BorderedMatrix.hs

module BorderedMatrix where

import Data.Array
import Data.List

{- | 
>>> mapM_ print sample
[1,2,3]
[4,5,6]
>>> mapM_ print (borderedMatrix 0 sample)
[0,0,0,0,0]
[0,1,2,3,0]
[0,4,5,6,0]
[0,0,0,0,0]
-}
borderedMatrix :: a -> [[a]] -> [[a]]
borderedMatrix e
    = map (bordered e) . transpose . map (bordered e) . transpose

bordered :: a -> [a] -> [a]
bordered e = (e:) . reverse . (e:) . reverse

sample :: [[Int]]
sample = [[1,2,3],[4,5,6]]

{- |
>>> print (matrixWithBorder 0 sample)
array ((-1,-1),(2,3)) [((-1,-1),0),((-1,0),0),((-1,1),0),((-1,2),0),((-1,3),0),((0,-1),0),((0,0),1),((0,1),2),((0,2),3),((0,3),0),((1,-1),0),((1,0),4),((1,1),5),((1,2),6),((1,3),0),((2,-1),0),((2,0),0),((2,1),0),((2,2),0),((2,3),0)]
-}
matrixWithBorder :: a -> [[a]] -> Array (Int, Int) a
matrixWithBorder e mat
    = listArray ((-1,-1), (h,w)) (concat (borderedMatrix e mat))
        where
            h = length mat
            w = length (head mat)