(define-module (catbird pushdown)
  #:use-module (chickadee data array-list)
  #:use-module (ice-9 match)
  #:use-module (oop goops)
  #:export (<pushdown-state>
            make-pushdown-state
            state-current
            state-previous
            state-push!
            state-pop!
            state-replace!))

(define-class <pushdown-state> ()
  (state-stack #:accessor state-stack #:init-form (make-array-list 2)))

(define (make-pushdown-state)
  (make <pushdown-state>))

(define-method (state-current (state <pushdown-state>))
  (let ((stack (state-stack state)))
    (if (array-list-empty? stack)
        #f
        (array-list-ref stack (- (array-list-size stack) 1)))))

(define-method (state-previous (state <pushdown-state>))
  (let* ((stack (state-stack state))
         (n (array-list-size stack)))
    (if (< n 2)
        #f
        (array-list-ref stack (- n 2)))))

(define-method (state-push! (state <pushdown-state>) obj)
  (array-list-push! (state-stack state) obj))

(define-method (state-pop! (state <pushdown-state>))
  (array-list-pop! (state-stack state)))

(define-method (state-replace! (state <pushdown-state>) obj)
  (let ((stack (state-stack state)))
    (if (array-list-empty? stack)
        (array-list-push! stack obj)
        (array-list-set! stack (- (array-list-size stack) 1) obj))))