diff --git a/21/input b/21/input
new file mode 100644
index 0000000..10276c8
--- /dev/null
+++ b/21/input
@@ -0,0 +1,5 @@
+140A
+169A
+170A
+528A
+340A
\ No newline at end of file
diff --git a/21/main.rb b/21/main.rb
new file mode 100755
index 0000000..e220ba4
--- /dev/null
+++ b/21/main.rb
@@ -0,0 +1,176 @@
+#!/usr/bin/env ruby
+
+require "debug"
+
+input = (ARGV.first.nil? ? DATA : ARGF)
+  .readlines(chomp: true)
+  .map(&:chars)
+
+@arrow_dirs = {
+  "<" => [0, -1],
+  "^" => [-1, 0],
+  ">" => [0, 1],
+  "v" => [1, 0],
+}
+
+@dir_arrows = @arrow_dirs.invert
+
+number_movements = {
+  "0" => ["^", ">"],
+  "A" => ["<", "^"],
+  "1" => ["^", ">"],
+  "2" => ["<", "^", ">", "v"],
+  "3" => ["<", "^", "v"],
+  "4" => ["^", ">", "v"],
+  "5" => ["<", "^", ">", "v"],
+  "6" => ["<", "^", "v"],
+  "7" => [">", "v"],
+  "8" => ["<", ">", "v"],
+  "9" => ["<", "v"],
+}
+
+arrow_movements = {
+  "^" => [">", "v"],
+  "A" => ["v", "<"],
+  "<" => [">"],
+  "v" => ["^", ">", "<"],
+  ">" => ["^", "<"],
+}
+
+number_grid = [
+  ["7", "8", "9"],
+  ["4", "5", "6"],
+  ["1", "2", "3"],
+  ["#", "0", "A"],
+]
+
+arrow_grid = [
+  ["#", "^", "A"],
+  ["<", "v", ">"],
+]
+
+class PriorityQueue < Array
+  alias_method :extract_min, :shift
+
+  def <<(v)
+    super
+
+    sort_by!(&:last)
+  end
+end
+
+def discover(char, graph)
+  (0...graph.size).each do |x|
+    (0...graph.first.size).each do |y|
+      return [x, y] if char == graph.dig(x, y)
+    end
+  end
+end
+
+def dijkstra_with_lookup(graph, movements, start_char, target_char)
+  start = discover(start_char, graph)
+  target = discover(target_char, graph)
+
+  dijkstra(graph, movements, start, target)
+end
+
+def dijkstra(graph, movements, start, target)
+  queue = PriorityQueue.new
+  dest = Hash.new
+  prev = Hash.new
+
+  queue << [start, 0]
+  dest[start] = 0
+  prev[start] = nil
+
+  until queue.empty?
+    node, moves = queue.extract_min
+
+    movements[graph.dig(*node)]
+      .map { @arrow_dirs[_1] }
+      .map { |dx, dy| [node.first + dx, node.last + dy] }
+      .each do |neighbor|
+        if dest[neighbor].nil? || moves + 1 < dest[neighbor]
+          dest[neighbor] = moves + 1
+          prev[neighbor] = node
+          queue << [neighbor, moves + 1]
+        end
+      end
+  end
+
+  path = Array.new
+  u = target
+  while prev[u]
+    path.unshift(u)
+    u = prev[u]
+  end
+
+  return dest, path
+end
+
+# calculate most efficient number pad paths
+# actually AN efficient path; ties not included :\
+number_cache = number_grid
+  .flatten.reject { _1 == "#" || _2 == "#" }
+  .permutation(2).to_a
+  .map { [[_1, _2], dijkstra_with_lookup(number_grid, number_movements, _1, _2)] }
+  .map do |(start, target), (dest, prev)|
+    presses = ([discover(start, number_grid)] + prev)
+      .each_cons(2).to_a
+      .map { @dir_arrows[[_2.first - _1.first, _2.last - _1.last]] }
+    [[start, target], presses]
+  end
+  .to_h
+
+# cache efficient direction movements
+arrow_cache = arrow_grid
+  .flatten.reject { _1 == "#" || _2 == "#" }
+  .permutation(2).to_a
+  .map { [[_1, _2], dijkstra_with_lookup(arrow_grid, arrow_movements, _1, _2)] }
+  .map do |(start, target), (dest, prev)|
+    presses = ([discover(start, arrow_grid)] + prev)
+      .each_cons(2).to_a
+      .map { @dir_arrows[[_2.first - _1.first, _2.last - _1.last]] }
+    [[start, target], presses]
+  end
+  .to_h
+arrow_cache[["<", "<"]] = ["A"]
+arrow_cache[["^", "^"]] = ["A"]
+arrow_cache[[">", ">"]] = ["A"]
+arrow_cache[["v", "v"]] = ["A"]
+arrow_cache[["A", "A"]] = ["A"]
+
+robot_2 = input
+  .map { ["A"] + _1 }
+  .map { _1.each_cons(2).to_a }
+  .map { _1.map { |move| number_cache[move].join }.join("A") + "A" }
+
+
+robot_3 = robot_2
+  .map { _1.chars }
+  .map { ["A"] + _1 }
+  .map { _1.each_cons(2).to_a }
+  .map { _1.map { |move| arrow_cache[move].join }.join("A") + "A" }
+
+me = robot_3
+  .map { _1.chars }
+  .map { ["A"] + _1 }
+  .map { _1.each_cons(2).to_a }
+  .map { _1.map { |move| arrow_cache[move].join }.join("A") + "A" }
+
+pp " <vA<AA>>^AvAA<^A>A<v<A>>^AvA^A<vA>^A<v<A>^A>AAvA^A<v<A>A>^AAAvA<^A>A"
+pp me
+
+me.each_with_index do |entry, i|
+  p "#{entry.length} * #{input[i].join.to_i} = #{entry.length * input[i].join.to_i}"
+end
+p me.map.with_index { |entry, i| entry.length * input[i].join.to_i }.sum
+
+#debugger
+
+__END__
+029A
+980A
+179A
+456A
+379A