June 2018 Update

Task/Parallel-calculations/Kotlin/parallel-calculations.kotlin

@@ -0,0 +1,52 @@
+// version 1.1.51
+
+import java.util.stream.Collectors
+
+/* returns the number itself, its smallest prime factor and all its prime factors */
+fun primeFactorInfo(n: Int): Triple<Int, Int, List<Int>> {
+    if (n <= 1) throw IllegalArgumentException("Number must be more than one")
+    if (isPrime(n)) return Triple(n, n, listOf(n))
+    val factors = mutableListOf<Int>()
+    var factor = 2
+    var nn = n
+    while (true) {
+        if (nn % factor == 0) {
+            factors.add(factor)
+            nn /= factor
+            if (nn == 1) return Triple(n, factors.min()!!, factors)
+            if (isPrime(nn)) factor = nn
+        }
+        else if (factor >= 3) factor += 2
+        else factor = 3
+    }
+}
+
+fun isPrime(n: Int) : Boolean {
+    if (n < 2) return false
+    if (n % 2 == 0) return n == 2
+    if (n % 3 == 0) return n == 3
+    var d = 5
+    while (d * d <= n) {
+        if (n % d == 0) return false
+        d += 2
+        if (n % d == 0) return false
+        d += 4
+    }
+    return true
+}
+
+fun main(args: Array<String>) {
+    val numbers = listOf(
+        12757923, 12878611, 12878893, 12757923, 15808973, 15780709, 197622519
+    )
+    val info = numbers.stream()
+                      .parallel()
+                      .map { primeFactorInfo(it) }
+                      .collect(Collectors.toList())
+    val maxFactor = info.maxBy { it.second }!!.second
+    val results = info.filter { it.second == maxFactor }
+    println("The following number(s) have the largest minimal prime factor of $maxFactor:")
+    for (result in results) {
+        println("  ${result.first} whose prime factors are ${result.third}")
+    }
+}