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+--- Day 25: The Halting Problem ---
+
+Following the twisty passageways deeper and deeper into the CPU, you
+finally reach the core of the computer. Here, in the expansive central
+chamber, you find a grand apparatus that fills the entire room,
+suspended nanometers above your head.
+
+You had always imagined CPUs to be noisy, chaotic places, bustling
+with activity. Instead, the room is quiet, motionless, and dark.
+
+Suddenly, you and the CPU's garbage collector startle each other.
+"It's not often we get many visitors here!", he says. You inquire
+about the stopped machinery.
+
+"It stopped milliseconds ago; not sure why. I'm a garbage collector,
+not a doctor." You ask what the machine is for.
+
+"Programs these days, don't know their origins. That's the Turing
+machine! It's what makes the whole computer work." You try to explain
+that Turing machines are merely models of computation, but he cuts you
+off. "No, see, that's just what they want you to think. Ultimately,
+inside every CPU, there's a Turing machine driving the whole thing!
+Too bad this one's broken. We're doomed!"
+
+You ask how you can help. "Well, unfortunately, the only way to get
+the computer running again would be to create a whole new Turing
+machine from scratch, but there's no way you can-" He notices the look
+on your face, gives you a curious glance, shrugs, and goes back to
+sweeping the floor.
+
+You find the Turing machine blueprints (your puzzle input) on a tablet
+in a nearby pile of debris. Looking back up at the broken Turing
+machine above, you can start to identify its parts:
+
+    A tape which contains 0 repeated infinitely to the left and right.
+
+    A cursor, which can move left or right along the tape and read or
+    write values at its current position.
+
+    A set of states, each containing rules about what to do based on
+    the current value under the cursor.
+
+
+Each slot on the tape has two possible values: 0 (the starting value
+for all slots) and 1. Based on whether the cursor is pointing at a 0
+or a 1, the current state says what value to write at the current
+position of the cursor, whether to move the cursor left or right one
+slot, and which state to use next.
+
+For example, suppose you found the following blueprint:
+
+Begin in state A.
+Perform a diagnostic checksum after 6 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 B.
+
+In state B:
+  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 right.
+    - Continue with state A.
+
+Running it until the number of steps required to take the listed
+diagnostic checksum would result in the following tape configurations
+(with the cursor marked in square brackets):
+
+... 0  0  0 [0] 0  0 ... (before any steps; about to run state A)
+... 0  0  0  1 [0] 0 ... (after 1 step;     about to run state B)
+... 0  0  0 [1] 1  0 ... (after 2 steps;    about to run state A)
+... 0  0 [0] 0  1  0 ... (after 3 steps;    about to run state B)
+... 0 [0] 1  0  1  0 ... (after 4 steps;    about to run state A)
+... 0  1 [1] 0  1  0 ... (after 5 steps;    about to run state B)
+... 0  1  1 [0] 1  0 ... (after 6 steps;    about to run state A)
+
+The CPU can confirm that the Turing machine is working by taking a
+diagnostic checksum after a specific number of steps (given in the
+blueprint). Once the specified number of steps have been executed, the
+Turing machine should pause; once it does, count the number of times 1
+appears on the tape. In the above example, the diagnostic checksum is
+3.
+
+Recreate the Turing machine and save the computer! What is the
+diagnostic checksum it produces once it's working again?
+
+Your puzzle answer was 2526.
+
+--- Part Two ---
+
+The Turing machine, and soon the entire computer, springs back to
+life. A console glows dimly nearby, awaiting your command.
+
+> reboot printer
+Error: That command requires priority 50. You currently have priority 0.
+You must deposit 50 stars to increase your priority to the required level.
+
+The console flickers for a moment, and then prints another message:
+
+Star accepted.
+You must deposit 49 stars to increase your priority to the required level.
+
+The garbage collector winks at you, then continues sweeping.
+
+If you like, you can [Reboot the Printer Again].
+
+Both parts of this puzzle are complete! They provide two gold stars: **