September 2017 Update

Task/Self-referential-sequence/Kotlin/self-referential-sequence.kotlin

@@ -0,0 +1,68 @@
+// version 1.1.2
+
+const val LIMIT = 1_000_000
+
+val sb = StringBuilder()
+
+fun selfRefSeq(s: String): String {
+    sb.setLength(0)  // faster than using a local StringBuilder object
+    for (d in '9' downTo '0') {
+        if (d !in s) continue
+        val count = s.count { it == d }
+        sb.append("$count$d")
+    }
+    return sb.toString()
+}
+
+fun permute(input: List<Char>): List<List<Char>> {
+    if (input.size == 1) return listOf(input)
+    val perms = mutableListOf<List<Char>>()
+    val toInsert = input[0]
+    for (perm in permute(input.drop(1))) {
+        for (i in 0..perm.size) {
+            val newPerm = perm.toMutableList()
+            newPerm.add(i, toInsert)
+            perms.add(newPerm)
+        }
+    }
+    return perms
+}
+
+fun main(args: Array<String>) {
+    val sieve = IntArray(LIMIT) // all zero by default
+    val elements = mutableListOf<String>()
+    for (n in 1 until LIMIT) {
+        if (sieve[n] > 0) continue
+        elements.clear()
+        var next = n.toString()
+        elements.add(next)
+        while (true) {
+            next = selfRefSeq(next)
+            if (next in elements) {
+                val size = elements.size
+                sieve[n] = size
+                if (n > 9) {
+                    val perms = permute(n.toString().toList()).distinct()
+                    for (perm in perms) {
+                        if (perm[0] == '0') continue
+                        val k = perm.joinToString("").toInt()
+                        sieve[k] = size
+                    }
+                }
+                break
+            }
+            elements.add(next)
+        }
+    }
+    val maxIterations = sieve.max()!!
+    for (n in 1 until LIMIT) {
+        if (sieve[n] < maxIterations) continue
+        println("$n -> Iterations = $maxIterations")
+        var next = n.toString()
+        for (i in 1..maxIterations) {
+            println(next)
+            next = selfRefSeq(next)
+        }
+        println()
+    }
+}