Find length of initial segment matching mask on Arrays

Question

Given an array with n values, for example:

$arr[] = \'ABCDEFABC\';
$arr[] = \'ABCDEFDEF\';
$arr[] = \'ABCDEFGHI\';
$arr[] = \'ABCDEFJKL\';

how

Answer 1

I've come up with a solution for my first problem:

EDIT: BUGGY!

$arr = array();

// Bug: ABCDEFX

$arr[] = 'ABCDEFAXC';
$arr[] = 'ABCDEFDXF';
$arr[] = 'ABCDEFGXI';
$arr[] = 'ABCDEFJXL';

/*
$arr[] = 'ABCDEFABC';
$arr[] = 'ABCDEFDEF';
$arr[] = 'ABCDEFGHI';
$arr[] = 'ABCDEFJKL';
*/

// ABCDEF    
$result = implode('', call_user_func_array('array_intersect_assoc', array_map('str_split', $arr)));

One left to go now...

Answer 2

If I understand correctly, you're trying to determine the set of longest common prefixes given a set of strings.

Breaking it down, the shared prefix between any two strings may be found as

function longestCommonPrefix($str1, $str2) {
  $minLen = min(strlen($str1), strlen($str2));
  for ($i = 0; $i < $minLen; $i++) {
      if ($str1[$i] != $str2[$i]) {
          return substr($str1, 0, $i);
      }
  }
  return substr($str1, 0, $minLen);
}

One way to get the set of prefixes could then be:

function longestCommonPrefixes($arr) {
  sort($arr);
  $prefixes = array();
  for ($i = 0; $i < count($arr); $i++) {
      for ($j = $i+1; $j < count($arr); $j++) {
          $prefix = longestCommonPrefix($arr[$i], $arr[$j]);
          if ($prefix == "") break;
          $prefixes[$prefix] = true;
      }
  }
  return array_keys($prefixes);
}

Notice that the returned prefixes may be prefixes of each other. That is, it is possible for the result to contain a set of strings such as array('A', 'AA', 'AAA').

Putting it all together:

$arr = array();
$arr[] = 'ABCDEFABC';
$arr[] = 'ABCDEFDEF';
$arr[] = 'ABCDEFGHI';
$arr[] = 'ABCDEFJKL';
$arr[] = 'DEFABCABC';
$arr[] = 'DEFABCDEF';
$arr[] = 'DEFABCGHI';
$arr[] = 'DEFABCJKL';

print_r(longestCommonPrefixes($arr));

yields

Array
(
    [0] => ABCDEF
    [1] => DEFABC
)