Ask HN: How do you get started writing "predicting" software?

I've seen some cell phones being able to predict the word you are trying to write when you slide your finger on the letters composing a word.

How do you get started writing software like that? Is it a computer software trained over data on finger movement?

I hope I am making myself clear, the one in which you slide your finger over the letters on a keyboard on a screen, and the Program comes up with the word you want to write.

Thanks!!!