计算相对角度的两个函数

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有时候需要计算点点之间的相对角度，使用arctan等需要考虑符号问题，而且比较慢，如果只需要用来比较或排序，可以完全不用计算精确大小。

下面是两个函数，可以完美解决，返回值是单调的~~

pseudoangle

// Returns the pseudoangle between the line p1 to (infinity, p1.y) // and the line from p1 to p2. The pseudoangle has the property // that the ordering of points by true angle anround p1 and // ordering of points by pseudoangle are the same The result is// in the range [0, 4) (or error -1). float pseudoangle(const WPoint& p1, const WPoint& p2) {   WVector delta = p2 - p1;   float result;      if ((delta.x == 0) && (delta.y == 0)) {      return -1;   } else {      result =(float) (delta.y / (abs(delta.x) + abs(delta.y)));      if (delta.x < 0.0) {         result = 2.0f - result;      } else {         result = 4.0f + result;      }   }   return result;}

Fowler Angles

from:http://paulbourke.net/geometry/fowler/

/*   This function is due to Rob Fowler.  Given dy and dx between 2 points   A and B, we calculate a number in [0.0, 8.0) which is a monotonic   function of the direction from A to B.    (0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0) correspond to   (  0,  45,  90, 135, 180, 225, 270, 315, 360) degrees, measured   counter-clockwise from the positive x axis.*/double FowlerAngle(dy,dx)double dy,dx;{    double adx, ady;    /* Absolute Values of Dx and Dy */    int    code;        /* Angular Region Classification Code */    adx = (dx < 0) ? -dx : dx;  /* Compute the absolute values. */    ady = (dy < 0) ? -dy : dy;    code = (adx < ady) ? 1 : 0;    if (dx < 0)  code += 2;    if (dy < 0)  code += 4;    switch (code)    {        case 0: return (dx==0) ? 0 : ady/adx;  /* [  0, 45] */        case 1: return (2.0 - (adx/ady));      /* ( 45, 90] */        case 3: return (2.0 + (adx/ady));      /* ( 90,135) */        case 2: return (4.0 - (ady/adx));      /* [135,180] */        case 6: return (4.0 + (ady/adx));      /* (180,225] */        case 7: return (6.0 - (adx/ady));      /* (225,270) */        case 5: return (6.0 + (adx/ady));      /* [270,315) */        case 4: return (8.0 - (ady/adx));      /* [315,360) */    }}