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)) }

h27:=collatz(27);
println("Hailstone(27)-->",h27[0,4].concat(","),"...",
	h27[-4,*].concat(",")," length ",h27.len());

[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)))
.println();