Problem Statement

The Hailstone sequence of numbers can be generated from a starting positive integer,   n   by:

The (unproven) Collatz conjecture is that the hailstone sequence for any starting number always terminates.

This sequence was named by Lothar Collatz in 1937   (or possibly in 1939),   and is also known as (the):

The hailstone sequence is also known as   hailstone numbers   (because the values are usually subject to multiple descents and ascents like hailstones in a cloud).

Let's start with the solution:

fcn collatz(n,z=L()){ z.append(n); if(n==1) return(z);
   if(n.isEven) return(self.fcn(n/2,z)); return(self.fcn(n*3+1,z)) }

[2..0d100_000].pump(Void,  // loop n from 2 to 100,000
   collatz,              // generate Collatz sequence(n)
   fcn(c,n){           // if new longest sequence, save length/C, return longest
      if(c.len()>n[0]) n.clear(c.len(),c[0]); n}.fp1(L(0,0)))