Add day 21: Scrambled Passwords

Author: petertseng

21_scrambled_passwords.rb

@@ -0,0 +1,62 @@
+def apply(instructions, input, undo: false)
+  instructions.reduce(input.dup) { |pw, (cmd, *args)|
+    case cmd
+    when :swap_letter
+      # Undo == do
+      pw.tr(args.join, args.join.reverse)
+    when :swap_position
+      # Undo == do
+      i, j = args
+      pw[i], pw[j] = [pw[j], pw[i]]
+      pw
+    when :rotate_right
+      pw.chars.rotate(args[0] * (undo ? 1 : -1)).join
+    when :rotate_left
+      pw.chars.rotate(args[0] * (undo ? -1 : 1)).join
+    when :rotate_based
+      i = pw.index(args[0])
+      if undo
+        # rotate_based needs the most work to undo.
+        # pos shift newpos
+        #   0     1      1
+        #   1     2      3
+        #   2     3      5
+        #   3     4      7
+        #   4     6      2
+        #   5     7      4
+        #   6     8      6
+        #   7     9      0
+        # all odds have a clear pattern, all evens have a clear pattern...
+        # except 0, which we'll just special-case.
+        rot = i / 2 + (i % 2 == 1 || i == 0 ? 1 : 5)
+      else
+        rot = -(i + 1 + (i >= 4 ? 1 : 0))
+      end
+      pw.chars.rotate(rot).join
+    when :reverse_positions
+      # Undo == do
+      c = pw.chars
+      s, e = args
+      c[s..e] = c[s..e].reverse
+      c.join
+    when :move_position
+      from, to = undo ? args.reverse : args
+      c = pw.chars
+      ch = c.delete_at(from)
+      c.insert(to, ch)
+      c.join
+    else raise "Unknown command #{cmd} #{args}"
+    end
+  }
+end
+
+instructions = ARGF.map { |l|
+  words = l.split
+  # All the args are either single letters or single digits.
+  [words[0..1].join(?_).to_sym] + words.select { |w| w.size == 1 }.map { |w|
+    w.match?(/\d+/) ? Integer(w) : w
+  }.freeze
+}.freeze
+
+puts apply(instructions, 'abcdefgh')
+puts apply(instructions.reverse, 'fbgdceah', undo: true)