100 REM BASIC Month: Terraspin
110 REM http://reddit.com/r/RetroBattlestations
120 REM written by FozzTexx - Commodore 64 port by FozzTexx

130 REM Commodore 64 BASIC has bitwise operators so much of the line
140 REM clipping logic is simplified compared to the Apple II version.

150 REM The Commodore 64 has *no* graphics commands at all. All graphics
160 REM has to be done using PEEKs and POKEs. Line drawing is handled by
170 REM a complicated routine at 1500. Individual pixels are then plotted
180 REM with the routine at 1700.

200 REM === Initialize variables
210 SW = 320:SH = 200: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 )"

460 REM ### If the program crashes it will
470 REM ### be stuck in graphics mode.
480 REM ### Type RUN 999 to get back to text.
490 GOSUB 6010

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

999 POKE 53272,PEEK(53272) AND 247:POKE 53265,PEEK(53265) AND 223:END

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 X1% = X1:X2% = X2:Y1% = Y1:Y2% = Y2
1520 DX% = ABS(X2% - X1%):SX% = -1:IF X1% < X2% THEN SX% = 1
1530 DY% = ABS(Y2% - Y1%):SY% = -1:IF Y1% < Y2% THEN SY% = 1
1540 ER% = -DY%:IF DX% > DY% THEN ER% = DX%
1550 ER% = ER% / 2
1560 GOSUB 1710
1570 IF X1% = X2% AND Y1% = Y2% THEN RETURN
1580 E2% = ER%
1590 IF E2% > -DX% THEN ER% = ER% - DY%:X1% = X1% + SX%
1600 IF X1% = X2% AND Y1% = Y2% THEN RETURN
1610 IF E2% < DY% THEN ER% = ER% + DX%:Y1% = Y1% + SY%
1620 IF X1% = X2% AND Y1% = Y2% THEN RETURN
1630 GOTO 1560

1700 REM === Plot a point
1710 CH% = INT(X1% / 8):RO% = INT(Y1% / 8):LN% = Y1% AND 7
1720 BY = BASE + RO% * 320 + 8 * CH% + LN%:BI% = 7 - (X1% AND 7)
1730 POKE BY,PEEK(BY) OR (2 ^ BI%)
1740 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 IF C1 AND C2 THEN RETURN
4560 C = C1:IF C = 0 THEN C = C2
4570 IF C AND 1 THEN X = X1 + (X2 - X1) * (SH - Y1) / (Y2 - Y1):Y = SH:GOTO 4610
4580 IF C AND 2 THEN X = X1 + (X2 - X1) * (0 - Y1) / (Y2 - Y1):Y = 0:GOTO 4610
4590 IF C AND 4 THEN Y = Y1 + (Y2 - Y1) * (SW - X1) / (X2 - X1):X = SW:GOTO 4610
4600 IF C AND 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 OR 1
5030 IF Y < 0 THEN C = C OR 2
5040 IF X > SW THEN C = C OR 4
5050 IF X < 0 THEN C = C OR 8
5060 RETURN

6000 REM === Enable graphics and clear screen
6010 BASE = 2 * 4096:POKE 53272,PEEK(53272) OR 8:POKE 53265,PEEK(53265) OR 32
6020 FOR I = 1024 to 2023:POKE I,3:NEXT
6030 FOR I = BASE TO BASE + 7999:POKE I,255:POKE I, 0:NEXT
6040 RETURN