    -- (c) 2000-2005 by Martin Erwig [see file COPYRIGHT]
    -- | Inward directed trees as lists of paths.
    module Data.Graph.Inductive.Internal.RootPath (
        -- * Types
        RTree,LRTree,
        -- * Operations
        getPath,getLPath,
        getDistance,
        getLPathNodes
    ) where

    import Data.Graph.Inductive.Graph

    type LRTree a = [LPath a]
    type RTree = [Path]

    first :: ([a] -> Bool) -> [[a]] -> [a]
    first p xss  = case filter p xss of
                     []   -> []
                     x:_  -> x

    -- | Find the first path in a tree that starts with the given node.
    --
    --   Returns an empty list if there is no such path.
    findP :: Node -> LRTree a -> [LNode a]
    findP _ []                                = []
    findP v (LP []:ps)                        = findP v ps
    findP v (LP (p@((w,_):_)):ps) | v==w      = p
                                  | otherwise = findP v ps

    getPath :: Node -> RTree -> Path
    getPath v = reverse . first (\(w:_)->w==v)

    getLPath :: Node -> LRTree a -> LPath a
    getLPath v = LP . reverse . findP v

    -- | Return the distance to the given node in the given tree.
    --
    --   Returns 'Nothing' if the given node is not reachable.
    getDistance :: Node -> LRTree a -> Maybe a
    getDistance v t = case findP v t of
      []      -> Nothing
      (_,d):_ -> Just d

    getLPathNodes :: Node -> LRTree a -> Path
    getLPathNodes v = (\(LP p)->map fst p) . getLPath v