2018 Day 21 Complete

2018/day21/problem

@@ -0,0 +1,84 @@
+Advent of Code
+
+--- Day 21: Chronal Conversion ---
+
+   You should have been watching where you were going, because as you wander the
+   new North Pole base, you trip and fall into a very deep hole!
+
+   Just kidding. You're falling through time again.
+
+   If you keep up your current pace, you should have resolved all of the temporal
+   anomalies by the next time the device activates. Since you have very little
+   interest in browsing history in 500-year increments for the rest of your life,
+   you need to find a way to get back to your present time.
+
+   After a little research, you discover two important facts about the behavior
+   of the device:
+
+   First, you discover that the device is hard-wired to always send you back in
+   time in 500-year increments. Changing this is probably not feasible.
+
+   Second, you discover the activation system (your puzzle input) for the time
+   travel module. Currently, it appears to run forever without halting.
+
+   If you can cause the activation system to halt at a specific moment, maybe you
+   can make the device send you so far back in time that you cause an integer
+   underflow in time itself and wrap around back to your current time!
+
+   The device executes the program as specified in manual section one and manual
+   section two.
+
+   Your goal is to figure out how the program works and cause it to halt. You can
+   only control register 0; every other register begins at 0 as usual.
+
+   Because time travel is a dangerous activity, the activation system begins with
+   a few instructions which verify that bitwise AND (via bani) does a numeric
+   operation and not an operation as if the inputs were interpreted as strings.
+   If the test fails, it enters an infinite loop re-running the test instead of
+   allowing the program to execute normally. If the test passes, the program
+   continues, and assumes that all other bitwise operations (banr, bori, and
+   borr) also interpret their inputs as numbers. (Clearly, the Elves who wrote
+   this system were worried that someone might introduce a bug while trying to
+   emulate this system with a scripting language.)
+
+   What is the lowest non-negative integer value for register 0 that causes the
+   program to halt after executing the fewest instructions? (Executing the same
+   instruction multiple times counts as multiple instructions executed.)
+
+   Your puzzle answer was 7967233.
+
+--- Part Two ---
+
+   In order to determine the timing window for your underflow exploit, you also
+   need an upper bound:
+
+   What is the lowest non-negative integer value for register 0 that causes the
+   program to halt after executing the most instructions? (The program must
+   actually halt; running forever does not count as halting.)
+
+   Your puzzle answer was 16477902.
+
+   Both parts of this puzzle are complete! They provide two gold stars: **
+
+References
+
+   Visible links
+   . https://adventofcode.com/
+   . https://adventofcode.com/2018/about
+   . https://adventofcode.com/2018/events
+   . https://adventofcode.com/2018/settings
+   . https://adventofcode.com/2018/auth/logout
+   . Advent of Code Supporter
+	https://adventofcode.com/2018/support
+   . https://adventofcode.com/2018
+   . https://adventofcode.com/2018
+   . https://adventofcode.com/2018/support
+   . https://adventofcode.com/2018/sponsors
+   . https://adventofcode.com/2018/leaderboard
+   . https://adventofcode.com/2018/stats
+   . https://adventofcode.com/2018/sponsors
+   . https://cwe.mitre.org/data/definitions/191.html
+   . https://adventofcode.com/2018/day/16
+   . https://adventofcode.com/2018/day/19
+   . https://adventofcode.com/2018
+   . https://adventofcode.com/2018/day/21/input