Do breadth first search

conda/models/prefix_graph.py

@@ -409,7 +409,23 @@ def __init__(self, records, specs=()):
             consolidated_graph[node.name] = cg
         self.graph_by_name = consolidated_graph
 
-    def depth_first_search_by_name(self, root_spec, spec_name, allowed_specs):
+    def breadth_first_search_by_name(self, root_spec, spec_name):
+        """Return shorted path from root_spec to spec_name"""
+        queue = []
+        queue.append([root_spec.name])
+        while queue:
+            path = queue.pop(0)
+            node = path[-1]
+            if node == spec_name:
+                return path
+            children = sorted(self.graph_by_name.get(node, set()),
+                              key=lambda x: list(self.graph_by_name.keys()).index(x))
+            for adj in children:
+                new_path = list(path)
+                new_path.append(adj)
+                queue.append(new_path)
+
+    def depth_first_search_by_name(self, root_spec, spec_name):
         """Return paths from root_spec to spec_name"""
         if root_spec.name == spec_name:
             return [[root_spec]]
@@ -434,8 +450,11 @@ def build_dependency_chain(node, spc, chains=None):
             return chains
 
         chains = build_dependency_chain(root_spec.name, spec_name)
+        return chains
 
+    def get_dependency_chains(self, root_spec, spec_name, allowed_specs):
         final_chains = []
+        chains = [self.breadth_first_search_by_name(root_spec, spec_name)]
         for chain in sorted(chains, key=len):
             if chain[0] == root_spec.name and chain[-1] == spec_name:
                 # remap to matchspecs

conda/resolve.py

@@ -415,7 +415,7 @@ def build_conflict_map(self, specs, specs_to_add=None, history_specs=None):
                                 key=lambda x: list(g.graph_by_name.keys()).index(x.name))
             for spec in spec_order:
                 # the DFS approach works well when things are actually in the graph
-                bad_deps.extend(g.depth_first_search_by_name(spec, dep, sdeps[spec]))
+                bad_deps.extend(g.get_dependency_chains(spec, dep, sdeps[spec]))
 
         if not bad_deps:
             # no conflicting nor missing packages found, return the bad specs

tests/test_resolve.py

@@ -355,6 +355,56 @@ def test_unsat_simple():
     assert "b -> c[version='>=2,<3']" in str(excinfo.value)
 
 
+def test_unsat_shortest_chain():
+    index = (
+        simple_rec(name='a', depends=['d', 'c <1.3.0']),
+        simple_rec(name='b', depends=['c']),
+        simple_rec(name='c', version='1.3.6',),
+        simple_rec(name='c', version='1.2.8',),
+        simple_rec(name='d', depends=['c >=0.8.0']),
+    )
+    r = Resolve(OrderedDict((prec, prec) for prec in index))
+    with pytest.raises(UnsatisfiableError) as excinfo:
+        r.install(['c=1.3.6', 'a', 'b'])
+    assert "a -> c[version='<1.3.0']" in str(excinfo.value)
+    assert "b -> c" in str(excinfo.value)
+    assert "c=1.3.6" in str(excinfo.value)
+
+
+def test_unsat_shortest_reverse_chain():
+    index = (
+        simple_rec(name='a', depends=['d', 'c >=0.8.0']),
+        simple_rec(name='b', depends=['c']),
+        simple_rec(name='c', version='1.3.6',),
+        simple_rec(name='c', version='1.2.8',),
+        simple_rec(name='d', depends=['c <1.3.0']),
+    )
+    r = Resolve(OrderedDict((prec, prec) for prec in index))
+    with pytest.raises(UnsatisfiableError) as excinfo:
+        r.install(['c=1.3.6', 'a', 'b'])
+    assert "a -> d -> c[version='<1.3.0']" in str(excinfo.value)
+    assert "b -> c" in str(excinfo.value)
+    assert "c=1.3.6" in str(excinfo.value)
+
+
+def test_unsat_shortest_long_chain():
+    index = (
+        simple_rec(name='a', depends=['f', 'e']),
+        simple_rec(name='b', depends=['c']),
+        simple_rec(name='c', version='1.3.6',),
+        simple_rec(name='c', version='1.2.8',),
+        simple_rec(name='d', depends=['c >=0.8.0']),
+        simple_rec(name='e', depends=['c <1.3.0']),
+        simple_rec(name='f', depends=['d']),
+    )
+    r = Resolve(OrderedDict((prec, prec) for prec in index))
+    with pytest.raises(UnsatisfiableError) as excinfo:
+        r.install(['c=1.3.6', 'a', 'b'])
+    assert "a -> e -> c[version='<1.3.0']" in str(excinfo.value)
+    assert "b -> c" in str(excinfo.value)
+    assert "c=1.3.6" in str(excinfo.value)
+
+
 def test_unsat_chain():
     # a -> b -> c=1.x -> d=1.x
     # e      -> c=2.x -> d=2.x