;; file: sumdigits.s
;; author: SS
;; date: Sep 11, 2019
;;
;; This program reads in a nonnegative integer n, then prints
;; the sum of the digits of n (when written in base-10).  It
;; demonstrates how to write recursive functions in SLIM.
;;
;; The SLIM code translates the following python code:
;;
;; def sumdigits(n):
;;    if n < 10:
;;      return n
;;    return sumdigits(n // 10) + n % 10

    allocate-registers sumdigits test n one ten cont sp s recursiveCase

    li sp 0
    li one 1
    li ten 10

    li sumdigits sumdigitsL
    li recursiveCase recursiveCaseL

    read n
    li cont backL
    j sumdigits
backL:
    write s
    halt

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; function: sumdigits
;; input registers: n
;; output register: s
;; return address register: cont
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
sumdigitsL:
    slt test n ten ; if n < 10:
    jeqz test recursiveCase
    li s 0  ; return n in s
    add s s n
    j cont

recursiveCaseL:
    ; save context: n, cont
    st n sp
    add sp sp one
    st cont sp
    add sp sp one

    li cont afterL
    div n n ten

    j sumdigits ; computes sumdigits(n // 10) and get the result in s

afterL:
    ; retrive context: cont, n
    sub sp sp one
    ld cont sp
    sub sp sp one
    ld n sp

    rem n n ten  
    add s s n   ; s now contains s + n % 10
    j cont