Task: Prime conspiracy
C: #include <assert.h>
#include <stdbool.h>
#include <stdio.h>

typedef unsigned char byte;

struct Transition {
    byte a, b;
    unsigned int c;
} transitions[100];

void init() {
    int i, j;
    for (i = 0; i < 10; i++) {
        for (j = 0; j < 10; j++) {
            int idx = i * 10 + j;
            transitions[idx].a = i;
            transitions[idx].b = j;
            transitions[idx].c = 0;
        }
    }
}

void record(int prev, int curr) {
    byte pd = prev % 10;
    byte cd = curr % 10;
    int i;

    for (i = 0; i < 100; i++) {
        int z = 0;
        if (transitions[i].a == pd) {
            int t = 0;
            if (transitions[i].b == cd) {
                transitions[i].c++;
                break;
            }
        }
    }
}

void printTransitions(int limit, int last_prime) {
    int i;

    printf("%d primes, last prime considered: %d\n", limit, last_prime);

    for (i = 0; i < 100; i++) {
        if (transitions[i].c > 0) {
            printf("%d->%d  count: %5d  frequency: %.2f\n", transitions[i].a, transitions[i].b, transitions[i].c, 100.0 * transitions[i].c / limit);
        }
    }
}

bool isPrime(int n) {
    int s, t, a1, a2;

    if (n % 2 == 0) return n == 2;
    if (n % 3 == 0) return n == 3;
    if (n % 5 == 0) return n == 5;
    if (n % 7 == 0) return n == 7;
    if (n % 11 == 0) return n == 11;
    if (n % 13 == 0) return n == 13;
    if (n % 17 == 0) return n == 17;
    if (n % 19 == 0) return n == 19;

    
    t = 23;
    a1 = 96;
    a2 = 216;
    s = t * t;
    while (s <= n) {
        if (n % t == 0) return false;

        
        s += a1;
        t += 2;
        a1 += 24;
        assert(t * t == s);

        if (s <= n) {
            if (n % t == 0) return false;

            
            s += a2;
            t += 4;
            a2 += 48;
            assert(t * t == s);
        }
    }

    return true;
}

#define LIMIT 1000000
int main() {
    int last_prime = 3, n = 5, count = 2;

    init();
    record(2, 3);

    while (count < LIMIT) {
        if (isPrime(n)) {
            record(last_prime, n);
            last_prime = n;
            count++;
        }
        n += 2;

        if (count < LIMIT) {
            if (isPrime(n)) {
                record(last_prime, n);
                last_prime = n;
                count++;
            }
            n += 4;
        }
    }

    printTransitions(LIMIT, last_prime);

    return 0;
}

VB: Option Explicit

Sub Main()
Dim Dict As Object, L() As Long
Dim t As Single

   Init Dict
   L = ListPrimes(100000000)
t = Timer
   PrimeConspiracy L, Dict, 1000000
Debug.Print "----------------------------"
Debug.Print "Execution time : " & Format(Timer - t, "0.000s.")
Debug.Print ""
   Init Dict
t = Timer
   PrimeConspiracy L, Dict, 5000000
Debug.Print "----------------------------"
Debug.Print "Execution time : " & Format(Timer - t, "0.000s.")
End Sub

Private Function ListPrimes(MAX As Long) As Long()

Dim t() As Boolean, L() As Long, c As Long, s As Long, i As Long, j As Long
    ReDim t(2 To MAX)
    ReDim L(MAX \ 2)
    s = Sqr(MAX)
    For i = 3 To s Step 2
        If t(i) = False Then
            For j = i * i To MAX Step i
                t(j) = True
            Next
        End If
    Next i
    L(0) = 2
    For i = 3 To MAX Step 2
        If t(i) = False Then
            c = c + 1
            L(c) = i
        End If
    Next i
    ReDim Preserve L(c)
    ListPrimes = L
End Function

Private Sub Init(d As Object)
   Set d = CreateObject("Scripting.Dictionary")
   d("1 to 1") = 0
   d("1 to 3") = 0
   d("1 to 7") = 0
   d("1 to 9") = 0
   d("2 to 3") = 0
   d("3 to 1") = 0
   d("3 to 3") = 0
   d("3 to 5") = 0
   d("3 to 7") = 0
   d("3 to 9") = 0
   d("5 to 7") = 0
   d("7 to 1") = 0
   d("7 to 3") = 0
   d("7 to 7") = 0
   d("7 to 9") = 0
   d("9 to 1") = 0
   d("9 to 3") = 0
   d("9 to 7") = 0
   d("9 to 9") = 0
End Sub

Private Sub PrimeConspiracy(Primes() As Long, Dict As Object, Nb)
Dim n As Long, temp As String, r, s, K
   For n = LBound(Primes) To Nb
      r = CStr((Primes(n)))
      s = CStr((Primes(n + 1)))
      temp = Right(r, 1) & " to " & Right(s, 1)
      If Dict.Exists(temp) Then Dict(temp) = Dict(temp) + 1
   Next
   Debug.Print Nb & " primes, last prime considered: " & Primes(Nb)
   Debug.Print "Transition  Count     Frequency"
   Debug.Print "==========  =======   ========="
   For Each K In Dict.Keys
      Debug.Print K & "      " & Right("      " & Dict(K), 6) & "    " & Dict(K) / Nb * 100 & "%"
   Next
End Sub