1000 REM STRANGE ATTRACTOR PROGRAM  BASIC Ver 2.0 (c) 1993 by J. C. Sprott
1070 D% = 2                     'Dimension of system
1090 ODE% = 0                   'System is map
1140 SAV% = 0                   'Don't save any data

1310 ON ERROR GOTO 6600         'Find legal graphics mode
1330 ON ERROR GOTO 0            'Resume default error trapping
1340 DEF SEG = 64: WID% = PEEK(74)  'Number of text columns
1360 CLS : LOCATE 13, WID% / 2 - 6: PRINT "Searching..."

2270    IF PJT% = 2 THEN GOSUB 6700     'Project onto a horizontal cylinder
2280    IF PJT% = 3 THEN GOSUB 6800     'Project onto a vertical cylinder
2290    IF PJT% = 4 THEN GOSUB 6900     'Project onto a torus

2500 IF SAV% > 0 THEN IF N > 1000 AND N < 17001 THEN GOSUB 7000 'Save data

3060    IF N > 1000 AND N MOD 10 = 0 THEN LOCATE 1, WID% - 4: PRINT USING "##.##"; L;

3360    IF WID% < 80 THEN GOTO 3390
3400 LOCATE 1, 1: IF LEN(CODE$) < WID% - 18 THEN PRINT CODE$
3410 IF LEN(CODE$) >= WID% - 18 THEN PRINT LEFT$(CODE$, WID% - 23) + "..."
3420 LOCATE 1, WID% - 17: PRINT "F =": LOCATE 1, WID% - 7: PRINT "L ="

3630 IF INSTR("ACDEHINPRSVX", Q$) = 0 THEN GOSUB 4200   'Display menu screen
3670 IF Q$ = "C" THEN IF N > 999 THEN N = 999
3740 IF Q$ = "N" THEN NMAX = 10 * (NMAX - 1000) + 1000: IF NMAX > 10 ^ 10 THEN NMAX = 2000
3750 IF Q$ = "P" THEN PJT% = (PJT% + 1) MOD 5: T% = 3: IF N > 999 THEN N = 999
3780 IF Q$ = "V" THEN SAV% = (SAV% + 1) MOD 5: FAV$ = CHR$(87 + SAV% MOD 4): T% = 3: IF N > 999 THEN N = 999

4000    LOCATE 1, WID% - 14: PRINT USING "##.##"; F;

4240    PRINT TAB(27); "IBM PC BASIC Version 2.0"
4250    PRINT TAB(27); "(c) 1993 by J. C. Sprott"
4280    PRINT TAB(27); "C: Clear screen and restart"
4380    PRINT TAB(27); "N: Number of iterations is 10^";
4390    PRINT USING "#"; CINT(LOG(NMAX - 1000) / LOG(10))
4430        IF PJT% = 2 THEN PRINT "horiz cyl"
4440        IF PJT% = 3 THEN PRINT "vert cyl "
4450        IF PJT% = 4 THEN PRINT "toroidal "
4570    PRINT TAB(27); "V: ";
4580        IF SAV% = 0 THEN PRINT "No data will be saved       "
4590        IF SAV% > 0 THEN PRINT FAV$; " will be saved in "; FAV$; "DATA.DAT"

5650 IF SM% > 2 THEN GOTO 5720  'Not in CGA mode
5660    WID% = 80: IF D% < 3 THEN SCREEN 2: GOTO 5720
5670    IF (TRD% = 0 OR TRD% > 4) AND (D% = 3 OR FTH% <> 2) THEN SCREEN 2: GOTO 5720
5680    WID% = 40: SCREEN 1
5690    COLR%(0) = 0: COLR%(1) = 2: COLR%(2) = 1: COLR%(3) = 3
5700    WH% = 3: BK% = 0: RD% = 2: CY% = 1
5710    FOR I% = 4 TO NC%: COLR%(I%) = COLR%(I% MOD 4 + 1): NEXT I%

5840 IF Q$ <> CHR$(27) AND INSTR("CHNPRVS", Q$) = 0 THEN Q$ = ""

6600 REM Find legal graphics mode
6610 SM% = SM% - 1
6620 IF SM% = 0 THEN PRINT "This program requires a graphics monitor": STOP
6630 RESUME

6700 REM Project onto a horizontal cylinder
6710 PH = PT * (YMAX - YP)
6720 YP = YA + .5 * (YH - YL) * COS(PH)
6730 RETURN

6800 REM Project onto a vertical cylinder
6810 TH = TT * (XMAX - XP)
6820 XP = XA + .5 * (XH - XL) * COS(TH)
6830 RETURN

6900 REM Project onto a torus (unity aspect ratio)
6910 TH = TT * (XMAX - XP)
6920 PH = 2 * PT * (YMAX - YP)
6930 XP = XA + .18 * (XH - XL) * (1 + COS(TH)) * SIN(PH)
6940 YP = YA + .25 * (YH - YL) * (1 + COS(TH)) * COS(PH)
6950 RETURN

7000 REM Save data
7010 IF N = 1001 THEN CLOSE #3: OPEN FAV$ + "DATA.DAT" FOR OUTPUT AS #3
7020 IF SAV% = 1 THEN DUM = XNEW
7030 IF SAV% = 2 THEN DUM = YNEW
7040 IF SAV% = 3 THEN DUM = ZNEW
7050 IF SAV% = 4 THEN DUM = WNEW
7060 PRINT #3, CSNG(DUM)
7070 RETURN