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