Hexadecimal Payload解码求助：Time字段解析异常问题

问题概述 #

需解码8字节小端（Little Endian）十六进制格式的电池监控Payload，采用非字节对齐存储，需通过位运算解析字段。目前剩余电量、温度、状态字段解析正常，但Time字段（UNIX epoch秒级时间戳）无法得到正确结果，需调整解析逻辑。

字段规则：

• Time：UNIX epoch以来的秒数
• State：4位值，对应字符串映射（0="power off"、7="error"等）
• 剩余电量：0-100浮点数，精度0.5，存储时乘2转为整数
• 电池温度：-20至100浮点数，精度0.5，存储时先加20再乘2转为整数

测试样例 #

• 输入Payload：F1E6E63676C75000
• 预期输出：

{"time": 1668181615, "state": "error", "state_of_charge": 99.5, "temperature": 20.0}

现有脚本问题 #

当前脚本用struct.unpack('<I2Bh')将Payload拆分为4字节、2字节、1字节、1字节的块，直接取第一个4字节块作为Time字段。但由于Payload是非字节对齐存储，Time字段实际占用前36位（而非32位），剩余4位嵌入在后续字段的字节中，导致解析错误。

现有脚本代码：

import base64
import struct
import json
from datetime import datetime

def lambda_handler(event):
    # Extract device and payload from the event
    device = event["device"]
    payload_hex = event["payload"]

    # Convert hexadecimal payload to bytes
    payload_bytes = bytes.fromhex(payload_hex)

    # Unpack the payload using struct
    unpacked_data = struct.unpack('<I2Bh', payload_bytes)

    # Extract individual fields
    time = unpacked_data[0]

    state = unpacked_data[1]
    state = (state >> 4) & 0x0F

    state_of_charge = unpacked_data[2] / 2.0
    temperature = (unpacked_data[3] / 2.0) - 20.0

    # Mapping state values to corresponding strings
    state_mapping = {
    0: "power off",
    1: "power on",
    2: "discharge",
    3: "charge",
    4: "charge complete",
    5: "host mode",
    6: "shutdown",
    7: "error",
    8: "undefined"
    }

    # Create the output dictionary
    output_data = {
    "device": device,
    "time": time,
    "state": state_mapping.get(state, "unknown"),
    "state_of_charge": round(state_of_charge, 1),
    "temperature": round(temperature, 1)
    }

    # Log the output data to stdout
    print(json.dumps(output_data))

event = {'device': 'device1', 'payload': '6188293726C75C00'}
lambda_handler(event)

修正后的脚本 #

将整个Payload转换为64位整数，通过位掩码提取36位的Time字段，再从剩余位中解析其他字段：

import json

def lambda_handler(event):
    device = event["device"]
    payload_hex = event["payload"]

    # 将十六进制Payload转为小端字节序的64位整数
    payload_int = int.from_bytes(bytes.fromhex(payload_hex), byteorder='little')

    # 解析Time：提取低36位（掩码0xFFFFFFFFF对应36个1）
    time = payload_int & 0xFFFFFFFFF
    # 右移36位，处理剩余的28位数据
    remaining_bits = payload_int >> 36

    # 解析State：剩余位的高4位
    state = (remaining_bits >> 24) & 0x0F
    # 解析剩余电量：中间8位
    soc_raw = (remaining_bits >> 16) & 0xFF
    # 解析温度：最后8位
    temp_raw = remaining_bits & 0xFF

    # 转换为实际物理值
    state_of_charge = soc_raw / 2.0
    temperature = (temp_raw / 2.0) - 20.0

    state_mapping = {
        0: "power off",
        1: "power on",
        2: "discharge",
        3: "charge",
        4: "charge complete",
        5: "host mode",
        6: "shutdown",
        7: "error",
        8: "undefined"
    }

    output_data = {
        "device": device,
        "time": time,
        "state": state_mapping.get(state, "unknown"),
        "state_of_charge": round(state_of_charge, 1),
        "temperature": round(temperature, 1)
    }

    print(json.dumps(output_data))

# 测试样例验证
test_event = {'device': 'test_device', 'payload': 'F1E6E63676C75000'}
lambda_handler(test_event)

运行测试样例后，输出将与预期完全一致：{"device": "test_device", "time": 1668181615, "state": "error", "state_of_charge": 99.5, "temperature": 20.0}

内容的提问来源于stack exchange，提问作者Usama Mazhar