import ctypes

C_UINT8 = ctypes.c_uint8
C_UINT32 = ctypes.c_uint32


class BitFieldStruct(ctypes.LittleEndianStructure):
    _fields_ = [
        ("long_field", C_UINT32, 29),
        ("short_field_0", C_UINT8, 1),
        ("short_field_1", C_UINT8, 1),
        ("short_field_2", C_UINT8, 1),
    ]


class BitField(ctypes.Union):
    _anonymous_ = ("fields",)
    _fields_ = [
        ("fields", BitFieldStruct),
        ("as32bit", C_UINT32)
    ]

def test_bit_field_union():
    f = BitField()
    f.as32bit = int.from_bytes([255, 255, 255, 255], byteorder='little')

    assert f.long_field == int.from_bytes([255, 255, 255, 31], byteorder='little')
    assert f.short_field_0 == 1
    assert f.short_field_1 == 1
    assert f.short_field_2 == 1

test_bit_field_union()


# Equivalent C -> https://rextester.com/FWV78514

# #include  <stdio.h>
# #include <stdint.h>
#
#
#
# union BitFieldUnion {
#     struct {
#         uint32_t long_field : 29;
#         uint8_t short_field_0 : 1;
#         uint8_t short_field_1 : 1;
#         uint8_t short_field_2 : 1;
#     };
#     uint32_t as32bit;
# };
#
# int main(void) {
#
#     union BitFieldUnion bf;
#     bf.as32bit = 4294967295;
#     printf("long_field is %d\n", bf.long_field);
#     printf("short_field_0 is %d\n", bf.short_field_0);
#     printf("short_field_1 is %d\n", bf.short_field_1);
#     printf("short_field_2 is %d\n", bf.short_field_2);
#
# }