adventofcode/2016/day23/problem

Advent of Code

--- Day 23: Safe Cracking ---

   This is one of the top floors of the nicest tower in EBHQ. The Easter
   Bunny's private office is here, complete with a safe hidden behind a
   painting, and who wouldn't hide a star in a safe behind a painting?

   The safe has a digital screen and keypad for code entry. A sticky note
   attached to the safe has a password hint on it: "eggs". The painting is of a
   large rabbit coloring some eggs. You see 7.

   When you go to type the code, though, nothing appears on the display;
   instead, the keypad comes apart in your hands, apparently having been
   smashed. Behind it is some kind of socket - one that matches a connector in
   your prototype computer! You pull apart the smashed keypad and extract the
   logic circuit, plug it into your computer, and plug your computer into the
   safe.

   Now, you just need to figure out what output the keypad would have sent to
   the safe. You extract the assembunny code from the logic chip (your puzzle
   input).

   The code looks like it uses almost the same architecture and instruction set
   that the monorail computer used! You should be able to use the same
   assembunny interpreter for this as you did there, but with one new
   instruction:

   tgl x toggles the instruction x away (pointing at instructions like jnz
   does: positive means forward; negative means backward):

     • For one-argument instructions, inc becomes dec, and all other
       one-argument instructions become inc.

     • For two-argument instructions, jnz becomes cpy, and all other
       two-instructions become jnz.

     • The arguments of a toggled instruction are not affected.

     • If an attempt is made to toggle an instruction outside the program,
       nothing happens.

     • If toggling produces an invalid instruction (like cpy 1 2) and an
       attempt is later made to execute that instruction, skip it instead.

     • If tgl toggles itself (for example, if a is 0, tgl a would target itself
       and become inc a), the resulting instruction is not executed until the
       next time it is reached.

   For example, given this program:

 cpy 2 a
 tgl a
 tgl a
 tgl a
 cpy 1 a
 dec a
 dec a

     • cpy 2 a initializes register a to 2.

     • The first tgl a toggles an instruction a (2) away from it, which changes
       the third tgl a into inc a.

     • The second tgl a also modifies an instruction 2 away from it, which
       changes the cpy 1 a into jnz 1 a.

     • The fourth line, which is now inc a, increments a to 3.

     • Finally, the fifth line, which is now jnz 1 a, jumps a (3) instructions
       ahead, skipping the dec a instructions.

   In this example, the final value in register a is 3.

   The rest of the electronics seem to place the keypad entry (the number of
   eggs, 7) in register a, run the code, and then send the value left in
   register a to the safe.

   What value should be sent to the safe?

   Your puzzle answer was 10584.

--- Part Two ---

   The safe doesn't open, but it does make several angry noises to express its
   frustration.

   You're quite sure your logic is working correctly, so the only other thing
   is... you check the painting again. As it turns out, colored eggs are still
   eggs. Now you count 12.

   As you run the program with this new input, the prototype computer begins to
   overheat. You wonder what's taking so long, and whether the lack of any
   instruction more powerful than "add one" has anything to do with it. Don't
   bunnies usually multiply?

   Anyway, what value should actually be sent to the safe?

   Your puzzle answer was 479007144.

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