001 package edu.harvard.deas.hyperenc.vsat;
002
003 import java.io.Serializable;
004 import java.security.SecureRandom;
005
006 import edu.harvard.deas.hyperenc.util.NNLookup;
007
008 // TODO:Fix the collision detection
009 // TODO:Sync access to the database
010
011 /** Contains an index of the pages we will provide. Does the
012 book-keeping to make sure that each page is only accessed twice,
013 returns pages based on the Nearest Neighbor lookup */
014 public class NNPageDatabase
015 extends Thread
016 implements Serializable, PageDatabase
017 {
018 private static final long serialVersionUID = 1L;
019
020 /** Controls debugging output. */
021 private static final boolean DEBUG = false;
022
023 /**
024 * The number of pages this database will contain
025 */
026 public static final int MAX_NUM_PAGES = 50;
027
028 /**
029 * Strong pseudo-random number generator for creating page id's
030 */
031 private SecureRandom securePRNG;
032
033 /**
034 * Our database of pages is a NNLookup table. Each page is a VSatPage object
035 **/
036 private NNLookup<VSatPage> pageDB;
037
038 /**
039 * Creates a new NNPageDatabase. Generates MAX_NUM_PAGES and inserts them
040 * into the database.
041 */
042 public NNPageDatabase()
043 {
044 pageDB = new NNLookup<VSatPage>();
045 securePRNG = new SecureRandom();
046
047 // generate our initial complement of pages
048 for ( int i = 0; i < MAX_NUM_PAGES; i++ ) {
049 if (DEBUG) System.out.println("Creating page " + i + " of " + MAX_NUM_PAGES);
050 generateNewPage();
051 }
052 }
053
054 /**
055 * @return Number of pages currently stored in the page database.
056 */
057 public int getNumPages() {
058 return pageDB.size();
059 }
060
061 /**
062 * @return Maximum number of pages that can be stored in the page database.
063 */
064 public int getMaxPages() {
065 return MAX_NUM_PAGES;
066 }
067
068 /**
069 * Equivalent to <code>getPage</code>.
070 */
071 public byte[] getClosestPage(int key) {
072 return getPage(key);
073 }
074
075 /** Use the key to return the closest available page.
076 @param key The key we are trying to match.
077 @return The page whose key was closest to the request, or
078 <code>null</code> if the database is empty.
079 */
080 public byte[] getPage(int key)
081 {
082 VSatPage thisPage;
083
084 //get closest page
085 synchronized (pageDB) {
086 thisPage = pageDB.get(key);
087 }
088
089 if (thisPage == null) { return null; }
090
091 // increment the counter for that page
092 thisPage.increment();
093
094 //if this page has been accessed twice, delete it from the db
095 int count = thisPage.getCount();
096 if ( count == 2 ) {
097 synchronized (pageDB) {
098 pageDB.delete(key);
099 }
100 // Generate a new page to replace the old one
101 this.generateNewPage();
102 } else if (count > 2) {
103 throw new IllegalStateException("Page access count is " + count
104 + ", which is greater than 2.");
105 }
106
107 return thisPage.getData();
108 }
109
110 /** Generate a new page and put it into the database */
111 private void generateNewPage()
112 {
113 // Spawn a thread to collect the page
114 PageCreator pc = new PageCreator();
115 pc.start();
116 }
117
118 private class PageCreator extends Thread {
119 public void run() {
120 boolean keepGoing;
121
122 //call a new TrueRandomSource to get file data
123 TrueRandomSource myFRS;
124 // XXX coerce into using DevRandomSource... (undo me later)
125 if (true && DevRandomSource.randomDevice() != null) {
126 myFRS = new DevRandomSource();
127 if (DEBUG)
128 System.out.println("Node: getting randomness from "+
129 DevRandomSource.randomDevice());
130 } else {
131 //System.out.println("Node: getting randomness from FileRandomSource");
132 myFRS = new FileRandomSource();
133 if (DEBUG)
134 System.out.println("Node: getting randomness from FileRandomSource");
135 //myFRS = new DevRandomSource();
136 }
137
138 VSatPage newPage = null;
139 int key;
140
141 do {
142 byte[] fileRandom = null; // Hold the random file data
143
144 do {
145 fileRandom = myFRS.genRandomness();
146 } while (fileRandom == null);
147
148 //create a new VSatPage with this data
149 newPage = new VSatPage(fileRandom);
150
151 //we cannot permit collisions; that is,
152 //two pages cannot have the same key value
153 //so we need to find a unique key or just
154 //replace the existing key-page pair
155 //Rather than waste pages, or cause page collection
156 // to fail for the client, we check for uniqueness
157 // make a new key using our strong prng
158 key = securePRNG.nextInt();
159 synchronized (pageDB) {
160 keepGoing = pageDB.getExact(key) != null;
161 }
162 } while (keepGoing);
163
164 synchronized (pageDB) {
165 pageDB.add(key, newPage);
166 if (DEBUG) System.out.println("Added a page");
167 }
168 }
169 }
170
171
172 }
173