## \-\-- 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]{title="Floor 17: cafeteria, printing department, and experimental organic digitization equipment."},
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](https://en.wikipedia.org/wiki/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 `1158`.

## \-\-- Part Two \-\-- {#part2}

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 `2`s 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 `1132`.

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

At this point, you should [return to your Advent calendar](/2017) and
try another puzzle.

If you still want to see it, you can [get your puzzle
input](1/input).