Advent of Code

--- Day 17: Spinlock ---

   Suddenly, whirling in the distance, you notice what looks like a massive,
   pixelated hurricane: a deadly spinlock. This spinlock isn't just consuming
   computing power, but memory, too; vast, digital mountains are being ripped
   from the ground and consumed by the vortex.

   If you don't move quickly, fixing that printer will be the least of your
   problems.

   This spinlock's algorithm is simple but efficient, quickly consuming
   everything in its path. It starts with a circular buffer containing only the
   value 0, which it marks as the current position. It then steps forward
   through the circular buffer some number of steps (your puzzle input) before
   inserting the first new value, 1, after the value it stopped on. The inserted
   value becomes the current position. Then, it steps forward from there the
   same number of steps, and wherever it stops, inserts after it the second new
   value, 2, and uses that as the new current position again.

   It repeats this process of stepping forward, inserting a new value, and using
   the location of the inserted value as the new current position a total of
   2017 times, inserting 2017 as its final operation, and ending with a total of
   2018 values (including 0) in the circular buffer.

   For example, if the spinlock were to step 3 times per insert, the circular
   buffer would begin to evolve like this (using parentheses to mark the current
   position after each iteration of the algorithm):

     * (0), the initial state before any insertions.
     * 0 (1): the spinlock steps forward three times (0, 0, 0), and then inserts
       the first value, 1, after it. 1 becomes the current position.
     * 0 (2) 1: the spinlock steps forward three times (0, 1, 0), and then
       inserts the second value, 2, after it. 2 becomes the current position.
     * 0  2 (3) 1: the spinlock steps forward three times (1, 0, 2), and then
       inserts the third value, 3, after it. 3 becomes the current position.

   And so on:

     * 0  2 (4) 3  1
     * 0 (5) 2  4  3  1
     * 0  5  2  4  3 (6) 1
     * 0  5 (7) 2  4  3  6  1
     * 0  5  7  2  4  3 (8) 6  1
     * 0 (9) 5  7  2  4  3  8  6  1

   Eventually, after 2017 insertions, the section of the circular buffer near
   the last insertion looks like this:

 1512  1134  151 (2017) 638  1513  851

   Perhaps, if you can identify the value that will ultimately be after the last
   value written (2017), you can short-circuit the spinlock. In this example,
   that would be 638.

   What is the value after 2017 in your completed circular buffer?

   Your puzzle answer was ________.

--- Part Two ---

   The spinlock does not short-circuit. Instead, it gets more angry. At least,
   you assume that's what happened; it's spinning significantly faster than it
   was a moment ago.

   You have good news and bad news.

   The good news is that you have improved calculations for how to stop the
   spinlock. They indicate that you actually need to identify the value after 0
   in the current state of the circular buffer.

   The bad news is that while you were determining this, the spinlock has just
   finished inserting its fifty millionth value (50000000).

   What is the value after 0 the moment 50000000 is inserted?

   Your puzzle answer was _____________.

   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.

   Your puzzle input was 301.

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