/*
* Modbus CRC-16 CFunction for Micromite Mk2 MMBasic
*
* Compile this with MPLAB X and XC32 compiler using the settings:
* - Position independent code (-mno-abicalls -fno-pic)
* - Optimization level -O1 or -O2
* - Target: PIC32MX470F512H (for Micromite Mk2)
*
* After compiling, extract the hex codes and insert into MMBasic as:
*
* CFunction modbus_crc(STRING) INTEGER
*   [hex codes here]
* End CFunction
*
* Usage in MMBasic:
*   data$ = CHR$(&h01) + CHR$(&h03) + CHR$(&h00) + CHR$(&h00) + CHR$(&h00) + CHR$(&h0A)
*   crc% = modbus_crc(data$)
*   PRINT "CRC: "; HEX$(crc%, 4)
*/

#include "CFunction.h"

/*
* Calculate Modbus CRC-16
* Input: Pointer to MMBasic string (first byte = length, followed by data)
* Returns: 16-bit CRC value as long long int
*/
long long int modbus_crc(unsigned char *str) {
   unsigned char len;
   unsigned char *data;
   unsigned short crc;
   int i, j;
   
   // Get string length (first byte)
   len = *str;
   
   // Point to actual data (skip length byte)
   data = str + 1;
   
   // Initialize CRC to 0xFFFF
   crc = 0xFFFF;
   
   // Process each byte
   for (i = 0; i < len; i++) {
       crc = crc ^ data[i];
       
       // Process each bit
       for (j = 0; j < 8; j++) {
           if (crc & 0x0001) {
               crc = (crc >> 1) ^ 0xA001;
           } else {
               crc = crc >> 1;
           }
       }
   }
   
   // Return as long long int (MMBasic integer type)
   return (long long int)crc;
}

/*
* Example usage in MMBasic:
*
* ' Define the CFunction (after compiling and extracting hex codes)
* CFunction modbus_crc(STRING) INTEGER
*   00000000
*   [your hex codes here]
* End CFunction
*
* ' Use it in your program
* data$ = CHR$(&h01) + CHR$(&h03) + CHR$(&h00) + CHR$(&h00) + CHR$(&h00) + CHR$(&h0A)
* crc% = modbus_crc(data$)
*
* PRINT "CRC value: "; HEX$(crc%, 4)
*
* ' Append CRC to message (Modbus uses little-endian: low byte first)
* message$ = data$ + CHR$(crc% AND &hFF) + CHR$((crc% >> 8) AND &hFF)
*
* ' Send the complete message with CRC
* PRINT "Complete message with CRC: ";
* FOR i = 1 TO LEN(message$)
*   PRINT HEX$(ASC(MID$(message$, i, 1)), 2); " ";
* NEXT i
* PRINT
*/