Question

Is there any command in stl that converts ascii data to the integer form of its hex representation? such as: “abc” -> 0x616263.

i have the most basic way i can think of:

uint64_t tointeger(std::string){
    std::string str = "abc";
    uint64_t value = 0;                  // allow max of 8 chars
    for(int x = 0; x < str.size(); x++)
        value = (value << 8) + str[x];
    return value;
}

as stated above: tointeger("abc"); returns the value 0x616263

but this is too slow. and because i have to use this function hundreds of thousands of times, it has slowed down my program significantly. there are 4 or 5 functions that rely on this one, and each of those are called thousands of times, in addition to this function being called thousands of times

what is a faster way to do this?

Answer 1

You want to pack ASCII characters from a string into a 64-bit integer.

Since std::string is not an intrinsic type, for safety, copy the data into a buffer:

uint_64 values[100]; // Allocate memory on a 64-bit boundary.

char * p = (char *) values; // Point to the memory as characters.

std::string example("beethoven");

std::copy(example.c_str(), p, example.length();

The copying is more safe as far as alignments go. To be faster, but more dangerous, just avoid the copy:

  uint_64 danger;
  danger = *((uint_64 *) example.c_str());

The std::string::c_str method returns a pointer to a c-style string representation of the text, but the text is not guaranteed to last forever, thus the need to copy. Also, the pointer is only guaranteed to be on a character alignment. Thus if it happens to reside at address 0x1003, the processor may generate an alighnment fault (or slow down because it has to fetch at an un-aligned boundary).

Edit 1:

This method does not take into consideration Endianness. The method uses the Endianness of the platform. Changing Endianness will slow the performance.