How to set custom sort order for product list

For some categories I want to set the default products sort order by most viewed.

I have a custom function to count unique pageviews by product and date. Now I need to apply it as the default sort order.

What file contains the sorting algorithms? Once I find it I can do a conditional sort by category.

I have a custom function to count unique pageviews by product and date
I can do a conditional sort by category
What do you want??
Product listing or category?

It's for a products listing, but I'll sort conditionally based on the category. I can probably figure out the conditional code once I find the spot just to do the sorting.

You will need to review what is standard mostly in

includes/index_filters/default_filter.php
includes/modules/product_listing.php

You will most likely need to create a file along the lines of this for your sorter

includes/modules/product_listing_alpha_sorter.php

And include calls to it for display in

includes/templates/template_default/templates/tpl_index_product_list.php
includes/templates/template_default/templates/tpl_modules_product_listing.php