{-# LANGUAGE Safe #-}
{-
Copyright (c) 2008-2011 John Goerzen <jgoerzen@complete.org>

All rights reserved.

For license and copyright information, see the file LICENSE
-}

{- |
   Module     : Data.BinPacking
   Copyright  : Copyright (C) 2008-2011 John Goerzen
   License    : BSD3

   Maintainer : John Goerzen <jgoerzen@complete.org> 
   Stability  : provisional
   Portability: portable

Tools for packing into bins

Written by John Goerzen, jgoerzen\@complete.org

This module is designed to solve this type of problem: Given a bunch of
objects of varying sizes, what is the best possible way to pack them into
fixed-size bins?  This can be used, for instance, by the datapacker program
to pack files onto CDs or DVDs; by manufacturing environments to pack
physical items into physicl bins; etc.

A description of bin packing algorithms can be found at
<http://en.wikipedia.org/wiki/Bin_packing_problem>.
-}

module Data.BinPacking (BinPacker,
                        BinPackerError(..),
                        packByOrder,
                        packLargeFirst
                       )

where
import Data.List
import Control.Monad.Error

{- | Potential errors returned as Left values by 'BinPacker' functions. 
Calling 'show' on this value will produce a nice error message suitable for
display. -}
data (Num size, Ord size, Show size, Show obj) => BinPackerError size obj = 
    BPTooFewBins [(size, obj)]                -- ^ Ran out of bins; attached value is the list of objects that do not fit
    | BPSizeTooLarge size (size, obj)   -- ^ Bin size1 exceeded by at least the given object and size
    | BPOther String                    -- ^ Other error
      deriving (Eq, Read)

instance (Num size, Ord size, Show size, Show obj) => Show (BinPackerError size obj) where
    show (BPTooFewBins _) = "Too few bins"
    show (BPSizeTooLarge binsize (objsize, obj)) =
        "Size " ++ show objsize ++ " greater than bin size " ++ show binsize
        ++ " at " ++ show obj
    show (BPOther x) = x

{- | Let us use this as part of the Either monad -}
instance (Num size, Ord size, Show size, Show obj) => Error (BinPackerError size obj) where
    strMsg = BPOther

{- | The primary type for bin-packing functions.

These functions take a list of size of bins.  If every bin is the same size,
you can pass @repeat binSize@ to pass an infinite list of bins if the
same size.  Any surplus bins will simply be ignored. 

> [size] is the sizes of bins
> [(size, obj)] is the sizes and objects
> result is Either error or results
-}
type BinPacker = forall size obj. (Num size, Ord size, Show size, Show obj) => 
                  [size]        -- The sizes of bins
               -> [(size, obj)] -- The sizes and objects
               -> Either (BinPackerError size obj) [[(size, obj)]] -- Either error or results


{- | Pack objects into bins, preserving order.  Objects will be taken from the
input list one by one, and added to each bin until the bin is full.  Work will
then proceed on the next bin.  No attempt is made to optimize allocations to
bins.  This is the simplest and most naive bin-packing algorithm, but
may not make very good use of bin space. -}
packByOrder :: BinPacker
packByOrder _ [] = Right []                     -- Ran out of sizes
packByOrder [] remainder = Left (BPTooFewBins remainder)
packByOrder (thisbinsize:otherbins) sizes =
    let fillBin _ [] = Right []
        fillBin accumsize ((s, o):xs) 
            | s > thisbinsize = Left $ BPSizeTooLarge thisbinsize (s, o)
            | s + accumsize > thisbinsize = Right []
            | otherwise = do next <- fillBin (accumsize + s) xs
                             return $ (s, o) : next
        in do thisset <- fillBin 0 sizes
              next <- packByOrder otherbins (drop (length thisset) sizes)
              return (thisset : next)

{- | Pack objects into bins.  For each bin, start with the largest objects,
and keep packing the largest object from the remainder until no object can
be found to put in the bin.  This is substantially more efficient than
'packByOrder', but requires sorting the input. -}
packLargeFirst :: BinPacker
packLargeFirst bins sizes = packLargeFirst' bins (sortBy fstSort sizes)
    where fstSort a b = compare (fst a) (fst b)

packLargeFirst' :: BinPacker
packLargeFirst' _ [] = Right []                     -- Ran out of sizes
packLargeFirst' [] remainder = Left (BPTooFewBins remainder)
packLargeFirst' (thisbinsize:otherbins) sizes =
    let fillBin _ [] = Right []
        fillBin accumsize sizelist =
            case break (\x -> (fst x) + accumsize <= thisbinsize) sizelist of
              (_, []) ->
                  if accumsize == 0
                     then Left $ BPSizeTooLarge thisbinsize (head sizelist)
                     else Right []
              (nonmatches, ((s, o):matchxs)) ->
                  do next <- fillBin (accumsize + s) (nonmatches ++ matchxs)
                     return $ (s, o) : next
        in do thisset <- fillBin 0 sizes
              next <- packLargeFirst' otherbins (drop (length thisset) sizes)
              return (thisset : next)
