collatz(n) := block([L], L: [n], while n > 1 do
(n: if evenp(n) then n/2 else 3*n + 1, L: endcons(n, L)), L)$

collatz_length(n) := block([m], m: 1, while n > 1 do
(n: if evenp(n) then n/2 else 3*n + 1, m: m + 1), m)$

collatz_max(n) := block([j, m, p], m: 0,
for i from 1 thru n do
   (p: collatz_length(i), if p > m then (m: p, j: i)),
[j, m])$

collatz(27);           /* [27, 82, 41, ..., 4, 2, 1] */
length(%);             /* 112 */
collatz_length(27);    /* 112 */
collatz_max(100000);   /* [77031, 351] */