switch README to org-mode / add ccls compile_commands.json generation

2020/day24/README.org

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+** --- Day 24: Lobby Layout ---
+Your raft makes it to the tropical island; it turns out that the small
+crab was an excellent navigator. You make your way to the resort.
+
+As you enter the lobby, you discover a small problem: the floor is being
+renovated. You can't even reach the check-in desk until they've finished
+installing the /new tile floor/.
+
+The tiles are all /hexagonal/; they need to be arranged in a
+[[https://en.wikipedia.org/wiki/Hexagonal_tiling][hex grid]] with a very
+specific color pattern. Not in the mood to wait, you offer to help
+figure out the pattern.
+
+The tiles are all /white/ on one side and /black/ on the other. They
+start with the white side facing up. The lobby is large enough to fit
+whatever pattern might need to appear there.
+
+A member of the renovation crew gives you a /list of the tiles that need
+to be flipped over/ (your puzzle input). Each line in the list
+identifies a single tile that needs to be flipped by giving a series of
+steps starting from a /reference tile/ in the very center of the room.
+(Every line starts from the same reference tile.)
+
+Because the tiles are hexagonal, every tile has /six neighbors/: east,
+southeast, southwest, west, northwest, and northeast. These directions
+are given in your list, respectively, as =e=, =se=, =sw=, =w=, =nw=, and
+=ne=. A tile is identified by a series of these directions with /no
+delimiters/; for example, =esenee= identifies the tile you land on if
+you start at the reference tile and then move one tile east, one tile
+southeast, one tile northeast, and one tile east.
+
+Each time a tile is identified, it flips from white to black or from
+black to white. Tiles might be flipped more than once. For example, a
+line like =esew= flips a tile immediately adjacent to the reference
+tile, and a line like =nwwswee= flips the reference tile itself.
+
+Here is a larger example:
+
+#+BEGIN_EXAMPLE
+  sesenwnenenewseeswwswswwnenewsewsw
+  neeenesenwnwwswnenewnwwsewnenwseswesw
+  seswneswswsenwwnwse
+  nwnwneseeswswnenewneswwnewseswneseene
+  swweswneswnenwsewnwneneseenw
+  eesenwseswswnenwswnwnwsewwnwsene
+  sewnenenenesenwsewnenwwwse
+  wenwwweseeeweswwwnwwe
+  wsweesenenewnwwnwsenewsenwwsesesenwne
+  neeswseenwwswnwswswnw
+  nenwswwsewswnenenewsenwsenwnesesenew
+  enewnwewneswsewnwswenweswnenwsenwsw
+  sweneswneswneneenwnewenewwneswswnese
+  swwesenesewenwneswnwwneseswwne
+  enesenwswwswneneswsenwnewswseenwsese
+  wnwnesenesenenwwnenwsewesewsesesew
+  nenewswnwewswnenesenwnesewesw
+  eneswnwswnwsenenwnwnwwseeswneewsenese
+  neswnwewnwnwseenwseesewsenwsweewe
+  wseweeenwnesenwwwswnew
+#+END_EXAMPLE
+
+In the above example, 10 tiles are flipped once (to black), and 5 more
+are flipped twice (to black, then back to white). After all of these
+instructions have been followed, a total of /=10=/ tiles are /black/.
+
+Go through the renovation crew's list and determine which tiles they
+need to flip. After all of the instructions have been followed, /how
+many tiles are left with the black side up?/
+
+Your puzzle answer was =450=.
+
+** --- Part Two ---
+The tile floor in the lobby is meant to be a living art exhibit. Every
+day, the tiles are all flipped according to the following rules:
+
+- Any /black/ tile with /zero/ or /more than 2/ black tiles immediately
+  adjacent to it is flipped to /white/.
+- Any /white/ tile with /exactly 2/ black tiles immediately adjacent to
+  it is flipped to /black/.
+
+Here, /tiles immediately adjacent/ means the six tiles directly touching
+the tile in question.
+
+The rules are applied /simultaneously/ to every tile; put another way,
+it is first determined which tiles need to be flipped, then they are all
+flipped at the same time.
+
+In the above example, the number of black tiles that are facing up after
+the given number of days has passed is as follows:
+
+#+BEGIN_EXAMPLE
+  Day 1: 15
+  Day 2: 12
+  Day 3: 25
+  Day 4: 14
+  Day 5: 23
+  Day 6: 28
+  Day 7: 41
+  Day 8: 37
+  Day 9: 49
+  Day 10: 37
+
+  Day 20: 132
+  Day 30: 259
+  Day 40: 406
+  Day 50: 566
+  Day 60: 788
+  Day 70: 1106
+  Day 80: 1373
+  Day 90: 1844
+  Day 100: 2208
+#+END_EXAMPLE
+
+After executing this process a total of 100 times, there would be
+/=2208=/ black tiles facing up.
+
+/How many tiles will be black after 100 days?/
+
+Your puzzle answer was =4059=.
+
+Both parts of this puzzle are complete! They provide two gold stars: **
+
+At this point, all that is left is for you to [[/2020][admire your
+Advent calendar]].
+
+If you still want to see it, you can [[file:24/input][get your puzzle
+input]].
+
+You can also [Shareon
+[[https://twitter.com/intent/tweet?text=I%27ve+completed+%22Lobby+Layout%22+%2D+Day+24+%2D+Advent+of+Code+2020&url=https%3A%2F%2Fadventofcode%2Ecom%2F2020%2Fday%2F24&related=ericwastl&hashtags=AdventOfCode][Twitter]]
+[[javascript:void(0);][Mastodon]]] this puzzle.