Added solution to rubyquiz 60

AStar.hs

@@ -1,3 +1,5 @@
+{- Copyright 2012 Abhinav Sarkar <abhinav@abhinavsarkar.net> -}
+
 {-# LANGUAGE MultiParamTypeClasses #-}
 
 module AStar where

NumericMaze.hs

@@ -0,0 +1,44 @@
+{-
+ A Solution to rubyquiz 60 (http://rubyquiz.com/quiz60.html)
+
+ You have a starting point and a target. You have a set of three operations:
+  double
+  halve (Odd numbers cannot be halved)
+  add_two
+ Problem: Move from the starting point to the target, minimizing the number of operations.
+
+ This solution finds the shortest path using A* search with cost of each operation
+ as one and the heuristic as the absolute of log of the ratio of the target
+ and start numbers.
+
+ Usage: bin/NumericMaze 2 9
+
+ Copyright 2012 Abhinav Sarkar <abhinav@abhinavsarkar.net>
+-}
+
+module Main (main) where
+
+import AStar
+import Control.Monad (when)
+import System.Environment (getArgs)
+
+data Op = Double | Halve | AddTwo
+
+execute :: Integral a => a -> Op -> a
+execute n Double = 2 * n
+execute n Halve  = let (q, r) = n `divMod` 2 in if r == 0 then q else n
+execute n AddTwo = n + 2
+
+solve start end =
+  astar start end
+    (\n -> zip (map (execute n) [Double, Halve, AddTwo]) (repeat 1))
+    (\a b -> abs $ logBase 2 (fromIntegral b / fromIntegral a))
+
+main = do
+  (start : end : _) <- fmap (map read) getArgs
+
+  when (start <= 0 || end <= 0) (error "Error: The numbers must be positive")
+
+  case solve start end of
+    Nothing -> putStrLn "No solution"
+    Just (_, solution) -> putStrLn . unwords . map show $ solution

rubyquiz.cabal

@@ -88,4 +88,12 @@ executable SudokuSolver
                       unordered-containers == 0.2.1.*
   main-is           : SudokuSolver.hs
   ghc-options       : -threaded -rtsopts
+  default-language  : Haskell2010
+
+executable NumericMaze
+  build-depends     : base == 4.*,
+                      containers == 0.4.*,
+                      mtl == 2.1.*,
+                      pqueue == 1.2.*
+  main-is           : NumericMaze.hs
   default-language  : Haskell2010