day21.rb

2 files changed, 186 insertions, 0 deletions

diff --git a/day21 b/day21
new file mode 100644
index 0000000..b39b884
--- /dev/null
+++ b/day21
@@ -0,0 +1,100 @@
+reverse positions 1 through 6
+rotate based on position of letter a
+swap position 4 with position 1
+reverse positions 1 through 5
+move position 5 to position 7
+swap position 4 with position 0
+swap position 4 with position 6
+rotate based on position of letter a
+swap position 0 with position 2
+move position 5 to position 2
+move position 7 to position 1
+swap letter d with letter c
+swap position 5 with position 3
+reverse positions 3 through 7
+rotate based on position of letter d
+swap position 7 with position 5
+rotate based on position of letter f
+swap position 4 with position 1
+swap position 3 with position 6
+reverse positions 4 through 7
+rotate based on position of letter c
+move position 0 to position 5
+swap position 7 with position 4
+rotate based on position of letter f
+reverse positions 1 through 3
+move position 5 to position 3
+rotate based on position of letter g
+reverse positions 2 through 5
+rotate right 0 steps
+rotate left 0 steps
+swap letter f with letter b
+rotate based on position of letter h
+move position 1 to position 3
+reverse positions 3 through 6
+rotate based on position of letter h
+swap position 4 with position 3
+swap letter b with letter h
+swap letter a with letter h
+reverse positions 1 through 6
+swap position 3 with position 6
+swap letter e with letter d
+swap letter e with letter h
+swap position 1 with position 5
+rotate based on position of letter a
+reverse positions 4 through 5
+swap position 0 with position 4
+reverse positions 0 through 3
+move position 7 to position 2
+swap letter e with letter c
+swap position 3 with position 4
+rotate left 3 steps
+rotate left 7 steps
+rotate based on position of letter e
+reverse positions 5 through 6
+move position 1 to position 5
+move position 1 to position 2
+rotate left 1 step
+move position 7 to position 6
+rotate left 0 steps
+reverse positions 5 through 6
+reverse positions 3 through 7
+swap letter d with letter e
+rotate right 3 steps
+swap position 2 with position 1
+swap position 5 with position 7
+swap letter h with letter d
+swap letter c with letter d
+rotate based on position of letter d
+swap letter d with letter g
+reverse positions 0 through 1
+rotate right 0 steps
+swap position 2 with position 3
+rotate left 4 steps
+rotate left 5 steps
+move position 7 to position 0
+rotate right 1 step
+swap letter g with letter f
+rotate based on position of letter a
+rotate based on position of letter b
+swap letter g with letter e
+rotate right 4 steps
+rotate based on position of letter h
+reverse positions 3 through 5
+swap letter h with letter e
+swap letter g with letter a
+rotate based on position of letter c
+reverse positions 0 through 4
+rotate based on position of letter e
+reverse positions 4 through 7
+rotate left 4 steps
+swap position 0 with position 6
+reverse positions 1 through 6
+rotate left 2 steps
+swap position 5 with position 3
+swap letter b with letter d
+swap letter b with letter d
+rotate based on position of letter d
+rotate based on position of letter c
+rotate based on position of letter h
+move position 4 to position 7
diff --git a/day21.rb b/day21.rb
new file mode 100644
index 0000000..dea375b
--- /dev/null
+++ b/day21.rb
@@ -0,0 +1,86 @@
+s = "abcdefgh"
+
+def rl(s, c)
+  s = s.dup
+  a = s.index c
+  s = s.slice!(-1) + s
+  a.times {
+    s = s.slice!(-1) + s
+  }
+  s = s.slice!(-1) + s  if a >= 4
+  s
+end
+
+File.readlines("day21").each { |line|
+  case line
+  when /reverse positions (\d+) through (\d+)/
+    a, b = $1.to_i, $2.to_i
+    s[a..b] = s[a..b].reverse
+  when /rotate based on position of letter (.)/
+    s = rl(s, $1)
+  when /rotate right (\d+) step/
+    $1.to_i.times {
+      s = s.slice!(-1) + s
+    }
+  when /rotate left (\d+) step/
+    $1.to_i.times {
+      s = s + s.slice!(0)
+    }
+  when /swap position (\d+) with position (\d+)/
+    a, b = $1.to_i, $2.to_i
+    s[a], s[b] = s[b], s[a]
+  when /move position (\d+) to position (\d+)/
+    s.insert($2.to_i, s.slice!($1.to_i))
+  when /swap letter (.) with letter (.)/
+    s.tr! $1+$2, $2+$1
+  else
+    abort "what is #{line.dump}"
+  end
+}
+
+p s # gfdhebac
+
+def brute(s, a)
+  s = s.dup
+  (1..s.size).each { |i|
+    t = s.dup
+    i.times {
+      t = t.slice!(-1) + t
+    }
+    if rl(t, a) == s
+      return t
+    end
+  }
+
+  abort "oops"
+end
+
+s = "fbgdceah"
+File.readlines("day21").reverse_each { |line|
+  case line
+  when /reverse positions (\d+) through (\d+)/  #idem
+    a, b = $1.to_i, $2.to_i
+    s[a..b] = s[a..b].reverse
+  when /rotate based on position of letter (.)/  #brute
+    s = brute(s, $1)
+  when /rotate left (\d+) step/  #swap
+    $1.to_i.times {
+      s = s.slice!(-1) + s
+    }
+  when /rotate right (\d+) step/  #swap
+    $1.to_i.times {
+      s = s + s.slice!(0)
+    }
+  when /swap position (\d+) with position (\d+)/  #idem
+    a, b = $1.to_i, $2.to_i
+    s[a], s[b] = s[b], s[a]
+  when /move position (\d+) to position (\d+)/  #swap
+    s.insert($1.to_i, s.slice!($2.to_i))
+  when /swap letter (.) with letter (.)/   #idem
+    s.tr! $1+$2, $2+$1
+  else
+    abort "what is #{line.dump}"
+  end
+}
+
+p s