Escape (1994 J. C. Sprott) - Printable Version

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Rem Program ESCAPE.BAS (c) 1994 by J. C. Sprott

Dim a(12) 'Array of coefficients

Randomize Timer 'Reseed random numbers

Screen 12 'Assume VGA graphics

n% = 0

While InKey$ = "" 'Loop until a key is pressed

    If n% = 0 Then Call setparams(x, y)

    Call advancexy(x, y, n%)

    Call testsoln(x, y, n%)

    If n% = 1000 Then Call display: n% = 0

Wend

End



Sub advancexy (x, y, n%) 'Advance (x, y) at step n%

    Shared a()

    xnew = a(1) + x * (a(2) + a(3) * x + a(4) * y) + y * (a(5) + a(6) * y)

    y = a(7) + x * (a(8) + a(9) * x + a(10) * y) + y * (a(11) + a(12) * y)

    x = xnew

    n% = n% + 1

End Sub



Sub display () 'Plot escape-time contours

    For i% = 0 To 639

        For j% = 0 To 479

            x = -5 + i% / 64

            y = 5 - j% / 48

            n% = 0

            While n% < 128 And x * x + y * y < 1000000

                Call advancexy(x, y, n%)

            Wend

            PSet (i%, j%), n% Mod 16

        Next j%

    Next i%

    _Display

End Sub



Sub setparams (x, y) 'Set a() and initialize (x,y)

    Shared a()

    x = 0: y = 0

    For i% = 1 To 12: a(i%) = (Int(25 * Rnd) - 12) / 10: Next i%

End Sub



Sub testsoln (x, y, n%) 'Test the solution

    If n% = 1000 Then n% = 0 'Solution is bounded

    If x * x + y * y > 1000000 Then 'Solution escaped

        If n% > 100 Then n% = 1000 Else n% = 0

    End If

End Sub