260 lines
8.1 KiB
Plaintext
260 lines
8.1 KiB
Plaintext
Advent of Code
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--- Day 20: Race Condition ---
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The Historians are quite pixelated again. This time, a massive, black
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building looms over you - you're [16]right outside the CPU!
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While The Historians get to work, a nearby program sees that you're idle
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and challenges you to a race. Apparently, you've arrived just in time for
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the frequently-held race condition festival!
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The race takes place on a particularly long and twisting code path;
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programs compete to see who can finish in the fewest picoseconds. The
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winner even gets their very own [17]mutex!
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They hand you a map of the racetrack (your puzzle input). For example:
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###############
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#...#...#.....#
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#.#.#.#.#.###.#
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#S#...#.#.#...#
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#######.#.#.###
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#######.#.#...#
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#######.#.###.#
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###..E#...#...#
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###.#######.###
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#...###...#...#
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#.#####.#.###.#
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#.#...#.#.#...#
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#.#.#.#.#.#.###
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#...#...#...###
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###############
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The map consists of track (.) - including the start (S) and end (E)
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positions (both of which also count as track) - and walls (#).
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When a program runs through the racetrack, it starts at the start
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position. Then, it is allowed to move up, down, left, or right; each such
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move takes 1 picosecond. The goal is to reach the end position as quickly
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as possible. In this example racetrack, the fastest time is 84
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picoseconds.
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Because there is only a single path from the start to the end and the
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programs all go the same speed, the races used to be pretty boring. To
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make things more interesting, they introduced a new rule to the races:
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programs are allowed to cheat.
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The rules for cheating are very strict. Exactly once during a race, a
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program may disable collision for up to 2 picoseconds. This allows the
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program to pass through walls as if they were regular track. At the end of
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the cheat, the program must be back on normal track again; otherwise, it
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will receive a [18]segmentation fault and get disqualified.
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So, a program could complete the course in 72 picoseconds (saving 12
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picoseconds) by cheating for the two moves marked 1 and 2:
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###############
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#...#...12....#
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#.#.#.#.#.###.#
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#S#...#.#.#...#
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#######.#.#.###
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#######.#.#...#
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#######.#.###.#
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###..E#...#...#
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###.#######.###
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#...###...#...#
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#.#####.#.###.#
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#.#...#.#.#...#
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#.#.#.#.#.#.###
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#...#...#...###
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###############
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Or, a program could complete the course in 64 picoseconds (saving 20
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picoseconds) by cheating for the two moves marked 1 and 2:
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###############
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#...#...#.....#
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#.#.#.#.#.###.#
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#S#...#.#.#...#
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#######.#.#.###
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#######.#.#...#
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#######.#.###.#
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###..E#...12..#
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###.#######.###
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#...###...#...#
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#.#####.#.###.#
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#.#...#.#.#...#
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#.#.#.#.#.#.###
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#...#...#...###
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###############
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This cheat saves 38 picoseconds:
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###############
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#...#...#.....#
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#.#.#.#.#.###.#
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#S#...#.#.#...#
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#######.#.#.###
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#######.#.#...#
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#######.#.###.#
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###..E#...#...#
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###.####1##.###
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#...###.2.#...#
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#.#####.#.###.#
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#.#...#.#.#...#
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#.#.#.#.#.#.###
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#...#...#...###
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###############
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This cheat saves 64 picoseconds and takes the program directly to the end:
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###############
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#...#...#.....#
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#.#.#.#.#.###.#
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#S#...#.#.#...#
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#######.#.#.###
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#######.#.#...#
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#######.#.###.#
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###..21...#...#
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###.#######.###
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#...###...#...#
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#.#####.#.###.#
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#.#...#.#.#...#
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#.#.#.#.#.#.###
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#...#...#...###
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###############
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Each cheat has a distinct start position (the position where the cheat is
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activated, just before the first move that is allowed to go through walls)
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and end position; cheats are uniquely identified by their start position
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and end position.
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In this example, the total number of cheats (grouped by the amount of time
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they save) are as follows:
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• There are 14 cheats that save 2 picoseconds.
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• There are 14 cheats that save 4 picoseconds.
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• There are 2 cheats that save 6 picoseconds.
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• There are 4 cheats that save 8 picoseconds.
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• There are 2 cheats that save 10 picoseconds.
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• There are 3 cheats that save 12 picoseconds.
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• There is one cheat that saves 20 picoseconds.
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• There is one cheat that saves 36 picoseconds.
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• There is one cheat that saves 38 picoseconds.
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• There is one cheat that saves 40 picoseconds.
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• There is one cheat that saves 64 picoseconds.
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You aren't sure what the conditions of the racetrack will be like, so to
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give yourself as many options as possible, you'll need a list of the best
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cheats. How many cheats would save you at least 100 picoseconds?
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Your puzzle answer was 1389.
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--- Part Two ---
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The programs seem perplexed by your list of cheats. Apparently, the
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two-picosecond cheating rule was deprecated several milliseconds ago! The
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latest version of the cheating rule permits a single cheat that instead
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lasts at most 20 picoseconds.
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Now, in addition to all the cheats that were possible in just two
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picoseconds, many more cheats are possible. This six-picosecond cheat
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saves 76 picoseconds:
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###############
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#...#...#.....#
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#.#.#.#.#.###.#
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#S#...#.#.#...#
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#1#####.#.#.###
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#2#####.#.#...#
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#3#####.#.###.#
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#456.E#...#...#
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###.#######.###
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#...###...#...#
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#.#####.#.###.#
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#.#...#.#.#...#
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#.#.#.#.#.#.###
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#...#...#...###
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###############
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Because this cheat has the same start and end positions as the one above,
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it's the same cheat, even though the path taken during the cheat is
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different:
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###############
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#...#...#.....#
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#.#.#.#.#.###.#
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#S12..#.#.#...#
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###3###.#.#.###
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###4###.#.#...#
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###5###.#.###.#
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###6.E#...#...#
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###.#######.###
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#...###...#...#
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#.#####.#.###.#
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#.#...#.#.#...#
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#.#.#.#.#.#.###
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#...#...#...###
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###############
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Cheats don't need to use all 20 picoseconds; cheats can last any amount of
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time up to and including 20 picoseconds (but can still only end when the
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program is on normal track). Any cheat time not used is lost; it can't be
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saved for another cheat later.
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You'll still need a list of the best cheats, but now there are even more
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to choose between. Here are the quantities of cheats in this example that
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save 50 picoseconds or more:
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• There are 32 cheats that save 50 picoseconds.
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• There are 31 cheats that save 52 picoseconds.
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• There are 29 cheats that save 54 picoseconds.
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• There are 39 cheats that save 56 picoseconds.
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• There are 25 cheats that save 58 picoseconds.
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• There are 23 cheats that save 60 picoseconds.
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• There are 20 cheats that save 62 picoseconds.
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• There are 19 cheats that save 64 picoseconds.
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• There are 12 cheats that save 66 picoseconds.
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• There are 14 cheats that save 68 picoseconds.
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• There are 12 cheats that save 70 picoseconds.
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• There are 22 cheats that save 72 picoseconds.
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• There are 4 cheats that save 74 picoseconds.
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• There are 3 cheats that save 76 picoseconds.
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Find the best cheats using the updated cheating rules. How many cheats
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would save you at least 100 picoseconds?
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Your puzzle answer was 1005068.
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Both parts of this puzzle are complete! They provide two gold stars: **
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At this point, you should [19]return to your Advent calendar and try
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another puzzle.
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If you still want to see it, you can [20]get your puzzle input.
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References
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Visible links
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1. https://adventofcode.com/
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2. https://adventofcode.com/2024/about
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3. https://adventofcode.com/2024/events
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4. https://cottonbureau.com/people/advent-of-code
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5. https://adventofcode.com/2024/settings
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6. https://adventofcode.com/2024/auth/logout
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7. Advent of Code Supporter
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https://adventofcode.com/2024/support
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8. https://adventofcode.com/2024
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9. https://adventofcode.com/2024
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10. https://adventofcode.com/2024/support
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11. https://adventofcode.com/2024/sponsors
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12. https://adventofcode.com/2024/leaderboard
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13. https://adventofcode.com/2024/stats
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14. https://adventofcode.com/2024/sponsors
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16. https://adventofcode.com/2017/day/24
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17. https://en.wikipedia.org/wiki/Lock_(computer_science)
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18. https://en.wikipedia.org/wiki/Segmentation_fault
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19. https://adventofcode.com/2024
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20. https://adventofcode.com/2024/day/20/input
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