/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.commons.collections4.trie.analyzer;
  
  import org.apache.commons.collections4.trie.KeyAnalyzer;
  
  /**
   * An {@link KeyAnalyzer} for {@link String}s.
   *
   * @since 4.0
   */
  public class StringKeyAnalyzer extends KeyAnalyzer<String> {
  
      private static final long serialVersionUID = -7032449491269434877L;
  
      /** A singleton instance of {@link StringKeyAnalyzer}. */
      public static final StringKeyAnalyzer INSTANCE = new StringKeyAnalyzer();
  
      /** The number of bits per {@link Character}. */
      public static final int LENGTH = Character.SIZE;
  
      /** A bit mask where the first bit is 1 and the others are zero. */
      private static final int MSB = 0x8000;
  
      /** Returns a bit mask where the given bit is set. */
      private static int mask(final int bit) {
          return MSB >>> bit;
      }
  
      @Override
      public int bitIndex(final String key, final int offsetInBits, final int lengthInBits,
                          final String other, final int otherOffsetInBits, final int otherLengthInBits) {
  
          boolean allNull = true;
  
          if (offsetInBits % LENGTH != 0 || otherOffsetInBits % LENGTH != 0
                  || lengthInBits % LENGTH != 0 || otherLengthInBits % LENGTH != 0) {
              throw new IllegalArgumentException("The offsets and lengths must be at Character boundaries");
          }
  
          final int beginIndex1 = offsetInBits / LENGTH;
          final int beginIndex2 = otherOffsetInBits / LENGTH;
  
          final int endIndex1 = beginIndex1 + lengthInBits / LENGTH;
          final int endIndex2 = beginIndex2 + otherLengthInBits / LENGTH;
  
          final int length = Math.max(endIndex1, endIndex2);
  
          // Look at each character, and if they're different
          // then figure out which bit makes the difference
          // and return it.
          char k = 0, f = 0;
          for (int i = 0; i < length; i++) {
              final int index1 = beginIndex1 + i;
              final int index2 = beginIndex2 + i;
  
              if (index1 >= endIndex1) {
                  k = 0;
              } else {
                  k = key.charAt(index1);
              }
  
              if (other == null || index2 >= endIndex2) {
                  f = 0;
              } else {
                  f = other.charAt(index2);
              }
  
              if (k != f) {
                  final int x = k ^ f;
                  return i * LENGTH + Integer.numberOfLeadingZeros(x) - LENGTH;
              }
  
              if (k != 0) {
                  allNull = false;
              }
          }
  
          // All bits are 0
          if (allNull) {
              return KeyAnalyzer.NULL_BIT_KEY;
          }
  
          // Both keys are equal
          return KeyAnalyzer.EQUAL_BIT_KEY;
     }
 
     @Override
     public int bitsPerElement() {
         return LENGTH;
     }
 
     @Override
     public boolean isBitSet(final String key, final int bitIndex, final int lengthInBits) {
         if (key == null || bitIndex >= lengthInBits) {
             return false;
         }
 
         final int index = bitIndex / LENGTH;
         final int bit = bitIndex % LENGTH;
 
         return (key.charAt(index) & mask(bit)) != 0;
     }
 
     @Override
     public boolean isPrefix(final String prefix, final int offsetInBits,
                             final int lengthInBits, final String key) {
         if (offsetInBits % LENGTH != 0 || lengthInBits % LENGTH != 0) {
             throw new IllegalArgumentException(
                     "Cannot determine prefix outside of Character boundaries");
         }
 
         final String s1 = prefix.substring(offsetInBits / LENGTH, lengthInBits / LENGTH);
         return key.startsWith(s1);
     }
 
     @Override
     public int lengthInBits(final String key) {
         return key != null ? key.length() * LENGTH : 0;
     }
 }