gecko-dev/grendel/storage/MessageID.java
1999-11-02 01:51:54 +00:00

118 lines
3.8 KiB
Java

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