159 lines
4.7 KiB
Markdown
159 lines
4.7 KiB
Markdown
## \-\-- 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 [right outside](/2017/day/24) the CPU!
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While The Historians get to work, a nearby program sees that you\'re
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idle and challenges you to a *race*. Apparently, you\'ve arrived just in
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time for 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
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even gets their very own
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[mutex](https://en.wikipedia.org/wiki/Lock_(computer_science))!
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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*
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(`E`) 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
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such move takes *1 picosecond*. The goal is to reach the end position as
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quickly 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
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the program to *pass through walls* as if they were regular track. At
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the end of the cheat, the program must be back on normal track again;
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otherwise, it will receive a [segmentation
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fault](https://en.wikipedia.org/wiki/Segmentation_fault)
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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
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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
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is activated, just before the first move that is allowed to go through
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walls) and *end position*; cheats are uniquely identified by their start
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position and end position.
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In this example, the total number of cheats (grouped by the amount of
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time 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
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to give yourself as many options as possible, you\'ll need a list of the
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best cheats. *How many cheats would save you at least 100 picoseconds?*
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To begin, [get your puzzle input](20/input).
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Answer:
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