Re: [Algorithms] How to get 3dvector largest coordinate index?

[Marc H. <ma...@gm...>]

  This problem felt a lot like you could somehow just do math on the
conditionals and get the answer.  The second best I could come up with
is:

  (Course, conditional moves makes much of this moot, but on with the show)

//I used 3 for values that can never happen for fun, always cool to go
1 past the end of an array
//Replace with 0, 1, or 2 in shipping code, assert in debug
static uint s_lookup[] = {
/*000*/ 2,
/*001*/ 2,
/*010*/ 3,
/*011*/ 0,
/*100*/ 1,
/*101*/ 3,
/*110*/ 1,
/*111*/ 0
}

inline uint LargestCoordinate(const Vector3d &v)
{
  uint first_bit = v.x > v.y;
  uint second_bit = (v.x > v.z) << 1;
  uint third_bit = (v.y > v.z) << 2;

  uint index = first_bit | second_bit | third_bit;

  return lookup[index];
}

  Then, I thought, I could just load a single uint, shift right by the
index, then and off the low 2 bits:
//Replace binary syntax with something to get around C's lack of
syntax. (like http://c-faq.com/misc/sd28.html )
inline uint LargestCoordinate(const Vector3d &v)
{
  uint val = 0b0001110100111010;

  uint rotate = (v.y > v.z) << 3 | (v.x > v.z) << 2 | (v.x > v.y) << 1;

  return ( val >> rotate ) & 0b11;
}

  I found I needed to use the int trick also, otherwise the (float >
float) statement generates a jump, blah.


  (Hire me!)


-- 
m   h
  arc ernandez
/ these are the days of lethal perfection /