> Then problem is that if then else is not linear so you can't express this.
It sounds like a limited set of discontinuous functions are needed, like how media queries are used in CSS. The constraints to satisfy are defined by a set of top-level discontinuous functions on environmental parameters, but the set is always fixed so it's pretty simple to detect when you need to switch to a different set of constraints. Wouldn't that suffice?
> The height of a text is not a linear function based on the width of the container. You can't say, the height of the text is 10px * number of characters * width of the container.
For simplicity, let's take the easiest case of a fixed width font and we'll hard wrap at X characters, regardless of whitespace positions. The number of lines L and the height of the text H in px is then given by:
L = char_px_width * char_count / container_px_per_line
H = L * char_px_height
Agreed? If so, then that case isn't difficult, but difficulties arise when you want better wrapping behaviour based on whitespace with a non-fixed width font. I agree that doing it precisely with linear equations doesn't seem feasible, but it seems like we can closely approximate it to minimize the reflows needed to converge on the final result.
Suppose we set char_px_height=the widest character of the typeface (ditto for char_px_height) so we overestimate the dimensions needed. A quick google search suggests that the average word length for ordinary English is around 5 characters. So with avg_word_length=5, the above equation changes to something like:
L = char_px_width * avg_word_length * ceiling(char_count / avg_word_length) / container_px_per_line
This should miss the actual number of lines needed at most by 1, except when dealing with highly irregular text. Then again, the avg_word_length can be constantly adjusted on the fly based on the reflows you have to do, so this should quickly converge too. Does this sound right?
Edit: sorry, that last calculation is obviously wrong, I was rushing out the door. The idea is to estimate number of lines from the average nnumber of words that would fit per line.