This:
private var alphabet:Array = [ "0" , "1" , "2" , "3" , "4" , "5" , "6" , "7" , "8" , "9" , "a" , "b" , "c" , "d" , "e" , "f" , "g" , "h" , "i" , "j" , "k" , "l" , "m" , "n" , "o" , "p" , "q" , "r" , "s" , "t" , "u" , "v" , "w" , "x" , "y" , "z" , "A" , "B" , "C" , "D" , "E" , "F" , "G" , "H" , "I" , "J" , "K" , "L" , "M" , "N" , "O" , "P" , "Q" , "R" , "S" , "T" , "U" , "V" , "W" , "X" , "Y" , "Z" , "!" , "@" , "#" , "$" , "%" , "^" , "_" , ":" , "&" , "*" , "(" , ")" , "|" , "+" , "-" , "=" , ">" , "<" ];
The second character's value is equal to its position in the array; the first character's value is its position in the array multiplied by the length of the array. Those two values are added together to get the integer number corresponding to the tile to be loaded.