problems while programming an edit

Now I'm programming an line edit program. I met some problems in calculating every caret position.
In my program, one line can have several parts and
each part have a different font or style. And because the line has fixed width, so I have to adjust interchar space and extra space(use
SetTextJustification and SetTextCharacterExtra).
This cause the difficult in calculating each char
start point of the line accurately. How to calculate each caret position? I need help.(or a sample). Thank you friend in advance.