/*
    * 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.collections.trie;
  
  /**
   * An {@link KeyAnalyzer} for {@link String}s.
   * 
   * @since 4.0
   * @version $Id: StringKeyAnalyzer.java 1429905 2013-01-07 17:15:14Z ggregory $
   */
  public class StringKeyAnalyzer extends AbstractKeyAnalyzer<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;
      }
      
      /**
       * {@inheritDoc}
       */
      public int bitsPerElement() {
          return LENGTH;
      }
      
      /**
       * {@inheritDoc}
       */
      public int lengthInBits(final String key) {
          return key != null ? key.length() * LENGTH : 0;
      }
      
      /**
       * {@inheritDoc}
       */
      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;
     }
     
     /**
      * {@inheritDoc}
      */
     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;
     }
     
     /**
      * {@inheritDoc}
      */
     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);
     }
 }