#include "modbus.h"

#include <stdio.h>

// frame spec
#define MB_MIN_LEN 4
#define MB_CRC_LEN 2
#define MB_WREG_LEN 8
#define MB_BCAST 0xFF

// resp len
#define MB_RSP_LEN 8
#define MB_ERR_LEN 5

static uint16_t modbus_crc16(const unsigned char* buf, unsigned int len) {
  static const uint16_t table[256] = {
      0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241, 0xC601,
      0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440, 0xCC01, 0x0CC0,
      0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40, 0x0A00, 0xCAC1, 0xCB81,
      0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841, 0xD801, 0x18C0, 0x1980, 0xD941,
      0x1B00, 0xDBC1, 0xDA81, 0x1A40, 0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01,
      0x1DC0, 0x1C80, 0xDC41, 0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0,
      0x1680, 0xD641, 0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081,
      0x1040, 0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
      0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441, 0x3C00,
      0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41, 0xFA01, 0x3AC0,
      0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840, 0x2800, 0xE8C1, 0xE981,
      0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41, 0xEE01, 0x2EC0, 0x2F80, 0xEF41,
      0x2D00, 0xEDC1, 0xEC81, 0x2C40, 0xE401, 0x24C0, 0x2580, 0xE541, 0x2700,
      0xE7C1, 0xE681, 0x2640, 0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0,
      0x2080, 0xE041, 0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281,
      0x6240, 0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
      0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41, 0xAA01,
      0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840, 0x7800, 0xB8C1,
      0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41, 0xBE01, 0x7EC0, 0x7F80,
      0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40, 0xB401, 0x74C0, 0x7580, 0xB541,
      0x7700, 0xB7C1, 0xB681, 0x7640, 0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101,
      0x71C0, 0x7080, 0xB041, 0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0,
      0x5280, 0x9241, 0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481,
      0x5440, 0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
      0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841, 0x8801,
      0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40, 0x4E00, 0x8EC1,
      0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41, 0x4400, 0x84C1, 0x8581,
      0x4540, 0x8701, 0x47C0, 0x4680, 0x8641, 0x8201, 0x42C0, 0x4380, 0x8341,
      0x4100, 0x81C1, 0x8081, 0x4040};

  uint8_t xor = 0;
  uint16_t crc = 0xFFFF;

  while (len--) {
    xor = (*buf++) ^ crc;
    crc >>= 8;
    crc ^= table[xor];
  }

  return crc;
}

// byte ops
static inline uint16_t mb_word(uint8_t hi, uint8_t lo) {
  return ((uint16_t)hi << 8) | lo;
}

static inline void mb_split(uint16_t val, uint8_t* hi, uint8_t* lo) {
  *hi = (val >> 8) & 0xFF;
  *lo = val & 0xFF;
}

static inline void mb_putcrc(uint8_t* buf, uint16_t len) {
  uint16_t crc = modbus_crc16(buf, len);
  buf[len] = crc & 0xFF;
  buf[len + 1] = (crc >> 8) & 0xFF;
}

// frame data
typedef struct {
  uint8_t addr;
  uint8_t func;
  uint16_t reg;
  uint16_t val;
} mb_frame_t;

static int mb_parse(const uint8_t* buf, uint16_t len, mb_frame_t* frame) {
  if (len < MB_MIN_LEN) {
    return 0;
  }

  frame->addr = buf[0];
  frame->func = buf[1];
  frame->reg = mb_word(buf[2], buf[3]);

  if (frame->func == MODBUS_FC_WRITE_SINGLE_REGISTER && len >= 6) {
    frame->val = mb_word(buf[4], buf[5]);
  }

  return 1;
}

uint8_t modbus_process_message(uint8_t* buf, uint16_t len, uint8_t slave_addr,
                               uint16_t* holding_registers,
                               uint16_t num_registers) {
  if (len < MB_MIN_LEN) {
    return MODBUS_ERROR_VALUE;
  }

  mb_frame_t frame;
  if (!mb_parse(buf, len, &frame)) {
    return MODBUS_ERROR_VALUE;
  }

  if (frame.addr != slave_addr && frame.addr != MB_BCAST) {
    return MODBUS_ERROR_ADDRESS;
  }

  uint16_t rcv_crc = mb_word(buf[len - 1], buf[len - 2]);
  uint16_t calc_crc = modbus_crc16(buf, len - 2);

#ifdef MB_DEBUG
  printf("RCV CRC: 0x%04X\n", rcv_crc);
  printf("CALC CRC: 0x%04X\n", calc_crc);
  printf("CRC %s\n", (rcv_crc == calc_crc) ? "OK" : "ERR");
#endif

  if (rcv_crc != calc_crc) {
    return MODBUS_ERROR_VALUE;
  }

  switch (frame.func) {
    case MODBUS_FC_WRITE_SINGLE_REGISTER:
      if (len != MB_WREG_LEN) {
        return MODBUS_ERROR_VALUE;
      }
      if (frame.reg >= num_registers) {
        return MODBUS_ERROR_ADDRESS;
      }

      holding_registers[frame.reg] = frame.val;
      return MODBUS_ERROR_NONE;

    default:
      return MODBUS_ERROR_FUNCTION;
  }
}

uint16_t modbus_create_response(uint8_t* rsp, uint8_t slave_addr,
                                uint8_t function, uint16_t address,
                                uint16_t value) {
  rsp[0] = slave_addr;
  rsp[1] = function;

  uint8_t hi, lo;
  mb_split(address, &hi, &lo);
  rsp[2] = hi;
  rsp[3] = lo;

  mb_split(value, &hi, &lo);
  rsp[4] = hi;
  rsp[5] = lo;

  mb_putcrc(rsp, 6);
  return MB_RSP_LEN;
}

uint16_t modbus_create_error_response(uint8_t* rsp, uint8_t slave_addr,
                                      uint8_t function, uint8_t error_code) {
  rsp[0] = slave_addr;
  rsp[1] = function | 0x80;  // Set MSB for error response
  rsp[2] = error_code;

  mb_putcrc(rsp, 3);
  return MB_ERR_LEN;
}