Data commit

Task/Circular-primes/Scala/circular-primes.scala

@@ -0,0 +1,108 @@
+object CircularPrimes {
+  def main(args: Array[String]): Unit = {
+    println("First 19 circular primes:")
+    var p = 2
+    var count = 0
+    while (count < 19) {
+      if (isCircularPrime(p)) {
+        if (count > 0) {
+          print(", ")
+        }
+        print(p)
+        count += 1
+      }
+      p += 1
+    }
+    println()
+
+    println("Next 4 circular primes:")
+    var repunit = 1
+    var digits = 1
+    while (repunit < p) {
+      repunit = 10 * repunit + 1
+      digits += 1
+    }
+    var bignum = BigInt.apply(repunit)
+    count = 0
+    while (count < 4) {
+      if (bignum.isProbablePrime(15)) {
+        if (count > 0) {
+          print(", ")
+        }
+        print(s"R($digits)")
+        count += 1
+      }
+      digits += 1
+      bignum = bignum * 10
+      bignum = bignum + 1
+    }
+    println()
+
+    testRepunit(5003)
+    testRepunit(9887)
+    testRepunit(15073)
+    testRepunit(25031)
+  }
+
+  def isPrime(n: Int): Boolean = {
+    if (n < 2) {
+      return false
+    }
+    if (n % 2 == 0) {
+      return n == 2
+    }
+    if (n % 3 == 0) {
+      return n == 3
+    }
+    var p = 5
+    while (p * p <= n) {
+      if (n % p == 0) {
+        return false
+      }
+      p += 2
+      if (n % p == 0) {
+        return false
+      }
+      p += 4
+    }
+    true
+  }
+
+  def cycle(n: Int): Int = {
+    var m = n
+    var p = 1
+    while (m >= 10) {
+      p *= 10
+      m /= 10
+    }
+    m + 10 * (n % p)
+  }
+
+  def isCircularPrime(p: Int): Boolean = {
+    if (!isPrime(p)) {
+      return false
+    }
+    var p2 = cycle(p)
+    while (p2 != p) {
+      if (p2 < p || !isPrime(p2)) {
+        return false
+      }
+      p2 = cycle(p2)
+    }
+    true
+  }
+
+  def testRepunit(digits: Int): Unit = {
+    val ru = repunit(digits)
+    if (ru.isProbablePrime(15)) {
+      println(s"R($digits) is probably prime.")
+    } else {
+      println(s"R($digits) is not prime.")
+    }
+  }
+
+  def repunit(digits: Int): BigInt = {
+    val ch = Array.fill(digits)('1')
+    BigInt.apply(new String(ch))
+  }
+}