tests: add test demonstrating optimizer

tests/test_optimizer.py

@@ -0,0 +1,65 @@
+# Copyright (C) 2021 FireEye, Inc. All Rights Reserved.
+# Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at: [package root]/LICENSE.txt
+# Unless required by applicable law or agreed to in writing, software distributed under the License
+#  is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and limitations under the License.
+
+import textwrap
+
+import pytest
+
+import capa.rules
+import capa.engine
+import capa.optimizer
+import capa.features.common
+from capa.engine import Or, And
+from capa.features.insn import Mnemonic
+from capa.features.common import Arch, Bytes, Substring
+
+
+def test_optimizer_order():
+    rule = textwrap.dedent(
+        """
+        rule:
+            meta:
+                name: test rule
+                scope: function
+            features:
+                - and:
+                    - substring: "foo"
+                    - arch: amd64
+                    - mnemonic: cmp
+                    - and:
+                      - bytes: 3
+                      - offset: 2
+                    - or:
+                      - number: 1
+                      - offset: 4
+        """
+    )
+    r = capa.rules.Rule.from_yaml(rule)
+
+    # before optimization
+    children = list(r.statement.get_children())
+    assert isinstance(children[0], Substring)
+    assert isinstance(children[1], Arch)
+    assert isinstance(children[2], Mnemonic)
+    assert isinstance(children[3], And)
+    assert isinstance(children[4], Or)
+
+    # after optimization
+    capa.optimizer.optimize_rules([r])
+    children = list(r.statement.get_children())
+
+    # cost: 0
+    assert isinstance(children[0], Arch)
+    # cost: 1
+    assert isinstance(children[1], Mnemonic)
+    # cost: 2
+    assert isinstance(children[2], Substring)
+    # cost: 3
+    assert isinstance(children[3], Or)
+    # cost: 4
+    assert isinstance(children[4], And)