Haskell Quiz/Numeric Maze/Solution Dolio: Difference between revisions
< Haskell Quiz | Numeric Maze
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import Control.Monad | import Control.Monad | ||
import Data.IntSet (fromList, insert, empty, notMember, union) | import Data.IntSet (fromList, insert, empty, notMember, union) | ||
import Data.Ratio | import Data.Ratio | ||
import System | import System | ||
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t = target % 1 | t = target % 1 | ||
search' ls s | search' ls s | ||
| | | (x:_) <- filter ((==t) . head) ls = x | ||
| otherwise = search' ls' s' | | otherwise = search' ls' s' | ||
where | where | ||
Latest revision as of 07:19, 13 December 2009
This is a simple breadth-first search with some pruning. It avoids visiting numbers that have already been seen in a shorter sequence, and trims in too-large values (I noticed that no shortest sequences seemed to use numbers greater than 2*max(init, target) + 2, so such values could be discarded).
{-# OPTIONS_GHC -fglasgow-exts #-}
module Main where
import Control.Monad
import Data.IntSet (fromList, insert, empty, notMember, union)
import Data.Ratio
import System
search init target = reverse . map numerator $ search' [[init % 1]] (fromList [init])
where
t = target % 1
search' ls s
| (x:_) <- filter ((==t) . head) ls = x
| otherwise = search' ls' s'
where
s' = s `union` fromList (map (numerator . head) ls')
ls' = do l@(h:_) <- ls
n <- [h + 2, h * 2, h / 2]
guard (denominator n == 1)
guard (numerator n `notMember` s)
guard (if target > init then n < 2*t + 1 else n < 2*(init%1) + 3)
return (n:l)
main = do [i, t] <- fmap (map read) getArgs
print $ search i t