Task: Hailstone sequence





Java: import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

class Hailstone {

  public static List<Long> getHailstoneSequence(long n) {
    if (n <= 0)
      throw new IllegalArgumentException("Invalid starting sequence number");
    List<Long> list = new ArrayList<Long>();
    list.add(Long.valueOf(n));
    while (n != 1) {
      if ((n & 1) == 0)
        n = n / 2;
      else
        n = 3 * n + 1;
      list.add(Long.valueOf(n));
    }
    return list;
  }
  
  public static void main(String[] args) {
    List<Long> sequence27 = getHailstoneSequence(27);
    System.out.println("Sequence for 27 has " + sequence27.size() + " elements: " + sequence27);
    
    long MAX = 100000;
    
    {
      long highestNumber = 1;
      int highestCount = 1;
      for (long i = 2; i < MAX; i++) {
        int count = getHailstoneSequence(i).size();
        if (count > highestCount) {
          highestCount = count;
          highestNumber = i;
        }
      }
      System.out.println("Method 1, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
    }
    
    
    {
      long highestNumber = 1;
      int highestCount = 1;
      for (long i = 2; i < MAX; i++) {
        int count = 1;
        long n = i;
        while (n != 1) {
          if ((n & 1) == 0)
            n = n / 2;
          else
            n = 3 * n + 1;
          count++;
        }
        if (count > highestCount) {
          highestCount = count;
          highestNumber = i;
        }
      }
      System.out.println("Method 2, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
    }
    
    
    {
      long highestNumber = 1;
      long highestCount = 1;
      Map<Long, Integer> sequenceMap = new HashMap<Long, Integer>();
      sequenceMap.put(Long.valueOf(1), Integer.valueOf(1));
      
      List<Long> currentList = new ArrayList<Long>();
      for (long i = 2; i < MAX; i++) {
        currentList.clear();
        Long n = Long.valueOf(i);
        Integer count = null;
        while ((count = sequenceMap.get(n)) == null) {
          currentList.add(n);
          long nValue = n.longValue();
          if ((nValue & 1) == 0)
            n = Long.valueOf(nValue / 2);
          else
            n = Long.valueOf(3 * nValue + 1);
        }
        int curCount = count.intValue();
        for (int j = currentList.size() - 1; j >= 0; j--)
          sequenceMap.put(currentList.get(j), Integer.valueOf(++curCount));
        if (curCount > highestCount) {
          highestCount = curCount;
          highestNumber = i;
        }
      }
      System.out.println("Method 3, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
    }
    return;
  }
}


PHP: function hailstone($n,$seq=array()){
	$sequence = $seq;
	$sequence[] = $n;
	if($n == 1){
		return $sequence;
	}else{
		$n = ($n%2==0) ? $n/2 : (3*$n)+1;
		return hailstone($n, $sequence);
	}
}

$result = hailstone(27);
echo count($result) . ' Elements.<br>';
echo 'Starting with : ' . implode(",",array_slice($result,0,4)) .' and ending with : ' . implode(",",array_slice($result,count($result)-4)) . '<br>';

$maxResult = array(0);

for($i=1;$i<=100000;$i++){
		$result = count(hailstone($i));
		if($result > max($maxResult)){
			$maxResult = array($i=>$result);
		}
}
foreach($maxResult as $key => $val){
echo 'Number < 100000 with longest Hailstone seq.: ' . $key . ' with length of ' . $val;
}