ancestor: add a way to remove ancestors of bases from a given set

This and missingancestors can share state, which will turn out to be perfect
for set discovery.

mercurial/ancestor.py

@@ -154,6 +154,32 b' class incrementalmissingancestors(object'
         '''grow the ancestor set by adding new bases'''
         self.bases.update(newbases)
 
+    def removeancestorsfrom(self, revs):
+        '''remove all ancestors of bases from the set revs (in place)'''
+        bases = self.bases
+        pfunc = self.pfunc
+        revs.difference_update(bases)
+        # nullrev is always an ancestor
+        revs.discard(nullrev)
+        if not revs:
+            return
+        # anything in revs > start is definitely not an ancestor of bases
+        # revs <= start needs to be investigated
+        start = max(bases)
+        keepcount = sum(1 for r in revs if r > start)
+        if len(revs) == keepcount:
+            # no revs to consider
+            return
+
+        for curr in xrange(start, min(revs) - 1, -1):
+            if curr not in bases:
+                continue
+            revs.discard(curr)
+            bases.update(pfunc(curr))
+            if len(revs) == keepcount:
+                # no more potential revs to discard
+                break
+
     def missingancestors(self, revs):
         '''return all the ancestors of revs that are not ancestors of self.bases
 

tests/test-ancestor.py

@@ -47,6 +47,11 b' class naiveincrementalmissingancestors(o'
         self.bases = set(bases)
     def addbases(self, newbases):
         self.bases.update(newbases)
+    def removeancestorsfrom(self, revs):
+        for base in self.bases:
+            if base != nullrev:
+                revs.difference_update(self.ancs[base])
+        revs.discard(nullrev)
     def missingancestors(self, revs):
         res = set()
         for rev in revs:
@@ -98,12 +103,25 b' def test_missingancestors(seed, rng):'
             # reference slow algorithm
             naiveinc = naiveincrementalmissingancestors(ancs, bases)
             seq = []
+            revs = []
             for _ in xrange(inccount):
                 if rng.random() < 0.2:
                     newbases = samplerevs(graphnodes)
                     seq.append(('addbases', newbases))
                     inc.addbases(newbases)
                     naiveinc.addbases(newbases)
+                if rng.random() < 0.4:
+                    # larger set so that there are more revs to remove from
+                    revs = samplerevs(graphnodes, mu=1.5)
+                    seq.append(('removeancestorsfrom', revs))
+                    hrevs = set(revs)
+                    rrevs = set(revs)
+                    inc.removeancestorsfrom(hrevs)
+                    naiveinc.removeancestorsfrom(rrevs)
+                    if hrevs != rrevs:
+                        err(seed, graph, bases, seq, sorted(hrevs),
+                            sorted(rrevs))
+                else:
                 revs = samplerevs(graphnodes)
                 seq.append(('missingancestors', revs))
                 h = inc.missingancestors(revs)