original_string: public function letter_frequency()
      {
          return $this->transform(
              function ($frequencies, $word) {
                  foreach (count_chars($word, 1) as $byte => $frequency) {
                      $letter = chr($byte);
                      if ($frequencies->has($letter)) {
                          $frequencies->set($letter, $frequencies->get($letter)+1);
                      } else {
                          $frequencies->set($letter, 1);
                      }
                  }
              },
              function (Map $original) { return new MapOfInts(); }
          );
      }
code: public function letter_frequency()
      {
          return $this->transform(
              function ($frequencies, $word) {
                  foreach (count_chars($word, 1) as $byte => $frequency) {
                      $letter = chr($byte);
                      if ($frequencies->has($letter)) {
                          $frequencies->set($letter, $frequencies->get($letter)+1);
                      } else {
                          $frequencies->set($letter, 1);
                      }
                  }
              },
              function (Map $original) { return new MapOfInts(); }
          );
      }
code_tokens: public function letter_frequency ( ) { return $ this -> transform ( function ( $ frequencies , $ word ) { foreach ( count_chars ( $ word , 1 ) as $ byte => $ frequency ) { $ letter = chr ( $ byte ) ; if ( $ frequencies -> has ( $ letter ) ) { $ frequencies -> set ( $ letter , $ frequencies -> get ( $ letter ) + 1 ) ; } else { $ frequencies -> set ( $ letter , 1 ) ; } } } , function ( Map $ original ) { return new MapOfInts ( ) ; } ) ; }
docstring: Calculate frequency of letters in all strings.
  
  Counts how many times each letter occurs within every string in this
  map.  Returns a new Map of letter to number of occurrences.  Only
  letters that appear will be in the resulting map.
  
  @return \Haldayne\Boost\MapOfIntegers
  @api
docstring_tokens: Calculate frequency of letters in all strings .