rank function for SQLite3 FTS3/FTS4

sqlite_fts4_rank.php

<?php

/**
 * SQLite search ranking user defined function
 * Converted from C from SQLite manual: https://www.sqlite.org/fts3.html#appendix_a
 */
function sqlite_rank(string $aMatchinfo, ...$weights): float
{
  /* Check that the number of arguments passed to this function is correct.
  ** If not, jump to wrong_number_args. Set aMatchinfo to point to the array
  ** of unsigned integer values returned by FTS function matchinfo. Set
  ** nPhrase to contain the number of reportable phrases in the users full-text
  ** query, and nCol to the number of columns in the table.
  */
  $nCol = 0;
  $nPhrase = 0;
  $match_info = unpack('V*', $aMatchinfo);
  $nMatchinfo = count($match_info);
  $match_info = array_values(array_map('intval', $match_info));

  if ($nMatchinfo >= 2) {
    $nPhrase = $match_info[0];
    $nCol = $match_info[1];
  }

  if ($nMatchinfo !== (2 + 3 * $nCol * $nPhrase)) {
    throw new \BadMethodCallException('invalid matchinfo blob passed to function rank()');
  }

  if (count($weights) !== $nCol) {
    throw new \BadMethodCallException('Invalid number of arguments: ' . $nCol);
  }

  $score = 0.0;
  $weights = array_map('floatval', $weights);

  // Iterate through each phrase in the users query. //
  for ($iPhrase = 0; $iPhrase < $nPhrase; $iPhrase++) {
    /* Now iterate through each column in the users query. For each column,
    ** increment the relevancy score by:
    **
    **   (<hit count> / <global hit count>) * <column weight>
    **
    ** aPhraseinfo[] points to the start of the data for phrase iPhrase. So
    ** the hit count and global hit counts for each column are found in
    ** aPhraseinfo[iCol*3] and aPhraseinfo[iCol*3+1], respectively.
    */

    $aPhraseinfoOffset = 2 + $iPhrase * $nCol * 3;

    for ($iCol = 0; $iCol < $nCol; $iCol++) {
      $idxHit = $aPhraseinfoOffset + $iCol * 3;
      $nHitCount = $match_info[$idxHit];
      $nGlobalHitCount = $match_info[$idxHit + 1];
      $weight = $weights[$iCol];

      if ($nHitCount > 0 && $nGlobalHitCount !== 0) {
        $score += ($nHitCount / $nGlobalHitCount) * $weight;
      }
    }
  }

  return $score;
}