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Day 17 - sped up to ~6s using strict recursion!

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Jonathan Chan 2017-12-17 15:13:38 -08:00
parent c585f7d7ba
commit ec07598f61
1 changed files with 16 additions and 17 deletions
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17.hs
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@ -1,23 +1,22 @@
{-# LANGUAGE BangPatterns #-}
import Data.List (foldl')
(%) = mod
steps = 312
ith :: Int -> Int -> Int
ith i steps =
let (position, list) = foldl' (\(currPos, currList) n ->
let newPos = (currPos + steps % n + 1) % n
in (newPos, take newPos currList ++ [n] ++ drop newPos currList))
(0, [0]) [1..i]
in list !! (position + 1)
postNth :: Int -> Int -> [Int] -> Int
postNth 2018 pos list = list !! (pos + 1)
postNth n pos list =
let !newPos = (pos + steps % n + 1) % n
!newList = take newPos list ++ [n] ++ drop newPos list
in postNth (n + 1) newPos newList
oneth :: Int -> Int -> Int
oneth i steps =
snd $ foldl' (\(currPos, currOneth) n ->
let newPos = (currPos + steps % n + 1) % n
in (newPos, if newPos == 0 then n else currOneth))
(0, 1) [1..i]
oneNth :: Int -> Int -> Int -> Int
oneNth 50000001 _ oneth = oneth
oneNth n pos oneth =
let !newPos = (pos + steps % n + 1) % n
!newOneth = if newPos == 0 then n else oneth
in oneNth (n + 1) newPos newOneth
main :: IO ()
main = do
print $ ith 2017 312
print $ oneth 50000000 312
print $ postNth 1 1 [0]
print $ oneNth 1 1 1