From da9b172b8abd44825d362a57c9209f8a6fdc6e5d Mon Sep 17 00:00:00 2001
From: Ian C <ianc@noddybox.co.uk>
Date: Fri, 19 May 2017 14:20:56 +0000
Subject: Tweaks to gravity demo.

---
 README |   2 +-
 xd.c   | 159 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 159 insertions(+), 2 deletions(-)

diff --git a/README b/README
index 3ab53a7..7a4dfb8 100644
--- a/README
+++ b/README
@@ -1,6 +1,6 @@
 BUILDING
 --------
-To build just run 'make'.  To run, just type 'xd'.
+To build just run 'make'.  To run, just type './xd'.
 
 
 DESCRIPTION
diff --git a/xd.c b/xd.c
index cb02ab5..8d043a7 100644
--- a/xd.c
+++ b/xd.c
@@ -169,10 +169,11 @@ static void	TriangleDemo(void);
 static void	TriangleDemo2(void);
 static void	FollowDemo(void);
 static void	CatwalkDemo(void);
+static void	GravityDemo(void);
 
 /* ---------------------------------------- GLOBAL VARS
 */
-#define NO_DEMOS	17
+#define NO_DEMOS	18
 
 static int		demo=NO_DEMOS-1;
 
@@ -265,6 +266,11 @@ static DemoInfo		demotbl[NO_DEMOS]=
 				CatwalkDemo,
 				UsesPixmap
 			    },
+			    {
+				"Gravity Demo (LMB to reset, RMB to move central mass)",
+				GravityDemo,
+				UsesPixmap
+			    },
 			};
 
 /* ---------------------------------------- X11 VARS
@@ -3895,4 +3901,155 @@ static void CatwalkDemo(void)
 #undef TRENCH_Z_MIN
 
 
+/* ---------------------------------------- GRAVITY DEMO
+ */
+
+#define GRAV_OBJ	20
+#define GRAV_TRAIL	200
+#define GRAV_SIZE	4
+
+static void GravityDemo(void)
+{
+    static int init = FALSE;
+
+    typedef struct
+    {
+    	double	x,y,dx,dy;
+	int	col;
+	int	mass;
+	double	tx[GRAV_TRAIL];
+	double	ty[GRAV_TRAIL];
+    } GravObj;
+
+    static GravObj gravobj[GRAV_OBJ];
+
+    int f,r;
+
+    if (!init || mouse_b&Button1Mask)
+    {
+	for(f = 0; f < GRAV_OBJ; f++)
+	{
+	    if (f == 0)
+	    {
+		gravobj[f].x = centre_x;
+		gravobj[f].y = centre_y;
+		gravobj[f].dx = 0;
+		gravobj[f].dy = 0;
+		gravobj[f].mass = 80;
+	    }
+	    else
+	    {
+		int a;
+		int d;
+
+		a = RND(3600);
+		d = RND2(400,200);
+
+		gravobj[f].x = centre_x + sintab[a] * d;
+		gravobj[f].y = centre_y + costab[a] * d;
+
+		a = (a + 900) % 3600;
+
+		gravobj[f].dx = sintab[a] * 10;
+		gravobj[f].dy = costab[a] * 10;
+
+		/*
+		gravobj[f].x = RND(width);
+		gravobj[f].y = RND(height);
+		gravobj[f].dx = (double)RND(10)/10.0;
+		gravobj[f].dy = (double)RND(10)/10.0;
+		*/
+
+		gravobj[f].mass = RND2(10, 2);
+	    }
+
+	    gravobj[f].col = GetCol(RND2(255,128),
+				    RND2(255,100),
+				    RND2(255,100));
+
+	    for(r = 0; r < GRAV_TRAIL; r++)
+	    {
+	    	gravobj[f].tx[r] = gravobj[f].x;
+	    	gravobj[f].ty[r] = gravobj[f].y;
+	    }
+	}
+
+	init=TRUE;
+    }
+
+    Cls();
+
+    if (mouse_b & Button3Mask)
+    {
+    	gravobj[0].x = mouse_x;
+    	gravobj[0].y = mouse_y;
+    }
+
+    for(f = 1; f < GRAV_OBJ; f++)
+    {
+	for(r = 0; r < GRAV_OBJ; r++)
+	{
+	    if (f != r)
+	    {
+		double d;
+		double g;
+		double dx;
+		double dy;
+
+		dx = gravobj[f].x - gravobj[r].x;
+		dy = gravobj[f].y - gravobj[r].y;
+
+		d = sqrt(dx*dx + dy*dy);
+
+		g = 1.0 / d*d;
+
+		g /= 100.0;
+		g /= gravobj[f].mass;
+
+		dx /= d;
+		dy /= d;
+
+		dx *= g;
+		dy *= g;
+
+		gravobj[f].dx -= dx;
+		gravobj[f].dy -= dy;
+	    }
+	}
+    }
+
+    for(f = 0; f < GRAV_OBJ; f++)
+    {
+	gravobj[f].x += gravobj[f].dx / 50.0;
+	gravobj[f].y += gravobj[f].dy / 50.0;
+
+	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];
+	}
+
+	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);
+    }
+}
+
+#undef GRAV_OBJ
+#undef GRAV_TRAIL
+#undef GRAV_SIZE
+
+
 /* END OF FILE */
-- 
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