ANSI C to VEX: 2D Line Segments Intersection

// Source: http://www.realtimerendering.com/resources/GraphicsGems/gemsii/xlines.c
// Translated to VEX language with some minor changes.
int x_line_line(float x1, y1, x2, y2, x3, y3, x4, y4; export float x, y)
{
    #define DONT_INTERSECT    0
    #define DO_INTERSECT      1
    #define COLLINEAR         2
    #define SAME_SIGNS(a, b) (sign(a) == sign(b))

    float a1, a2, b1, b2, c1, c2; // Coefficients of line eqns.
    float r1, r2, r3, r4;         // "Sign" values.
    float denom;                  // Intermediate values.

    // Compute a1, b1, c1, where line joining points 1 and 2
    // is "a1x + b1y + c1 = 0".
    a1 = y2 - y1;
    b1 = x1 - x2;
    c1 = x2 * y1 - x1 * y2;

    // Compute r3 and r4.
    r3 = a1 * x3 + b1 * y3 + c1;
    r4 = a1 * x4 + b1 * y4 + c1;

    // Check signs of r3 and r4. If both point 3 and point 4 lie on same side
    // of line 1, the line segments do not intersect.
    if (r3 != 0 && r4 != 0 && SAME_SIGNS(r3, r4))
    {
        return (DONT_INTERSECT);
    }

    // Compute a2, b2, c2.
    a2 = y4 - y3;
    b2 = x3 - x4;
    c2 = x4 * y3 - x3 * y4;

    // Compute r1 and r2.
    r1 = a2 * x1 + b2 * y1 + c2;
    r2 = a2 * x2 + b2 * y2 + c2;

    // Check signs of r1 and r2. If both point 1 and point 2 lie on same side
    // of second line segment, the line segments do not intersect.
    if (r1 != 0 && r2 != 0 && SAME_SIGNS(r1, r2))
    {
        return (DONT_INTERSECT);
    }

    // Line segments intersect: compute intersection point.
    denom = a1 * b2 - a2 * b1;

    if (denom == 0)
    {
        return (COLLINEAR);
    }

    x = (b1 * c2 - b2 * c1) / denom;
    y = (a2 * c1 - a1 * c2) / denom;
    return (DO_INTERSECT);
}