static PPATH() { // 10000 because zero-based indices. This isn't great as we don't need 10000 // (as most #s don't represent primes), but hopefully it doesn't matter. Edges // exist between primes (a vertex whose index is prime) and other primes, // when there's a one-digit swap to go between them (swaps are reversible). _primeGraph = new SimpleGraph(10000); var primeDecider = new SieveOfEratosthenesDecider(9999); // If n is a prime, connect to the primes greater than it, within a one-digit swap. Only // greater than because lesser primes were already connected to it earlier in the loop. for (int n = 1001; n <= 9999; n += 2) { if (!primeDecider.IsOddPrime(n)) continue; int nSwapped = n + 1000; while (nSwapped % 10000 > n) { if (primeDecider.IsOddPrime(nSwapped)) _primeGraph.AddEdge(n, nSwapped); nSwapped += 1000; } nSwapped = n + 100; while (nSwapped % 1000 > n % 1000) { if (primeDecider.IsOddPrime(nSwapped)) _primeGraph.AddEdge(n, nSwapped); nSwapped += 100; } nSwapped = n + 10; while (nSwapped % 100 > n % 100) { if (primeDecider.IsOddPrime(nSwapped)) _primeGraph.AddEdge(n, nSwapped); nSwapped += 10; } nSwapped = n + 2; while (nSwapped % 10 > n % 10) { if (primeDecider.IsOddPrime(nSwapped)) _primeGraph.AddEdge(n, nSwapped); nSwapped += 2; } } }
public static string Solve() { var decider = new SieveOfEratosthenesDecider(_100Million); var output = new StringBuilder(); output.Append(2); output.AppendLine(); int count = 1; for (int n = 3; n <= _100Million; n += 2) { if (decider.IsOddPrime(n) && ++count == 101) { output.Append(n); output.AppendLine(); count = 1; } } return output.ToString(); }
public bool IsOddPrime(int n) => _decider.IsOddPrime(n);
static PPATH() { // 10000 because zero-based indices. This isn't great as we don't need 10000 // (as most #s don't represent primes), but hopefully it doesn't matter. Edges // exist between primes (a vertex whose index is prime) and other primes, // when there's a one-digit swap to go between them (swaps are reversible). _primeGraph = new SimpleGraph(10000); var primeDecider = new SieveOfEratosthenesDecider(9999); // If n is a prime, connect to the primes greater than it, within a one-digit // swap. Only greater than because lesser primes were already connected to it // earlier in the loop. for (int n = 1001; n <= 9999; n += 2) { if (!primeDecider.IsOddPrime(n)) { continue; } int nSwapped = n + 1000; while (nSwapped % 10000 > n) { if (primeDecider.IsOddPrime(nSwapped)) { _primeGraph.AddEdge(n, nSwapped); } nSwapped += 1000; } nSwapped = n + 100; while (nSwapped % 1000 > n % 1000) { if (primeDecider.IsOddPrime(nSwapped)) { _primeGraph.AddEdge(n, nSwapped); } nSwapped += 100; } nSwapped = n + 10; while (nSwapped % 100 > n % 100) { if (primeDecider.IsOddPrime(nSwapped)) { _primeGraph.AddEdge(n, nSwapped); } nSwapped += 10; } nSwapped = n + 2; while (nSwapped % 10 > n % 10) { if (primeDecider.IsOddPrime(nSwapped)) { _primeGraph.AddEdge(n, nSwapped); } nSwapped += 2; } } }