/**
 * <copyright> Copyright 2006 by Know-Center, Graz, Austria </copyright>
 * PDF-AS has been contracted by the E-Government Innovation Center EGIZ, a
 * joint initiative of the Federal Chancellery Austria and Graz University of
 * Technology.
 *
 * Licensed under the EUPL, Version 1.1 or - as soon they will be approved by
 * the European Commission - subsequent versions of the EUPL (the "Licence");
 * You may not use this work except in compliance with the Licence.
 * You may obtain a copy of the Licence at:
 * http://www.osor.eu/eupl/
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the Licence is distributed on an "AS IS" basis,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the Licence for the specific language governing permissions and
 * limitations under the Licence.
 *
 * This product combines work with different licenses. See the "NOTICE" text
 * file for details on the various modules and licenses.
 * The "NOTICE" text file is part of the distribution. Any derivative works
 * that you distribute must include a readable copy of the "NOTICE" text file.
 *
 * $Id: AbsoluteTextSignature.java,v 1.1 2006/10/31 08:08:33 wprinz Exp $
 */
package at.knowcenter.wag.egov.egiz.pdf;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.Vector;

import org.apache.log4j.Logger;

import at.knowcenter.wag.egov.egiz.PdfAS;
import at.knowcenter.wag.egov.egiz.cfg.ConfigLogger;
import at.knowcenter.wag.egov.egiz.exceptions.SignatureException;
import at.knowcenter.wag.egov.egiz.exceptions.SignatureTypesException;
import at.knowcenter.wag.egov.egiz.framework.FoundBlock;
import at.knowcenter.wag.egov.egiz.framework.FoundKey;
import at.knowcenter.wag.egov.egiz.sig.SignatureObject;
import at.knowcenter.wag.egov.egiz.sig.SignatureTypeDefinition;
import at.knowcenter.wag.egov.egiz.sig.SignatureTypes;

/**
 * Contains methods and helpers that implement the absolute text signature.
 * 
 * @author wprinz
 */
public class AbsoluteTextSignature
{

  /**
   * The logger definition.
   */
  private static final Logger logger = ConfigLogger.getLogger(AbsoluteTextSignature.class);

  /**
   * Returns a List of SignatureTypeDefinitions that can be extracted from text.
   * 
   * <p>
   * These SignatureTypeDefinitions are all text extractable, which means that they define all required fields as visible.
   * </p>
   * <p>
   * This method filters out all SignatureTypeDefinitions that are not text extractable.
   * </p>
   * 
   * @return Returns a List of SignatureTypeDefinitions that can be extracted from text.
   * @throws SignatureTypesException F.e.
   */
  public static List getSignatureTypesForTextAnalysis() throws SignatureTypesException
  {
    SignatureTypes sig_types = SignatureTypes.getInstance();
    List allSignatureTypes = sig_types.getSignatureTypeDefinitions();
    
    List textSignatureTypes = new ArrayList(allSignatureTypes.size());
    Iterator it = allSignatureTypes.iterator();
    while (it.hasNext())
    {
      SignatureTypeDefinition std = (SignatureTypeDefinition) it.next();
      if (!std.isTextExtractable())
      {
        logger.debug("The profile " + std.getType() + " is not text extractable and is thereby not used for text analysis.");
        continue;
      }
      textSignatureTypes.add(std);
    }

    return textSignatureTypes;
  }
  
  /**
   * Extracts all signature holders from a given text.
   * 
   * <p>
   * First the latest signature holder is extracted. Then the latest signature
   * holder in the rest text, which is the second latest one, is extracted. Then
   * the third latest signature holder is extracted and so forth until no more
   * signature holders are found.
   * </p>
   * 
   * @param text
   *          The text.
   * @return Returns the List of extracted signature holders ordered by their
   *         date ascendingly (the lowest, earliest date first, the latest,
   *         newest date last). An empty list is returned if no signature
   *         holders were found.
   * @throws SignatureException
   *           F.e.
   * @throws SignatureTypesException
   *           F.e.
   */
  public static List extractSignatureHoldersFromText(String text) throws SignatureException, SignatureTypesException
  {
    List holders = new ArrayList();
    String current_text = text;
    for (;;)
    {
      TextualSignatureHolder signature_holder = extractLatestBlock(current_text);
      if (signature_holder == null)
      {
        break;
      }
      holders.add(0, signature_holder);
      current_text = signature_holder.getSignedText();
    }
    return holders;
  }

  /**
   * Extracts the latest signature block from the given text and creates a
   * SignatureHolder object that can be verified.
   * 
   * @param text
   *          The text.
   * @return Returns the SignatureObject extracted from the text, or null, if no
   *         latest block was found.
   * @throws SignatureException
   *           F.e.
   * @throws SignatureTypesException
   *           F.e.
   */
  public static TextualSignatureHolder extractLatestBlock(String text) throws SignatureException, SignatureTypesException
  {
    FoundBlock latest_block = findLatestBlock(text);
    if (latest_block == null)
    {
      return null;
    }
    String reconstructed_text = cutOutBlock(text, latest_block);
    SignatureObject so = createSignatureObjectFromFoundBlock(text, latest_block);
    TextualSignatureHolder tsh = new TextualSignatureHolder(reconstructed_text, so);
    return tsh;
  }

  /**
   * Finds the latest signature block for a given text.
   * 
   * <p>
   * The latest block is the one with the highest, most recent date. Usually
   * this block will be extracted (cut out) of the text which will result in the
   * originally signed text of this signature to be verified using the cut out
   * data.
   * </p>
   * 
   * @param text
   *          The text to be analyzed.
   * @return Returns the latest found block or null, if there was none.
   * @throws SignatureException
   *           F.e.
   * @throws SignatureTypesException
   *           F.e.
   */
  public static FoundBlock findLatestBlock(String text) throws SignatureException, SignatureTypesException
  {
    List signatureTypes_ = getSignatureTypesForTextAnalysis();
    
    List found_potential_candidates = new ArrayList();
 
    for (int i = 0; i < signatureTypes_.size(); i++)
    {
      SignatureTypeDefinition block_type = (SignatureTypeDefinition) signatureTypes_.get(i);
      List found_candidates_for_type = findPotentialSignaturesForProfile(text, block_type);

      found_potential_candidates.addAll(found_candidates_for_type);
    }

    if (found_potential_candidates.isEmpty())
    {
      logger.debug("no candidates found at all");
      return null;
    }

    List found_candidates = new ArrayList();
    logger.debug("checking block integrity");
    for (int i = 0; i < found_potential_candidates.size(); i++)
    {
      FoundBlock found_block = (FoundBlock) found_potential_candidates.get(i);
      String date_value = getDateValue(text, found_block);
      try
      {
        EGIZDate date = EGIZDate.parseFromString(date_value);

        logger.debug("found_block = " + date + " - " + found_block);

        checkBlockIntegrity(text, found_block);
        found_candidates.add(found_block);
      }
      catch (Exception e)
      {
        logger.debug("Exception while checking the integrity of the found block " + found_block + ". Ignoring this block.", e);
      }
    }

    sortFoundBlocksByDate(text, found_candidates);
    if (logger.isDebugEnabled())
    {
      logger.debug("sorted blocks:");
      for (int i = 0; i < found_candidates.size(); i++)
      {
        FoundBlock found_block = (FoundBlock) found_candidates.get(i);

        String date_value = getDateValue(text, found_block);
        EGIZDate date = EGIZDate.parseFromString(date_value);

        logger.debug("  #" + i + ": " + date + " - " + found_block);
      }
    }

    List latest_blocks = filterLastDateEqualBlocks(text, found_candidates);
    if (logger.isDebugEnabled())
    {
      logger.debug("latest blocks:");
      for (int i = 0; i < latest_blocks.size(); i++)
      {
        FoundBlock found_block = (FoundBlock) latest_blocks.get(i);

        String date_value = getDateValue(text, found_block);
        EGIZDate date = EGIZDate.parseFromString(date_value);

        logger.debug("  #" + i + ": " + date + " - " + found_block);
      }
    }

    // The semantic equality check has been outdated by the
    // advanced choosing algorithm.
    // boolean semantic_equality =
    // PdfAS.checkForSemanticEquality(latest_blocks);
    // logger.debug("semantic_equality = " + semantic_equality);
    // if (!semantic_equality)
    // {
    // throw new SignatureException(314, "The latest blocks weren't semantically
    // equal.");
    // }

    FoundBlock latest_block = chooseMostPossibleBlock(latest_blocks);

    logger.debug("latest block = " + latest_block);
    return latest_block;
  }

  /**
   * Finds the List of potential blocks within the given text for the given
   * profile.
   * 
   * @param text
   *          The text, in which potential block are to be sought.
   * @param block_type
   *          The profile for which the text is to be sought.
   * @return Returns the List of potential FoundBlocks or an empty List if none
   *         could be found.
   */
  public static List findPotentialSignaturesForProfile(String text,
      SignatureTypeDefinition block_type)
  {
    logger.debug("find potential signatures for " + block_type.getType());
    List found_blocks = new ArrayList();

    final boolean old_style = false;

    Vector keys = block_type.getRevertSortedKeys();
    Vector captions = block_type.getRevertSortedCaptions();

    String last_key = (String) keys.get(0);
    logger.debug("last_key = " + last_key);
    String last_caption = (String) captions.get(0);
    logger.debug("last_caption = " + last_caption);
    String current_last_caption= last_caption;
    List found_last_captions = findIndicesWithStartingNL(text, last_caption);
    if (last_key.equals(SignatureTypes.SIG_ID))
    {
      logger.debug("Last key is SIG_ID, so it may not be present. Searching for the previous to last key.");
      String prevlast_key = (String) keys.get(1);
      String prevlast_caption = (String) captions.get(1);
      current_last_caption = prevlast_caption; 
      List found_prevlast_captions = findIndicesWithStartingNL(text, prevlast_caption);
      if (!found_prevlast_captions.isEmpty())
      {
        found_last_captions.addAll(found_prevlast_captions);
      }
    }
    if (logger.isDebugEnabled())
    {
      logger.debug("found " + found_last_captions.size() + " last captions.");
      for (int i = 0; i < found_last_captions.size(); i++)
      {
        logger.debug("  found last caption at index " + found_last_captions.get(i));
      }
    }

    for (int lci = 0; lci < found_last_captions.size(); lci++)
    {
      int last_caption_index = ((Integer) found_last_captions.get(lci)).intValue();
      logger.debug("resolving signature block from last caption index " + last_caption_index);
      int potential_block_end = findEndOfValue(text, last_caption_index);
      if (potential_block_end == (last_caption_index + current_last_caption.length()+1))
      {
    	  potential_block_end = findEndOfValue(text, potential_block_end);
      }
      
      // FIXME: complete HOTFIX
      /*
      int extendedValueEnd = potential_block_end;
      String cv;
      do {
         extendedValueEnd = findEndOfValue(text, extendedValueEnd);
         cv = text.substring(last_caption_index + current_last_caption.length()+1, extendedValueEnd);
      } while (extendedValueEnd < text.length());
      */
      
      logger.debug("potential_block_end = " + potential_block_end);
      List found_keys = PdfAS.findBlockInText(text.substring(0, potential_block_end), block_type, old_style); // findRestKeys(text,
      // keys,
      // captions,
      // last_caption_index);

      if (found_keys == null)
      {
        logger.debug("Not all other captions could be found for the last_caption_index " + last_caption_index + " ==> discarding this index.");

        continue;
      }

      // sort found keys ascendingly
      PdfAS.sortFoundKeysAscendingly(found_keys);

      boolean reverse_check_ok = reverseCheckFoundKeys(text, found_keys);
      if (!reverse_check_ok)
      {
        logger.debug("The reverse check ruled this list of found keys out ==> they are discarded.");

        continue;
      }

      logger.debug("The reverse check proved this list of found keys out ==> adding them as potential candidates.");

      FoundBlock found_block = new FoundBlock();
      found_block.std = block_type;
      found_block.found_keys = found_keys;
      found_block.end_index = potential_block_end;//findEndOfValue(text, last_caption_index);
      found_blocks.add(found_block);
    }

    logger.debug("found " + found_blocks.size() + " potential signatures for " + block_type.getType());
    return found_blocks;
  }

  /**
   * Finds all indices of the given subtext (starting at a new line) within a
   * given text.
   * 
   * <p>
   * This is usually used to find the indices of the last captions.
   * </p>
   * 
   * @param text
   *          The text to be searched.
   * @param subtext
   *          The subtext to be sought.
   * @return Returns the List of found indices.
   */
  public static List findIndicesWithStartingNL(String text, String subtext)
  {
    List found_indices = new ArrayList();

    // // for some reason "^" + subtext doesn't work as a pattern
    // String pattern = "\n" + subtext;
    // Pattern p = Pattern.compile(pattern);
    // Matcher m = p.matcher(text);
    //    
    // while (m.find())
    // {
    // int found_index = m.start() + 1; // +1 removes the newline
    // found_indices.add(new Integer(found_index));
    // }

    int search_from_index = 0;
    for (;;)
    {
      int found_index = text.indexOf("\n" + subtext, search_from_index);
      if (found_index < 0)
      {
        break;
      }
      found_index += 1; // The +1 compensates the "\n"
      found_indices.add(new Integer(found_index));
      search_from_index = found_index + subtext.length();
    }
    return found_indices;
  }

  /**
   * Finds the other keys/captions according to their order starting from the
   * last_caption index upwards.
   * 
   * @param text
   *          The text.
   * @param keys
   *          The list of keys.
   * @param captions
   *          The list of captions.
   * @param last_caption_index
   *          The index of the last caption.
   * @return Returns the List of found keys, if all keys could be found, or null
   *         if not all keys could be found.
   */
  public static List findRestKeys(String text, List keys, List captions,
      int last_caption_index)
  {
    List found_keys = new ArrayList();

    FoundKey last_caption_found_key = new FoundKey((String) keys.get(0), (String) captions.get(0), last_caption_index);
    found_keys.add(last_caption_found_key);

    String rest_text = text.substring(0, last_caption_index);

    for (int i = 1; i < captions.size(); i++)
    {
      String sought_caption = (String) captions.get(i);
      int index = rest_text.lastIndexOf(sought_caption);

      if (index < 0)
      {
        return null;
      }
      FoundKey found_key = new FoundKey((String) keys.get(i), (String) captions.get(i), index);
      found_keys.add(0, found_key);

      rest_text = rest_text.substring(0, index);
    }

    return found_keys;
  }

  /**
   * Performs a reverse (top to bottom) search for the found keys and checks
   * that these indices are the same as those that were found during the regular
   * (bottom up) search.
   * <p>
   * If a reverse check proves that the found keys are not at the same positions
   * as during regular search, this list of found keys should be discarded.
   * </p>
   * 
   * @param text
   *          The text.
   * @param found_keys
   *          The found keys to be reversely checked.
   * @return Returns true, if all (also the non required) captions could be
   *         found at the same indices as during regular search, false
   *         otherwise.
   */
  public static boolean reverseCheckFoundKeys(String text, List found_keys)
  {
    int search_from_index = ((FoundKey) found_keys.get(0)).start_index;
    for (int i = 0; i < found_keys.size(); i++)
    {
      FoundKey found_key = (FoundKey) found_keys.get(i);

      int reverse_found_index = text.indexOf(found_key.caption, search_from_index);
      
      // dferbas fix #331 ??
      
      if (reverse_found_index < 0)
      {
        throw new RuntimeException("The caption " + found_key.caption + " wasn't found in the text during reverse checking - there is something wrong.");
      }
      
      if (reverse_found_index != found_key.start_index)
      {
        logger.debug("The index for caption " + found_key.caption + " wasn't proved during reverse checking.");
        return false;
      }
      search_from_index = found_key.start_index + found_key.caption.length();
    }
    return true;
  }

  /**
   * Finds the end of the value in the text.
   * 
   * <p>
   * This simply scans for a '\n' from a given start index. The line up to and
   * inclusive the '\n' is considered to be the value.
   * </p>
   * <p>
   * Note that this method does NOT find the accurate value, if the value goes
   * over multiple lines! This may bear a serious problem. Usually this method
   * is only used to finding the end of the last value in a found block, because
   * mid- values are exactly determined by their start index and the start of
   * the next caption. Nevertheless, if the last value spans over multiple
   * lines, this method will not retrieve it completely.
   * </p>
   * 
   * @param text
   *          The text.
   * @param start_index
   *          The start index from where the end of the value is sought.
   * @return Returns the end index of the value, which is the index of the first
   *         character not belonging to the value anymore (the character after
   *         the '\n').
   */
  public static int findEndOfValue(String text, int start_index)
  {
    int newline_index = text.indexOf('\n', start_index);
    if (newline_index < 0)
    {
      return text.length();
    }   
    return newline_index + 1;
  }
 
  /**
   * Checks the integrity of a found block.
   * 
   * <p>
   * This is an assertive function.
   * </p>
   * 
   * @param text
   *          The text.
   * @param found_block
   *          The found block.
   */
  public static void checkBlockIntegrity(String text, FoundBlock found_block)
  {
    for (int i = 0; i < found_block.found_keys.size() - 1; i++)
    {
      FoundKey this_key = (FoundKey) found_block.found_keys.get(i);
      FoundKey next_key = (FoundKey) found_block.found_keys.get(i + 1);

      int this_end_index = findEndOfValue(text, this_key.start_index);
      if (this_end_index != next_key.start_index)
      {
        logger.debug("multi line value: " + this_key);
        // throw new RuntimeException("The end index of found key " + this_key +
        // " doesn't match the start index of found key " + next_key);
      }
    }

    FoundKey last_key = (FoundKey) found_block.found_keys.get(found_block.found_keys.size() - 1);
    int end_of_block =  findEndOfValue(text, last_key.start_index);
    if (end_of_block == (last_key.start_index+last_key.caption.length()+1))
    {
    	end_of_block = findEndOfValue(text,end_of_block);
    }
    if (end_of_block != found_block.end_index)
    {
        throw new RuntimeException("The end index of last key " + last_key + " doesn't match the end index of the block " + found_block);
    }
  }

  /**
   * Cuts out the given found block from the text.
   * 
   * @param text
   *          The text.
   * @param block
   *          The found block.
   * @return Returns the rest text without the block.
   */
  public static String cutOutBlock(String text, FoundBlock block)
  {
    int block_start_index = ((FoundKey) block.found_keys.get(0)).getStartIndex();
    int block_end_index = block.end_index;
    
    if (block_start_index == 0 && block_end_index == text.length())
    {
      // the block is the whole text - the rest text is empty.
      // This may happen if a (no-text) empty document contains a binary signature.
      // Then the "signed text" of the binary signature is empty.
      return "";
    }

    if (block_end_index == text.length())
    {
      // if the block is at the end of the text, remove the "\n" before the
      // block as well.
      String pre = text.substring(0, block_start_index - 1);
      return pre;
    }

    String pre = text.substring(0, block_start_index);
    String post = text.substring(block_end_index);

    String rest_text = pre + post;
    return rest_text;
  }

  /**
   * Returns the value of the date field as String.
   * 
   * @param text
   *          The text.
   * @param block
   *          The found block.
   * @return Returns the date value.
   */
  public static String getDateValue(String text, FoundBlock block)
  {
    FoundKey date_key = block.getDateFoundKey();
    int date_value_start_index = date_key.start_index + date_key.caption.length();
    int date_value_end_index = findEndOfValue(text, date_value_start_index);
    if (date_value_end_index == (date_value_start_index+1))
    {
    	date_value_end_index = findEndOfValue(text, date_value_end_index);	
    }
    String date_value = text.substring(date_value_start_index, date_value_end_index).trim();
    logger.debug("DateString="+date_value);
    return date_value;
  }

  /**
   * Creates a SignatureObject from a found block by extracting the
   * corresponding values.
   * 
   * @param text
   *          The text.
   * @param found_block
   *          The found block.
   * @return Returns the created SignatureObject.
   * @throws SignatureTypesException
   *           F.e.
   * @throws SignatureException
   *           F.e.
   */
  public static SignatureObject createSignatureObjectFromFoundBlock(
      String text, FoundBlock found_block) throws SignatureTypesException, SignatureException
  {
    SignatureObject signatureObject = new SignatureObject();

    signatureObject.setSigType(found_block.std.getType());
    signatureObject.initByType();

    int end_index = found_block.end_index;
    for (int i = found_block.found_keys.size() - 1; i >= 0; i--)
    {
      FoundKey cur_key = (FoundKey) found_block.found_keys.get(i);
      int start_index = cur_key.getStartIndex() + cur_key.caption.length();

      String value = text.substring(start_index, end_index);

      signatureObject.setSigValueCaption(cur_key.getKey(), value, cur_key.caption);

      end_index = cur_key.getStartIndex();
    }

    return signatureObject;

  }

  /**
   * Parses the EGIZDate from a found block and the given text.
   * 
   * @param text
   *          The text.
   * @param found_block
   *          The found block.
   * @return Returns the parsed EGIZDate.
   */
  public static EGIZDate getDateFromFoundBlock(String text,
      FoundBlock found_block)
  {
    String date_value = getDateValue(text, found_block);
    EGIZDate date = EGIZDate.parseFromString(date_value);
    return date;
  }

  /**
   * Sorts the List of found blocks by date.
   * 
   * @param text
   *          The text.
   * @param found_blocks
   *          The List of found blocks.
   */
  public static void sortFoundBlocksByDate(final String text, List found_blocks)
  {
    Collections.sort(found_blocks, new Comparator()
    {
      public int compare(Object arg0, Object arg1)
      {
        FoundBlock fb0 = (FoundBlock) arg0;
        FoundBlock fb1 = (FoundBlock) arg1;

        EGIZDate date0 = getDateFromFoundBlock(text, fb0);
        EGIZDate date1 = getDateFromFoundBlock(text, fb1);
        return date0.compareTo(date1);
      }
    });
  }

  /**
   * Given a List of FoundBlock objects, this method returns the last blocks of
   * this list that have the same date.
   * 
   * <p>
   * Usually a date sorted list (earliest first, latest last) will be provided
   * to this method. Then the last date equal blocks are returned, which are the
   * last blocks.
   * </p>
   * 
   * @param text
   *          The text to retrieve the values of the fields from.
   * @param found_blocks
   *          The List of FoundBlock objects.
   * @return Returns the List of the last date equal blocks.
   */
  public static List filterLastDateEqualBlocks(String text, List found_blocks)
  {
    List latest_blocks = new ArrayList();
    latest_blocks.add(found_blocks.get(found_blocks.size() - 1));
    for (int i = found_blocks.size() - 2; i >= 0; i--)
    {
      FoundBlock this_block = (FoundBlock) found_blocks.get(i);
      FoundBlock succ_block = (FoundBlock) found_blocks.get(i + 1);

      EGIZDate this_date = getDateFromFoundBlock(text, this_block);
      EGIZDate succ_date = getDateFromFoundBlock(text, succ_block);

      if (!this_date.equals(succ_date))
      {
        break;
      }
      latest_blocks.add(0, this_block);
    }

    return latest_blocks;
  }
  /**
   * Chooses the most possible (best choice) block of the list of blocks.
   * 
   * <p>
   * The strategy to find the most possible block is to choose the very one
   * block with the maximum number of captions. This block has extracted most
   * information from the text.
   * </p>
   * <p>
   * If there are still multiple blocks with the same number of cations, the
   * blocks are compared caption-wise. The block with all captions being longer
   * or equal to all other blocks' captions wins.
   * </p>
   * 
   * @param found_blocks
   *          The List of semantically equal blocks.
   * @return Returns the best choice FoundBlock.
   * @throws SignatureException
   */
  public static FoundBlock chooseMostPossibleBlock(List found_blocks) throws SignatureException
  {
    // int largest_block_index = 0;
    // FoundBlock largest_block = (FoundBlock) found_blocks.get(0);
    //
    // for (int i = 1; i < found_blocks.size(); i++)
    // {
    // FoundBlock current_block = (FoundBlock) found_blocks.get(i);
    //
    // if (current_block.found_keys.size() > largest_block.found_keys.size())
    // {
    // largest_block = current_block;
    // largest_block_index = i;
    // }
    // }

    List vertically_largest = filterVerticallyLargestBlocks(found_blocks);
    if (logger.isDebugEnabled())
    {
      logger.debug("vertically largest blocks:");
      for (int i = 0; i < vertically_largest.size(); i++)
      {
        FoundBlock found_block = (FoundBlock) vertically_largest.get(i);
        logger.debug("  #" + i + ": " + found_block);
      }
    }

    List horizontally_largest = filterHorizontallyLargestBlocks(vertically_largest);
    if (logger.isDebugEnabled())
    {
      logger.debug("horizontally largest blocks:");
      for (int i = 0; i < horizontally_largest.size(); i++)
      {
        FoundBlock found_block = (FoundBlock) horizontally_largest.get(i);
        logger.debug("  #" + i + ": " + found_block);
      }
    }
    FoundBlock largest_block = (FoundBlock) horizontally_largest.get(0);
    logger.debug("Chose largest block: " + largest_block);
    return largest_block;
  }

  /**
   * Filters out all blocks but the vertically largest ones.
   * 
   * <p>
   * A vertically largest block has the most found keys.
   * </p>
   * 
   * @param found_blocks
   *          The List of FoundBlock objects to be filtered.
   * @return Returns the List of the vertically largest FoundBlock objects.
   */
  public static List filterVerticallyLargestBlocks(List found_blocks)
  {
    // determine the size of the largest block(s)
    int largest_size = Integer.MIN_VALUE;
    for (int i = 0; i < found_blocks.size(); i++)
    {
      FoundBlock fb = (FoundBlock) found_blocks.get(i);
      final int current_size = fb.found_keys.size();
      if (current_size > largest_size)
      {
        largest_size = current_size;
      }
    }

    // keep all blocks that have the largest_size
    List largest_blocks = new ArrayList();
    for (int i = 0; i < found_blocks.size(); i++)
    {
      FoundBlock fb = (FoundBlock) found_blocks.get(i);
      if (fb.found_keys.size() < largest_size)
      {
        continue;
      }
      largest_blocks.add(fb);
    }
    return largest_blocks;
  }

  /**
   * Filters out all blocks but the horizonally largest ones.
   * 
   * <p>
   * A vertically largest block has the most found keys.
   * </p>
   * 
   * @param found_blocks
   *          The List of FoundBlock objects to be filtered. All of these
   *          FoundBlock objects must have the same number of found keys.
   * @return Returns the List of the horizontally largest FoundBlock objects.
   * @throws SignatureException
   */
  public static List filterHorizontallyLargestBlocks(List found_blocks) throws SignatureException
  {
    List horizontally_largest = new ArrayList();
    FoundBlock largest_block = (FoundBlock) found_blocks.get(0);
    horizontally_largest.add(largest_block);

    for (int i = 1; i < found_blocks.size(); i++)
    {
      FoundBlock fb = (FoundBlock) found_blocks.get(i);

      if (isHorizontallyEqual(fb, largest_block))
      {
        horizontally_largest.add(fb);
        continue;
      }

      if (isHorizontallyLarger(fb, largest_block))
      {
        horizontally_largest = new ArrayList();
        largest_block = fb;
        horizontally_largest.add(largest_block);
      }
      else
      {
        if (!isHorizontallyLarger(largest_block, fb))
        {
          // The block is neither equal nor larger nor lower.
          // We cannot exactly determine which one to use.
          throw new SignatureException(315, "The blocks are neither larger nor lower nor equal. Cannot decide which one to pick. fb = " + fb + ", largest_block = " + largest_block);
        }
      }

    }

    return horizontally_largest;
  }

  protected static boolean isHorizontallyEqual(FoundBlock fb0, FoundBlock fb1)
  {
    final int num_keys = fb0.found_keys.size();
    if (num_keys != fb1.found_keys.size())
    {
      throw new IllegalArgumentException("Cannot compare FoundBlock keys: fb0 doesn't have the same number of keys as fb1. " + fb0.found_keys.size() + " vs. " + fb1.found_keys.size());
    }

    for (int i = 0; i < num_keys; i++)
    {
      FoundKey fk0 = (FoundKey) fb0.found_keys.get(i);
      FoundKey fk1 = (FoundKey) fb1.found_keys.get(i);

      if (fk0.caption.length() != fk1.caption.length())
      {
        return false;
      }
    }

    return true;
  }

  protected static boolean isHorizontallyLarger(FoundBlock fb0, FoundBlock fb1)
  {
    final int num_keys = fb0.found_keys.size();
    if (num_keys != fb1.found_keys.size())
    {
      throw new IllegalArgumentException("Cannot compare FoundBlock keys: fb0 doesn't have the same number of keys as fb1. " + fb0.found_keys.size() + " vs. " + fb1.found_keys.size());
    }

    boolean larger = false;

    for (int i = 0; i < num_keys; i++)
    {
      FoundKey fk0 = (FoundKey) fb0.found_keys.get(i);
      FoundKey fk1 = (FoundKey) fb1.found_keys.get(i);

      if (fk0.caption.length() == fk1.caption.length())
      {
        continue;
      }

      if (fk0.caption.length() > fk1.caption.length())
      {
        larger = true;
        continue;
      }

      // if (fk0.caption.length() < fk1.caption.length())
      return false;
    }

    return larger;
  }

}