Colin Putney wrote:
> 
> On Wednesday, July 23, 2003, at 12:36  PM, Avi Bryant wrote:
> 
>> The way Monticello currently does a three-way merge is by combining
>> two patches - so, for example, if you have
>>
>>   B
>>  /
>> A
>>  \
>>   B'
>>
>> and you want to produce C by merging B and B', you take the patches (B-A)
>> and (B'-A) and combine them (possibly resolving conflicts in the process)
>> into the patch (C-A).  You then apply (C-A) to A and get C.
>>
>> Now, your 4-way case probably isn't as simple as combining the patches
>> (E'-D') and (D-A), and then applying the resulting patch to A - or is it?
>> What are the cases where that wouldn't work?  (As you can see, I'm
>> still being lazy).
> 
> 
> Yup, I think it is that simple. The one caveat is that it's possible to 
> produce an inconsistent snapshot (ie, one with unfulfilled 
> dependencies), which you can't do with a three way merge.

Well, in CVS or SVN with, let's say C code, it is definitely not that 
simple - but I can't remember the details and don't have time to look it 
up right now - I will do so at some point.  Like I said in my other 
email, though, it is possible that it *is* that simple in this case 
because of the extra information we maintain (SVN doesn't do anything 
with ancestry yet).

> Let's say that (C'-B') added a class, and (E'-D') added a method on that 
> class. If you apply (D-A) and (E'-D') to A, you end up with the method, 
> but not the class. That's semantically correct, in that it is what you 
> asked for, but you can't actually load it into the image.

Yes, when you start merging stuff you absolutely need to know what you 
are doing.  But this is also the case if you merge two whole branches 
together: you probably won't end up with missing classes but you may 
have other incompatibilities that aren't detectable as conflicts but do 
still break.

This is why I always make a big deal when people commit unlrelated 
things together or make line ending changes while making code changes. 
If your commits are small, you can say that you need, version X, Y, Q, 
and R to get a particular piece of functionality that you want out of a 
discarded branch.  Or you can revert a particular change by applying 
those changes in reverse.

Julian