AOC2020Day8inGWBASIC

 10 DIM oc$(1000), of%(1000), v%(1000)
 20 OPEN "I", 1, "data08.txt"
 30 FOR i = 0 TO 1000
 40 IF EOF(1) GOTO 70
 50 oc$(i) = INPUT$(3, # 1): INPUT #1, of%(i)
 60 NEXT i
 70 ic = i
 80 PRINT "Part 1"
 90 GOSUB 500
100 PRINT "Final value of accumulator:"; r
200 PRINT "Part 2"
210 FOR i = 0 TO ic - 1
220 IF oc$(i) = "acc" THEN GOTO 280
230 PRINT "Changing instruction"; i; "...";
240 IF oc$(i) = "nop" THEN oc$(i) = "jmp": GOSUB 500: oc$(i) = "nop"
250 IF oc$(i) = "jmp" THEN oc$(i) = "nop": GOSUB 500: oc$(i) = "jmp"
260 IF c = 1 GOTO 290
270 PRINT "Loop detected."
280 NEXT i
290 PRINT "Program finished!"
300 PRINT "Final accumulator value:"; r
310 END
500 FOR j = 0 TO 1000: v%(j) = 0: NEXT j
510 ip = 0: acc = 0
520 IF v%(ip) THEN c = 0: r = acc: RETURN
530 IF ip >= ic THEN c = 1: r = acc: RETURN
540 v%(ip) = -1
550 IF oc$(ip) = "jmp" THEN ip = ip + of%(ip) - 1
560 IF oc$(ip) = "acc" THEN acc = acc + of%(ip)
570 ip = ip + 1
580 GOTO 520