A-M baby

Task/Find-largest-left-truncatable-prime-in-a-given-base/Java/find-largest-left-truncatable-prime-in-a-given-base.java

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+import java.math.BigInteger;
+import java.util.*;
+
+class LeftTruncatablePrime
+{
+  private static List<BigInteger> getNextLeftTruncatablePrimes(BigInteger n, int radix, int millerRabinCertainty)
+  {
+    List<BigInteger> probablePrimes = new ArrayList<BigInteger>();
+    String baseString = n.equals(BigInteger.ZERO) ? "" : n.toString(radix);
+    for (int i = 1; i < radix; i++)
+    {
+      BigInteger p = new BigInteger(Integer.toString(i, radix) + baseString, radix);
+      if (p.isProbablePrime(millerRabinCertainty))
+        probablePrimes.add(p);
+    }
+    return probablePrimes;
+  }
+
+  public static BigInteger getLargestLeftTruncatablePrime(int radix, int millerRabinCertainty)
+  {
+    List<BigInteger> lastList = null;
+    List<BigInteger> list = getNextLeftTruncatablePrimes(BigInteger.ZERO, radix, millerRabinCertainty);
+    while (!list.isEmpty())
+    {
+      lastList = list;
+      list = new ArrayList<BigInteger>();
+      for (BigInteger n : lastList)
+        list.addAll(getNextLeftTruncatablePrimes(n, radix, millerRabinCertainty));
+    }
+    if (lastList == null)
+      return null;
+    Collections.sort(lastList);
+    return lastList.get(lastList.size() - 1);
+  }
+
+  public static void main(String[] args)
+  {
+    int maxRadix = Integer.parseInt(args[0]);
+    int millerRabinCertainty = Integer.parseInt(args[1]);
+    for (int radix = 3; radix <= maxRadix; radix++)
+    {
+      BigInteger largest = getLargestLeftTruncatablePrime(radix, millerRabinCertainty);
+      System.out.print("n=" + radix + ": ");
+      if (largest == null)
+        System.out.println("No left-truncatable prime");
+      else
+        System.out.println(largest + " (in base " + radix + "): " + largest.toString(radix));
+    }
+  }
+
+}