Fastest way to sum digits in a number

Question

Given a large number, e.g. 9223372036854775807 (Int64.MaxValue), what is the quickest way to sum the digits?

Currently I am ToStringing and reparsing each char into an int:

num.ToString().Sum(c => int.Parse(new String(new char[] { c })));

Which is surely horrifically inefficent. Any suggestions?

And finally, how would you make this work with BigInteger?

Thanks

Answer 1

Well, another option is:

int sum = 0;
while (value != 0)
{
    int remainder;
    value = Math.DivRem(value, 10, out remainder);
    sum += remainder;
}

BigInteger has a DivRem method as well, so you could use the same approach.

Note that I've seen DivRem not be as fast as doing the same arithmetic "manually", so if you're really interested in speed, you might want to consider that.

Also consider a lookup table with (say) 1000 elements precomputed with the sums:

int sum = 0;
while (value != 0)
{
    int remainder;
    value = Math.DivRem(value, 1000, out remainder);
    sum += lookupTable[remainder];
}

That would mean fewer iterations, but each iteration has an added array access...