Mock Protobuf Binary Encoding in Python

import struct
from dataclasses import dataclass


@dataclass
class Event:
    id: int
    user_id: int
    action: str

    def encode(self) -> bytes:
        # Mock protobuf-like binary encoding using varint and length-delimited fields
        buf = bytearray()
        # field 1: varint id (tag = (1 << 3) | 0 = 8)
        buf.append(8)
        buf.extend(_encode_varint(self.id))
        # field 2: varint user_id (tag = (2 << 3) | 0 = 16)
        buf.append(16)
        buf.extend(_encode_varint(self.user_id))
        # field 3: string action (tag = (3 << 3) | 2 = 26)
        buf.append(26)
        action_bytes = self.action.encode("utf-8")
        buf.extend(_encode_varint(len(action_bytes)))
        buf.extend(action_bytes)
        return bytes(buf)


def _encode_varint(value: int) -> bytes:
    if value < 0:
        raise ValueError("only non-negative integers supported")
    result = bytearray()
    while value > 127:
        result.append((value & 0x7F) | 0x80)
        value >>= 7
    result.append(value)
    return bytes(result)


def decode(binary: bytes) -> dict:
    # Minimal decoder for the mock format (supports fields 1, 2, 3)
    offset = 0
    result = {}
    while offset < len(binary):
        tag = binary[offset]
        offset += 1
        field_number = tag >> 3
        wire_type = tag & 0x07
        if wire_type == 0:
            value, offset = _decode_varint(binary, offset)
            result[field_number] = value
        elif wire_type == 2:
            length, offset = _decode_varint(binary, offset)
            value = binary[offset : offset + length].decode("utf-8")
            offset += length
            result[field_number] = value
        else:
            raise ValueError(f"unsupported wire type {wire_type}")
    return result


def _decode_varint(data: bytes, offset: int) -> tuple[int, int]:
    result = 0
    shift = 0
    while True:
        byte = data[offset]
        offset += 1
        result |= (byte & 0x7F) << shift
        if not byte & 0x80:
            break
        shift += 7
    return result, offset


if __name__ == "__main__":
    event = Event(id=150, user_id=42, action="login")
    encoded = event.encode()
    print(f"Encoded bytes: {encoded.hex()}")
    decoded = decode(encoded)
    print(f"Decoded dict: {decoded}")