX
+  dy = $headY - $tailY
+  
+  case [dx.abs, dy.abs]
+  when [0, 0], [1, 1], [1, 0], [0, 1] then
+    
+  when [2, 0], [0, 2], [2, 2] then
+    $tailX += dx <=> 0
+    $tailY += dy <=> 0
+  when [2, 1] then
+    $tailX += dx <=> 0
+    $tailY += dy <=> 0
+  when [1, 2] then
+    $tailX += dx <=> 0
+    $tailY += dy <=> 0
+  else 
+    puts "Unknown: (#{dx}, #{dy})"
+    raise 
+  end
+
+  $tailPositions.add [$tailX, $tailY]
+end
+
+
+for line in File.readlines('day9.input') 
+  unless /^(?<dir>[RLUD]) (?<steps>\d+)$/ =~ line
+    puts "Failed to parse #{line}"
+    raise
+  end
+  
+  steps = steps.to_i
+  
+  case dir
+  when "R" then
+    steps.times { moveStep 1, 0 }
+  when "L" then
+    steps.times { moveStep -1, 0 }
+  when "U" then
+    steps.times { moveStep 0, 1 }
+  when "D" then
+    steps.times { moveStep 0, -1 }
+  else
+    raise
+  end
+end
+
+puts $tailPositions.count
+
+