100 lines
3.4 KiB
Plaintext
100 lines
3.4 KiB
Plaintext
Advent of Code
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--- Day 8: Two-Factor Authentication ---
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You come across a door implementing what you can only assume is an
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implementation of two-factor authentication after a long game of
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requirements telephone.
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To get past the door, you first swipe a keycard (no problem; there was one
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on a nearby desk). Then, it displays a code on a little screen, and you type
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that code on a keypad. Then, presumably, the door unlocks.
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Unfortunately, the screen has been smashed. After a few minutes, you've
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taken everything apart and figured out how it works. Now you just have to
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work out what the screen would have displayed.
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The magnetic strip on the card you swiped encodes a series of instructions
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for the screen; these instructions are your puzzle input. The screen is 50
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pixels wide and 6 pixels tall, all of which start off, and is capable of
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three somewhat peculiar operations:
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• rect AxB turns on all of the pixels in a rectangle at the top-left of
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the screen which is A wide and B tall.
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• rotate row y=A by B shifts all of the pixels in row A (0 is the top row)
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right by B pixels. Pixels that would fall off the right end appear at the
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left end of the row.
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• rotate column x=A by B shifts all of the pixels in column A (0 is the
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left column) down by B pixels. Pixels that would fall off the bottom
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appear at the top of the column.
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For example, here is a simple sequence on a smaller screen:
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• rect 3x2 creates a small rectangle in the top-left corner:
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###....
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###....
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.......
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• rotate column x=1 by 1 rotates the second column down by one pixel:
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#.#....
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###....
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.#.....
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• rotate row y=0 by 4 rotates the top row right by four pixels:
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....#.#
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###....
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.#.....
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• rotate column x=1 by 1 again rotates the second column down by one
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pixel, causing the bottom pixel to wrap back to the top:
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.#..#.#
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#.#....
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.#.....
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As you can see, this display technology is extremely powerful, and will soon
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dominate the tiny-code-displaying-screen market. That's what the
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advertisement on the back of the display tries to convince you, anyway.
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There seems to be an intermediate check of the voltage used by the display:
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after you swipe your card, if the screen did work, how many pixels should be
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lit?
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Your puzzle answer was 119.
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--- Part Two ---
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You notice that the screen is only capable of displaying capital letters; in
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the font it uses, each letter is 5 pixels wide and 6 tall.
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After you swipe your card, what code is the screen trying to display?
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Your puzzle answer was ZFHFSFOGPO.
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References
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Visible links
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. http://adventofcode.com/
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. http://adventofcode.com/2016/about
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. http://adventofcode.com/2016/support
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. http://adventofcode.com/2016/events
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. http://adventofcode.com/2016/settings
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. http://adventofcode.com/2016/auth/logout
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016/leaderboard
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. http://adventofcode.com/2016/stats
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. http://adventofcode.com/2016/sponsors
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. http://adventofcode.com/2016/sponsors
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. https://en.wikipedia.org/wiki/Multi-factor_authentication
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. https://en.wikipedia.org/wiki/Requirement
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. https://en.wikipedia.org/wiki/Chinese_whispers
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. https://www.google.com/search?q=tiny+lcd&tbm=isch
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016/day/8/input
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