1/*
2 * Copyright 2009 Mike Cumings
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *   http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.kenai.jbosh;
18
19import java.security.SecureRandom;
20import java.util.concurrent.atomic.AtomicLong;
21import java.util.concurrent.locks.Lock;
22import java.util.concurrent.locks.ReentrantLock;
23
24/**
25 * Request ID sequence generator.  This generator generates a random first
26 * RID and then manages the sequence from there on out.
27 */
28final class RequestIDSequence {
29
30    /**
31     * Maximum number of bits available for representing request IDs, according
32     * to the XEP-0124 spec.s
33     */
34    private static final int MAX_BITS = 53;
35
36    /**
37     * Bits devoted to incremented values.
38     */
39    private static final int INCREMENT_BITS = 32;
40
41    /**
42     * Minimum number of times the initial RID can be incremented before
43     * exceeding the maximum.
44     */
45    private static final long MIN_INCREMENTS = 1L << INCREMENT_BITS;
46
47    /**
48     * Max initial value.
49     */
50    private static final long MAX_INITIAL = (1L << MAX_BITS) - MIN_INCREMENTS;
51
52    /**
53     * Max bits mask.
54     */
55    private static final long MASK = ~(Long.MAX_VALUE << MAX_BITS);
56
57    /**
58     * Random number generator.
59     */
60    private static final SecureRandom RAND = new SecureRandom();
61
62    /**
63     * Internal lock.
64     */
65    private static final Lock LOCK = new ReentrantLock();
66
67    /**
68     * The last reqest ID used, or &lt;= 0 if a new request ID needs to be
69     * generated.
70     */
71    private AtomicLong nextRequestID = new AtomicLong();
72
73    ///////////////////////////////////////////////////////////////////////////
74    // Constructors:
75
76    /**
77     * Prevent direct construction.
78     */
79    RequestIDSequence() {
80        nextRequestID = new AtomicLong(generateInitialValue());
81    }
82
83    ///////////////////////////////////////////////////////////////////////////
84    // Public methods:
85
86    /**
87     * Calculates the next request ID value to use.  This number must be
88     * initialized such that it is unlikely to ever exceed 2 ^ 53, according
89     * to XEP-0124.
90     *
91     * @return next request ID value
92     */
93    public long getNextRID() {
94        return nextRequestID.getAndIncrement();
95    }
96
97    ///////////////////////////////////////////////////////////////////////////
98    // Private methods:
99
100    /**
101     * Generates an initial RID value by generating numbers until a number is
102     * found which is smaller than the maximum allowed value and greater
103     * than zero.
104     *
105     * @return random initial value
106     */
107    private long generateInitialValue() {
108        long result;
109        LOCK.lock();
110        try {
111            do {
112                result = RAND.nextLong() & MASK;
113            } while (result > MAX_INITIAL);
114        } finally {
115            LOCK.unlock();
116        }
117        return result;
118    }
119
120}
121