<?php

/**
 * Takes a well formed list of tokens and fixes their nesting.
 *
 * HTML elements dictate which elements are allowed to be their children,
 * for example, you can't have a p tag in a span tag.  Other elements have
 * much more rigorous definitions: tables, for instance, require a specific
 * order for their elements.  There are also constraints not expressible by
 * document type definitions, such as the chameleon nature of ins/del
 * tags and global child exclusions.
 *
 * The first major objective of this strategy is to iterate through all the
 * nodes (not tokens) of the list of tokens and determine whether or not
 * their children conform to the element's definition.  If they do not, the
 * child definition may optionally supply an amended list of elements that
 * is valid or require that the entire node be deleted (and the previous
 * node rescanned).
 *
 * The second objective is to ensure that explicitly excluded elements of
 * an element do not appear in its children.  Code that accomplishes this
 * task is pervasive through the strategy, though the two are distinct tasks
 * and could, theoretically, be seperated (although it's not recommended).
 *
 * @note Whether or not unrecognized children are silently dropped or
 *       translated into text depends on the child definitions.
 *
 * @todo Enable nodes to be bubbled out of the structure.
 *
 * @warning This algorithm (though it may be hard to see) proceeds from
 *          a top-down fashion.  Thus, parents are processed before
 *          children.  This is easy to implement and has a nice effiency
 *          benefit, in that if a node is removed, we never waste any
 *          time processing it, but it also means that if a child
 *          changes in a non-encapsulated way (e.g. it is removed), we
 *          need to go back and reprocess the parent to see if those
 *          changes resulted in problems for the parent.  See
 *          [BACKTRACK] for an example of this.  In the current
 *          implementation, this backtracking can only be triggered when
 *          a node is removed and if that node was the sole node, the
 *          parent would need to be removed.  As such, it is easy to see
 *          that backtracking only incurs constant overhead.  If more
 *          sophisticated backtracking is implemented, care must be
 *          taken to avoid nontermination or exponential blowup.
 */

class HTMLPurifier_Strategy_FixNesting extends HTMLPurifier_Strategy
{

    public function execute($tokens, $config, $context) {
        //####################################################################//
        // Pre-processing

        // get a copy of the HTML definition
        $definition = $config->getHTMLDefinition();

        $excludes_enabled = !$config->get('Core.DisableExcludes');

        // insert implicit "parent" node, will be removed at end.
        // DEFINITION CALL
        $parent_name = $definition->info_parent;
        array_unshift($tokens, new HTMLPurifier_Token_Start($parent_name));
        $tokens[] = new HTMLPurifier_Token_End($parent_name);

        // setup the context variable 'IsInline', for chameleon processing
        // is 'false' when we are not inline, 'true' when it must always
        // be inline, and an integer when it is inline for a certain
        // branch of the document tree
        $is_inline = $definition->info_parent_def->descendants_are_inline;
        $context->register('IsInline', $is_inline);

        // setup error collector
        $e =& $context->get('ErrorCollector', true);

        //####################################################################//
        // Loop initialization

        // stack that contains the indexes of all parents,
        // $stack[count($stack)-1] being the current parent
        $stack = array();

        // stack that contains all elements that are excluded
        // it is organized by parent elements, similar to $stack,
        // but it is only populated when an element with exclusions is
        // processed, i.e. there won't be empty exclusions.
        $exclude_stack = array();

        // variable that contains the start token while we are processing
        // nodes. This enables error reporting to do its job
        $start_token = false;
        $context->register('CurrentToken', $start_token);

        //####################################################################//
        // Loop

        // iterate through all start nodes. Determining the start node
        // is complicated so it has been omitted from the loop construct
        for ($i = 0, $size = count($tokens) ; $i < $size; ) {

            //################################################################//
            // Gather information on children

            // child token accumulator
            $child_tokens = array();

            // scroll to the end of this node, report number, and collect
            // all children
            for ($j = $i, $depth = 0; ; $j++) {
                if ($tokens[$j] instanceof HTMLPurifier_Token_Start) {
                    $depth++;
                    // skip token assignment on first iteration, this is the
                    // token we currently are on
                    if ($depth == 1) continue;
                } elseif ($tokens[$j] instanceof HTMLPurifier_Token_End) {
                    $depth--;
                    // skip token assignment on last iteration, this is the
                    // end token of the token we're currently on
                    if ($depth == 0) break;
                }
                $child_tokens[] = $tokens[$j];
            }

            // $i is index of start token
            // $j is index of end token

            $start_token = $tokens[$i]; // to make token available via CurrentToken

            //################################################################//
            // Gather information on parent

            // calculate parent information
            if ($count = count($stack)) {
                $parent_index = $stack[$count-1];
                $parent_name  = $tokens[$parent_index]->name;
                if ($parent_index == 0) {
                    $parent_def   = $definition->info_parent_def;
                } else {
                    $parent_def   = $definition->info[$parent_name];
                }
            } else {
                // processing as if the parent were the "root" node
                // unknown info, it won't be used anyway, in the future,
                // we may want to enforce one element only (this is
                // necessary for HTML Purifier to clean entire documents
                $parent_index = $parent_name = $parent_def = null;
            }

            // calculate context
            if ($is_inline === false) {
                // check if conditions make it inline
                if (!empty($parent_def) && $parent_def->descendants_are_inline) {
                    $is_inline = $count - 1;
                }
            } else {
                // check if we're out of inline
                if ($count === $is_inline) {
                    $is_inline = false;
                }
            }

            //################################################################//
            // Determine whether element is explicitly excluded SGML-style

            // determine whether or not element is excluded by checking all
            // parent exclusions. The array should not be very large, two
            // elements at most.
            $excluded = false;
            if (!empty($exclude_stack) && $excludes_enabled) {
                foreach ($exclude_stack as $lookup) {
                    if (isset($lookup[$tokens[$i]->name])) {
                        $excluded = true;
                        // no need to continue processing
                        break;
                    }
                }
            }

            //################################################################//
            // Perform child validation

            if ($excluded) {
                // there is an exclusion, remove the entire node
                $result = false;
                $excludes = array(); // not used, but good to initialize anyway
            } else {
                // DEFINITION CALL
                if ($i === 0) {
                    // special processing for the first node
                    $def = $definition->info_parent_def;
                } else {
                    $def = $definition->info[$tokens[$i]->name];

                }

                if (!empty($def->child)) {
                    // have DTD child def validate children
                    $result = $def->child->validateChildren(
                        $child_tokens, $config, $context);
                } else {
                    // weird, no child definition, get rid of everything
                    $result = false;
                }

                // determine whether or not this element has any exclusions
                $excludes = $def->excludes;
            }

            // $result is now a bool or array

            //################################################################//
            // Process result by interpreting $result

            if ($result === true || $child_tokens === $result) {
                // leave the node as is

                // register start token as a parental node start
                $stack[] = $i;

                // register exclusions if there are any
                if (!empty($excludes)) $exclude_stack[] = $excludes;

                // move cursor to next possible start node
                $i++;

            } elseif($result === false) {
                // remove entire node

                if ($e) {
                    if ($excluded) {
                        $e->send(E_ERROR, 'Strategy_FixNesting: Node excluded');
                    } else {
                        $e->send(E_ERROR, 'Strategy_FixNesting: Node removed');
                    }
                }

                // calculate length of inner tokens and current tokens
                $length = $j - $i + 1;

                // perform removal
                array_splice($tokens, $i, $length);

                // update size
                $size -= $length;

                // there is no start token to register,
                // current node is now the next possible start node
                // unless it turns out that we need to do a double-check

                // this is a rought heuristic that covers 100% of HTML's
                // cases and 99% of all other cases. A child definition
                // that would be tricked by this would be something like:
                // ( | a b c) where it's all or nothing. Fortunately,
                // our current implementation claims that that case would
                // not allow empty, even if it did
                if (!$parent_def->child->allow_empty) {
                    // we need to do a double-check [BACKTRACK]
                    $i = $parent_index;
                    array_pop($stack);
                }

                // PROJECTED OPTIMIZATION: Process all children elements before
                // reprocessing parent node.

            } else {
                // replace node with $result

                // calculate length of inner tokens
                $length = $j - $i - 1;

                if ($e) {
                    if (empty($result) && $length) {
                        $e->send(E_ERROR, 'Strategy_FixNesting: Node contents removed');
                    } else {
                        $e->send(E_WARNING, 'Strategy_FixNesting: Node reorganized');
                    }
                }

                // perform replacement
                array_splice($tokens, $i + 1, $length, $result);

                // update size
                $size -= $length;
                $size += count($result);

                // register start token as a parental node start
                $stack[] = $i;

                // register exclusions if there are any
                if (!empty($excludes)) $exclude_stack[] = $excludes;

                // move cursor to next possible start node
                $i++;

            }

            //################################################################//
            // Scroll to next start node

            // We assume, at this point, that $i is the index of the token
            // that is the first possible new start point for a node.

            // Test if the token indeed is a start tag, if not, move forward
            // and test again.
            $size = count($tokens);
            while ($i < $size and !$tokens[$i] instanceof HTMLPurifier_Token_Start) {
                if ($tokens[$i] instanceof HTMLPurifier_Token_End) {
                    // pop a token index off the stack if we ended a node
                    array_pop($stack);
                    // pop an exclusion lookup off exclusion stack if
                    // we ended node and that node had exclusions
                    if ($i == 0 || $i == $size - 1) {
                        // use specialized var if it's the super-parent
                        $s_excludes = $definition->info_parent_def->excludes;
                    } else {
                        $s_excludes = $definition->info[$tokens[$i]->name]->excludes;
                    }
                    if ($s_excludes) {
                        array_pop($exclude_stack);
                    }
                }
                $i++;
            }

        }

        //####################################################################//
        // Post-processing

        // remove implicit parent tokens at the beginning and end
        array_shift($tokens);
        array_pop($tokens);

        // remove context variables
        $context->destroy('IsInline');
        $context->destroy('CurrentToken');

        //####################################################################//
        // Return

        return $tokens;

    }

}

// vim: et sw=4 sts=4
