I have written a solution to Hackerrank’s problem “While Language” in Common Lisp, solely for fun. Several years ago, I have solved it using OCaml.

Where does ‘fun’ come from? In Lisp, it’s really easy to construct ASTs. In a lot of scenarios, you can effortlessly transpile source code into Lisp forms and then ask your Lisp implementation to analyse, optimise and execute it. For example, after lexing and parsing, I have written a function gen-lisp-code to expose the transpiled Lisp code:

* (pprint (gen-lisp-code "fact := 1 ;
val := 10000 ;
cur := val ;
mod := 1000000007 ;

while ( cur > 1 )
  do
   {
      fact := fact * cur ;
      fact := fact - fact / mod * mod ;
      cur := cur - 1
   } ;

cur := 0"))

(LAMBDA ()
  (LET* ((|cur| 0) (|fact| 0) (|mod| 0) (|val| 0))
    (DECLARE (OPTIMIZE SPEED (SPACE 0) (SAFETY 1))
             (TYPE (INTEGER 0 2000000000000000000) |cur| |fact| |mod| |val|))
    (SETF |fact| 1)
    (SETF |val| 10000)
    (SETF |cur| |val|)
    (SETF |mod| 1000000007)
    (DO ()
        ((NOT (> |cur| 1)))
      (SETF |fact| (* |fact| |cur|))
      (SETF |fact| (- |fact| (* (FLOOR (/ |fact| |mod|)) |mod|)))
      (SETF |cur| (- |cur| 1)))
    (SETF |cur| 0)
    (FORMAT T "~a ~d~%" "cur" |cur|)
    (FORMAT T "~a ~d~%" "fact" |fact|)
    (FORMAT T "~a ~d~%" "mod" |mod|)
    (FORMAT T "~a ~d~%" "val" |val|)))

Afterwards, the lambda definition can be sent to Lisp’s compile function and generate a Lisp function which can be excuted to print the desired results:

* (funcall (compile nil (gen-lisp-code "fact := 1 ;
val := 10000 ;
cur := val ;
mod := 1000000007 ;

while ( cur > 1 )
  do
   {
      fact := fact * cur ;
      fact := fact - fact / mod * mod ;
      cur := cur - 1
   } ;

cur := 0")))

cur 0
fact 531950728
mod 1000000007
val 10000
NIL

Therefore, my solution is actually not an interpreter but a compiler that transpiles While language code into Lisp forms. In other languages, writing a compiler arguably requires more code; but in Lisp, with its comprehensive run-time system, it becomes much easier.

The full solution is shown below (see from Gist):

;;;; while.lisp
;;;; A solution to Hackerrank's While Language problem --
;;;; https://www.hackerrank.com/challenges/while-language-fp/problem
;;;; Author: David Gu ([email protected])
(in-package #:cl-user)
(defpackage #:while
  (:use #:cl)
  (:nicknames #:while-lang)
  (:export #:gen-lisp-code #:run-program))
(in-package #:while)

;;; Part I. Lex
;;; Variable, Numeral,
;;; AOp: Plus, Minus, Mul, Div, Assign
;;; BOp: And, Or,
;;; ROp('<' | '>'): Gt, Lt,
;;; Other Keywords: True, False, Lparen, Rparen, If, Then, Else, 
;;; --------------- Semicolon, While, Do, Lbracket, Rbracket
(defstruct token type val)

(defmethod print-object ((obj token) stream)
  (with-slots (type val) obj
    (if val
        (format stream "~A(~A)~%" type val)
      (format stream "~A~%" type))))

(defun make-var (var)
  (make-token :type :var :val var))

(defun make-num (num)
  (make-token :type :num :val num))

(defconstant +keywords-table+
  (loop with table = (make-hash-table :test 'equal)
        for (k . v) in '((#\+ . :plus) (#\- . :minus) (#\* . :mul) (#\/ . :div)
                         (":=" . :assign) ("and" . :and) ("or" . :or) (#\> . :gt) (#\< . :lt)
                         ("true" . :true) ("false" . :false) (#\( . :lparen) (#\) . :rparen)
                         ("if" . :if) ("then" . :then) ("else" . :else) (#\; . :semicolon)
                         ("while" . :while) ("do" . :do) (#\{ . :lbracket) (#\} . :rbracket))
        do (setf (gethash k table) v)
        finally (return table)))

(defun make-keyword (part)
  (make-token :type (gethash part +keywords-table+)))

(defstruct token-stream tokens len ptr)

(defmethod print-object ((obj token-stream) stream)
  (with-slots (len ptr) obj
    (format stream "<~d tokens in total, ~d consumed, ~d left.>~%"
            len ptr (- len ptr))))

(defun next-token (tokens)
  (with-slots (tokens len ptr) tokens
    (unless (= ptr len)
      (let ((rt (aref tokens ptr)))
        (incf ptr)
        rt))))

(defun peek-token (tokens)
  (with-slots (tokens len ptr) tokens
    (unless (= ptr len)
      (aref tokens ptr))))

(defun expect-token (tokens expect)
  (let ((next (next-token tokens)))
    (unless (eq (token-type next) expect)
      (error "Parsing error: unexpected token ~A" next))))

(defun token-stream-empty-p (tokens)
  (with-slots (len ptr) tokens
    (= len ptr)))

(defun lex-by-pred (in pred)
  (declare (type stream in))
  (with-output-to-string (o)
    (loop for c = (peek-char nil in nil nil)
          while (and c (funcall pred c))
          do (write-char (read-char in) o))))

(defun lex-str (in)
  (declare (type stream in))
  (lex-by-pred in (lambda (c) (char<= #\a c #\z))))

(defun lex-num (in)
  (declare (type stream in))
  (let ((str (lex-by-pred in (lambda (c) (char<= #\0 c #\9)))))
    (parse-integer str :junk-allowed nil)))

(defun lex (in)
  (declare (type stream in))
  (let ((tokens (make-array 0 :element-type 'token :adjustable t :fill-pointer 0)))
    (loop for c = (peek-char t in nil nil)
          while c do (cond (;; keywords
                            (find c #(#\+ #\- #\* #\/ #\> #\< #\( #\) #\; #\{ #\}) :test 'char=)
                            (vector-push-extend (make-keyword (read-char in)) tokens))
                           (;; Assign
                            (char= c #\:)
                            (read-char in) ;; #\:
                            (read-char in) ;; #\=
                            (vector-push-extend (make-keyword ":=") tokens))
                           (;; a var or a keyword
                            (char<= #\a c #\z)
                            (let ((str (lex-str in)))
                              (if (find str #("and" "or" "true" "false" "if" "then" "else" "while" "do") :test 'string=)
                                  (vector-push-extend (make-keyword str) tokens)
                                (vector-push-extend (make-var str) tokens))))
                           (;; a numeral
                            (char<= #\0 c #\z)
                            (vector-push-extend (make-num (lex-num in)) tokens)))
          finally (return (make-token-stream :tokens tokens :len (length tokens) :ptr 0)))))

;;; Part II. Parse (recursive descent)
(defun parse (tokens)
  (let (stmts)
    (tagbody
     start
     (push (parse-statement tokens) stmts)
     (if (and (peek-token tokens)
              (eq :semicolon (token-type (peek-token tokens))))
         (progn 
           (next-token tokens)
           (go start))
       (return-from parse (nreverse stmts))))))

(defun parse-statement (tokens)
  (let ((token (peek-token tokens)))
    (case (token-type token)
      (:var (parse-assign tokens))
      (:if (next-token tokens)
          (parse-if tokens))
      (:while (next-token tokens)
       (parse-while tokens))
      (t (error "Parsing error.")))))

(defun parse-assign (tokens)
  (let (var val)
    (setq var (intern (token-val (next-token tokens)) :while))
    (expect-token tokens :assign)
    (setq val (parse-aexpr tokens))
    `(setf ,var ,val)))

(defun parse-aexpr (tokens)
  (labels ((parse-factor (tokens)
             (let ((token (next-token tokens)))
               (case (token-type token)
                 (:var (intern (token-val token) :while))
                 (:num (token-val token))
                 (:lparen (let ((arith (parse-aexpr tokens)))
                            (expect-token tokens :rparen)
                            arith))
                 (t (error "Parsing error")))))
           (parse-term (tokens)
             (let ((left (parse-factor tokens)))
               (tagbody
                start
                (cond ((and (peek-token tokens)
                            (eq :mul (token-type (peek-token tokens))))
                       (next-token tokens)
                       (setq left `(* ,left ,(parse-factor tokens)))
                       (go start))
                      ((and (peek-token tokens)
                            (eq :div (token-type (peek-token tokens))))
                       (next-token tokens)
                       (setq left `(floor (/ ,left ,(parse-factor tokens))))
                       (go start))
                      (t (return-from parse-term left)))))))
    (let ((left (parse-term tokens)))
      (tagbody
       start
       (cond ((and (peek-token tokens)
                   (eq :plus (token-type (peek-token tokens))))
              (next-token tokens)
              (setq left `(+ ,left ,(parse-term tokens)))
              (go start))
             ((and (peek-token tokens)
                   (eq :minus (token-type (peek-token tokens))))
              (next-token tokens)
              (setq left `(- ,left ,(parse-term tokens)))
              (go start))
             (t (return-from parse-aexpr left)))))))

(defun parse-if (tokens)
  (let (test then else)
    (setq test (parse-bexpr tokens))
    (expect-token tokens :then)
    (expect-token tokens :lbracket)
    (setq then (parse tokens))
    (expect-token tokens :rbracket)
    (expect-token tokens :else)
    (expect-token tokens :lbracket)
    (setq else (parse tokens))
    (expect-token tokens :rbracket)
    `(if ,test
         (progn ,@then)
       (progn ,@else))))

(defun parse-while (tokens)
  (let (test body)
    (setq test (parse-bexpr tokens))
    (expect-token tokens :do)
    (expect-token tokens :lbracket)
    (setq body (parse tokens))
    (expect-token tokens :rbracket)
    `(do ()
         ((not ,test))
       ,@body)))

(defun parse-bexpr (tokens)
  (labels ((parse-bexpr/1 ()
             (case (token-type (peek-token tokens))
               (:true (next-token tokens) t)
               (:false (next-token tokens) nil)
               (:lparen (next-token tokens)
                (let ((bexpr (parse-bexpr tokens)))
                  (expect-token tokens :rparen)
                  bexpr))
               (t (let ((left (parse-aexpr tokens)))
                    (case (token-type (next-token tokens))
                      (:gt `(> ,left ,(parse-aexpr tokens)))
                      (:lt `(< ,left ,(parse-aexpr tokens)))
                      (t (error "Parsing error"))))))))
    (let ((left (parse-bexpr/1)))
      (tagbody
       start
       (case (token-type (peek-token tokens))
         (:and (next-token tokens)
          (setq left `(and ,left ,(parse-bexpr/1)))
          (go start))
         (:or (next-token tokens)
          (setq left `(or ,left ,(parse-bexpr/1)))
          (go start))
         (t (return-from parse-bexpr left)))))))
  
    
;;; Part III: Compile (transpile, actually)
(defun generate-symbol-table (stmts)
  (when stmts
    (loop with syms = (list)
          for stmt in stmts
          do (case (car stmt)
               (setf (pushnew (second stmt) syms :test 'eq))
               (do (let ((res (generate-symbol-table (cdddr stmt))))
                     (when res
                       (setf syms (concatenate 'list syms res)))))
               (if (let ((res1 (generate-symbol-table (cdr (third stmt))))
                         (res2 (generate-symbol-table (cdr (fourth stmt)))))
                     (setf syms (concatenate 'list syms res1 res2)))))
          finally (return (sort (delete-duplicates syms :test 'eq)
                                'string< :key 'symbol-name)))))

(defun gen-lisp-code (program)
  (let* ((stmts (parse (with-input-from-string (in program)
                         (lex in))))
         (syms (generate-symbol-table stmts)))
    `(lambda ()
       (let* ,(mapcar (lambda (sym) `(,sym 0)) syms)
         (declare (optimize speed (space 0) (safety 1))
                  (type (integer 0 #.(* 2 (expt 10 18))) ,@syms))
         ,@stmts
         ,@(mapcar (lambda (sym)
                     `(format t "~a ~d~%" ,(symbol-name sym) ,sym))
                   syms)))))

(defun run-program (program)
  (let ((fn (compile 'nil (gen-lisp-code program))))
    (funcall fn)))

;;; Entry point
(in-package #:cl-user)

(defun main ()
  (while:run-program
   (with-output-to-string (o)
     (loop for line = (read-line t nil nil)
           while line do (write-line line o)))))

;;; uncomment this line if you wanto to submit it to Hackerrank
;; (main)

;;; test case
(defvar *test-0*
  "base := 2 ;
power := 100 ;
prime := 1000000007 ;

res := 1 ;

while ( power > 0 ) do {
        parity := power - ( power / 2 * 2 ) ;
        if ( power - power / 2 * 2 > 0 ) then
        {
            res := res * base ;
            res := res - res / prime * prime
        }
        else
        {
            res := res
        } ;

    base := base * base ;
    base := base - base / prime * prime ;
    power :=  power / 2
}")

(defvar *test-1*
  "fact := 1 ;
val := 10000 ;
cur := val ;
mod := 1000000007 ;

while ( cur > 1 )
  do
   {
      fact := fact * cur ;
      fact := fact - fact / mod * mod ;
      cur := cur - 1
   } ;

cur := 0")

(defvar *test-2*
  "a := 267815000 ;
b := 556456000 ;
while ( b > 0 ) do
 {
	t := b ;
    b := a - ( a / b ) * b ;
	a := t
} ;

res  := a")

(defvar *test-3*
  "a := 10 ;
b := 100 ;
c := 1000 ;

if ( a > b and a > c ) then {
    largest := a
}
else {
    if ( b > a and b > c ) then {
        largest := b
    }
    else {
        largest := c
    }
 }
;

if ( a > b and a < c ) then {
    middle := a
}
else {
    if ( b > a and b < c ) then {
        middle := b
    }
    else {
        middle := c
    }
} ;

if ( a < b and a < c ) then {
    smallest := a
}
else {
    if ( b < a and b < c ) then {
        smallest := b
    }
    else {
        smallest := c
    }
}")

(defvar *test-4*
  "sum := 0 ;
cur := 0 ;
while ( cur < 10000 ) do
{
    cur := cur + 1 ;
    sum := sum + cur
} ;

p := 1000000007 ;
cur := 0 ;
prod := 1 ;

while ( cur < 10000 ) do
{
    cur := cur + 1 ;
    prod := prod * cur ;
    prod := prod - prod / p * p
}")

(defvar *test-5*
  "a := 1000 ;
b := 2000 ;

c := b ;
b := a ;
a := c ;

c := 0")

(defvar *test-6*
  "a := 10 ;
b := 100 ;

if ( a < b ) then
    {
        min := a ;
        max := b
    }
else {
    min := b ;
    max := a
    }")

(defvar *test-7*
  "i := 0 ;

oddsum := 0 ;
evensum := 0 ;

while ( i < 100 ) do
{
    j := 0 ;

    while ( j < i ) do {
        if ( j - j / 2 * 2 > 0 ) then {
            oddsum := oddsum + j
        }
        else {
            evensum := evensum + j
        } ;
        j := j + 1
    } ;

    i := i + 1
}")