From b7bb6135ac5d61c3c11079f8a526bfb3e091b04e Mon Sep 17 00:00:00 2001
From: Ian C <ianc@noddybox.co.uk>
Date: Tue, 16 Oct 2018 07:29:18 +0000
Subject: Added collisions to grav demo.

---
 xd.c | 86 +++++++++++++++++++++++++++++++++++++++++---------------------------
 1 file changed, 52 insertions(+), 34 deletions(-)

diff --git a/xd.c b/xd.c
index f2502e0..b695107 100644
--- a/xd.c
+++ b/xd.c
@@ -3919,6 +3919,7 @@ static void GravityDemo(void)
 	int	mass;
 	double	tx[GRAV_TRAIL];
 	double	ty[GRAV_TRAIL];
+	int	alive;
     } GravObj;
 
     static GravObj gravobj[GRAV_OBJ];
@@ -3936,6 +3937,7 @@ static void GravityDemo(void)
 		gravobj[f].dx = 0;
 		gravobj[f].dy = 0;
 		gravobj[f].mass = 1000;
+		gravobj[f].alive = TRUE;
 	    }
 	    else
 	    {
@@ -3954,6 +3956,7 @@ static void GravityDemo(void)
 		gravobj[f].dy = costab[a] * 0.1;
 
 		gravobj[f].mass = 1;
+		gravobj[f].alive = TRUE;
 	    }
 
 	    gravobj[f].col = GetCol(RND2(255,128),
@@ -3980,63 +3983,78 @@ static void GravityDemo(void)
 
     for(f = 1; f < GRAV_OBJ; f++)
     {
-	for(r = 0; r < GRAV_OBJ; r++)
+	if (gravobj[f].alive)
 	{
-	    if (f != r)
+	    for(r = 0; r < GRAV_OBJ; r++)
 	    {
-		double d;
-		double g;
-		double dx;
-		double dy;
+		if (f != r && gravobj[r].alive)
+		{
+		    double d;
+		    double g;
+		    double dx;
+		    double dy;
 
-		dx = gravobj[f].x - gravobj[r].x;
-		dy = gravobj[f].y - gravobj[r].y;
+		    dx = gravobj[f].x - gravobj[r].x;
+		    dy = gravobj[f].y - gravobj[r].y;
 
-		d = sqrt(dx*dx + dy*dy);
+		    d = sqrt(dx*dx + dy*dy);
+
+		    if (d < (gravobj[r].mass / GRAV_SIZE))
+		    {
+			gravobj[f].alive = FALSE;
+			gravobj[r].mass += gravobj[f].mass;
+		    }
 
-		g = 1.0 / d*d;
+		    g = 1.0 / d*d;
 
-		g /= 10000.0;
-		g *= gravobj[f].mass;
+		    g /= 10000.0;
+		    g *= gravobj[f].mass;
 
-		dx /= d;
-		dy /= d;
+		    dx /= d;
+		    dy /= d;
 
-		dx *= g;
-		dy *= g;
+		    dx *= g;
+		    dy *= g;
 
-		gravobj[f].dx -= dx;
-		gravobj[f].dy -= dy;
+		    gravobj[f].dx -= dx;
+		    gravobj[f].dy -= dy;
+		}
 	    }
 	}
     }
 
     for(f = 0; f < GRAV_OBJ; f++)
     {
-	gravobj[f].x += gravobj[f].dx;
-	gravobj[f].y += gravobj[f].dy;
-
-	for(r = GRAV_TRAIL - 1; r > 0; r--)
+	if (gravobj[f].alive)
 	{
-	    gravobj[f].tx[r] = gravobj[f].tx[r-1];
-	    gravobj[f].ty[r] = gravobj[f].ty[r-1];
-	}
+	    gravobj[f].x += gravobj[f].dx;
+	    gravobj[f].y += gravobj[f].dy;
 
-	gravobj[f].tx[0] = gravobj[f].x;
-	gravobj[f].ty[0] = gravobj[f].y;
+	    for(r = GRAV_TRAIL - 1; r > 0; r--)
+	    {
+		gravobj[f].tx[r] = gravobj[f].tx[r-1];
+		gravobj[f].ty[r] = gravobj[f].ty[r-1];
+	    }
 
-	for(r = 0; r < GRAV_TRAIL - 1; r++)
-	{
-	    Line(gravobj[f].tx[r], gravobj[f].ty[r],
-		 gravobj[f].tx[r+1], gravobj[f].ty[r+1],
-		 gravobj[f].col);
+	    gravobj[f].tx[0] = gravobj[f].x;
+	    gravobj[f].ty[0] = gravobj[f].y;
+
+	    for(r = 0; r < GRAV_TRAIL - 1; r++)
+	    {
+		Line(gravobj[f].tx[r], gravobj[f].ty[r],
+		     gravobj[f].tx[r+1], gravobj[f].ty[r+1],
+		     gravobj[f].col);
+	    }
 	}
     }
 
     for(f = 0; f < GRAV_OBJ; f++)
     {
-	FCircle(gravobj[f].x, gravobj[f].y,
-		gravobj[f].mass/GRAV_SIZE, gravobj[f].col);
+	if (gravobj[f].alive)
+	{
+	    FCircle(gravobj[f].x, gravobj[f].y,
+		    gravobj[f].mass/GRAV_SIZE, gravobj[f].col);
+	}
     }
 }
 
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