typedef unsigned int uint16_t;
typedef unsigned char uint8_t;
typedef unsigned int size_t;

uint16_t mod255(uint16_t a)
{

//Modulus without Division, Mersenne Numbers

//http://homepage.divms.uiowa.edu/~jones/bcd/mod.shtml
   //a = (a >> 16) + (a & 0xFFFF); /* sum base 2**16 digits - нужно, если использовать uint32_t */
   a = (a >>  8) + (a & 0xFF);   /* sum base 2**8 digits */
   if (a < 255) return a;
   if (a < (2 * 255)) return a - 255;
   return a - (2 * 255);
}

uint16_t fletcher16(const uint8_t *data, size_t len)
{

//Found by solving for c1 overflow:

//n > 0 and n * (n+1) / 2 * (2^8-1) < (2^16-1)

//len <= 22 при использовании uint16_t

uint16_t c0 = 0, c1 = 0;

while(len--)

{
       c0 = c0 + *data++;
       c1 = c1 + c0;
   }
   //c0 = c0 % 255;
   //c1 = c1 % 255;
   c0 = mod255(c0);
   c1 = mod255(c1);
   return (c1 << 8 | c0);
}

volatile static uint8_t buf[5] = {1,2,3,4,5};


int foo()
{
   uint16_t fletcher = *((uint16_t *) &buf[3]);
   //uint16_t fletcher = (((uint16_t)buf[3])…