' version 10-12-2016
' compile with: fbc -s console

Dim Shared As Byte isPrime()

Sub sieve(m As UInteger)

    Dim As Integer i, j
    ReDim isPrime(m)

    For i = 4 To m Step 2
        isPrime(i) = 1
    Next

    For i = 3 To Sqr(m) Step 2
        If isPrime(i) = 0 Then
            For j = i * i To m Step i * 2
                isPrime(j) = 1
            Next
        End If
    Next

End Sub

' ------=< MAIN >=------

#Define max 1000000 'upto 2^30 max for 32bit OS

Dim As UInteger a(), lt_prime(5000), rt_prime(100)
Dim As UInteger i, j, j1, p1, p2, left_max, right_max

sieve(max)

' left truncatable primes
' if odd and ends with 3 or 7, never ends 1 or 9 (no prime
' never ends on a 2 or 5 and starts with 1 to 9
lt_prime(1) = 3 : lt_prime(2) = 7
p1 = 1 : p2 = 2

Do
    For i = 1 To 9
        j = Val( Str(i) + Str(lt_prime(p1)) )
        If j > max Then Exit Do
        If isPrime(j) = 0 Then ' if prime then add to the list
            p2 += 1
            lt_prime(p2) = j
            If Left_max < j Then left_max = j
        End If
    Next
    p1 += 1
Loop Until p1 > p2   ' no more numbers to process

' right truncatable prime
' start with 2, 3, 5 or 7 and end with 1, 3, 7 or 9
rt_prime(1) = 2 : rt_prime(2) = 3 : rt_prime(3) = 5 : rt_prime(4) = 7
p1 = 1 : p2 = 4
Dim As UInteger end_num(1 To 4) => {1, 3, 7, 9}

Do
    j1 = rt_prime(p1) * 10
    If j1 > max Then Exit Do
    For i = 1 To 4
        j = j1 + End_num(i)
        If isprime(j) = 0 Then  ' if prime then add to the list
            p2 += 1
            rt_prime(p2) = j
           ' If right_max < j Then right_max = j
        End If
    Next
    p1 += 1
Loop Until p1 > p2   ' no more numbers to process
' the last one added is the biggest
right_max = rt_prime(p2)

Print
Print "The biggest  left truncatable prime below"; max; " is "; left_max
Print "The biggest right truncatable prime below"; max; " is "; right_max

' empty keyboard buffer
While Inkey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End