14.wsf

# Project Euler
# Problem 14: Longest Collatz sequence
# https://projecteuler.net/problem=14

    0 ^ readi retrieve 1-
    0 ^ store
    1
.loop:
    2dup j< .end
    ^ call collatz_memo
    2dup store
    1 retrieve ^1 j< .longer
    drop
.loop_next:
    1+
    jmp .loop
.end:
    0 retrieve printi end
.longer:
    1 swap store
    0 ^1 store
    jmp .loop_next

# collatz_memo returns the number of steps for n to reach 1 in the
# Collatz sequence, with counts for values less than n memoized. The
# sequence is defined as follows: if n is even, the next term is n/2,
# otherwise 3*n+1.
# (n -- steps)
collatz_memo:
    0 ^1
.collatz_loop:
    ^ 1 j= .collatz_1
    ^ 2% jz .collatz_even
# .collatz_odd:
    3* 1+ 2/
    swap 2+ swap
    jmp .collatz_next
.collatz_even:
    2/
    swap 1+ swap
.collatz_next:
    ^ ^2 j< .collatz_stored
    jmp .collatz_loop
.collatz_stored:
    retrieve + 1slide ret
.collatz_1:
    2slide ret