string to integer fast conversion

nixmans_atoi.cpp

#include <cstdint>
#include <cassert>
#include <cstring>

#if !defined(__fallthrough) && defined(__has_cpp_attribute)
  #if __has_cpp_attribute(fallthrough)
    #define __FJ__FALLTHROUGH [[fallthrough]]
  #elif __has_cpp_attribute(gnu::fallthrough)
    #define __FJ__FALLTHROUGH [[gnu::fallthrough]]
  #endif
#endif

#if !defined(__FJ__FALLTHROUGH) && defined(__has_attribute)
  #if __has_attribute(__fallthrough__)
    #define __FJ__FALLTHROUGH __attribute__((__fallthrough__))
  #endif
#endif

#if !defined(__FJ__FALLTHROUGH) /* well, we tried */
  #define __FJ__FALLTHROUGH ((void)0)
#endif

std::uint64_t atou64(const char *ptr, std::size_t len) {
#   define __FJ__PER_CHAR_EXPR(n) ((res << 1) + (res << 3) + (str[len - n] - '0'))

    const auto *str = reinterpret_cast<const std::uint8_t *>(ptr);
    std::uint64_t res = 0;
    switch ( len ) {
        case 20: res = __FJ__PER_CHAR_EXPR(20); __FJ__FALLTHROUGH;
        case 19: res = __FJ__PER_CHAR_EXPR(19); __FJ__FALLTHROUGH;
        case 18: res = __FJ__PER_CHAR_EXPR(18); __FJ__FALLTHROUGH;
        case 17: res = __FJ__PER_CHAR_EXPR(17); __FJ__FALLTHROUGH;
        case 16: res = __FJ__PER_CHAR_EXPR(16); __FJ__FALLTHROUGH;
        case 15: res = __FJ__PER_CHAR_EXPR(15); __FJ__FALLTHROUGH;
        case 14: res = __FJ__PER_CHAR_EXPR(14); __FJ__FALLTHROUGH;
        case 13: res = __FJ__PER_CHAR_EXPR(13); __FJ__FALLTHROUGH;
        case 12: res = __FJ__PER_CHAR_EXPR(12); __FJ__FALLTHROUGH;
        case 11: res = __FJ__PER_CHAR_EXPR(11); __FJ__FALLTHROUGH;
        case 10: res = __FJ__PER_CHAR_EXPR(10); __FJ__FALLTHROUGH;
        case 9 : res = __FJ__PER_CHAR_EXPR( 9); __FJ__FALLTHROUGH;
        case 8 : res = __FJ__PER_CHAR_EXPR( 8); __FJ__FALLTHROUGH;
        case 7 : res = __FJ__PER_CHAR_EXPR( 7); __FJ__FALLTHROUGH;
        case 6 : res = __FJ__PER_CHAR_EXPR( 6); __FJ__FALLTHROUGH;
        case 5 : res = __FJ__PER_CHAR_EXPR( 5); __FJ__FALLTHROUGH;
        case 4 : res = __FJ__PER_CHAR_EXPR( 4); __FJ__FALLTHROUGH;
        case 3 : res = __FJ__PER_CHAR_EXPR( 3); __FJ__FALLTHROUGH;
        case 2 : res = __FJ__PER_CHAR_EXPR( 2); __FJ__FALLTHROUGH;
        case 1 : res = __FJ__PER_CHAR_EXPR( 1);
    }

#   undef __FJ__PER_CHAR_EXPR

    return res;
}

int main(int, char **argv) {
    return atou(argv[1], std::strlen(argv[1]));
}