/**
    * Logback: the reliable, generic, fast and flexible logging framework.
    * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
    *
    * This program and the accompanying materials are dual-licensed under
    * either the terms of the Eclipse Public License v1.0 as published by
    * the Eclipse Foundation
    *
    *   or (per the licensee's choosing)
   *
   * under the terms of the GNU Lesser General Public License version 2.1
   * as published by the Free Software Foundation.
   */
  package ch.qos.logback.core.joran.spi;
  
  import java.util.ArrayList;
  import java.util.List;
  
  public class CaseCombinator {
  
      List<String> combinations(String in) {
          int length = in.length();
          List<String> permutationsList = new ArrayList<String>();
  
          int totalCombinations = computeTotalNumerOfCombinations(in, length);
  
          for (int j = 0; j < totalCombinations; j++) {
              StringBuilder newCombination = new StringBuilder();
              int pos = 0;
              for (int i = 0; i < length; i++) {
                  char c = in.charAt(i);
                  if (isEnglishLetter(c)) {
                      c = permute(c, j, pos);
                      pos++;
                  }
                  newCombination.append(c);
              }
              permutationsList.add(newCombination.toString());
          }
          return permutationsList;
  
      }
  
      private char permute(char c, int permutation, int position) {
          int mask = 1 << position;
          boolean shouldBeInUpperCase = (permutation & mask) != 0;
          boolean isEffectivelyUpperCase = isUpperCase(c);
          if (shouldBeInUpperCase && !isEffectivelyUpperCase)
              return toUpperCase(c);
          if (!shouldBeInUpperCase && isEffectivelyUpperCase)
              return toLowerCase(c);
          return c;
      }
  
      private int computeTotalNumerOfCombinations(String in, int length) {
          int count = 0;
          for (int i = 0; i < length; i++) {
              char c = in.charAt(i);
              if (isEnglishLetter(c))
                  count++;
          }
          // return 2^count (2 to the power of count)
          return (1 << count);
      }
  
      private char toUpperCase(char c) {
          if ('A' <= c && c <= 'Z') {
              return c;
          }
          if ('a' <= c && c <= 'z') {
              return (char) ((int) c + 'A' - 'a');
          }
          // code should never reach this point
          return c;
      }
  
      private char toLowerCase(char c) {
          if ('a' <= c && c <= 'z') {
              return c;
          }
          if ('A' <= c && c <= 'Z') {
              return (char) ((int) c + 'a' - 'A');
          }
          // code should never reach this point
          return c;
      }
  
      private boolean isEnglishLetter(char c) {
          if ('a' <= c && c <= 'z')
              return true;
  
          if ('A' <= c && c <= 'Z')
              return true;
          return false;
      }
  
      private boolean isUpperCase(char c) {
          return ('A' <= c && c <= 'Z');
      }
 }