Located in File: /vendors/swiftMailer/classes/Swift/CharacterReader.php
For fixed width character sets this should be the number of octets-per-character. For multibyte character sets this will probably be 1.
A positive integer indicates the number of more bytes to fetch before invoking this method again. A value of zero means this is already a valid character. A value of -1 means this cannot possibly be a valid character.