"bzr update" performance analysis
=================================

There are 5 different slightly different situations in which bzr update 
can be used:

* local only (no-op)
* lightweight checkout
* heavy checkout
* heavy checkout w/ local changes
* bzr update could work on "bound branch" w/no wt

No new revisions
================
Should be O(1) to determine
Tree base is up to date
wt.last-rev == wt.b.last-rev

No local changes, only new revisions
====================================
1) Need to move wt.last_rev (O(1))
2) apply delta from base to new rev (O(changes))
   applying changes to files is approx (O(lines-in-files ^ 2))
3) update meta-info (executable bits, etc) about modified files (O(changes))

2/3 could be concurrent (but that may not necessarily be faster)

potential issue w/ serialized is having 50k files in limbo/

the limbo/ directory could be avoided in some cases, for example when 
adding new files in new directories.

modifying in place: reduces fragmentation of fs, not atomic
w/ local modification, potential of data loss
w/o should be safe

"local mod" is diff between disk and last commit, not merge base

Detecting name conflicts should be O(siblings). Alternatively, conflicts 
with existing files can be detected using stat() and conflicts with new files 
can be detected by examining the pending transform. This changes 
complexity to O(changes).

out of date heavyweight checkout, out of date w/master
=======================================================
1) open working tree, check latest revision
2) open working tree branch, check latest revision
3) mismatch => update wt => wt.b.lastrev
   apply delta to tree O(changed file size)
   ---- conflicts
   stop on conflicts
   stop always -> inform user they need to repeat (why not?, GFD)
4) pull new revs M => L O(newrevs)
5) apply delta to wt
   local committed changes become a pending merge
   local uncommitted stay uncommitted
   local pending merges are retained (should be gc'd)

offtopic:
should bzr update report where the source is ?
should bzr update handle both cases (local tree out-of-date w/local branch, checkout out-of-date w/master) ?

if updating would diverge, give opportuniuty to branch/unbind instead
local ahead, "push to master"

ideas:
1) can this be done as a single logical step?
2) can this be done w/o modifying working tree until end? possible performance improvements
3) if the pulling revision step could deliver full texts, that may help for the merge (same thing as "bzr pull")
