Re: [PATCH v2] rev-list docs: clarify --topo-order description

Thomas Rast [off-list ref] writes:

Junio C Hamano [off-list ref] writes:

The topo order algorithm can be modified to take advantage of
[generation numbers], in order to provide incremental processing:

  Let S be the set of tentative sources

  Let U be the set of vertices whose out-edges are no known yet
    (i.e., the set of commits which haven't been loaded yet)

[...]

  while there are any vertices left:

    pick any tentative source C from S that we "want to emit"

    # Ascertain that no unknown commit (from U or further beyond) can be
    # a descendant of C
    while there is a D in U such that g(D) > g(C):
      load D
      remove D from U
      add the parents of D to U if they were not already loaded
      possibly remove some elements of S if their indegree became nonzero

    if C was removed from S:
      continue

    remove C from the graph and emit it

By the way, this does bump the runtime of the algorithm a bit, depending
on the data structure used for U.  Recall that ordinary topo-sort with a
stack for S (i.e., --topo-order) runs linearly with the number of
vertices.

If we use a priority queue for U, which lets us get at the
highest-generation unknown commits easily, it potentially goes to n log n 
if U reaches linear size at some point.

That shouldn't hurt too much of course, since on the one hand it should
rarely actually get that big, and OTOH --date-order has n log n runtime
anyway (using a priority queue for S).

Thanks for challenging me on my "it should work" feeling.  It was quite
interesting to actually think it through and write down a workable
algorithm.

-- 
Thomas Rast
trast@{inf,student}.ethz.ch