C++11 Fletcher checksum

fletcher.cpp

#include <stdio.h>
#include <stdint.h>
#include <cstring>
#include <type_traits>

constexpr const uint16_t csum(const char* s, size_t n, size_t i, uint16_t sum1, uint16_t sum2) {
  return (i <= n) ? csum(s, n, i+1, (sum1 + s[i]) % 255, (sum2 + sum1) % 255) : ((sum2 << 8) | sum1);
}

constexpr const uint16_t operator "" _csum(const char* s, size_t n) {
  return csum(s, n, 0, 0, 0);
}

#define CSUM(a) std::integral_constant<uint16_t, a##_csum>::value

int main() {
  printf("%d\n", CSUM("asdfada"));

  return 0;
}