revlogdeltas: extract _getcandidaterevs in a function

revlogdeltas: extract _getcandidaterevs in a function

The logic barely uses the object it is attached to. This is an important
function that we will clean up in the coming changesets. Moving it at the top
level helps us with that cleanup.

--- a/mercurial/revlogutils/deltas.py	Thu Aug 16 04:38:57 2018 +0200
+++ b/mercurial/revlogutils/deltas.py	Sat Aug 18 07:27:34 2018 +0200
@@ -568,58 +568,57 @@
 
     return True
 
+def _candidategroups(revlog, p1, p2, cachedelta):
+    """
+    Provides revisions that present an interest to be diffed against,
+    grouped by level of easiness.
+    """
+    gdelta = revlog._generaldelta
+    curr = len(revlog)
+    prev = curr - 1
+    p1r, p2r = revlog.rev(p1), revlog.rev(p2)
+
+    # should we try to build a delta?
+    if prev != nullrev and revlog._storedeltachains:
+        tested = set()
+        # This condition is true most of the time when processing
+        # changegroup data into a generaldelta repo. The only time it
+        # isn't true is if this is the first revision in a delta chain
+        # or if ``format.generaldelta=true`` disabled ``lazydeltabase``.
+        if cachedelta and gdelta and revlog._lazydeltabase:
+            # Assume what we received from the server is a good choice
+            # build delta will reuse the cache
+            yield (cachedelta[0],)
+            tested.add(cachedelta[0])
+
+        if gdelta:
+            # exclude already lazy tested base if any
+            parents = [p for p in (p1r, p2r)
+                       if p != nullrev and p not in tested]
+
+            if not revlog._deltabothparents and len(parents) == 2:
+                parents.sort()
+                # To minimize the chance of having to build a fulltext,
+                # pick first whichever parent is closest to us (max rev)
+                yield (parents[1],)
+                # then the other one (min rev) if the first did not fit
+                yield (parents[0],)
+                tested.update(parents)
+            elif len(parents) > 0:
+                # Test all parents (1 or 2), and keep the best candidate
+                yield parents
+                tested.update(parents)
+
+        if prev not in tested:
+            # other approach failed try against prev to hopefully save us a
+            # fulltext.
+            yield (prev,)
+            tested.add(prev)
+
 class deltacomputer(object):
     def __init__(self, revlog):
         self.revlog = revlog
 
-    def _getcandidaterevs(self, p1, p2, cachedelta):
-        """
-        Provides revisions that present an interest to be diffed against,
-        grouped by level of easiness.
-        """
-        revlog = self.revlog
-        gdelta = revlog._generaldelta
-        curr = len(revlog)
-        prev = curr - 1
-        p1r, p2r = revlog.rev(p1), revlog.rev(p2)
-
-        # should we try to build a delta?
-        if prev != nullrev and revlog._storedeltachains:
-            tested = set()
-            # This condition is true most of the time when processing
-            # changegroup data into a generaldelta repo. The only time it
-            # isn't true is if this is the first revision in a delta chain
-            # or if ``format.generaldelta=true`` disabled ``lazydeltabase``.
-            if cachedelta and gdelta and revlog._lazydeltabase:
-                # Assume what we received from the server is a good choice
-                # build delta will reuse the cache
-                yield (cachedelta[0],)
-                tested.add(cachedelta[0])
-
-            if gdelta:
-                # exclude already lazy tested base if any
-                parents = [p for p in (p1r, p2r)
-                           if p != nullrev and p not in tested]
-
-                if not revlog._deltabothparents and len(parents) == 2:
-                    parents.sort()
-                    # To minimize the chance of having to build a fulltext,
-                    # pick first whichever parent is closest to us (max rev)
-                    yield (parents[1],)
-                    # then the other one (min rev) if the first did not fit
-                    yield (parents[0],)
-                    tested.update(parents)
-                elif len(parents) > 0:
-                    # Test all parents (1 or 2), and keep the best candidate
-                    yield parents
-                    tested.update(parents)
-
-            if prev not in tested:
-                # other approach failed try against prev to hopefully save us a
-                # fulltext.
-                yield (prev,)
-                tested.add(prev)
-
     def buildtext(self, revinfo, fh):
         """Builds a fulltext version of a revision
 
@@ -711,7 +710,7 @@
                  depending on whether it is inlined or not
 
         Returns the first acceptable candidate revision, as ordered by
-        _getcandidaterevs
+        _candidategroups
 
         If no suitable deltabase is found, we return delta info for a full
         snapshot.
@@ -735,7 +734,8 @@
 
         deltainfo = None
         deltas_limit = revinfo.textlen * LIMIT_DELTA2TEXT
-        for candidaterevs in self._getcandidaterevs(p1, p2, cachedelta):
+        groups = _candidategroups(self.revlog, p1, p2, cachedelta)
+        for candidaterevs in groups:
             # filter out delta base that will never produce good delta
             candidaterevs = [r for r in candidaterevs
                              if self.revlog.length(r) <= deltas_limit]