Advent of Code

--- Day 1: Inverse Captcha ---

   The night before Christmas, one of Santa's Elves calls you in a panic. "The
   printer's broken! We can't print the Naughty or Nice List!" By the time you
   make it to sub-basement 17, there are only a few minutes until midnight. "We
   have a big problem," she says; "there must be almost fifty bugs in this
   system, but nothing else can print The List. Stand in this square, quick!
   There's no time to explain; if you can convince them to pay you in stars,
   you'll be able to--" She pulls a lever and the world goes blurry.

   When your eyes can focus again, everything seems a lot more pixelated than
   before. She must have sent you inside the computer! You check the system
   clock: 25 milliseconds until midnight. With that much time, you should be
   able to collect all fifty stars by December 25th.

   Collect stars by solving puzzles. Two puzzles will be made available on each
   day millisecond in the advent calendar; the second puzzle is unlocked when
   you complete the first. Each puzzle grants one star. Good luck!

   You're standing in a room with "digitization quarantine" written in LEDs
   along one wall. The only door is locked, but it includes a small interface.
   "Restricted Area - Strictly No Digitized Users Allowed."

   It goes on to explain that you may only leave by solving a captcha to prove
   you're not a human. Apparently, you only get one millisecond to solve the
   captcha: too fast for a normal human, but it feels like hours to you.

   The captcha requires you to review a sequence of digits (your puzzle input)
   and find the sum of all digits that match the next digit in the list. The
   list is circular, so the digit after the last digit is the first digit in
   the list.

   For example:

     • 1122 produces a sum of 3 (1 + 2) because the first digit (1) matches the
       second digit and the third digit (2) matches the fourth digit.
     • 1111 produces 4 because each digit (all 1) matches the next. 
     • 1234 produces 0 because no digit matches the next.
     • 91212129 produces 9 because the only digit that matches the next one is 
       the last digit, 9.

   What is the solution to your captcha?

   Your puzzle answer was 1144.

--- Part Two ---

   You notice a progress bar that jumps to 50% completion. Apparently, the door
   isn't yet satisfied, but it did emit a star as encouragement. The
   instructions change:

   Now, instead of considering the next digit, it wants you to consider the
   digit halfway around the circular list. That is, if your list contains 10
   items, only include a digit in your sum if the digit 10/2 = 5 steps forward
   matches it. Fortunately, your list has an even number of elements.

   For example:

     • 1212 produces 6: the list contains 4 items, and all four digits match
       the digit 2 items ahead.
     • 1221 produces 0, because every comparison is between a 1 and a 2.
     • 123425 produces 4, because both 2s match each other, but no other digit
       has a match.
     • 123123 produces 12.
     • 12131415 produces 4.

   What is the solution to your new captcha?

   Your puzzle answer was 1194.

   Both parts of this puzzle are complete! They provide two gold stars: **

   At this point, you should return to your advent calendar and try another
   puzzle.

   If you still want to see it, you can get your puzzle input.

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