diff --git a/25.hs b/25.hs
new file mode 100644
index 0000000..0289818
--- /dev/null
+++ b/25.hs
@@ -0,0 +1,36 @@
+{-# LANGUAGE BangPatterns #-}
+import Data.IntMap.Strict (IntMap, findWithDefault, insert, empty)
+
+type Tape = IntMap Int
+type Step = (Tape, Int, State)
+data State = State {
+    zeroVal :: Int,
+    oneVal  :: Int,
+    zeroPos :: Int,
+    onePos  :: Int,
+    zeroState :: State,
+    oneState  :: State
+}
+
+a, b, c, d, e, f :: State
+a = State 1 0   1  (-1) b e
+b = State 1 0 (-1)   1  c a
+c = State 1 0 (-1)   1  d c
+d = State 1 0 (-1) (-1) e f
+e = State 1 1 (-1) (-1) a c
+f = State 1 1 (-1)   1  e a
+
+step :: Step -> Step
+step (tape, position, (State zval oval zpos opos zstate ostate)) =
+    case findWithDefault 0 position tape of
+        0 -> (insert position zval tape, position + zpos, zstate)
+        1 -> (insert position oval tape, position + opos, ostate)
+
+stepN :: Int -> Step -> Step
+stepN 0 s = s
+stepN n s = let !nextStep = step s in stepN (n - 1) nextStep
+
+main :: IO ()
+main = do
+    let (tape, _, _) = stepN 12386363 (empty, 0, a)
+    print $ foldr (+) 0 tape
\ No newline at end of file
diff --git a/25.txt b/25.txt
new file mode 100644
index 0000000..db47644
--- /dev/null
+++ b/25.txt
@@ -0,0 +1,62 @@
+Begin in state A.
+Perform a diagnostic checksum after 12386363 steps.
+
+In state A:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state B.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state E.
+
+In state B:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state C.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the right.
+    - Continue with state A.
+
+In state C:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state D.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the right.
+    - Continue with state C.
+
+In state D:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state E.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state F.
+
+In state E:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state C.
+
+In state F:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state E.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
diff --git a/README.md b/README.md
index 4e46f03..24443db 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,8 @@ These are the runtimes of only one trial but the variances are fairly small and
 | 22  | 12.867      |  7.880
 | 23  |  0.274      |
 | 24  |  6.887      |
+| 25  |  6.072      |
 
-Problems that could use some work: 05, 22, 14, 15, 21, 17
+Problems that could use some work: 05, 22, 24, 25, 14, 15, 21, 17
 
 Problems that are good enough with optimizations: 13