sized packing

packer/_types/__init__.py

@@ -1,3 +1,4 @@
 from .base import *
 from .float_types import *
 from .int_types import *
+from .sized_packers import *

packer/_types/sized_packers.py

@@ -0,0 +1,32 @@
+__all__ = ("SizedData",)
+
+from typing import Type, Callable
+from .base import TypeDescriptor
+
+def _pack_sized_data_wrapper(size: int) -> Callable:
+    def inner_packer(val: bytes) -> bytes:
+        if (data_len:=len(val)) < size:
+            raise ValueError(f"Attempted to pack data of size {data_len}, expected data of size {size}.")
+        return val
+    return inner_packer
+
+def _unpack_sized_data_wrapper(size: int) -> Callable:
+    def inner_unpack(data: bytearray) -> tuple[int, bytearray]:
+        return (size, data[:size])
+    return inner_unpack
+
+class SizedDataMeta(type):
+    def __getitem__(cls, data_size: int) -> Type["SizedData"]:
+        return type(
+            f"{cls.__name__}({data_size})", 
+            (TypeDescriptor,), 
+            {
+                "__data_size__": data_size,
+                "pack": _pack_sized_data_wrapper(data_size),
+                "unpack": _unpack_sized_data_wrapper(data_size),
+            }
+        )
+
+class SizedData(metaclass=SizedDataMeta):
+    def pack(val: bytes) -> bytes: ...
+    def unpack(data: bytearray) -> tuple[int, bytearray]: ...

tests/test_sized_packing.py

@@ -0,0 +1,31 @@
+import math
+import struct
+from dataclasses import (
+    dataclass,
+)
+
+from packer import (
+    SizedData,
+    Pack,
+    packable,
+)
+
+
+@packable
+@dataclass
+class SizedDataStruct:
+    sized_data: Pack[SizedData[10]] = None
+    sized_data2: Pack[SizedData[50]] = None
+
+def test_sized_packing() -> bool:
+    t = SizedDataStruct()
+    t.sized_data = bytearray(b"h"*5 + b"i"*5)
+    t.sized_data2 = bytearray(50)
+
+    assert len(t.pack()) == 60 and t.pack() == bytearray(b"hhhhhiiiii\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00")
+    assert t.unpack(bytearray(b"x"*5 + b"d"*5) + bytearray(50)) == 60
+    assert t.sized_data == bytearray(b"x"*5 + b"d"*5)
+    assert t.sized_data2 == bytearray(50)
+
+if __name__ == "__main__":
+    test_sized_packing()