The following is intersects(Rectangle r)
method from awt Rectangle source code.
I add some comments starting with //*
related to my question:
Since the code verifies that rw (for example) is positive then rw rw+rx
must be bigger than rx
.
If that is the case rw < rx
is always false, so why is it checked?
/**
* Determines whether or not this {@code Rectangle} and the specified
* {@code Rectangle} intersect. Two rectangles intersect if
* their intersection is nonempty.
*
* @param r the specified {@code Rectangle}
* @return {@code true} if the specified {@code Rectangle}
* and this {@code Rectangle} intersect;
* {@code false} otherwise.
*/
public boolean intersects(Rectangle r) {
int tw = width;
int th = height;
int rw = r.width;
int rh = r.height;
if (rw <= 0 || rh <= 0 || tw <= 0 || th <= 0) return false;
int tx = x;
int ty = y;
int rx = r.x;
int ry = r.y;
//*at this point rw must be positive
rw += rx; //*after this assignment rw must be bigger than rx
rh += ry;
tw += tx;
th += ty;
// overflow || intersect
return (rw < rx || rw > tx) && //*rw < rx should always be false
(rh < ry || rh > ty) &&
(tw < tx || tw > rx) &&
(th < ty || th > ry);
}
The same applies to rh < ry
and tw < tx
and th < ty
.
The test for rw < rx
, rh < ry
, tw < tx
and th < ty
are redundant and can be eliminated:
public boolean intersects(Rectangle r) {
int tw = width;
int th = height;
int rw = r.width;
int rh = r.height;
if (rw <= 0 || rh <= 0 || tw <= 0 || th <= 0)return false;
rw += r.x;
rh += r.y;
tw += x;
th += y;
return rw > x &&
rh > y &&
tw > r.x &&
th > r.y;
}
After doing that we can take it one step further, make the code more readable and concise:
public boolean intersects(Rectangle other) {
//to intersect both vertical and horizontal edges need to cross
return
//check if horizontal edge cross
other.width + other.x > x && //other max x is bigger than min x and
other.x < width + x && //other min x is smaller than max x
//check if vertical edge cross
other.height+ other.y > y &&
other.y <height + y ;
}
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