From 3c57ba6e5e8af43c2e30dea7458f7b8792ec90eb Mon Sep 17 00:00:00 2001
From: Jimmy Hedman <jimmy.hedman@gmail.com>
Date: Sun, 8 Nov 2020 16:20:13 +0100
Subject: [PATCH] Add support to put calculated field allways last.

---
 binmap/__init__.py   | 30 +++++++++++++++------
 tests/test_binmap.py | 63 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+), 8 deletions(-)

diff --git a/binmap/__init__.py b/binmap/__init__.py
index 908f1b1..c8c98c7 100644
--- a/binmap/__init__.py
+++ b/binmap/__init__.py
@@ -178,14 +178,14 @@ def enumfield(
     return dataclasses.field(default=default, metadata={"enum": enumclass})
 
 
+def calculatedfield(function: Callable, last=False) -> dataclasses.Field:
     """
     Field generator function for calculated fields
 
     :param Callable function: function that calculates the field.
     :return: dataclass field
     """
-def calculatedfield(function: Callable) -> dataclasses.Field:
-    return dataclasses.field(default=0, metadata={"function": function})
+    return dataclasses.field(default=0, metadata={"function": function, "last": last})
 
 
 @dataclasses.dataclass
@@ -213,6 +213,7 @@ def __init_subclass__(cls, byteorder: str = ">"):
 
         cls.__formatstring = byteorder
 
+        lastfield = ""
         for field_ in dataclasses.fields(cls):
             if field_.name.startswith("_BinmapDataclass__"):
                 continue
@@ -230,7 +231,13 @@ def __init_subclass__(cls, byteorder: str = ">"):
                 _type = field_.default * _type
             if type_hints[field_.name] in (types.string, types.pascalstring, str):
                 _type = str(field_.metadata["length"]) + _type
-            cls.__formatstring += _type
+            if "last" in field_.metadata and field_.metadata["last"]:
+                if lastfield != "":
+                    raise ValueError("Can't have more than one last")
+                lastfield = _type
+            else:
+                cls.__formatstring += _type
+        cls.__formatstring += lastfield
 
     def __bytes__(self):
         """
@@ -239,12 +246,19 @@ def __bytes__(self):
         :rtype: bytes
         """
         values = []
+        lastvalue = None
         for k, v in self.__dict__.items():
-            if not k.startswith("_BinmapDataclass__"):
-                if callable(v):
-                    values.append(v(self))
-                else:
-                    values.append(v)
+            if k.startswith("_BinmapDataclass__"):
+                continue
+            if callable(v):
+                v = v(self)
+            if "last" in self.__datafieldsmap[k].metadata and self.__datafieldsmap[k].metadata["last"]:
+                lastvalue = v
+                continue
+            values.append(v)
+        if lastvalue is not None:
+            values.append(lastvalue)
+
         return struct.pack(
             self.__formatstring,
             *values,
diff --git a/tests/test_binmap.py b/tests/test_binmap.py
index 0d3e46e..b77fdc6 100644
--- a/tests/test_binmap.py
+++ b/tests/test_binmap.py
@@ -602,6 +602,21 @@ def chk(self) -> types.unsignedchar:
     checksum: types.unsignedchar = binmap.calculatedfield(chk)
 
 
+class CalculatedFieldLast(binmap.BinmapDataclass):
+    temp: types.signedchar = 0
+
+    def chk_last(self):
+        checksum = 0
+        for k, v in self.__dict__.items():
+            if k.startswith("_") or callable(v):
+                continue
+            checksum += v
+        return checksum & 0xFF
+
+    checksum: types.unsignedchar = binmap.calculatedfield(chk_last, last=True)
+    hum: types.unsignedchar = 0
+
+
 class TestCalculatedField:
     def test_calculated_field(self):
         cf = CalculatedField()
@@ -620,3 +635,51 @@ def test_calculated_field_binary(self):
         with pytest.raises(ValueError) as excinfo:
             CalculatedField(b"\xe4\x18\x00")
         assert "Wrong calculated value" in str(excinfo)
+
+    def test_calculated_field_last(self):
+
+        cfl = CalculatedFieldLast()
+        cfl.temp = 10
+        cfl.hum = 20
+
+        assert cfl.checksum == 30
+        assert bytes(cfl) == b"\x0a\x14\x1e"
+
+    def test_calculated_field_last_inherit(self):
+        class CalculatedFieldLastInherit(CalculatedFieldLast):
+            lux: types.unsignedinteger = 0
+
+        cfli = CalculatedFieldLastInherit()
+        cfli.temp = 10
+        cfli.hum = 20
+        cfli.lux = 401
+        assert bytes(cfli) == b"\x0a\x14\x00\x00\x01\x91\xaf"
+
+        with pytest.raises(ValueError) as excinfo:
+            CalculatedFieldLastInherit(b"\x0b\x20\x00\x00\x01\x30\x00")
+        assert "Wrong calculated value" in str(excinfo)
+
+    def test_calculated_field_multi_last(self):
+        with pytest.raises(ValueError) as excinfo:
+
+            class CalculatedFieldMultiLast(binmap.BinmapDataclass):
+                temp: types.unsignedchar = 0
+
+                def chk(self):
+                    return 0
+
+                checksum1: types.unsignedchar = binmap.calculatedfield(chk, last=True)
+                checksum2: types.unsignedchar = binmap.calculatedfield(chk, last=True)
+
+        assert "Can't have more than one last" in str(excinfo)
+
+    def test_calculated_field_multi_last_inherit(self):
+        with pytest.raises(ValueError) as excinfo:
+
+            class CalculatedFieldMultiLastInherit(CalculatedFieldLast):
+                def chk2(self):
+                    return 0
+
+                checksum2: types.unsignedchar = binmap.calculatedfield(chk2, last=True)
+
+        assert "Can't have more than one last" in str(excinfo)