What's the fastest C++ class or C library to convert latitude and longitude from decimal degrees to string and back

Question

I'm looking for the best C or C++ code to encode and decode decimal latitude and longitude values from/to double/char. I'd prefer the code convert from double to char[] and vice-versa rather than c++ strings.

If you have a code snippet that would be great too.

To clarify: I need to convert from a string Degrees/Minutes/Seconds to double and back to string. I have 300 million records, so speed is a big concern.

See: http://en.wikipedia.org/wiki/Geographic_coordinate_conversion

Answer 1

Here's the code I developed:

double Str2LatLong(char* coord)
{
    // char* testInput = "47,26'14\"";

    int i = 0;
    int parts[3] = {0};  // parts[0] is degrees, parts[1] is minutes, parts[2] is seconds
    int* pCurr = parts;

    do
    {
        if (coord[i] == '\0')
        {
            break;
        }
        if (!isdigit(coord[i]))
        {
            *pCurr++; // skip from parts[0] ==> [1], or [1] ==> [2]
            continue;
        }
        *pCurr = 10* (*pCurr) + coord[i] - '0';
        ++i;
    } while (1);

    return parts[0] + ((double)parts[1])/60.0 + ((double)parts[2])/3600.0;
}

Because it is written for speed, there is NO input validation.
You must supply proper input, or it will mess up badly.

I kept everything to integer math, and sequential memory as best I could.
It doesn't check for "proper" delimiters. Rather, anytime something is not a digit, it assumes that is the transition from degrees to minutes, or minutes to seconds.

It is only at the very last line that it converts to double with some very simple floating point operations.

I suspect you'll want to modify it to handle positive/negative values and North/South, East/West indicators, and decimal places after the seconds. But I think this code is a good foundation for a really fast conversion routine.

I hope this will test out very fast. Please let me know how it goes.

Answer 2

Here's another variation.

The logic is the same, but it uses a switch-case statement for better organization, fewer break/continue statements, and a faster return.

You should also be able to enhance this one with

case 'N':
case 'S':
case 'E':
case 'W': 

as needed.

double Str2LatLong(char* coord)
{
    // char* testInput = "47,26'14\"";

    int i = 0;
    int parts[3] = {0};
    int* pCurr = parts;

    while(1)
    {
        switch (coord[i])
        {
            /* Any digit contributes to either degrees,minutes, or seconds (as *pCurr) */
            case '0':
            case '1':
            case '2':
            case '3':
            case '4':
            case '5':
            case '6':
            case '7':
            case '8':
            case '9':
                *pCurr = 10* (*pCurr) + coord[i] - '0';
                break;

            /* A Null terminator triggers return (no break necessary) */
            case '\0':
                return parts[0] + ((double)parts[1])/60.0 + ((double)parts[2])/3600.0;

            /* Any other symbol moves pCurr from degrees to minutes, or minutes to seconds */
            default:
                *pCurr++;
        }
        i++;
    }

    return -1.0;  /* Should never reach this point! */
}

Answer 3

The hard part is going to be representing all the format variations with a single grammar. Once you do, you can use a lexer generator tool to spit out a highly optimized DFA that will be competitive with the best hand-tuned code.