builder: make group ids deterministic

This makes the order of sites in finalized builders reproducible.

kwant/builder.py

@@ -91,9 +91,8 @@ class Site(object):
             raise ValueError('Dimensionality mismatch')
         return group(*tuple(a + b for a, b in izip(tag, delta)))
 
-
     def __hash__(self):
-        return id(self.group) ^ hash(self.tag)
+        return self.group.group_id ^ hash(self.tag)
 
     def __eq__(self, other):
         return self.group is other.group and self.tag == other.tag
@@ -110,6 +109,10 @@ class Site(object):
         return self.group.pos(self.tag)
 
 
+# Counter used to give each newly created group an unique id.
+next_group_id = 0
+
+
 class SiteGroup(object):
     """
     Abstract base class for site groups.
@@ -125,10 +128,14 @@ class SiteGroup(object):
     __metaclass__ = abc.ABCMeta
 
     def __init__(self):
-        self.packed_group_id = pgid_of_group(self)
+        global next_group_id
+        self.group_id = next_group_id
+        next_group_id += 1
+        self.packed_group_id = struct.pack(gid_pack_fmt, self.group_id)
 
     def __repr__(self):
-        return '<{0} at {1}>'.format(self.__class__.__name__, hex(id(self)))
+        return '<{0} object: Site group {1}>'.format(
+            self.__class__.__name__, self.group_id)
 
     @abc.abstractmethod
     def pack_tag(self, tag):
@@ -183,11 +190,7 @@ class SimpleSiteGroup(SiteGroup):
 
 # This is used for packing and unpacking group ids (gids).
 gid_pack_fmt = '@P'
-gid_pack_size = len(struct.pack(gid_pack_fmt, id(None)))
-
-def pgid_of_group(group):
-    assert isinstance(group, SiteGroup)
-    return struct.pack(gid_pack_fmt, id(group))
+gid_pack_size = len(struct.pack(gid_pack_fmt, 0))
 
 
 # The reason why this is a global function and not a method of Builder is that

kwant/tests/test_builder.py

@@ -54,8 +54,6 @@ def test_graph():
 
 
 def test_site_groups():
-    pgid = builder.pgid_of_group
-
     sys = builder.Builder()
     assert_equal(sys._group_by_pgid, {})
     sg = builder.SimpleSiteGroup()
@@ -72,9 +70,10 @@ def test_site_groups():
     assert_equal(sys[sg(1)], 123)
     assert_raises(KeyError, sys.__getitem__, osg(1))
 
-    assert_equal(sys._group_by_pgid, {pgid(sg) : sg})
+    assert_equal(sys._group_by_pgid, {sg.packed_group_id : sg})
     sys[osg(1)] = 321
-    assert_equal(sys._group_by_pgid, {pgid(sg) : sg, pgid(osg) : osg})
+    assert_equal(sys._group_by_pgid, {sg.packed_group_id : sg,
+                                      osg.packed_group_id : osg})
     assert_equal(sys[osg(1)], 321)
 
     assert_equal(sg(-5).shifted((-2,), osg), osg(-7))

kwant/tests/test_lattice.py

@@ -19,7 +19,7 @@ def test_make_lattice():
 def test_pack_unpack():
     for dim in [1, 2, 3, 5, 10, 99]:
         group = lattice.make_lattice(np.identity(dim))
-        group_by_pgid = {builder.pgid_of_group(group) : group}
+        group_by_pgid = {group.packed_group_id : group}
         tag = tuple(xrange(dim))
         site = group(*tag)
         psite = site.packed()