source: CIVL/examples/loop_invariants/loop_assigns_gen/lcp2.cvl

/* 
 * Verify that the abstraction of function 'lcp2' is sound. 
 *
 * Note: requires Why3 with TIMEOUT > 10 seconds!
 * 
 * Command: civl verify -collectSymbolicConstants lcp2.cvl
 */
#include <stdlib.h>
#include <string.h>
#include <civlc.cvh>
#include <assert.h>

#pragma CIVL ACSL

$input int N;
$assume(N > 0);
$input int X1[N];

/* string 'x' is not greater than string 'y' with the specific length */
/*@
  @  predicate ngt_len(int * X1, int N, int x, int y, int l) = 
  @                   0 <= l && l <= N && x + l <= N && y + l <= N 
  @                          && (x + l == N || (y + l < N ==> X1[y+l] > X1[x+l]))
  @                          && (\forall int k1; 0 <= k1 && k1 < l ==> X1[(x)+k1] == X1[(y)+k1]);
 */

/* string 'x' is NOT greater than string 'y' */
/*@ 
  @  predicate ngt(int * X1, int N, int x, int y) = 
  @            (\exists int l; ngt_len(X1, N, x, y, l) &&
  @             !(\exists int o; ngt_len(X1, N, x, y, o) && o != l)
  @            );
  @*/

int lcp(int *arr, int n, int x, int y) {
  int l = 0;

  /*@ loop invariant 0 <= l && l <= n;
    @ loop invariant 0 <= x + l && x + l <= n;
    @ loop invariant 0 <= y + l && y + l <= n;
    @ loop invariant \forall int i; 0 <= i && i < l ==> arr[x+i]==arr[y+i];
    @*/
  while (x+l<n && y+l<n && arr[x+l]==arr[y+l]) {
      l++;
  }
  return l;
}

/* returns:
 *   0 if x and y points to the same suffix;
 *   poistive value if x "is greater than" y;
 *   negative or zero value if x "is NO greater than" y;
 */
int compare(int *a, int n, int x, int y) {
  if (x == y) return 0;
  
  int c = $choose_int(2);
  int ret;      

  $havoc(&ret); 
  if (c == 0) {
    $assume(ngt(X1, N, x, y) && ret <= 0);
    return ret;
  } else {
    $assume(ngt(X1, N, y, x) && ret > 0);
    return ret;
  }
}

void sort(int *a, int n, int *data) {
  /*@ loop invariant 1 <= i && i <= n;
    @ loop invariant \forall int k; 1 <= k && k < i ==> ngt(a, n, data[k-1],data[k]);
    @ loop invariant \forall int k; 0 <= k && k < n ==> 0 <= data[k] && data[k] < n;           // data elements are valid
    @ loop invariant \forall int t, k; 0 <= k && k < n && 0 <= t && t < n && k != t ==> 
    @                                  data[k] != data[t];
    @ loop assigns i, data[0 .. n-1];
    @*/
  for(int i = 1; i < n; i++) {
    int comp = compare(a, n, data[i - 1], data[i]);

    /*@ loop invariant 0 <= j && j <= i;
      @ loop invariant \forall int k; 0 <= k && k < n ==> 0 <= data[k] && data[k] < n;           // data elements are valid
      @ loop invariant \forall int t, k; 0 <= k && k < n && 0 <= t && t < n && k != t ==> 
      @                                  data[k] != data[t];
      @ loop invariant \forall int k; j < k && k <= i ==> ngt(a, n, data[k-1],data[k]);
      @ loop invariant \forall int k; 0 < k && k < j ==> ngt(a, n, data[k-1],data[k]);
      @ loop invariant comp <= 0 ==> (\forall int k; 0 < k && k <= j ==> ngt(a, n, data[k-1],data[k]));
      @ loop invariant (comp > 0  && 0 < j) ==>  ngt(a, n, data[j], data[j-1]);
      @ loop invariant (comp > 0 && 0 < j && j < i) ==> ngt(a, n, data[j-1], data[j+1]) ;
      @ loop assigns j, comp, data[0 .. n-1];
      @*/
    for(int j = i; j > 0 && comp > 0;) {
      // swap:
      int tmp = data[j];
      
      data[j] = data[j-1];
      data[j-1] = tmp;
      j--;
      if (j > 0)
        comp = compare(a, n, data[j - 1], data[j]);
    }
  }
  $assert($forall (int i : 1 .. N-1) ngt(a, n, data[i - 1], data[i]));
}

/* An abstraction of the 'sort' function */
void sort2(int *a, int n, int *data) {
  int new_data[n];

  memcpy(data, new_data, sizeof(int) * n);
  $assume($forall (int i : 1 .. N-1) ngt(a, n, data[i - 1], data[i]));
  $assume($forall (int i : 0 .. N-1) 0 <= data[i] && data[i] < n);
  $assume($forall (int i : 0 .. N-1) 
          ($forall (int j | 0 <= j && j < N && j != i) data[i] != data[j])
          );
}

int lcp2(int *a, int n, int index, int* suffixes) {
  return lcp(a,n,suffixes[index], suffixes[index-1]);
}

/**
result[0]: index
result[1]: length
*/
void lrs(int* a, int n, int *result) {
  int suffixes[n];

  /*@ loop invariant 0 <= i  && i <= n;
    @ loop invariant \forall int t; 0 <= t && t < i ==> suffixes[t] == t;
    @ loop assigns suffixes[0 .. n-1], i;
    @*/
  for(int i=0; i<n; i++) {
    suffixes[i] = i;
  }

  sort2(a, n, suffixes);

  int arbi_idx;
  int len;

  $havoc(&arbi_idx);
  $assume(0 < arbi_idx && arbi_idx < n);
  len = lcp2(a, n, arbi_idx, suffixes);
  $assert(ngt_len(a, n, suffixes[arbi_idx-1], suffixes[arbi_idx], len));
}

int main() {
  int result[2] = {0, 0};
  lrs(X1, N, result);
}