/* 
 * An interface to arrays
 * 
 * 15-122 Principles of Imperative Computation
 */

#use <util>
#use <string>

/* Interface */

typedef struct arr_header* arr_t; 
// typedef ______* arr_t;

int arr_len(arr_t A)
  /*@requires A != NULL; @*/;

arr_t arr_new(int limit)
  /*@requires 0 <= limit; @*/
  /*@ensures \result != NULL; @*/
  /*@ensures arr_len(\result) == limit; @*/;

string arr_get(arr_t A, int i)
  /*@requires A != NULL; @*/
  /*@requires 0 <= i && i < arr_len(A); @*/;

void arr_set(arr_t A, int i, string x)
  /*@requires A != NULL; @*/
  /*@requires 0 <= i && i < arr_len(A); @*/;

/* Implementation */

typedef struct arr_header arr;
struct arr_header {
  int limit;
  string[] data;
};

bool is_arr_expected_length(string[] A, int length) {
  //@assert \length(A) == length;
  return true;
}

bool is_arr(arr* AH) {
  return AH != NULL 
    && is_arr_expected_length(AH->data, AH->limit);
}

int arr_len(arr* A) 
//@requires is_arr(A);
//@ensures \result == \length(A->data);
{
  return A->limit;
}

string arr_get(arr* A, int i) 
//@requires is_arr(A);
//@requires 0 <= i && i < arr_len(A);
{
  return A->data[i];
}

void arr_set(arr* A, int i, string x)
//@requires is_arr(A);
//@requires 0 <= i && i < arr_len(A);
//@ensures is_arr(A);
{
  A->data[i] = x;
}

arr* arr_new(int limit) 
//@requires 0 <= limit;
//@ensures is_arr(\result);
//@ensures arr_len(\result) == limit; 
{
  arr* A = alloc(arr);
  A->data = alloc_array(string, limit);
  A->limit = limit;
  return A;
}