# HG changeset patch
# User Yuya Nishihara <yuya@tcha.org>
# Date 1498313157 -32400
# Node ID a53bfc2845f2a8f74f89f40a7a76f1d5351133f9
# Parent  550c390cd9b2da53457fbf28d09155de15a2c850
revset: add depth limit to descendants() (issue5374)

This is naive implementation using two-pass scanning. Tracking descendants
isn't an easy problem if both start and stop depths are specified. It's
impractical to remember all possible depths of each node while scanning from
roots to descendants because the number of depths explodes. Instead, we could
cache (min, max) depths as a good approximation and track ancestors back when
needed, but that's likely to have off-by-one bug.

Since this implementation appears not significantly slower, and is quite
straightforward, I think it's good enough for practical use cases. The time
and space complexity is O(n) ish.

  revisions:
  0) 1-pass scanning with (min, max)-depth cache (worst-case quadratic)
  1) 2-pass scanning (this version)

  repository:
  mozilla-central

  # descendants(0) (for reference)
  *) 0.430353

  # descendants(0, depth=1000)
  0) 0.264889
  1) 0.398289

  # descendants(limit(tip:0, 1, offset=10000), depth=1000)
  0) 0.025478
  1) 0.029099

  # descendants(0, depth=2000, startdepth=1000)
  0) painfully slow (due to quadratic backtracking of ancestors)
  1) 1.531138

diff -r 550c390cd9b2 -r a53bfc2845f2 mercurial/dagop.py
--- a/mercurial/dagop.py	Sat Jun 24 23:35:03 2017 +0900
+++ b/mercurial/dagop.py	Sat Jun 24 23:05:57 2017 +0900
@@ -125,10 +125,38 @@
                     yield i
                     break
 
-def revdescendants(repo, revs, followfirst):
+def _builddescendantsmap(repo, startrev, followfirst):
+    """Build map of 'rev -> child revs', offset from startrev"""
+    cl = repo.changelog
+    nullrev = node.nullrev
+    descmap = [[] for _rev in xrange(startrev, len(cl))]
+    for currev in cl.revs(startrev + 1):
+        p1rev, p2rev = cl.parentrevs(currev)
+        if p1rev >= startrev:
+            descmap[p1rev - startrev].append(currev)
+        if not followfirst and p2rev != nullrev and p2rev >= startrev:
+            descmap[p2rev - startrev].append(currev)
+    return descmap
+
+def _genrevdescendantsofdepth(repo, revs, followfirst, startdepth, stopdepth):
+    startrev = revs.min()
+    descmap = _builddescendantsmap(repo, startrev, followfirst)
+    def pfunc(rev):
+        return descmap[rev - startrev]
+    return _walkrevtree(pfunc, revs, startdepth, stopdepth, reverse=False)
+
+def revdescendants(repo, revs, followfirst, startdepth=None, stopdepth=None):
     """Like revlog.descendants() but supports additional options, includes
-    the given revs themselves, and returns a smartset"""
-    gen = _genrevdescendants(repo, revs, followfirst)
+    the given revs themselves, and returns a smartset
+
+    Scan ends at the stopdepth (exlusive) if specified. Revisions found
+    earlier than the startdepth are omitted.
+    """
+    if startdepth is None and stopdepth is None:
+        gen = _genrevdescendants(repo, revs, followfirst)
+    else:
+        gen = _genrevdescendantsofdepth(repo, revs, followfirst,
+                                        startdepth, stopdepth)
     return generatorset(gen, iterasc=True)
 
 def _reachablerootspure(repo, minroot, roots, heads, includepath):
diff -r 550c390cd9b2 -r a53bfc2845f2 mercurial/revset.py
--- a/mercurial/revset.py	Sat Jun 24 23:35:03 2017 +0900
+++ b/mercurial/revset.py	Sat Jun 24 23:05:57 2017 +0900
@@ -595,23 +595,42 @@
     return subset.filter(lambda r: matcher(repo[r].description()),
                          condrepr=('<desc %r>', ds))
 
-def _descendants(repo, subset, x, followfirst=False):
+def _descendants(repo, subset, x, followfirst=False, startdepth=None,
+                 stopdepth=None):
     roots = getset(repo, fullreposet(repo), x)
     if not roots:
         return baseset()
-    s = dagop.revdescendants(repo, roots, followfirst)
+    s = dagop.revdescendants(repo, roots, followfirst, startdepth, stopdepth)
     return subset & s
 
-@predicate('descendants(set)', safe=True)
+@predicate('descendants(set[, depth])', safe=True)
 def descendants(repo, subset, x):
     """Changesets which are descendants of changesets in set, including the
     given changesets themselves.
+
+    If depth is specified, the result only includes changesets up to
+    the specified generation.
     """
-    args = getargsdict(x, 'descendants', 'set')
+    # startdepth is for internal use only until we can decide the UI
+    args = getargsdict(x, 'descendants', 'set depth startdepth')
     if 'set' not in args:
         # i18n: "descendants" is a keyword
         raise error.ParseError(_('descendants takes at least 1 argument'))
-    return _descendants(repo, subset, args['set'])
+    startdepth = stopdepth = None
+    if 'startdepth' in args:
+        n = getinteger(args['startdepth'],
+                       "descendants expects an integer startdepth")
+        if n < 0:
+            raise error.ParseError("negative startdepth")
+        startdepth = n
+    if 'depth' in args:
+        # i18n: "descendants" is a keyword
+        n = getinteger(args['depth'], _("descendants expects an integer depth"))
+        if n < 0:
+            raise error.ParseError(_("negative depth"))
+        stopdepth = n + 1
+    return _descendants(repo, subset, args['set'],
+                        startdepth=startdepth, stopdepth=stopdepth)
 
 @predicate('_firstdescendants', safe=True)
 def _firstdescendants(repo, subset, x):
diff -r 550c390cd9b2 -r a53bfc2845f2 tests/test-revset.t
--- a/tests/test-revset.t	Sat Jun 24 23:35:03 2017 +0900
+++ b/tests/test-revset.t	Sat Jun 24 23:05:57 2017 +0900
@@ -980,6 +980,60 @@
   7
   8
 
+test descendants with depth limit
+
+ (depth=0 selects the node itself)
+
+  $ log 'descendants(0, depth=0)'
+  0
+  $ log 'null: & descendants(null, depth=0)'
+  -1
+
+ (p2 = null should be ignored)
+
+  $ log 'null: & descendants(null, depth=2)'
+  -1
+  0
+  1
+
+ (multiple paths: depth(6) = (2, 3))
+
+  $ log 'descendants(1+3, depth=2)'
+  1
+  2
+  3
+  4
+  5
+  6
+
+ (multiple paths: depth(5) = (1, 2), depth(6) = (2, 3))
+
+  $ log 'descendants(3+1, depth=2, startdepth=2)'
+  4
+  5
+  6
+
+ (multiple depths: depth(6) = (0, 2, 4), search for depth=2)
+
+  $ log 'descendants(0+3+6, depth=3, startdepth=1)'
+  1
+  2
+  3
+  4
+  5
+  6
+  7
+
+ (multiple depths: depth(6) = (0, 4), no match)
+
+  $ log 'descendants(0+6, depth=3, startdepth=1)'
+  1
+  2
+  3
+  4
+  5
+  7
+
 test author
 
   $ log 'author(bob)'