day 9 part b

Author: schoblaska

09/a.rb

@@ -10,11 +10,8 @@
   dist.times do
     h = [h, dir].transpose.map(&:sum)
     xdiff, ydiff = h[0] - t[0], h[1] - t[1]
-
-    unless xdiff.abs < 2 && ydiff.abs < 2 # unless adjacent
-      t = [t[0] + xdiff.clamp(-1, 1), t[1] + ydiff.clamp(-1, 1)]
-    end
-
+    next if xdiff.abs < 2 && ydiff.abs < 2 # adjacent; do nothing
+    t = [t[0] + xdiff.clamp(-1, 1), t[1] + ydiff.clamp(-1, 1)]
     visited << t
   end
 end

09/b.rb

@@ -0,0 +1,25 @@
+require "set"
+
+knots = Array.new(10, [0, 0])
+visited = Set.new([[0, 0]])
+dirs = { "U" => [0, 1], "D" => [0, -1], "R" => [1, 0], "L" => [-1, 0] }
+
+File.foreach("09/input.txt", chomp: true) do |line|
+  dir, dist = dirs[line[/[UDLR]/]], line[/\d+/].to_i
+
+  dist.times do
+    knots.each_with_index do |knot, i|
+      if i == 0
+        knots[i] = [knot, dir].transpose.map(&:sum)
+      else
+        xdiff, ydiff = knots[i - 1][0] - knot[0], knots[i - 1][1] - knot[1]
+        next if xdiff.abs < 2 && ydiff.abs < 2 # adjacent, do nothing
+        knots[i] = [knot[0] + xdiff.clamp(-1, 1), knot[1] + ydiff.clamp(-1, 1)]
+      end
+    end
+
+    visited << knots.last
+  end
+end
+
+puts visited.count