copies: split up _chain() in naive chaining and filtering steps

The function now has two clearly defined steps. The first step is the
actual chaining. This step is very cheap. The second step is filtering
out invalid copies. This step is expensive. For changeset-centric copy
tracing, I want to do the filtering step only at the end. This patch
prepares for that.

Differential Revision: https://phab.mercurial-scm.org/D6418

mercurial/copies.py

@@ -107,8 +107,8 b' def _findlimit(repo, ctxa, ctxb):'
     # This only occurs when a is a descendent of b or visa-versa.
     return min(limit, a, b)
 
-def _chain(src, dst, a, b):
-    """chain two sets of copies 'a' and 'b'"""
+def _chainandfilter(src, dst, a, b):
+    """chain two sets of copies 'a' and 'b' and filter result"""
 
     # When chaining copies in 'a' (from 'src' via some other commit 'mid') with
     # copies in 'b' (from 'mid' to 'dst'), we can get the different cases in the
@@ -123,31 +123,37 b' def _chain(src, dst, a, b):'
     #   4   x   y   z   x->z
     #   5   -   x   y    -
     #   6   x   x   y   x->y
+    #
+    # _chain() takes care of chaining the copies in 'a' and 'b', but it
+    # cannot tell the difference between cases 1 and 2, between 3 and 4, or
+    # between 5 and 6, so it includes all cases in its result.
+    # Cases 1, 3, and 5 are then removed by _filter().
 
-    # Initialize result ('t') from 'a'. This catches cases 1 & 2. We'll remove
-    # case 1 later. We'll also catch cases 3 & 4 here. Case 4 will be
-    # overwritten later, and case 3 will be removed later.
+    t = _chain(a, b)
+    _filter(src, dst, t)
+    return t
+
+def _filter(src, dst, t):
+    """filters out invalid copies after chaining"""
+    for k, v in list(t.items()):
+        # remove copies from files that didn't exist
+        if v not in src:
+            del t[k]
+        # remove criss-crossed copies
+        elif k in src and v in dst:
+            del t[k]
+        # remove copies to files that were then removed
+        elif k not in dst:
+            del t[k]
+
+def _chain(a, b):
+    """chain two sets of copies 'a' and 'b'"""
     t = a.copy()
     for k, v in b.iteritems():
         if v in t:
-            # Found a chain, i.e. cases 3 & 4. We'll remove case 3 later.
             t[k] = t[v]
         else:
-            # Renamed only in 'b', i.e. cases 5 & 6. We'll remove case 5 later.
             t[k] = v
-
-    for k, v in list(t.items()):
-        # remove copies from files that didn't exist, i.e. case 5
-        if v not in src:
-            del t[k]
-        # remove criss-crossed copies, i.e. case 3
-        elif k in src and v in dst:
-            del t[k]
-        # remove copies to files that were then removed, i.e. case 1
-        # and file 'y' in cases 3 & 4 (in case of rename)
-        elif k not in dst:
-            del t[k]
-
     return t
 
 def _tracefile(fctx, am, limit):
@@ -307,7 +313,7 b' def _changesetforwardcopies(a, b, match)'
             if not match.always():
                 childcopies = {dst: src for dst, src in childcopies.items()
                                if match(dst)}
-            childcopies = _chain(a, childctx, copies, childcopies)
+            childcopies = _chainandfilter(a, childctx, copies, childcopies)
             heapq.heappush(work, (c, parent, childcopies))
     assert False
 
@@ -323,7 +329,7 b' def _forwardcopies(a, b, match=None):'
 
         cm = _committedforwardcopies(a, b.p1(), match)
         # combine copies from dirstate if necessary
-        return _chain(a, b, cm, _dirstatecopies(b._repo, match))
+        return _chainandfilter(a, b, cm, _dirstatecopies(b._repo, match))
     return _committedforwardcopies(a, b, match)
 
 def _backwardrenames(a, b, match):
@@ -367,8 +373,8 b' def pathcopies(x, y, match=None):'
         return _backwardrenames(x, y, match=match)
     if debug:
         repo.ui.debug('debug.copies: search mode: combined\n')
-    return _chain(x, y, _backwardrenames(x, a, match=match),
-                  _forwardcopies(a, y, match=match))
+    return _chainandfilter(x, y, _backwardrenames(x, a, match=match),
+                           _forwardcopies(a, y, match=match))
 
 def mergecopies(repo, c1, c2, base):
     """