100 REM BASIC Month: Terraspin
110 REM http://reddit.com/r/RetroBattlestations
120 REM written by FozzTexx - Apple II port by FozzTexx

130 REM Apple II doesn't have bitwise operators so they are
140 REM simulated. User defined function FN OD checks if a number is odd
150 REM by checking the LSB. Bit shifting is simulated by dividing by a
160 REM multiple of two. A bitwise AND is done in the subroutine at 5500.

200 REM === Initialize variables
210 SW = 280:SH = 192:XS = SW / 1000:YS = (SH * 4) / (1000 * 3)
220 SW = SW - 1:SH = SH - 1
230 PI = 3.14159
240 DEF FN OD(VV) = INT(VV) / 2 - INT(VV / 2) > 0

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 HGR

500 REM === Command interpreter
510 IP = 1:SP = 0
520 C$ = MID$(CM$, IP, 1)
530 IF C$ = " " THEN IP = IP + 1:GOTO 520
540 IF C$ >= "-" AND C$ <= "9" THEN V = VAL(MID$(CM$, 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 END
630 GOTO 520

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$) THEN 1250
1230 C2$ = MID$(CM$, IP, 1)
1240 IF C2$ >= "-" AND C2$ <= "9" THEN GOTO 1210
1250 IP = IP - 1
1260 RETURN

1500 REM === Plot a line
1510 HCOLOR = 3
1520 HPLOT X1,Y1 TO X2,Y2
1530 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) * PI / 180):LY = V * SIN((360 - TA) * PI / 180)
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 IF FN OD(C) THEN X = X1 + (X2 - X1) * (SH - Y1) / (Y2 - Y1):Y = SH:GOTO 4610
4580 IF FN OD(C / 2) THEN X = X1 + (X2 - X1) * (0 - Y1) / (Y2 - Y1):Y = 0:GOTO 4610
4590 IF FN OD(C / 4) THEN Y = Y1 + (Y2 - Y1) * (SW - X1) / (X2 - X1):X = SW:GOTO 4610
4600 IF FN OD(C / 8) 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
5520 C = C + FN OD(C1 / EP) * FN OD(C2 / EP) * EP
5530 EP = EP * 2:NEXT
5540 RETURN