    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.collections4.trie.analyzer;
    018
    019import org.apache.commons.collections4.trie.KeyAnalyzer;
    020
    021/**
    022 * An {@link KeyAnalyzer} for {@link String}s.
    023 *
    024 * @since 4.0
    025 */
    026public class StringKeyAnalyzer extends KeyAnalyzer<String> {
    027
    028    private static final long serialVersionUID = -7032449491269434877L;
    029
    030    /** A singleton instance of {@link StringKeyAnalyzer}. */
    031    public static final StringKeyAnalyzer INSTANCE = new StringKeyAnalyzer();
    032
    033    /** The number of bits per {@link Character}. */
    034    public static final int LENGTH = Character.SIZE;
    035
    036    /** A bit mask where the first bit is 1 and the others are zero. */
    037    private static final int MSB = 0x8000;
    038
    039    /** Returns a bit mask where the given bit is set. */
    040    private static int mask(final int bit) {
    041        return MSB >>> bit;
    042    }
    043
    044    @Override
    045    public int bitIndex(final String key, final int offsetInBits, final int lengthInBits,
    046                        final String other, final int otherOffsetInBits, final int otherLengthInBits) {
    047
    048        boolean allNull = true;
    049
    050        if (offsetInBits % LENGTH != 0 || otherOffsetInBits % LENGTH != 0
    051                || lengthInBits % LENGTH != 0 || otherLengthInBits % LENGTH != 0) {
    052            throw new IllegalArgumentException("The offsets and lengths must be at Character boundaries");
    053        }
    054
    055        final int beginIndex1 = offsetInBits / LENGTH;
    056        final int beginIndex2 = otherOffsetInBits / LENGTH;
    057
    058        final int endIndex1 = beginIndex1 + lengthInBits / LENGTH;
    059        final int endIndex2 = beginIndex2 + otherLengthInBits / LENGTH;
    060
    061        final int length = Math.max(endIndex1, endIndex2);
    062
    063        // Look at each character, and if they're different
    064        // then figure out which bit makes the difference
    065        // and return it.
    066        char k = 0, f = 0;
    067        for (int i = 0; i < length; i++) {
    068            final int index1 = beginIndex1 + i;
    069            final int index2 = beginIndex2 + i;
    070
    071            if (index1 >= endIndex1) {
    072                k = 0;
    073            } else {
    074                k = key.charAt(index1);
    075            }
    076
    077            if (other == null || index2 >= endIndex2) {
    078                f = 0;
    079            } else {
    080                f = other.charAt(index2);
    081            }
    082
    083            if (k != f) {
    084                final int x = k ^ f;
    085                return i * LENGTH + Integer.numberOfLeadingZeros(x) - LENGTH;
    086            }
    087
    088            if (k != 0) {
    089                allNull = false;
    090            }
    091        }
    092
    093        // All bits are 0
    094        if (allNull) {
    095            return NULL_BIT_KEY;
    096        }
    097
    098        // Both keys are equal
    099        return EQUAL_BIT_KEY;
    100    }
    101
    102    @Override
    103    public int bitsPerElement() {
    104        return LENGTH;
    105    }
    106
    107    @Override
    108    public boolean isBitSet(final String key, final int bitIndex, final int lengthInBits) {
    109        if (key == null || bitIndex >= lengthInBits) {
    110            return false;
    111        }
    112
    113        final int index = bitIndex / LENGTH;
    114        final int bit = bitIndex % LENGTH;
    115
    116        return (key.charAt(index) & mask(bit)) != 0;
    117    }
    118
    119    @Override
    120    public boolean isPrefix(final String prefix, final int offsetInBits,
    121                            final int lengthInBits, final String key) {
    122        if (offsetInBits % LENGTH != 0 || lengthInBits % LENGTH != 0) {
    123            throw new IllegalArgumentException(
    124                    "Cannot determine prefix outside of Character boundaries");
    125        }
    126
    127        final String s1 = prefix.substring(offsetInBits / LENGTH, lengthInBits / LENGTH);
    128        return key.startsWith(s1);
    129    }
    130
    131    @Override
    132    public int lengthInBits(final String key) {
    133        return key != null ? key.length() * LENGTH : 0;
    134    }
    135}