android如何制作文本文件查看器？

This is a concept problem. I see many text file viewer in android market. They are able to open a txt file with thousand of words. The viewer shows the text content in hundred of pages, and allow go back to previous page, go to next page, etc. My problem is, as the length of word are different. If you want to go to page 50, do you have to start from page 1 and check how many words is needed to fit in page 1 to page 49, then you know the first English word in page 50? And when you go back one page, you don't know the first word of page 49. Again, how can I find it out efficiently? Thank you.This is a concept problem. I see many text file