This topic should help get you started.

thanks, but it looks very complicated
i use distance to check if to elements intersect
but this time x, y don't change

rotation changes thou, full circle will be 2*PI
maybe i could test if n is bigger or smaller than certain portion of 2*PI
and then act appropriately...
just don't know how right now

and that probably will not be the most precise method

https://www.google.com/search?q=line+circle+intersection
...

thanks, but it looks very complicated

1. Do you mean the actual calculations inside the method are complicated or
2. using the method (i.e. the actual call) is complicated or
3. both

If (1) then why does it matter because you don't need to understand how it works, you don't need to modify the code all you need to do is call the method with the correct values and have confidence it produces the correct result. After all you are using complex methods all the time it just so happens you can't normally see the source code that does the work. Look at the Processing reference section - complicated methods galore but it doesn't stop you using them!

If (2) then try the method in the code below, it simple determines whether a line crosses the circumference of a circle returning true or false.

If (3) then both the above apply.

Why don't you try these sketches and if you problems calling/using the methods then you can always post a question on this forum. Experimenting with code is a good way of learning to program.

1. void setup() {
2.   size(400, 400);
3.   cursor(CROSS);
4.   smooth();
5. }
6. 
   
7. void draw() {
8.   background(255);
9.   stroke(0);
10.   fill(0, 255, 0, 48);
11.   line(30, 30, 250, 250);
12.   // Draw ellipse of diameter 50
13.   ellipse(mouseX, mouseY, 50, 50);
14.   // This fucntion uses the circle radius so 50/2 = 25
15.   boolean intersects = line_circle(30, 30, 250, 250, mouseX, mouseY, 25);
16.   fill(0);
17.   if (intersects)
18.     text("INTERSECTION", 20, 20);
19.   else
20.     text("MISSED", 20, 20);
21. }
22. 
    
23. /**
24.  * Sees if a line intersects with the circumference of a circle.
25.  *
26.  * @param x0
27.  * @param y0
28.  * @param x1
29.  * @param y1
30.  * @param cx centre of circle x position
31.  * @param cy centre of circle y position
32.  * @param r radius of circle
33.  * @return true if the line intersects the circle else false
34.  */
35. public static boolean line_circle(float x0, float y0, float x1, float y1, float cx, float cy, float r) {
36.   float f = (x1 - x0);
37.   float g = (y1 - y0);
38.   float fSQ = f*f;
39.   float gSQ = g*g;
40.   float fgSQ = fSQ + gSQ;
41.   float rSQ = r*r;
42.   float xc0 = cx - x0;
43.   float yc0 = cy - y0;
44.   float xc1 = cx - x1;
45.   float yc1 = cy - y1;
46.   boolean lineInside = xc0*xc0 + yc0*yc0 < rSQ && xc1*xc1 + yc1*yc1 < rSQ;
47.   float fygx = f*yc0 - g*xc0;
48.   float root = r*r*fgSQ - fygx*fygx;
49.   if (root > 1E5 && !lineInside) {
50.     float fxgy = f*xc0 + g*yc0;
51.     float t = fxgy / fgSQ;
52.     if (t >= 0 && t <= 1)
53.       return true;
54.     // Circle intersects with one end then return true
55.     if ( (xc0*xc0 + yc0*yc0 < rSQ) || (xc1*xc1 + yc1*yc1 < rSQ) )
56.       return true;
57.   }
58.   return false;
59. }