Question

So I have the following function:

static int calcDTSize( int depth )
{
    if ( depth <= 8 )
    {
        return 1;
    }
    if ( depth <= 16 )
    {
        return 2;
    }
    if ( depth <= 32 )
    {
        return 4;
    }
    if ( depth <= 64 )
    {
        return 8;
    }

    throw std::exception( "Invalid bit count" );
}

Which calculates the size of datatype required for the specified number of bits. Originally I just had:

return ( (int) std::ceil( (double) depth / 8.0 ) );

However, on most machines that I know of, there isn’t a datatype which is 3 bytes long.

I’m sure there must be a neater way of doing the calculation without the if statements but I can’t think how.

Anyone got a better solution?

Answer 1

Divide by 8 and round up to closest power of 2.

Considering that the input is limited and the information is completely static, I would however probably put it in a look up array and do

if (depth <= 64)
    return lut[depth];

throw std::exception( "Invalid bit count" );

---

If you don’t want 64 entries in the lut, you could do something like

static int calcDTSize( int depth )
{
  static int lut[] = { 0, 1, 2, 4, 4, 8, 8, 8, 8 };

  if (depth <= 64)
    return lut[(depth - 1 >> 3) + 1];

  throw std::exception();
}