gistfile1.hs

import Data.List.Zipper as Z

main = do
    let string = "yabbadabbadoo"
    print $ maximum $ palindromeSizes string

-- Find the size of the largest palindrome centered at each location in
-- a string. There will be 2*n-1 locations, because we count even and odd
-- centers. Go along the zipper, checking the sizes of palindromes to the left
-- and right as we go.
palindromeSizes :: Eq a => [a] -> [Int]
palindromeSizes = sizes . Z.fromList
    where sizes :: Eq a => Zipper a -> [Int]
          sizes z = if Z.endp z then []
                    else evenSize z : oddSize z : sizes (right z)

evenSize :: Eq a => Zipper a -> Int
evenSize (Zip left (right)) = 2 * (equalPrefixLength left right)

oddSize :: Eq a => Zipper a -> Int
oddSize (Zip left (_:right)) = 2 * (equalPrefixLength left right) + 1

-- Check equal prefix length of the lists. The left list is already reversed
-- because of the Zipper.
equalPrefixLength :: Eq a => [a] -> [a] -> Int
equalPrefixLength xs ys = length $ takeWhile (uncurry (==)) $ zip xs ys