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118 lines
3.8 KiB
Java
118 lines
3.8 KiB
Java
/* -*- Mode: java; indent-tabs-mode: nil; c-basic-offset: 2 -*-
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*
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* The contents of this file are subject to the Mozilla Public
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* License Version 1.1 (the "License"); you may not use this file
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* except in compliance with the License. You may obtain a copy of
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* the License at http://www.mozilla.org/MPL/
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*
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* Software distributed under the License is distributed on an "AS
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* IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
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* implied. See the License for the specific language governing
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* rights and limitations under the License.
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*
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* The Original Code is the Grendel mail/news client.
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*
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* The Initial Developer of the Original Code is Netscape Communications
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* Corporation. Portions created by Netscape are
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* Copyright (C) 1997 Netscape Communications Corporation. All
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* Rights Reserved.
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*
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* Contributor(s):
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*
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* Created: Jamie Zawinski <jwz@netscape.com>, 19 Sep 1997.
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*/
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package grendel.storage;
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/** This class represents a Message-ID.
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It does not contain the ID itself: there is no way to reconstruct the
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original string from this object; however, it contains a hash of that
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string which should be sufficiently unique so that the odds of a
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collision are 1 in 2^32 (since it is a 64-bit hash.)
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<P>
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We store the hash instead of the original string because it takes up
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<I>significantly</I> less memory, and in order to do threading, we
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tend to need a huge number of Message-IDs in memory at once.
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@see MessageIDTable
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*/
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class MessageID {
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long hash; // not protected for the benfit of a MessageIDTable kludge.
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// String debug_id_string;
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/** Constructs a MessageID object with a null hash. */
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MessageID() {
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}
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/** Constructs a MessageID object with the given hash. */
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MessageID(long hash) {
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this.hash = hash;
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}
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/** Constructs a MessageID object for the given sub-sequence of bytes.
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A pointer to the bytes is not retained.
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*/
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MessageID(byte bytes[], int start, int length) {
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hash = hashBytes(bytes, start, length);
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// debug_id_string = new String(bytes, start, length);
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}
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MessageID(String chars) {
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byte bytes[] = chars.getBytes();
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hash = hashBytes(bytes, 0, bytes.length);
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// debug_id_string = chars;
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}
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/** Converts it to a string, for debugging. */
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public String toString() {
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return "<" + this.getClass() + " hash=" + Long.toHexString(hash) +
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// " string=" + debug_id_string +
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">";
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}
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public int hashCode() {
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// Note that this is a 32-bit hash, not the 64-bit internal hash;
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// it's good enough for hash tables, but not for equality checks.
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return (int) ((hash >> 32) ^ (hash & 0xFFFFFFFF));
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}
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public boolean equals(Object x) {
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if (x instanceof MessageID)
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return (hash == ((MessageID) x).hash);
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else
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return false;
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}
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/** Computes a 64-bit hash of the given subsequence of bytes.
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<P>
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Note: this hash value is written out to mail summary files by the
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MailSummaryFileGrendel class. If the hash algorithm is changed,
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it will invalidate all existing mail summary files, and necessitate
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a change of the file format version number.
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@see MailSummaryFileGrendel
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*/
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protected long hashBytes(byte bytes[], int start, int length) {
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long h = 0;
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int end = start + length;
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int i = start;
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// Just in case, don't count a leading < or trailing > in the hash.
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// All callers should strip those first, but be safe...
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while (i < end && (bytes[i] == '<' || bytes[i] <= ' ')) i++;
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while (end > i && (bytes[end-1] == '>' || bytes[end-1] <= ' ')) end--;
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while (i < end) {
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long hh = 0;
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for (int j = 0; i < end && j < 16; j++) {
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hh = (hh * 37) + bytes[i++]; // the guts of String.hashCode().
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}
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h = (h << 8) ^ hh; // fill in high-high-order bits too.
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}
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return h;
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}
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}
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