Fix Message'First handling for checksums

ref #503

rflx/pyrflx/typevalue.py

@@ -551,11 +551,13 @@ def __init__(
             }
         )
 
+        self.__message_first_name = First("Message")
         initial = self._fields[INITIAL.name]
         initial.first = Number(0)
         initial.typeval.assign(bytes())
         self._simplified_mapping: Dict[Name, Expr] = dict.fromkeys(
-            [initial.name_size, initial.name_last, initial.name_first], Number(0)
+            [initial.name_size, initial.name_last, initial.name_first, self.__message_first_name],
+            Number(0),
         )
         self.accessible_fields: List[str] = []
         if self._skip_verification:
@@ -903,7 +905,12 @@ def set_checksum_function(self, checksums: Dict[str, Callable]) -> None:
 
     def _is_checksum_settable(self, checksum: "MessageValue.Checksum") -> bool:
         def valid_path(value_range: ValueRange) -> bool:
-            expr: Dict[Expr, str] = dict.fromkeys([value_range.lower, value_range.upper])
+            lower = value_range.lower.substituted(
+                func=lambda e: self._fields[self._next_field(INITIAL.name)].name_first
+                if e == self.__message_first_name
+                else e
+            )
+            expr: Dict[Expr, str] = dict.fromkeys([lower, value_range.upper])
 
             for e in expr:
                 if isinstance(e, Sub):
@@ -916,8 +923,9 @@ def valid_path(value_range: ValueRange) -> bool:
                 else:
                     assert isinstance(e, (First, Last))
                     expr[e] = str(e.prefix)
+                print(str(e), ":", expr[e])
 
-            field = expr.get(value_range.lower)
+            field = expr.get(lower)
             assert isinstance(field, str)
             upper_field_name = expr[value_range.upper]
             if upper_field_name == "Message":
@@ -1113,7 +1121,7 @@ def __update_simplified_mapping(self, field: Optional[Field] = None) -> None:
             self._simplified_mapping[self.__message_last_name] = field.last
             return
 
-        self._simplified_mapping = {}
+        self._simplified_mapping = {self.__message_first_name: Number(0)}
         for v in self._fields.values():
             if isinstance(v.typeval, ScalarValue) and v.set:
                 self._simplified_mapping[v.name_variable] = v.typeval.expr

tests/data/fixtures/pyrflx.py

@@ -86,6 +86,37 @@ def fixture_icmp_checksum_message_value(icmp_message: model.Message) -> pyrflx.M
     )
 
 
+@pytest.fixture(name="icmp_checksum_message_first")
+def fixture_icmp_checksum_message_first(icmp_message: model.Message) -> pyrflx.MessageValue:
+    return pyrflx.MessageValue(
+        icmp_message.copy(
+            structure=[
+                model.Link(
+                    l.source,
+                    l.target,
+                    condition=expr.And(l.condition, expr.ValidChecksum("Checksum")),
+                )
+                if l.target == model.FINAL
+                else l
+                for l in icmp_message.structure
+            ],
+            aspects={
+                ID("Checksum"): {
+                    ID("Checksum"): [
+                        expr.ValueRange(
+                            expr.First("Message"), expr.Sub(expr.First("Checksum"), expr.Number(1))
+                        ),
+                        expr.Size("Checksum"),
+                        expr.ValueRange(
+                            expr.Add(expr.Last("Checksum"), expr.Number(1)), expr.Last("Message")
+                        ),
+                    ]
+                }
+            },
+        )
+    )
+
+
 @pytest.fixture(name="ipv4_package", scope="session")
 def fixture_ipv4_package(pyrflx_: pyrflx.PyRFLX) -> pyrflx.Package:
     return pyrflx_["IPv4"]

tests/unit/pyrflx_test.py

@@ -1,3 +1,4 @@
+# pylint: disable=too-many-lines
 from pathlib import Path
 
 import pytest
@@ -790,6 +791,8 @@ def test_array_assign_invalid(
 
 def icmp_checksum_function(message: bytes, **kwargs: object) -> int:
     first_arg = kwargs.get("Tag'First .. Checksum'First - 1")
+    if first_arg is None:
+        first_arg = kwargs.get("Message'First .. Checksum'First - 1")
     assert isinstance(first_arg, tuple)
     tag_first, checksum_first_minus_one = first_arg
     assert tag_first == 0 and checksum_first_minus_one == 15
@@ -926,6 +929,75 @@ def test_checksum_parse_invalid(icmp_checksum_message_value: MessageValue) -> No
     assert not icmp_checksum_message_value.valid_message
 
 
+def test_checksum_message_first(icmp_checksum_message_first: MessageValue) -> None:
+    test_data = (
+        b"\x47\xb4\x67\x5e\x00\x00\x00\x00"
+        b"\x4a\xfc\x0d\x00\x00\x00\x00\x00\x10\x11\x12\x13\x14\x15\x16\x17"
+        b"\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27"
+        b"\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37"
+    )
+    icmp_checksum_message_first.set_checksum_function({"Checksum": icmp_checksum_function})
+    icmp_checksum_message_first.set("Tag", "Echo_Request")
+    icmp_checksum_message_first.set("Code_Zero", 0)
+    icmp_checksum_message_first.set("Identifier", 5)
+    icmp_checksum_message_first.set("Sequence_Number", 1)
+    icmp_checksum_message_first.set("Data", test_data)
+    assert icmp_checksum_message_first.get("Checksum") == 12824
+    assert icmp_checksum_message_first.bytestring == b"\x08\x00\x32\x18\x00\x05\x00\x01" + test_data
+    assert icmp_checksum_message_first.valid_message
+
+
+def test_checksum_no_verification() -> None:
+    # pylint: disable = protected-access
+    pyrflx_ = PyRFLX.from_specs(
+        [f"{EX_SPEC_DIR}/icmp.rflx"], skip_model_verification=True, skip_message_verification=True
+    )
+    icmp_message = pyrflx_["ICMP"]["Message"]._type
+    icmp_msg = MessageValue(
+        icmp_message.copy(
+            structure=[
+                Link(
+                    l.source,
+                    l.target,
+                    condition=expr.And(l.condition, expr.ValidChecksum("Checksum")),
+                )
+                if l.target == FINAL
+                else l
+                for l in icmp_message.structure
+            ],
+            aspects={
+                ID("Checksum"): {
+                    ID("Checksum"): [
+                        expr.ValueRange(
+                            expr.First("Message"), expr.Sub(expr.First("Checksum"), expr.Number(1))
+                        ),
+                        expr.Size("Checksum"),
+                        expr.ValueRange(
+                            expr.Add(expr.Last("Checksum"), expr.Number(1)), expr.Last("Message")
+                        ),
+                    ]
+                }
+            },
+        )
+    )
+    test_data = (
+        b"\x47\xb4\x67\x5e\x00\x00\x00\x00"
+        b"\x4a\xfc\x0d\x00\x00\x00\x00\x00\x10\x11\x12\x13\x14\x15\x16\x17"
+        b"\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27"
+        b"\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37"
+    )
+    icmp_msg.set_checksum_function({"Checksum": icmp_checksum_function})
+    icmp_msg.set("Tag", "Echo_Request")
+    icmp_msg.set("Code_Zero", 0)
+    icmp_msg.set("Identifier", 5)
+    icmp_msg.set("Sequence_Number", 1)
+    icmp_msg.set("Data", test_data)
+    icmp_msg.update_checksums()
+    assert icmp_msg.get("Checksum") == 12824
+    assert icmp_msg.bytestring == b"\x08\x00\x32\x18\x00\x05\x00\x01" + test_data
+    assert icmp_msg.valid_message
+
+
 @pytest.fixture(name="tlv_checksum_package", scope="session")
 def fixture_tlv_checksum_package(pyrflx_: PyRFLX) -> Package:
     return pyrflx_["TLV_With_Checksum"]