from ctypes import Structure,c_uint8,Union
import struct

class saej1939_message_id(Structure):
    #_fields_ = [('data_page',c_uint8,1),#does work correct with inversed bit fields
                #('extended_data_page',c_uint8,1),
                #('priority',c_uint8,3),
                #('reserved',c_uint8,3),
                #('pdu_format',c_uint8),
                #('pdu_specific',c_uint8),
                #('source_address',c_uint8),
                #]
#output with reversed bit fields
#python test_structure.py 
#1c000003
#(0, 0, 0, 0, 3, 7)



    _fields_ = [('reserved',c_uint8,3),#does not work correct but is common c notation afaik
                ('priority',c_uint8,3),
                ('extended_data_page',c_uint8,1),
                ('data_page',c_uint8,1),
                ('pdu_format',c_uint8),
                ('pdu_specific',c_uint8),
                ('source_address',c_uint8)
                ]
#output with common c notation
#python test_structure.py 
#38000003
#(0, 0, 0, 0, 3, 7)

    
    def __init__(self,prio=None, pgn=None, da = None, sa=None):
        self.reserved = 0
        self.priority = 0
        self.extended_data_page = 0
        self.data_page = 0
        self.pdu_format = 0
        self.pdu_specific = 0
        self.source_address = 0
        if prio:
            self.priority = prio
        if pgn:
            if pgn < 0xF000:
                self.pdu_format = pgn >> 8
                if da:
                    self.pdu_specific = da
                else:
                    self.pdu_specific = 0xFF
            else:
                self.pdu_format = (pgn >> 8) & 0xFF
                self.pdu_specific = pgn & 0xFF
        if sa:
            self.source_address = sa
        
    def from_integer(self,msg_id):
        msg_union = saej1939_message_id_union()
        if isinstance(msg_id,int):
            msg_id_as_bytes = struct.pack('>I',msg_id)
        elif isinstance(msg_id,bytes):
            msg_id_as_bytes = msg_id
        else: 
            raise NotImplementedError('expected integer as msg_id')
        assert(len(msg_id_as_bytes) == 4)
        for idx,b in enumerate(msg_id_as_bytes):
            msg_union.bytes[idx] = b
        return msg_union.obj


    def to_integer(self): 
        msg_union = saej1939_message_id_union()
        msg_union.obj = self
        msg_id_as_bytes = bytearray()
        for i in range(len(msg_union.bytes)):
            msg_id_as_bytes.append(msg_union.bytes[i])
        msg_id_as_int = struct.unpack('>I',msg_id_as_bytes)[0]
        return msg_id_as_int

        
class saej1939_message_id_union(Union):
    _fields_ = [('bytes',c_uint8*4),
                ('obj',saej1939_message_id),
                ]

if __name__ == '__main__':
    a = saej1939_message_id(prio=7, pgn=0, da=0, sa=3)
    print('{0:08x}'.format(a.to_integer()))
    print(
    a.extended_data_page,
    a.data_page,
    a.pdu_format,
    a.pdu_specific,
    a.source_address,
    a.priority)