-- Columnwise.hs
-- Tom Moertel <tom@moertel.com>
-- CVS $Id: Columnwise.hs,v 1.2 2004/11/12 22:13:13 thor Exp $

-- | This module provides functions for folding over matrix
-- data column wise.  The primary advantage over transposing
-- the matrix to expose the columns is that this method
-- requires less space, especially if you are computing a
-- single summary value per column.

module Columnwise ( foldC, foldC' )
where

import Data.List

-- | Folds a function over each column of a matrix.  (foldC f z rows)
-- is equivalent to (map (foldl f z) (transpose rows)) but requires
-- less space.
--
-- Example: foldC (+) 0 [[0, 1], [2, 3]] ==> [2,4]

foldC :: (b -> a -> b) -> b -> [[a]] -> [b]
foldC f z rows =
    foldl (zipWith f) (repeat z) rows

-- | Strict version of foldC.

foldC' :: (b -> a -> b) -> b -> [[a]] -> [b]
foldC' f z rows =
    foldl' (zipWith f) (repeat z) rows
  where
    foldl' f z []       = z
    foldl' f z (x:xs)   = (foldl' f $! lseq (f z x)) xs
    lseq xs             = foldr seq () xs `seq` xs

-- =================================================================
--
-- Copyright (C) 2004 Thomas Moertel.
-- 
-- This program is free software; you can redistribute it and/or
-- modify it under the terms of the GNU General Public License
-- as published by the Free Software Foundation; either version 2
-- of the License, or (at your option) any later version.
-- 
-- The text of the GNU GPL may be found in the LICENSE file,
-- included with this software, or online at the following URL:
-- 
--     http://www.gnu.org/copyleft/gpl.html
-- 
-- This program is distributed in the hope that it will be useful,
-- but WITHOUT ANY WARRANTY; without even the implied warranty of
-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-- GNU General Public License for more details.
-- 
-- Except as provided for under the terms of the GNU GPL, all rights
-- are reserved worldwide.
--
-- =================================================================