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#lang racket/base
(require racket/match)
(provide (rename-out [array* array])
array-ref array-set
array-length array->list)
;; TODO: array-for-each, array-map, array-foldl, array-foldr
;; TODO: array, list->array
;; (data a)
(define-struct array ([data #:mutable]))
;; exact-nonnegative-integer * a * (data a)
(define-struct diff (index value data))
;; (data a) ::= (vector a) | (diff a)
;; exact-nonnegative-integer * a -> (array a)
(define array*
(lambda (n [x #f])
(make-array (make-vector n x)))
;; exact-nonnegative-integer * (exact-nonnegative-integer -> a) -> (array a)
(define (build-array n proc)
(make-array (build-vector n proc)))
;; (array a) -> void
(define (reroot! t)
(match (array-data t)
[(? vector?) (void)]
[(struct diff (i v t*))
(reroot! t*)
(let* ([n (array-data t*)]
[v* (vector-ref n i)])
(vector-set! n i v)
(set-array-data! t n)
(set-array-data! t* (make-diff i v* t)))]))
;; (array a) * exact-nonnegative-integer -> a
(define (array-ref t i)
(let ([a (array-data t)])
(if (vector? a)
(vector-ref a i)
(reroot! t)
(vector-ref (array-data t) i)))))
;; (array a) * exact-nonnegative-integer * a -> (array a)
(define (array-set t i v)
(reroot! t)
(let* ([n (array-data t)]
[old (vector-ref n i)])
(if (equal? old v)
(vector-set! n i v)
(let ([res (make-array n)])
(set-array-data! t (make-diff i old res))
(define (impure f t)
(reroot! t)
(f (array-data t)))
(define (apply-impure f ts)
(for-each reroot! ts)
(apply f ts))
(define (array-length t)
(impure vector-length t))
(define (array->list t)
(impure vector->list t))
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