From f95ba52ef9a14acac4167779a930c919c2efa163 Mon Sep 17 00:00:00 2001
From: Alpha Chen <alpha.chen@gmail.com>
Date: Wed, 21 Dec 2016 00:20:15 -0500
Subject: [PATCH] [2016][ruby][21.1]

---
 2016/ruby/day_21.rb | 79 ++++++++++++++++++++++++++-------------------
 1 file changed, 45 insertions(+), 34 deletions(-)

diff --git a/2016/ruby/day_21.rb b/2016/ruby/day_21.rb
index 8b9c137..d24f75f 100644
--- a/2016/ruby/day_21.rb
+++ b/2016/ruby/day_21.rb
@@ -1,37 +1,48 @@
-input = 'abcdefgh'
-ARGF.read.split("\n").each do |instruction|
-  case instruction
-  when /swap position (\d+) with position (\d+)/
-    x = $1.to_i
-    y = $2.to_i
-    input[x], input[y] = input[y], input[x]
-  when /swap letter (\w+) with letter (\w+)/
-    x = $1
-    y = $2
-    input.tr!("#{x}#{y}", "#{y}#{x}")
-  when /rotate (left|right) (\d+) steps?/
-    steps = $2.to_i
-    steps = -steps if $1 == 'right'
-    input = input.chars.rotate(steps).join
-  when /rotate based on position of letter (\w+)/
-    index = input.index($1)
-    index += 1 if index >= 4
-    index += 1
-    index = -index
-    input = input.chars.rotate(index).join
-  when /reverse positions (\d+) through (\d+)/
-    x = $1.to_i
-    y = $2.to_i
-    input[x..y] = input[x..y].reverse
-  when /move position (\d+) to position (\d+)/
-    x = $1.to_i
-    x = input[x]
-    input[x] = ''
+INSTRUCTIONS = ARGF.read.split("\n")
 
-    y = $2.to_i
-    input.insert(y, x)
-  else
-    raise "invalid instruction: '#{instruction}'"
+def scramble(input)
+  INSTRUCTIONS.each do |instruction|
+    case instruction
+    when /swap position (\d+) with position (\d+)/
+      x = $1.to_i
+      y = $2.to_i
+      input[x], input[y] = input[y], input[x]
+    when /swap letter (\w+) with letter (\w+)/
+      x = $1
+      y = $2
+      input.tr!("#{x}#{y}", "#{y}#{x}")
+    when /rotate (left|right) (\d+) steps?/
+      steps = $2.to_i
+      steps = -steps if $1 == 'right'
+      input = input.chars.rotate(steps).join
+    when /rotate based on position of letter (\w+)/
+      index = input.index($1)
+      index += 1 if index >= 4
+      index += 1
+      index = -index
+      input = input.chars.rotate(index).join
+    when /reverse positions (\d+) through (\d+)/
+      x = $1.to_i
+      y = $2.to_i
+      input[x..y] = input[x..y].reverse
+    when /move position (\d+) to position (\d+)/
+      x = $1.to_i
+      x = input[x]
+      input[x] = ''
+
+      y = $2.to_i
+      input.insert(y, x)
+    else
+      raise "invalid instruction: '#{instruction}'"
+    end
+  end
+  input
+end
+
+input = 'abcdefgh'.chars
+input.permutation.each do |candidate|
+  if scramble(candidate.join) == 'fbgdceah'
+    puts candidate.join
+    exit
   end
 end
-puts input