    -- ------------------------------------------------------------

    {- |
       Module     : Text.XML.HXT.DTDValidation.DocTransformation
       Copyright  : Copyright (C) 2008 Uwe Schmidt
       License    : MIT

       Maintainer : Uwe Schmidt (uwe@fh-wedel.de)
       Stability  : experimental
       Portability: portable

       This module provides functions for transforming XML documents represented as
       XmlTree with respect to its DTD.

       Transforming an XML document with respect to its DTD means:

        - add all attributes with default values

        - normalize all attribute values

        - sort all attributes in lexical order

       Note: Transformation should be started after validation.

       Before the document is validated, a lookup-table is build on the basis of
       the DTD which maps element names to their transformation functions.
       After this initialization phase the whole document is traversed in preorder
       and every element is transformed by the XmlFilter from the lookup-table.

    -}

    -- ------------------------------------------------------------

    module Text.XML.HXT.DTDValidation.DocTransformation
        ( transform
        )
    where

    import Text.XML.HXT.DTDValidation.TypeDefs
    import Text.XML.HXT.DTDValidation.AttributeValueValidation

    import Data.Maybe
    import Data.List
    import Data.Ord

    -- ------------------------------------------------------------

    -- |
    -- Lookup-table which maps element names to their transformation functions. The
    -- transformation functions are XmlArrows.

    type TransEnvTable      = [TransEnv]
    type TransEnv           = (ElemName, TransFct)
    type ElemName           = String
    type TransFct           = XmlArrow

    -- ------------------------------------------------------------

    -- |
    -- filter for transforming the document.
    --
    --    * 1.parameter dtdPart :  the DTD subset (Node @DOCTYPE@) of the XmlTree
    --
    --    - 2.parameter doc :  the document subset of the XmlTree
    --
    --    - returns : a list of errors

    transform :: XmlTree -> XmlArrow
    transform dtdPart
        = traverseTree transTable
        where
        transTable = buildAllTransformationFunctions (runLA getChildren dtdPart)

    -- |
    -- Traverse the XmlTree in preorder.
    --
    --    * 1.parameter transEnv :  lookup-table which maps element names to their transformation functions
    --
    --    - returns : the whole transformed document

    traverseTree :: TransEnvTable -> XmlArrow
    traverseTree transEnv
        = processTopDown
          ( ( (transFct $< getName)
              >>>
              processChildren (traverseTree transEnv)
            )
           `when`
           isElem
          )
        where
        transFct            :: String -> XmlArrow
        transFct name       = fromMaybe this . lookup name $ transEnv

    -- |
    -- Build all transformation functions.
    --
    --    * 1.parameter dtdPart :  the DTD subset, root node should be of type @DOCTYPE@
    --
    --    - returns : lookup-table which maps element names to their transformation functions

    buildAllTransformationFunctions :: XmlTrees -> TransEnvTable
    buildAllTransformationFunctions dtdNodes
        = (t_root, this)
          :
          concatMap (buildTransformationFunctions dtdNodes) dtdNodes

    -- |
    -- Build transformation functions for an element.
    --
    --    * 1.parameter dtdPart :  the children of the @DOCTYPE@ node
    --
    --    * 1.parameter nd :  element declaration for which the transformation functions are
    --                    created
    --
    --    - returns : entry for the lookup-table

    buildTransformationFunctions :: XmlTrees -> XmlTree -> [TransEnv]

    buildTransformationFunctions dtdPart dn
        | isDTDElementNode dn       = [(name, transFct)]
        | otherwise                 = []
        where
        al          = getDTDAttributes dn
        name        = dtd_name al
        transFct    = setDefaultAttributeValues dtdPart dn
                      >>>
                      normalizeAttributeValues dtdPart dn
                      >>>
                      lexicographicAttributeOrder

    -- ------------------------------------------------------------

    -- |
    -- Sort the attributes of an element in lexicographic order.
    --
    --    * returns : a function which takes an element (XTag), sorts its
    --                  attributes in lexicographic order and returns the changed element

    lexicographicAttributeOrder :: XmlArrow
    lexicographicAttributeOrder
        = setAttrl (getAttrl >>. sortAttrl)
          where
          sortAttrl         :: XmlTrees -> XmlTrees
          sortAttrl         = sortBy (comparing nameOfAttr)

    -- |
    -- Normalize attribute values.
    --
    --    * returns : a function which takes an element (XTag), normalizes its
    --                  attribute values and returns the changed element

    normalizeAttributeValues :: XmlTrees -> XmlTree -> XmlArrow
    normalizeAttributeValues dtdPart dn
        | isDTDElementNode dn       = processAttrl (normalizeAttr $< getName)
        | otherwise                 = this
        where
        al           = getDTDAttributes dn
        elemName     = dtd_name al
        declaredAtts = isAttlistOfElement elemName $$ dtdPart

        normalizeAttr :: String -> XmlArrow
        normalizeAttr nameOfAtt
            = normalizeAttrValue ( if null attDescr
                                   then Nothing
                                   else Just (head attDescr)
                                 )
              where
              attDescr = filter ((== nameOfAtt) . valueOfDTD a_value) declaredAtts

        normalizeAttrValue :: Maybe XmlTree -> XmlArrow
        normalizeAttrValue descr
            = replaceChildren ((xshow getChildren >>^ normalizeAttributeValue descr) >>> mkText)

    -- |
    -- Set default attribute values if they are not set.
    --
    --    * returns : a function which takes an element (XTag), adds missing attribute
    --                  defaults and returns the changed element

    setDefaultAttributeValues :: XmlTrees -> XmlTree -> XmlArrow
    setDefaultAttributeValues dtdPart dn
        | isDTDElementNode dn       = seqA (map setDefault defaultAtts)
        | otherwise                 = this
        where
        elemName    = dtd_name . getDTDAttributes $ dn
        defaultAtts = ( isAttlistOfElement elemName
                        >>>
                        ( isFixedAttrKind           -- select attributes with default values
                          `orElse`
                          isDefaultAttrKind
                        )
                      ) $$ dtdPart

        setDefault  :: XmlTree -> XmlArrow
        setDefault attrDescr                        -- add the default attributes
              = ( addAttr attName defaultValue      -- to tag nodes with missing attributes
                  `whenNot`
                  hasAttr attName
                )
                `when`
                isElem
            where
            al              = getDTDAttributes attrDescr
            attName         = dtd_value   al
            defaultValue    = dtd_default al

    -- ------------------------------------------------------------