14

euler14.php

@@ -0,0 +1,37 @@
+<?php
+/*
+* Gnieark's anwser to euler problem 14
+* https://projecteuler.net/problem=14
+*/
+$longestSequence=0;
+$startOfLongestSequence=0;
+for($i=1;$i<1000000;$i++){
+  //echo $i."\n";
+  $long=countCollatzSequence($i);
+  if($long>$longestSequence){
+   $longestSequence=$long;
+   $startOfLongestSequence=$i;
+  }
+}
+echo $startOfLongestSequence;
+
+function countCollatzSequence($start){
+  $count=1;
+  $number=$start;
+  $next=nextCollatzNumber($number);
+  while($number<>1){
+    $number=nextCollatzNumber($number);
+    $count++;
+  }
+  return $count;
+}
+
+function nextCollatzNumber($previous){
+  if(fmod($previous,2)==0){
+    //odd
+    return $previous/2;
+  }else{
+    //even
+    return 3 * $previous + 1;
+  }
+}