.

wcfs/internal/xbtree.py

@@ -495,7 +495,12 @@ def Restructure(ztree, newStructure):
 # AllStructs generates subset of all possible BTree structures for BTrees with
 # specified keys and btree depth up-to maxdepth. Each tree node is split by
 # up-to maxsplit points.
-def AllStructs(keys, maxdepth, maxsplit): # -> i[] of Tree
+#
+# If allowEmptyBuckets=y tree structures with empty buckets are also generated.
+# By default, except for empty-tree case, only tree structures with non-empty
+# buckets are generated, because ZODB BTree package misbehaves if it finds an
+# empty bucket in a trees.  ZODB.BTree._check also asserts if bucket is empty.
+def AllStructs(keys, maxdepth, maxsplit, allowEmptyBuckets=False): # -> i[] of Tree
     assert isinstance(maxdepth, int); assert maxdepth >= 0
     assert isinstance(maxsplit, int); assert maxsplit >= 0
     ks = set(keys)
@@ -517,11 +522,16 @@ def AllStructs(keys, maxdepth, maxsplit): # -> i[] of Tree
         klo = 0
         khi = 0
 
-    for tree in _allStructs(klo, khi, keyv, maxdepth, maxsplit):
+        # the only possible case for empty tree is T/B
+        if not allowEmptyBuckets:
+            yield Tree([], Bucket())
+            return
+
+    for tree in _allStructs(klo, khi, keyv, maxdepth, maxsplit, allowEmptyBuckets):
         yield tree
 
 
-def _allStructs(klo, khi, keyv, maxdepth, maxsplit):
+def _allStructs(klo, khi, keyv, maxdepth, maxsplit, allowEmptyBuckets):
     assert klo <= khi
     _assertIncv(keyv)
     if len(keyv) > 0:
@@ -539,8 +549,12 @@ def _allStructs(klo, khi, keyv, maxdepth, maxsplit):
             # emit Tree -> Buckets
             children = []
             for (xlo, xhi) in zip(ksplitv[:-1], ksplitv[1:]): # (klo, s1), (s1, s2), ..., (sN, khi)
-                children.append(Bucket(*_keyvSliceBy(keyv, xlo, xhi)))
-            yield Tree(ksplitv[1:-1], *children) # (s1, s2, ..., sN)
+                bkeyv = _keyvSliceBy(keyv, xlo, xhi)
+                if len(bkeyv) == 0 and (not allowEmptyBuckets):
+                    break
+                children.append(Bucket(*bkeyv))
+            else:
+                yield Tree(ksplitv[1:-1], *children) # (s1, s2, ..., sN)
 
             # emit Tree -> Trees -> ...
             if maxdepth == 0:
@@ -548,7 +562,7 @@ def _allStructs(klo, khi, keyv, maxdepth, maxsplit):
             ichildrenv = [] # of _allStructs for each child link
             for (xlo, xhi) in zip(ksplitv[:-1], ksplitv[1:]): # (klo, s1), (s1, s2), ..., (sN, khi)
                 ichildrenv.append( _allStructs(
-                    xlo, xhi, _keyvSliceBy(keyv, xlo, xhi), maxdepth - 1, maxsplit))
+                    xlo, xhi, _keyvSliceBy(keyv, xlo, xhi), maxdepth - 1, maxsplit, allowEmptyBuckets))
 
             for children in itertools.product(*ichildrenv):
                 yield Tree(ksplitv[1:-1], *children) # (s1, s2, ..., sN)

wcfs/internal/xbtree_test.py

@@ -116,10 +116,14 @@ def test_topoEncoding():
 def test_allStructs():
     T = xbtree.Tree
     B = xbtree.Bucket
-    def X(keys, maxdepth, maxsplit):
-        return list(xbtree.AllStructs(keys, maxdepth, maxsplit))
+    def X(keys, maxdepth, maxsplit, allowEmptyBuckets=True):
+        return list(xbtree.AllStructs(keys, maxdepth, maxsplit, allowEmptyBuckets))
+    def Y(keys, maxdepth, maxsplit):
+        return X(keys, maxsplit, maxsplit, allowEmptyBuckets=False)
 
     assert X([],    0, 0) == [ T([], B()) ]
+    assert Y([],    0, 0) == [ T([], B()) ]
+
     assert X([1],   0, 0) == [ T([], B(1)) ]
     assert X([1,3], 0, 0) == [ T([], B(1,3)) ]