Why is my collision prediction not detecting any collisions? - objective-c

Question

Bullet objects are created with .l for location and .vel for velocity. I'm trying with a large radius of 30 and still they never collide.
- (void)bulletsFired:(NSArray *)bullets li:(int)li {
[self playBulletSound:li];
for (Bullet *b in bullets) {
[self addChild:b];
b.tag = -1;
b.shotNumber = shotsFired;
}
for (Bullet *b in bullets) {
for (Bullet *bb in self.bullets) {
float timeOfApproach = TimeOfClosestApproach(b.l, b.vel, bb.l, bb.vel, 30, 30);
if (timeOfApproach > 0) {
NSLog(#"time of : %f", timeOfApproach);
NSString *keyName = [self.collisions objectForKey:[self keyNameForTime:(int)timeOfApproach]];
NSMutableArray *timedCollisions = [self.collisions objectForKey:keyName];
if (timedCollisions == nil) {
NSMutableArray *newCollisions = [NSMutableArray array];
[self.collisions setObject:newCollisions forKey:keyName];
}
NSDictionary *collision = #{#"b1" : [NSString stringWithFormat:#"%d", bb.shotNumber], #"b2" : [NSString stringWithFormat:#"%d", b.shotNumber]};
[timedCollisions addObject:collision];
}
}
}
[self.bullets addObjectsFromArray:bullets];
[self.scoreCycler score:1];
}
I check for timeOfClosestApproach with this function:
float TimeOfClosestApproach(CGPoint Pa, CGPoint Pb, CGPoint Va, CGPoint Vb, float Ra, float Rb) {
CGPoint Pab = ccpSub(Pa, Pb);
CGPoint Vab = ccpSub(Va, Vb);
float a = ccpDot(Vab, Vab);
float b = 2 * ccpDot(Pab, Vab);
float c = ccpDot(Pab, Pab) - (Ra + Rb) * (Ra + Rb);
// The quadratic discriminant.
float discriminant = b * b - 4 * a * c;
// Case 1:
// If the discriminant is negative, then there are no real roots, so there is no collision. The time of
// closest approach is then given by the average of the imaginary roots, which is: t = -b / 2a
float t;
if (discriminant < 0) {
t = -b / (2 * a);
return -1;
} else {
// Case 2 and 3:
// If the discriminant is zero, then there is exactly one real root, meaning that the circles just grazed each other. If the
// discriminant is positive, then there are two real roots, meaning that the circles penetrate each other. In that case, the
// smallest of the two roots is the initial time of impact. We handle these two cases identically.
float t0 = (-b + (float)sqrt(discriminant)) / (2 * a);
float t1 = (-b - (float)sqrt(discriminant)) / (2 * a);
t = min(t0, t1);
// We also have to check if the time to impact is negative. If it is negative, then that means that the collision
// occured in the past. Since we're only concerned about future events, we say that no collision occurs if t < 0.
if (t < 0) {
return -1;
} else {
}
}
// Finally, if the time is negative, then set it to zero, because, again, we want this function to respond only to future events.
if (t < 0) {
t = 0;
}
return t;
}
I keep getting -1 returned, and the bullets are never predicted to collide, even when they should based on my eyesight.
if (t < 0) {
return -1;
} else {
}
Keeps getting triggered.
What's wrong with my timeOfClosestApproach function?