in eden/scm/edenscm/mercurial/dagop.py [0:0]
def toposort(revs, parentsfunc, firstbranch=()):
"""Yield revisions from heads to roots one (topo) branch at a time.
This function aims to be used by a graph generator that wishes to minimize
the number of parallel branches and their interleaving.
Example iteration order (numbers show the "true" order in a changelog):
o 4
|
o 1
|
| o 3
| |
| o 2
|/
o 0
Note that the ancestors of merges are understood by the current
algorithm to be on the same branch. This means no reordering will
occur behind a merge.
"""
### Quick summary of the algorithm
#
# This function is based around a "retention" principle. We keep revisions
# in memory until we are ready to emit a whole branch that immediately
# "merges" into an existing one. This reduces the number of parallel
# branches with interleaved revisions.
#
# During iteration revs are split into two groups:
# A) revision already emitted
# B) revision in "retention". They are stored as different subgroups.
#
# for each REV, we do the following logic:
#
# 1) if REV is a parent of (A), we will emit it. If there is a
# retention group ((B) above) that is blocked on REV being
# available, we emit all the revisions out of that retention
# group first.
#
# 2) else, we'll search for a subgroup in (B) awaiting for REV to be
# available, if such subgroup exist, we add REV to it and the subgroup is
# now awaiting for REV.parents() to be available.
#
# 3) finally if no such group existed in (B), we create a new subgroup.
#
#
# To bootstrap the algorithm, we emit the tipmost revision (which
# puts it in group (A) from above).
revs.sort(reverse=True)
# Set of parents of revision that have been emitted. They can be considered
# unblocked as the graph generator is already aware of them so there is no
# need to delay the revisions that reference them.
#
# If someone wants to prioritize a branch over the others, pre-filling this
# set will force all other branches to wait until this branch is ready to be
# emitted.
unblocked = set(firstbranch)
# list of groups waiting to be displayed, each group is defined by:
#
# (revs: lists of revs waiting to be displayed,
# blocked: set of that cannot be displayed before those in 'revs')
#
# The second value ('blocked') correspond to parents of any revision in the
# group ('revs') that is not itself contained in the group. The main idea
# of this algorithm is to delay as much as possible the emission of any
# revision. This means waiting for the moment we are about to display
# these parents to display the revs in a group.
#
# This first implementation is smart until it encounters a merge: it will
# emit revs as soon as any parent is about to be emitted and can grow an
# arbitrary number of revs in 'blocked'. In practice this mean we properly
# retains new branches but gives up on any special ordering for ancestors
# of merges. The implementation can be improved to handle this better.
#
# The first subgroup is special. It corresponds to all the revision that
# were already emitted. The 'revs' lists is expected to be empty and the
# 'blocked' set contains the parents revisions of already emitted revision.
#
# You could pre-seed the <parents> set of groups[0] to a specific
# changesets to select what the first emitted branch should be.
groups = [([], unblocked)]
pendingheap = []
pendingset = set()
heapq.heapify(pendingheap)
heappop = heapq.heappop
heappush = heapq.heappush
for currentrev in revs:
# Heap works with smallest element, we want highest so we invert
if currentrev not in pendingset:
heappush(pendingheap, -currentrev)
pendingset.add(currentrev)
# iterates on pending rev until after the current rev have been
# processed.
rev = None
while rev != currentrev:
rev = -heappop(pendingheap)
pendingset.remove(rev)
# Seek for a subgroup blocked, waiting for the current revision.
matching = [i for i, g in enumerate(groups) if rev in g[1]]
if matching:
# The main idea is to gather together all sets that are blocked
# on the same revision.
#
# Groups are merged when a common blocking ancestor is
# observed. For example, given two groups:
#
# revs [5, 4] waiting for 1
# revs [3, 2] waiting for 1
#
# These two groups will be merged when we process
# 1. In theory, we could have merged the groups when
# we added 2 to the group it is now in (we could have
# noticed the groups were both blocked on 1 then), but
# the way it works now makes the algorithm simpler.
#
# We also always keep the oldest subgroup first. We can
# probably improve the behavior by having the longest set
# first. That way, graph algorithms could minimise the length
# of parallel lines their drawing. This is currently not done.
targetidx = matching.pop(0)
trevs, tparents = groups[targetidx]
for i in matching:
gr = groups[i]
trevs.extend(gr[0])
tparents |= gr[1]
# delete all merged subgroups (except the one we kept)
# (starting from the last subgroup for performance and
# sanity reasons)
for i in reversed(matching):
del groups[i]
else:
# This is a new head. We create a new subgroup for it.
targetidx = len(groups)
groups.append(([], {rev}))
gr = groups[targetidx]
# We now add the current nodes to this subgroups. This is done
# after the subgroup merging because all elements from a subgroup
# that relied on this rev must precede it.
#
# we also update the <parents> set to include the parents of the
# new nodes.
if rev == currentrev: # only display stuff in rev
gr[0].append(rev)
gr[1].remove(rev)
parents = [p for p in parentsfunc(rev) if p > node.nullrev]
gr[1].update(parents)
for p in parents:
if p not in pendingset:
pendingset.add(p)
heappush(pendingheap, -p)
# Look for a subgroup to display
#
# When unblocked is empty (if clause), we were not waiting for any
# revisions during the first iteration (if no priority was given) or
# if we emitted a whole disconnected set of the graph (reached a
# root). In that case we arbitrarily take the oldest known
# subgroup. The heuristic could probably be better.
#
# Otherwise (elif clause) if the subgroup is blocked on
# a revision we just emitted, we can safely emit it as
# well.
if not unblocked:
if len(groups) > 1: # display other subset
targetidx = 1
gr = groups[1]
elif not gr[1] & unblocked:
gr = None
if gr is not None:
# update the set of awaited revisions with the one from the
# subgroup
unblocked |= gr[1]
# output all revisions in the subgroup
for r in gr[0]:
yield r
# delete the subgroup that you just output
# unless it is groups[0] in which case you just empty it.
if targetidx:
del groups[targetidx]
else:
gr[0][:] = []
# Check if we have some subgroup waiting for revisions we are not going to
# iterate over
for g in groups:
for r in g[0]:
yield r