include c:\cxpl\codes;  \intrinsic 'code' declarations
int Seq(1000);          \more than enough for longest sequence

func Hailstone(N);      \Return length of Hailstone sequence starting at N
int  N;                 \ also fills Seq array with sequence
int  I;
[I:= 0;
loop [Seq(I):= N;  I:= I+1;
     if N=1 then return I;
     N:= if N&1 then N*3+1 else N/2;
     ];
];

int N, SN, Len, MaxLen;
[Len:= Hailstone(27);
Text(0, "27's Hailstone length = ");  IntOut(0, Len);  CrLf(0);

Text(0, "Sequence = ");
for N:= 0 to 3 do [IntOut(0, Seq(N));  ChOut(0, ^ )];
Text(0, "... ");
for N:= Len-4 to Len-1 do [IntOut(0, Seq(N));  ChOut(0, ^ )];
CrLf(0);

MaxLen:= 0;
for N:= 1 to 100_000-1 do
    [Len:= Hailstone(N);
    if Len > MaxLen then [MaxLen:= Len;  SN:= N];       \save N with max length
    ];
IntOut(0, SN);  Text(0, "'s Hailstone length = ");  IntOut(0, MaxLen);
]