RE: [External] : [PATCH] Completion preview common

[Drew Adams <drew.adams@oracle.com>]

> The goal is to preview a common prefix
> when available and longer than the prefix
> already inserted.

Related perhaps:

https://www.emacswiki.org/emacs/Icicles_-_Expanded-Common-Match_Completion

The "expanded common match" described there
is the "longest match of your input pattern
that is common to all candidates and also
contains the first input match in the first
or second candidate, whichever is longer".

This notion and behavior apply to not only
prefix completion but also regexp completion
(and substring completion).

This kind of common-match expansion is quick
but it doesn't always find the longest match.
(And of course there can be more than one
longest common match.)

The expansion typically does find a longest
common match, however, because your input
typically matches the first or the second
candidate in only one place.

And the longer the input you type, the more
likely this is.  In practice, it's only with
very short input such as 'a' that expansion
sometimes misses a longest common match.

The algorithm tries two candidates (1st and
2nd) as its starting point, to increase the
probability of finding a longest common match.

The expanded common match isn't just a common
substring among all of the candidates matched
by your input pattern.  It's a substring
common to all candidates matched by your input,
but a substring that also matches your input.

For example, with regexp completion input
`a.z' and candidates 'abz' and 'apz', there's
no expanded common match.  The substring 'a'
is common to both candidates, but it's not
matched by the (complete) input pattern.