100 REM BASIC Month: Terraspin
110 REM http://reddit.com/r/RetroBattlestations
120 REM written by FozzTexx - Atari 8-bit port by Gibstov

130 REM Atari doesn't have bitwise operators or DEF FN so they are
140 REM simulated. Routine at 6010 gets the LSB. Bit shifting is
150 REM simulated by dividing by a multiple of two. A bitwise AND is done
160 REM in the subroutine at 5500.

200 REM === Initialize variables
205 DIM SK(16), CM$(300), C$(1),C2$(1):DEG
210 SW = 320:SH = 192:XS = SW / 1000:YS = (SH * 4) / (1000 * 3)
220 SW = SW - 1:SH = SH - 1
230 PI = 3.14159

250 REM === Set turtle start at center of screen pointing up, pen up
260 TX = 500:TY = SH / YS / 2:TA = 90:TP = 0

300 REM === Get command to execute
310 CM$="5 ( D 12 ( 210 M 5 ( 30 T 10 M ) ) U 100 T 150 M )"

490 GR.8+16:COLOR 1:POKE 712,0

500 REM === Command interpreter
510 IP = 1:SP = 0
520 C$ = CM$(IP, IP)
530 IF C$ = " " THEN IP = IP + 1:GOTO 520
540 IF C$ >= "-" AND C$ <= "9" THEN V = VAL(CM$(IP)):GOSUB 1010:GOSUB 1210
550 IF C$ = "(" THEN V = IP:GOSUB 1010
560 IF C$ = ")" THEN GOSUB 2010
570 IF C$ = "M" THEN GOSUB 2510
580 IF C$ = "T" THEN GOSUB 3010
590 IF C$ = "U" THEN GOSUB 3510
600 IF C$ = "D" THEN GOSUB 4010
610 IP = IP + 1
620 IF IP > LEN(CM$) THEN POKE 712,96:GOTO 700
630 GOTO 520

700 GOTO 700

1000 REM === Push onto stack
1010 SP = SP + 1:SK(SP) = V
1020 RETURN

1100 REM === Pop from stack
1110 V = SK(SP):SP = SP - 1
1120 RETURN

1200 REM === Skip over number
1210 IP = IP + 1
1220 IF IP > LEN(CM$) THEN END
1230 C2$ = CM$(IP,IP)
1240 IF C2$ >= "-" AND C2$ <= "9" THEN GOTO 1210
1250 IP = IP - 1
1260 RETURN

1500 REM === Plot a line
1510 PLOT X1,Y1:DR. X2,Y2
1520 RETURN

2000 REM === Loop instruction end
2010 GOSUB 1110:BP = V
2020 GOSUB 1110:LR = V
2030 LR = LR - 1
2040 IF LR < 1 THEN RETURN
2050 V = LR:GOSUB 1010
2060 V = BP:GOSUB 1010
2070 IP = BP
2080 RETURN

2500 REM === Move
2510 GOSUB 1110
2520 LX = V * COS(360-TA):LY = V * SIN(360-TA)
2530 IF TP > 0 THEN X1 = TX:Y1 = TY:X2 = X1 + LX:Y2 = Y1 + LY:GOSUB 4510
2540 TX = TX + LX:TY = TY + LY
2550 RETURN

3000 REM === Turn
3010 GOSUB 1110
3020 TA = TA + V
3030 IF TA < 0 THEN TA = TA + 360:GOTO 3030
3040 IF TA >= 360 THEN TA = TA - 360:GOTO 3040
3050 RETURN

3500 REM === Pen up
3510 TP = 0
3520 RETURN

4000 REM === Pen down
4010 TP = 1
4020 RETURN

4500 REM === Draw line, clipping if needed
4510 X1 = X1 * XS:X2 = X2 * XS:Y1 = Y1 * YS:Y2 = Y2 * YS
4520 X = X1:Y = Y1:GOSUB 5010:C1 = C
4530 X = X2:Y = Y2:GOSUB 5010:C2 = C
4540 IF C1 = 0 AND C2 = 0 THEN GOSUB 1510:RETURN
4550 GOSUB 5510:IF C THEN RETURN
4560 C = C1:IF C = 0 THEN C = C2
4570 VV=C:GOSUB 6010:IF VV THEN X = X1 + (X2 - X1) * (SH - Y1) / (Y2 - Y1):Y = SH:GOTO 4610
4580 VV=C / 2:GOSUB 6010:IF VV THEN X = X1 + (X2 - X1) * (0 - Y1) / (Y2 - Y1):Y = 0:GOTO 4610
4590 VV=C / 4:GOSUB 6010:IF VV THEN Y = Y1 + (Y2 - Y1) * (SW - X1) / (X2 - X1):X = SW:GOTO 4610
4600 VV=C / 8:GOSUB 6010:IF VV THEN Y = Y1 + (Y2 - Y1) * (0 - X1) / (X2 - X1):X = 0:GOTO 4610
4610 IF C = C1 THEN X1 = X:Y1 = Y:GOTO 4630
4620 X2 = X:Y2 = Y
4630 GOTO 4520

5000 REM === Calculate region code
5010 C = 0
5020 IF Y > SH THEN C = C + 1
5030 IF Y < 0 THEN C = C + 2
5040 IF X > SW THEN C = C + 4
5050 IF X < 0 THEN C = C + 8
5060 RETURN

5500 REM === Bitwise AND of C1 and C2
5510 C = 0:EP = 1:FOR I = 1 TO 4
5515 VV = (C1 / EP):GOSUB 6010: V1=VV: VV= (C2 / EP):GOSUB 6010
5520 C = C + V1 * VV * EP
5530 EP = EP * 2:NEXT I
5540 RETURN

6000 REM === Get LSB
6010 VV= INT(VV) / 2 - INT(VV / 2) > 0
6020 RETURN