Re^3: [OT] Merging messages between users on different machines

[BrowserUk]

MD5(Timestamp+message)

I strongly suggest that you keep the timestamps outside of (separate from) whichever digest function you use. Digests are inherently unorderable with respect to time.

The open part of the question is: which shortcuts I can apply to avoid exchanging full lists of messages?

In the following assume: messageNo is a monotonically increase number from oldest to newest; messageId is timestamp+digest[+deviceId+deviceId].

1. Swap counts; same and you're done.
2. Otherwise; swap messageId of the message at messageNo (smaller count/2) from the beginning.
3. Same; add (smaller count/4) to previous messageNo (smaller count/2) and exchange messageIds. goto step 3.
4. Different; subtract (smaller count/4) from previous messageNo and exchange messageIds. goto step 3.

Basically; do a binary search (oldest->newest) to locate the first difference; and then move forward from that point; exchanging first messageIds; then full messages if needed.

[bitingduck]

I strongly suggest that you keep the timestamps outside of (separate from) whichever digest function you use. Digests are inherently unorderable with respect to time.

I've been a little busy this week so didn't bother looking back at this thread, but my (unstated) reasoning behind putting the timestamps inside the hash was for easy sortability on a single field, at the expense of easy time ordering. It also guarantees that short messages that may be have dupes in the past or future get unique digests. It might be reasonable to include the timestamp both inside the digest and outside as a separate column, depending on the application.