Randomize Timer

Const RAND_MAX = 32767
Const FILL = 1
Const RWALL = 2
Const BWALL = 4

Dim Shared As Integer x = 10, y = 10
Dim Shared As Integer grid(x * (y + 2))
Dim Shared As Integer m, n
Dim Shared As Integer cells, endPos

Sub makeGrid(p As Double)
    Dim As Integer i, j, thresh, r1, r2, r3

    thresh = Int(p * RAND_MAX)
    m = x
    n = y

    For i = 0 To Ubound(grid) :  grid(i) = 0 : Next

    For i = 0 To m-1: grid(i) = BWALL Or RWALL : Next

    cells = m
    endPos = m

    For i = 0 To y-1
        For j = x-1 To 1 Step -1
            r1 = Int(Rnd * (RAND_MAX + 1))
            r2 = Int(Rnd * (RAND_MAX + 1))
            grid(endPos) = Iif(r1 < thresh, BWALL, 0) Or Iif(r2 < thresh, RWALL, 0)
            endPos += 1
        Next
        r3 = Int(Rnd * (RAND_MAX + 1))
        grid(endPos) = RWALL Or Iif(r3 < thresh, BWALL, 0)
        endPos += 1
    Next
End Sub

Sub showGrid()
    Dim As Integer i, j

    For j = 0 To m-1
        Print "+--";
    Next
    Print "+"

    For i = 0 To n
        Print Iif(i = n, " ", "|");
        For j = 0 To m-1
            Print Iif((grid(i * m + j + cells) And FILL) <> 0, "[]", "  ");
            Print Iif((grid(i * m + j + cells) And RWALL) <> 0, "|", " ");
        Next
        Print
        If i = n Then Exit Sub
        For j = 0 To m-1
            Print Iif((grid(i * m + j + cells) And BWALL) <> 0, "+--", "+  ");
        Next
        Print "+"
    Next
End Sub

Function filled(p As Integer) As Boolean
    If (grid(p) And FILL) <> 0 Then Return False
    grid(p) = grid(p) Or FILL
    If p >= endPos Then Return True

    Return ((grid(p + 0) And BWALL) = 0 Andalso filled(p + m)) Orelse _
    ((grid(p + 0) And RWALL) = 0 Andalso filled(p + 1)) Orelse _
    ((grid(p - 1) And RWALL) = 0 Andalso filled(p - 1)) Orelse _
    ((grid(p - m) And BWALL) = 0 Andalso filled(p - m))
End Function

Function percolate() As Boolean
    Dim i As Integer = 0
    While i < m Andalso Not filled(cells + i)
        i += 1
    Wend
    Return i < m
End Function

' Main program
makeGrid(0.5)
percolate()
showGrid()

Print !"\nRunning " & x & " x " & y & " grids 10,000 times for each p:"
For p As Integer = 1 To 9
    Dim As Integer cnt = 0
    Dim As Double pp = p / 10
    For i As Integer = 0 To 9999
        makeGrid(pp)
        If percolate() Then cnt += 1
    Next
    Print Using "p = #.#    : #.####"; pp; cnt / 10000
Next

Sleep