GW-BASIC program that displays two balls bouncing around on a tiled field "eating into" each other's spaces.

BALLS.BAS

80 SCREEN 9:VIEW
90 DEFINT A-Z
95 X0=200:Y0=30
100 LINE (X0,Y0)-(X0+299,Y0+299),3,BF
120 LINE (X0+150,Y0)-(X0+299,Y0+299),1,BF
150 DIM A%(200):DIM B%(200)
155 LINE (543,23)-(557,37),3,BF:LINE(563,23)-(577,37),1,BF
160 CIRCLE (550,30),7,1,0,2*3.14,1:CIRCLE (570,30),7,3,0,2*3.14,1
170 PAINT (550,30),1:PAINT(570,30),3
180 GET (543,23)-(557,37),A%:GET (563,23)-(577,37),B%
194 VIEW SCREEN (X0,Y0)-(X0+299,Y0+299)
195 X1=X0:Y1=Y0+30:DX1=1:DY1=1
196 X2=X0+180:Y2=Y0+60:DX2=-1:DY2=1
198 DEF FNA(X)=X*(SGN(X+1)*14+1):DEF FNB(X)=X*15
200 IF POINT(X1+FNA(DX1),Y1) <> 3 THEN LINE (X1+FNB(DX1),Y1)-(X1+FNB(DX1)+14,Y1+14),3,BF:DX1=-DX1
205 IF POINT(X1,Y1+FNA(DY1)) <> 3 THEN LINE (X1,Y1+FNB(DY1))-(X1+14,Y1+FNB(DY1)+14),3,BF:DY1=-DY1
210 IF POINT(X1+FNA(DX1),Y1+FNA(DY1)) <> 3 THEN LINE(X1+FNB(DX1),Y1+FNB(DY1))-(X1+FNB(DX1)+14,Y1+FNB(DY1)+14),3,BF:DX1=-DX1:DY1=-DY1
215 IF POINT(X2+FNA(DX2),Y2) <> 1 THEN LINE (X2+FNB(DX2),Y2)-(X2+FNB(DX2)+14,Y2+14),1,BF:DX2=-DX2:GOTO 215
220 IF POINT(X2,Y2+FNA(DY2)) <> 1 THEN LINE (X2,Y2+FNB(DY2))-(X2+14,Y2+FNB(DY2)+14),1,BF:DY2=-DY2:GOTO 215
225 IF POINT(X2+FNA(DX2),Y2+FNA(DY2)) <> 1 THEN LINE(X2+FNB(DX2),Y2+FNB(DY2))-(X2+FNB(DX2)+14,Y2+FNB(DY2)+14),1,BF:DX2=-DX2:DY2=-DY2:GOTO 215
250 LINE (X1,Y1)-(X1+14,Y1+14),3,B
255 LINE (X2,Y2)-(X2+14,Y2+14),1,B
260 PUT (X1+DX1,Y1+DY1),A%,PSET
265 PUT (X2+DX2,Y2+DY2),B%,PSET
270 X1=X1+DX1:Y1=Y1+DY1
275 X2=X2+DX2:Y2=Y2+DY2
300 GOTO 200