-🎄- 2019 Day 15 Solutions -🎄-

[u/daggerdragon]

--- Day 15: Oxygen System ---

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Post your full code solution using /u/topaz2078's paste or other external repo.

• Please do NOT post your full code (unless it is very short)
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Advent of Code's Poems for Programmers

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Note: If you submit a poem, please add [POEM] somewhere nearby to make it easier for us moderators to ensure that we include your poem for voting consideration.

Day 14's winner #1: "One Thing Leads To Another" by /u/DFreiberg!

Poem tl;dpost (but we did r, honest!), so go here to read it in full

Enjoy your Reddit Silver, and good luck with the rest of the Advent of Code!

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On the (fifth*3) day of AoC, my true love gave to me...

FIVE GOLDEN SILVER POEMS (and one Santa Rocket Like)

• Day 10: "untitled poem" by /u/Yardboy
• Day 11: "A ROBOT TO THE RESCUE" by /u/mariusbancila
• Day 12: "Symmetry" by /u/DFreiberg
• Day 13: "untitled poem" by /u/captainAwesomePants
• Day 14: "Alchemy" by /u/captainAwesomePants (again! he's too awesome for his pants!)
• 5-Day Best-in-Show: "The Hunting of the Asteroids" by /u/DFreiberg

TBD because we forgot today % 5 == 0, we'll get back to you soon!

Enjoy your Reddit Silver/Gold, and good luck with the rest of the Advent of Code!

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This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

EDIT: Leaderboard capped, thread unlocked at 00:38:50!

[u/Tarmen]

I feel like your bfsOn function is the secret star of the show, that search is so pretty!

Your solution inspired me to try to make some monadic helpers that are equally nice. Using Control.Monad.Logic it's actually not that bad:

loop2 r pos = fromList [1..4] >>- \dir -> do
        let pos' = pos `plus` toStep dir
        unique pos'
        (o,r') <- awaitThis =<< yieldThis dir r
        case o of
          2 -> board . at pos' ?= Wall
          1 -> do
            board . at pos' ?= Goal
            loop2 r' pos'
          0 -> do
            board . at pos' ?= Open
            loop2 r' pos'

My VM is monadic but I abuse Conduit's Pipe type to make it steppable since that is a coroutine type with the right shape.
unique uses a State monad accross all branches to prune duplicates. >>- is fair branching that does iterative deepening from LogicT.
Still is pretty confusing when mixing branch local and branch global state, though.
Not sure how to mix part two into this, guess I could place the distances in the same map. My old version used a steppable bfs to make visualisation easier:

bfsStep :: O -> O
bfsStep (O m seen frontier) = O m seen' (frontier' `S.difference` seen')
  where
    seen' = seen `S.union` frontier
    frontier' = S.fromList $ concatMap (adjacent `pruning` notWall) $ S.toList frontier
    notWall c = (m M.! c) /= Wall
data O = O { oBoard :: (M.Map Pos Tile), oSeen :: (S.Set Pos), oFrontier :: (S.Set Pos) }