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<?php
/**
 * Standard diff function plus some extras for handling XHTML diffs.
 * @copyright © 2007 The Open University
 * @author s.marshall@open.ac.uk
 * @license http://www.gnu.org/copyleft/gpl.html GNU Public License
 * @package ouwiki
 *//** */
 
// Standard diff
//////////////// 
 
/**
 * Basic diff utility function, using standard diff algorithm.
 *
 * Based on Bell Laboratories Computing Science Technical Report #41, 
 * July 1976, Hunt & McIlroy, Appendix A.1 and A.3.
 * 
 * http://www.cs.dartmouth.edu/~doug/diff.ps
 *
 * @param array $file1 Array of lines in file 1. The first line in the file
 *   MUST BE INDEX 1 NOT ZERO!!
 * @param array $file2 Array of lines in file 2, again starting from 1.
 * @return array An array with one entry (again 1-based) for each line in  
 *   file 1, with its corresponding position in file 2 or 0 if it isn't there.
 */
function ouwiki_diff_internal($file1,$file2) {
	// Basic variables
	$n=count($file2);
	$m=count($file1);
    
    // Special-case for empty file2 which otherwise causes error
    if($n==0) 
    {
        $result=array();
        for($i=1;$i<=$m;$i++) 
        {
            $result[$i]=0;
        }
        return $result;
    }
	
	// Step 1   Build list of elements
	/////////
	
	$V=array(); 
	for($j=1;$j<=$n;$j++) {
		$V[$j]=new StdClass;
		$V[$j]->serial=$j;
		$V[$j]->hash=crc32($file2[$j]);
	}
    
	// Step 2   Sort by hash,serial
	/////////
	
	usort($V,"ouwiki_diff_sort_v");
    
    // Make it start from 1 again
    array_unshift($V,'bogus');
    unset($V[0]);
	
	// $V is now an array including the line number 'serial' and hash
	// of each line in file 2, sorted by hash and then serial.	
	
	// Step 3   Equivalence classes
	/////////
	
	$E=array();
	$E[0]=new StdClass;
	$E[0]->serial=0;
	$E[0]->last=true;
	for($j=1;$j<=$n;$j++) {
		$E[$j]=new StdClass;
		$E[$j]->serial=$V[$j]->serial;
		$E[$j]->last=$j===$n || $V[$j]->hash!==$V[$j+1]->hash;
	}

// E is now an array sorted the same way as $V which includes 
	// the line number 'serial' and whether or not that is the 'last'
	// line in the given equivalence class, i.e. set of identical lines
	
	// Step 4   For each line in file1, finds start of equivalence class
	/////////
    $P=array();
	for($i=1;$i<=$m;$i++) {
		// Find matching last entry from equivalence list    
		$P[$i]=ouwiki_diff_find_last($V,$E,crc32($file1[$i]));
	}
	
	// P is now an array that finds the index (within $V) of the *first*
	// matching line in $V (referencing file 2, but not a line number,
	// because sorted in $V order) for each line in file 1. In other words
	// if you were to start at the P-value in $V and continue through, you
	// would find all the lines from file 2 that are equal to the given line 
	// from file 1.	
	
	// Step 5   Initialise vector of candidates
	/////////
	
	// I do not trust PHP references further than I can throw them (preferably
	// at the idiot who came up with the idea) so I am using a separate array
	// to store candidates and all references are integers into that.
	
	$candidates=array();
	$candidates[0]=new StdClass;
	$candidates[0]->a=0;
	$candidates[0]->b=0;
	$candidates[0]->previous=null;
	$candidates[1]=new StdClass;
	$candidates[1]->a=$m+1; 
	$candidates[1]->b=$n+1;
	$candidates[1]->previous=null;
	
	$K=array();
	$K[0]=0; // Ref to candidate 0
	$K[1]=1; // Ref to candidate 1
	$k=0;
	
	// Step 6   Merge stage
	/////////
	
	for($i=1;$i<=$m;$i++) {
		if($P[$i]!==0) {
			ouwiki_diff_merge($K,$k,$i,$E,$P[$i],$candidates);
		}
	}
	
	// Step 7
	/////////
	
	$J=array();
	for($i=1;$i<=$m;$i++) {
		$J[$i]=0;
	}
	
	// Step 8   Follow candidate chain to make nice representation
	/////////
    
	$index=$K[$k];
	while(!is_null($index)) {
        // Stop when we reach the first, dummy candidate
        if($candidates[$index]->a!=0) {
		    $J[$candidates[$index]->a]=$candidates[$index]->b;
        }
		$index=$candidates[$index]->previous;
	}	
	
	// Step 9   Get rid of 'jackpots' (hash collisions)
	/////////
	
	for($i=1;$i<=$m;$i++) {
		if($J[$i]!=0 && $file1[$i]!=$file2[$J[$i]]) {
			$J[$i]=0;
		}
	}
	
	// Done! (Maybe.)
	return $J;
}

// Functions needed by parts of the algorithm
/////////////////////////////////////////////

// Merge, from step 7 (Appendix A.3)
function ouwiki_diff_merge(&$K,&$k,$i,&$E,$p,&$candidates) {
	$r=0;
	$c=$K[0];
	
	while(true) {
	    $j=$E[$p]->serial; // Paper says 'i' but this is wrong (OCR)
	    
	    // Binary search in $K from $r to $k
	    $min=$r;
	    $max=$k+1;
	    
		while(true) {
			$try = (int)(($min+$max)/2);
			if($candidates[$K[$try]]->b >= $j) {
				$max=$try;
			} else if($candidates[$K[$try+1]]->b <= $j) {
				$min=$try+1;
			} else { // $try is less and $try+1 is more
			    $s=$try;
				break;
			}
			if($max<=$min) {
			    $s=-1;
			    break;
			}
		}

if($s>-1) {
			if($candidates[$K[$s+1]]->b > $j) {
				// Create new candidate
				$index=count($candidates);
				$candidates[$index]=new StdClass;
				$candidates[$index]->a=$i;
				$candidates[$index]->b=$j;
				$candidates[$index]->previous=$K[$s];
				$K[$r]=$c;
				$r=$s+1;
				$c=$index; // Or should this go before?
			}	
		    
		    if($s===$k) {
		        $K[$k+2]=$K[$k+1];
		        $k++;
     		    break;
		    }		    
		}
		
		if($E[$p]->last) {
		    break;
		}
		
		$p++;
	}
	$K[$r]=$c;
	
}

// From Step 2
function ouwiki_diff_sort_v($a,$b) {
    if($a->hash < $b->hash) {
    	return -1;
    } else if($a->hash > $b->hash) {
        return 1;    
    } else if($a->serial < $b->serial) {
        return -1;
    } else if($a->serial > $b->serial) {
        return 1;
    } else {
        return 0;
    }
}

// From Step 4
function ouwiki_diff_find_last(&$V,&$E,$hash) {
    // Binary search in $V until we find something with $hash
    
    // Min = 1, array is 1-indexed
    $min=1;
    // Max = 1 higher than highest key
    end($V);
    $max=key($V)+1;
    while(true) {
        $try = (int)(($min+$max)/2);
        if($V[$try]->hash > $hash) {
            $max=$try;
        } else if($V[$try]->hash < $hash) {
            $min=$try+1;
        } else { // Equal
            break;
        }
        if($max<=$min) {
        	// No matching line
            return 0;
        }
    }
    
    // Now check back in $E to find the first line of that equivalence class
    for($j=$try;!$E[$j-1]->last;$j--) ;
	return $j;
}

///////////////////////////

/**
 * Class representing one 'line' of HTML content for the purpose of 
 * text comparison. 
 */
class ouwiki_line {
    /** Array of ouwiki_words */
    var $words=array();
    
    /**
     * Construct line object based on a chunk of text.
     * @param string $data Text data that makes up this 'line'. (May include line breaks etc.)
     * @param int $linepos Position number for first character in text
     */
    public function __construct($data,$linepos) {
        // 1. Turn things we don't want into spaces (so that positioning stays same)
        
        // Whitespace replaced with space
        $data=preg_replace('/\s/',' ',$data);
        
        // Various ways of writing non-breaking space replaced with space
        // Note that using a single param for replace only works because all
        // the search strings are 6 characters long
        $data=str_replace(array(' ',' ',' '),'      ',$data);

// Tags replaced with equal number of spaces
        $data = preg_replace_callback('/<.*?'.'>/', function($matches) {
            return preg_replace("/./", " ", $matches[0]);
        }, $data);

// 2. Analyse string so that each space-separated thing 
        // is counted as a 'word' (note these may not be real words,
        // for instance words may include punctuation at either end)
        $pos=0;
        while(true) {
            // Find a non-space
            $strlendata = strlen($data);
            for(;$pos < $strlendata && substr($data,$pos,1)===' ';$pos++) ;
            if($pos==$strlendata) {
                // No more content
                break;
            }
            
            // Aaaand find the next space after that
            $space2=strpos($data,' ',$pos);
            if($space2===false) {
                // No more spaces? Everything left must be a word
                $this->words[]=new ouwiki_word(substr($data,$pos),$pos+$linepos);
                break;
            } else {
                $this->words[]=new ouwiki_word(substr($data,$pos,$space2-$pos),$pos+$linepos);
                $pos=$space2;
            }
        }
    }

/**
     * Old syntax of class constructor. Deprecated in PHP7.
     *
     * @deprecated since Moodle 3.1
     */
    public function ouwiki_line($data, $linepos) {
        debugging('Use of class name as constructor is deprecated', DEBUG_DEVELOPER);
        self::__construct($data, $linepos);
    }

/**
     * @return string Normalised string representation of this line object
     */
    function get_as_string() {
        $result='';
        foreach($this->words as $word) {
            if($result!=='') {
                $result.=' ';
            }
            $result.=$word->word;
        }
        return $result;
    }
    
    /**
     * Static function converts lines to strings.
     * @param array $lines Array of ouwiki_line
     * @return array Array of strings
     */
    static function get_as_strings($lines) {
        $strings=array();
        foreach($lines as $key=>$value) {
            $strings[$key]=$value->get_as_string();        
        }
        return $strings;
    }
    
    
    /**
     * @return True if there are no words in the line
     */
    function is_empty() {
        return count($this->words)===0;
    }
}

/**
 * Represents single word for html comparison. Note that words
 * are just chunks of plain text and may not be actual words;
 * they could include punctuation or (if there was e.g. a span
 * in the middle of something) even be part-words.
 */
class ouwiki_word {
    /** Word as plain string */
    var $word;
    /** Start position in original xhtml */
    var $start;
    
    public function __construct($word,$start) {
        $this->word=$word;
        $this->start=$start;
    }

/**
     * Old syntax of class constructor. Deprecated in PHP7.
     *
     * @deprecated since Moodle 3.1
     */
    public function ouwiki_word($word, $start) {
        debugging('Use of class name as constructor is deprecated', DEBUG_DEVELOPER);
        self::__construct($word, $start);
    }
}

/**
 * Prepares XHTML content for text difference comparison. 
 * @param string $content XHTML content [NO SLASHES]
 * @return array Array of ouwiki_line objects
 */
function ouwiki_diff_html_to_lines($content) {
    // These functions are a pain mostly because PHP preg_* don't provide
    // proper information as to the start/end position of matches. As a
    // consequence there is a lot of hackery going down. At every point we
    // replace things with spaces rather than getting rid, in order to store
    // positions within original content.
    
    // Get rid of all script, style, object tags (that might contain non-text
    // outside tags)
    $content=preg_replace_callback(
        '^(<script .*?</script>)|(<object .*?</object>)|(<style .*?</style>)^i', function($matches) {
        return preg_replace("/./", " ", $matches[0]);
    }, $content);

// Get rid of all ` symbols as we are going to use these for a marker later.
    $content=preg_replace('/[`]/',' ',$content);
    
    // Put line breaks on block tags. Mark each line break with ` symbol
    $blocktags=array('p','div','h1','h2','h3','h4','h5','h6','td','li');
    $taglist='';
    foreach($blocktags as $blocktag) {
        if($taglist!=='') {
            $taglist.='|';
        }
        $taglist.="<$blocktag>|<\\/$blocktag>";
    }
    $content = preg_replace_callback('/((' . $taglist . ')\s*)+/i', function($matches) {
        return "`" . preg_replace("/./", " ", substr($matches[0], 1));
    }, $content);
        
    // Now go through splitting each line
    $lines=array(); $index=1;
    $pos=0;
    while($pos<strlen($content)) {
        $nextline=strpos($content,'`',$pos);
        if($nextline===false) {
            // No more line breaks? Take content to end
            $nextline=strlen($content);
        }
        
        $linestr=substr($content,$pos,$nextline-$pos);
        $line=new ouwiki_line($linestr,$pos);
        if(!$line->is_empty()) {
            $lines[$index++]=$line;
        }
        $pos=$nextline+1;
    }
    return $lines; 
}

/** 
 * Represents a changed area of file and where it is located in the 
 * two source files. 
 */ 
class ouwiki_change_range {
    var $file1start,$file1count;
    var $file2start,$file2count;
}

/**
 * A more logical representation of the results from ouwiki_internal_diff()
 */
class ouwiki_changes {
    
    /** Array of indexes (in file 2) of added lines */
    var $adds;
    
    /** Array of indexes (in file 1) of deleted lines */
    var $deletes;
    
    /** Array of changed ranges */
    var $changes;
    
    /** 
     * @param array $diff Array from line indices in file1
     *   to indices in file2. All indices 1-based.
     * @param int $count2 Number of lines in file2
     */
    public function __construct($diff,$count2) {
        // Find deleted lines
        $this->deletes=self::internal_find_deletes($diff,$count2);
        
        // Added lines work the same way after the comparison is
        // reversed.
        $this->adds=self::internal_find_deletes(
            ouwiki_diff_internal_flip($diff,$count2),count($diff));
        
        // Changed ranges are all the other lines from file 1 that
        // weren't found in file 2 but aren't deleted, and the 
        // corresponding lines from file 2 (between the equivalent
        // 'found' lines).        
        $this->changes=array();
        $matchbefore=0;
        $inrange=-1; $lastrange=-1;
        foreach($diff as $index1=>$index2) {
            // Changed line if this isn't in 'deleted' section and
            // doesn't have a match in file2.
            if($index2===0 && !in_array($index1,$this->deletes)) {
                if($inrange===-1) {
                    // Not already in a range, start a new one at array end
                    $inrange=count($this->changes);
                    $this->changes[$inrange]=new ouwiki_change_range;
                    $this->changes[$inrange]->file1start=$index1;
                    $this->changes[$inrange]->file1count=1;                    
                    $this->changes[$inrange]->file2start=$matchbefore+1; // Last valid from file2                    
                    $this->changes[$inrange]->file2count=0;
                    $lastrange=$inrange;
                } else {
                    // One more line that gets added to the range
                    $this->changes[$inrange]->file1count++;
                }
            } else {
                // Not in a range any more
                $inrange=-1;
                // If we have a line match...
                if($index2!==0) {
                    // Remember this line as next range must start after it
                    $matchbefore=$index2;
                    // If last range is still looking for a number, fill that in too
                    if($lastrange!==-1) {
                        $this->changes[$lastrange]->file2count=$index2
                            -$this->changes[$lastrange]->file2start;
                        $lastrange=-1;
                    }
                }
            }
        }
        // Unfinished range in file2 gets end of file
        if($lastrange!==-1) {
            $this->changes[$lastrange]->file2count=$count2
                -$this->changes[$lastrange]->file2start+1;
        }
    }

/**
     * Old syntax of class constructor. Deprecated in PHP7.
     *
     * @deprecated since Moodle 3.1
     */
    public function ouwiki_changes($diff, $count2) {
        debugging('Use of class name as constructor is deprecated', DEBUG_DEVELOPER);
        self::__construct($diff, $count2);
    }

/**
     * Find deleted lines. These are lines in file1 that
     * cannot be present even in modified form in file2
     * because we have matching lines around them.
     * O(n) algorithm.
     * @param array $diff Array of file1->file2 indexes
     * @param int $count2 Count of lines in file2
     */
    function internal_find_deletes($diff,$count2) {
        $deletes=array();
        
        // 1. Create a new array that includes the lowest-valued
        //    index2 value below each run of 0s.
        //    I.e. if our array is say 1,2,0,0,0,3,0 then the
        //    resulting array will be -,-,3,3,3,-,0
        $squidges=array();
        $lowest=0;
        $countdiff = count($diff);
        for($index1=$countdiff;$index1>=1;$index1--) {
            $index2=$diff[$index1];
            if($index2===0) {
                $squidges[$index1]=$lowest;
            } else {
                $lowest=$index2;
            }
        }
        
        // 2. OK now we can use this new array to work out 
        //    items that are known to be deleted because we
        //    have matching items either side        
        $highest=0;
        foreach($diff as $index1=>$index2) {
            if($index2===0) {
                if($highest===$count2 || $highest+1===$squidges[$index1]) {
                    // Yep! Definitely deleted.
                    $deletes[]=$index1;                    
                } 
            } else {
                $highest=$index2;                
            }
        }
        return $deletes;        
    }
}

/**
 * Flips around the array returned by ouwiki_diff_internal
 * so that it refers to lines from the other file. 
 * @param array $diff Array of index1=>index2
 * @param int $count2 Count of lines in file 2
 * @return array Flipped version
 */
function ouwiki_diff_internal_flip($diff,$count2) {
    $flip=array();
    for($i=1;$i<=$count2;$i++) {
        $flip[$i]=0;
    }
    foreach($diff as $index1=>$index2) {
        if($index2!==0) {
            $flip[$index2]=$index1;
        }
    }
    return $flip;
}

/**
 * Compares two files based initially on lines and then on words within the lines that
 * differ.
 * @param array $lines1 Array of ouwiki_line
 * @param array $lines2 Array of ouwiki_line
 * @return array (deleted,added); deleted and added are arrays of ouwiki_word with
 *   position numbers from $lines1 and $lines2 respectively 
 */
function ouwiki_diff_words($lines1,$lines2) {
    // Prepare arrays
    $deleted=array();
    $added=array();
    // Get line difference
    $linediff=ouwiki_diff(
        ouwiki_line::get_as_strings($lines1),
        ouwiki_line::get_as_strings($lines2));
        
    // Handle lines that were entirely deleted
    foreach($linediff->deletes as $deletedline) {
        $deleted = array_merge($deleted, $lines1[$deletedline]->words);
    }
    // And ones that were entirely added
    foreach($linediff->adds as $addedline) {
        $added = array_merge($added, $lines2[$addedline]->words);
    }
    
    // Changes get diffed at the individual-word level
    foreach($linediff->changes as $changerange) {
        // Build list of all words in each side of the range
        $file1words=array();
        for($index=$changerange->file1start;
            $index<$changerange->file1start+$changerange->file1count;$index++) {
            foreach($lines1[$index]->words as $word) {
                $file1words[]=$word;
            }
        }
        $file2words=array();
        for($index=$changerange->file2start;
            $index<$changerange->file2start+$changerange->file2count;$index++) {
            foreach($lines2[$index]->words as $word) {
                $file2words[]=$word;
            }
        }
                
        // Make arrays 1-based
        array_unshift($file1words,'dummy');
        unset($file1words[0]);
        array_unshift($file2words,'dummy');
        unset($file2words[0]);
        
        // Convert word lists into plain strings
        $file1strings=array();
        foreach($file1words as $index=>$word) {
            $file1strings[$index]=$word->word;
        }
        $file2strings=array();
        foreach($file2words as $index=>$word) {
            $file2strings[$index]=$word->word;
        }
        
        // Run diff on strings
        $worddiff=ouwiki_diff($file1strings,$file2strings);
        foreach($worddiff->adds as $index) {
            $added[]=$file2words[$index];
        }
        foreach($worddiff->deletes as $index) {
            $deleted[]=$file1words[$index];
        }
        foreach($worddiff->changes as $changerange) {
            for($index=$changerange->file1start;
                $index<$changerange->file1start+$changerange->file1count;$index++) {
                $deleted[]=$file1words[$index];
            }
            for($index=$changerange->file2start;
                $index<$changerange->file2start+$changerange->file2count;$index++) {
                $added[]=$file2words[$index];
            }
        }
    }
    
    return array($deleted,$added);
}

/**
 * Runs diff and interprets results into ouwiki_changes object.
 * @param array $file1 Array of lines in file 1. The first line in the file
 *   MUST BE INDEX 1 NOT ZERO!!
 * @param array $file2 Array of lines in file 2, again starting from 1.
 * @return ouwiki_changes Object describing changes
 */
function ouwiki_diff($file1,$file2) {
    return new ouwiki_changes(ouwiki_diff_internal($file1,$file2),count($file2));
}

/**
 * Adds HTML span elements to $html around the words listed in $words.
 * @param string $html HTML content
 * @param array $words Array of ouwiki_word to mark
 * @param string $markerclass Name of class for span element
 * @return HTML with markup added
 */
function ouwiki_diff_add_markers($html,$words,$markerclass,$beforetext,$aftertext) {
    // Sort words by start position
    usort($words, function($a, $b) {
        return $a->start - $b->start;
    });

// Add marker for each word. We use an odd tag name which will
    // be replaced by span later, this for ease of replacing 
    $spanstart="<ouwiki_diff_add_markers>";
    $pos=0;
    $result='';
    foreach($words as $word) {
        // Add everything up to the word
        $result.=substr($html,$pos,$word->start-$pos);
        // Add word
        $result.=$spanstart.$word->word.'</ouwiki_diff_add_markers>';
        // Update position
        $pos=$word->start+strlen($word->word);
    }

// Add everything after last word
    $result.=substr($html,$pos);
    
    // If we end a marker then immediately start one, get rid of
    // both the end and start
    $result=preg_replace('^</ouwiki_diff_add_markers>(\s*)<ouwiki_diff_add_markers>^','$1',$result);
    
    // Turn markers into proper span
    $result=preg_replace('^<ouwiki_diff_add_markers>^',$beforetext.'<span class="'.$markerclass.'">',$result);
    $result=preg_replace('^</ouwiki_diff_add_markers>^','</span>'.$aftertext,$result);
    
    return $result;
}

/**
 * Compares two HTML files. (This is the main function that everything else supports.)
 * @param string $html1 XHTML for file 1  
 * @param string $html2 XHTML for file 2
 * @return array ($result1,$result2) to be displayed indicating the differences  
 */
function ouwiki_diff_html($html1,$html2) {
    $lines1=ouwiki_diff_html_to_lines($html1);
    $lines2=ouwiki_diff_html_to_lines($html2);
    list($deleted,$added)=ouwiki_diff_words($lines1,$lines2);
    $result1=ouwiki_diff_add_markers($html1,$deleted,'ouw_deleted',
        '<strong class="accesshide">'.get_string('deletedbegins','wiki').'</strong>',
        '<strong class="accesshide">'.get_string('deletedends','wiki').'</strong>');
    $result2=ouwiki_diff_add_markers($html2,$added,'ouw_added',
        '<strong class="accesshide">'.get_string('addedbegins','wiki').'</strong>',
        '<strong class="accesshide">'.get_string('addedends','wiki').'</strong>');
    return array($result1,$result2);    
}