sound visualization + zoom

wazdc

Hello,
I'm trying to write code for a simple sound editor.
I want to have some basic operations such as zoom-out
(and zoom-in as a reverse op) there, but I have a problem with implementation of it: sound files are very large (that's the aim - to be able to edit large files of about 400MB) and I can't have them loaded to RAM (I also don't want to do that). My question is:
How to zoom-out? I draw samples as points among x-axis (time axis)
and connect them with lines. Zoom-out should draw more samples (values) onto one y-position (probably can be called "time zoom").
I want to do this in a bar graph manner, as is common.
Pro programs do this very fast. I found on the internet that there's
some algorithm to compute peaks or something similar and use them
in a fairly smaller amount of memory but I didn't find out
how to program it ;-(...
Any help appreciated...