source: CIVL/examples/verifyThis/lcp.c

/*
Author: Yihao Yan

Download LCP.zip from: http://fm2012.verifythis.org/challenges

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Problem decription:

Longest Common Prefix (LCP)
Input:  an integer array X1[n], and two indices x and y into this array
Output: length of the longest common prefix of the subarrays of a
        starting at x and y respectively.
        
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Verification task:

Implement the lcp function, and verify that it behaves in the way described above.
      
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Result:

For arrays with length less than 5, lcp function returns an integer n which is
the length of the longest common prefix between its two suffixes: The two suffixes
are the same with indexes from 0 to n-1 while they are different at index n. Therefore
the lcp function behave correctly.

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command: civl verify lcp.c
*/

#include <civlc.cvh>

$input int N_BOUND=4;
$input int n;
$input int x;
$input int y;
$assume (x < n && y < n && x >=0 && y>=0 && n > 0 && n <= N_BOUND);
$input int X1[n];

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

  while (x+l<n && y+l<n && arr[x+l]==arr[y+l]) {
      l++;
  }
  return l;
}

void main(){
  int result = lcp(X1, n, x, y);

  $assert($forall (int i: 0 .. result-1) X1[x+i] == X1[y+i]);

  int maxXY = x > y ? x : y;

  if(result + maxXY < n){
    $assert(X1[x+result] != X1[y+result]);
  }
}