Data structure behind T9 type of dictionary

Question

How does a T9 dictionary work? What is the data structure behind it. If we type \'4663\' we get \'good\' when we press down button we get \'gone\' then \'home\' etc...

Answer 1

I think that T9 is using a bit-map-based Trie. On the first level there is a 32-bit (I assume they expanded to 32) word where each bit represents a letter. All letters that are valid as start letters for a word have their bit set.

In the next level there is one 32-bit map for each of those bits that were set in the previous level. In these maps each bit is set if the corresponding letter is valid as a second letter in a word, with the starting letter decided by the first level.

The structure then continues. Using one-bit-per-letter makes a very efficient storage. The addressing scheme however must be challenging. There might also be some kind of optimization using the above schema for short words while using something else for longer words.