Re: Line diff handled badly (by old algorithm)

Stefan Küng wrote:
> On 6/6/06, Simon Large <simon@skirridsystems.co.uk> wrote:
>
>> I like it :-) It has certainly fixed the spurious matching in the old
>> algorithm.
>>
>> I'm not sure the line diff algorithm is correct yet though, or at least
>> the colour coding. Try these 2 lines:
>>
>> void EventWrite(U8 EventType, U8 Device)
>> void EventWrite(U8 EventType, U8 Ident)
>>
>> I get some very odd colour coding rather than a straight delete of
>> 'Device' and an add of 'Ident'.
>
> Well, it is 'correct'.

I disagree; look at the screenshot again - it seems to be showing the
wrong information in the colouring. 'Device' is changed to 'Ident', and
in my Tmerge it shows Device with 'D' deleted, 'evi' unchanged and 'ce'
added. Ident is shown with 'I' unchanged 'd' added, 'e' unchanged and
'nt' deleted. That makes no sense at all to me. The bits marked as
unchanged should be the same on both sides, and in the same order.

The algorithm seems to work when there is a plain addition or deletion
of text, but gets confused by changed text.

> The problem is that in such cases, it's hard
> for the algorithm to know if it would be better to just show a
> complete replace or still try to show inline (in this case: inword)
> diffs.

I know that, and I can see why it is difficult. Where there are several
fragments close together you could perhaps merge them into one if the
matching parts in the middle are shorter than say 3 characters. For example:

int LoadInternal()
int ReadExternal()
----##--##--------

# shows a difference, so you might highlight the Lo/Re and the In/Ex
separately. But because they are so close you could merge them and show
LoadIn/ReadEx. That's just an idea, and there may be counter-examples
where this makes the display worse, so it's not really important.

Simon

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