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REM Advent of Code 2022: Day 9, part 1
REM Written in Applesoft BASIC
REM HS is a "set" of unique tail coordinates.
10 DIM HS(7000)
20 HX = 0:HY = 0
30 TX = 0:TY = 0
REM Read a line of input.
100 S$ = ""
110 GET C$
120 IF ASC (C$) < 32 GOTO 200
130 S$ = S$ + C$
140 GOTO 110
REM If empty, we're done. Otherwise, extract direction and count.
200 IF LEN (S$) = 0 THEN GOTO 1000
210 D$ = LEFT$ (S$,1)
220 S = VAL ( MID$ (S$,3, LEN (S$) - 2))
230 PRINT ">>",D$;" ";S
REM For each step...
REM Handle head movement, then GOTO 400 to handle the tail.
300 FOR I = 1 TO S
310 IF D$ = "U" THEN HY = HY + 1: GOTO 400
320 IF D$ = "D" THEN HY = HY - 1: GOTO 400
330 IF D$ = "R" THEN HX = HX + 1: GOTO 400
340 IF D$ = "L" THEN HX = HX - 1: GOTO 400
350 PRINT "ERROR: ";D$
360 END
REM If no movement is necessary, GOTO 460. Otherwise, GOSUB 500 and/or 550
REM to move the tail as needed.
400 DX = HX - TX:DY = HY - TY
410 IF ABS (DX) < = 1 AND ABS (DY) < = 1 GOTO 460
420 IF HX < > TX THEN GOSUB 500
430 IF HY < > TY THEN GOSUB 550
REM Create a hash of the tail coordinates, then try placing it in the "set".
440 HH = TX * 1000 + TY
450 GOSUB 600
REM Handle next step for the current direction, or GOTO 100 for next input.
460 NEXT I
470 GOTO 100
REM Move tail one place closer to head in the X direction.
500 Z = - 1
510 IF HX > TX THEN Z = 1
520 TX = TX + Z
530 RETURN
REM Move tail one place closer to head in the Y direction.
550 Z = - 1
560 IF HY > TY THEN Z = 1
570 TY = TY + Z
580 RETURN
REM "Set" insertion:
REM If hash is already in the set, then exit.
REM Otherwise, if an empty slot is found, add the hash to the set.
600 FOR J = 0 TO 6999
610 IF HS(J) = HH THEN J = 6999: GOTO 630
620 IF HS(J) = 0 THEN HS(J) = HH:J = 9999
630 NEXT J
640 RETURN
REM All input has been read. Count up entries in the "set" and display.
1000 CO = 0
1010 FOR I = 0 TO 9999
1020 CO = CO + 1
1030 IF HS(I) = 0 THEN I = 9999
1040 NEXT I
1050 PRINT "TOTAL: ";CO
1060 END
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