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+--- 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 1306.
+
+--- 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 20430489.
+
+Both parts of this puzzle are complete! They provide two gold stars: **