How to solve Snarky Hangman

huge list of words 
user types in length of word, say 6
change the list to only words of length 6

Now the user types a letter, say t

want to create a dictionary of all the categories for where
that letter could appear, use that as the key and the value is a list of all the words with the letter in that place. 

1) one function to write
    given a word and a letter, return a string with the letter 
      in the corresponding spots, and _ in all the other spots
   f("smarty", "t") return "_ _ _ _ t _" (ignore the space)
   f("testy", "t") return "t _ _ t _"

2) a possible function to write
   give it a letter and a list and you create a dictionary of
    of categories  mapping of strings with placement of letters, 
    "_ _ t _ _" to a list of words that fit that format
    could return dictionary
    OR calculate the longest list in the dictionary and return   the longest list

Back to playing... user has typed t

reset the list to the longest list from the dictionary made with letter t

user picks e (2cd letter)

send e and the current list and create a dictionary based on e and return a new list (the largest category)

repeat repeat repeat