This Is Making My Head Spin

I have a table that contains categories, users, modifydates . All three of those fields may be repeated many times.

I need to build a table that contains user, modifydate, oldcategory, newcategory, duration (number of days between modifydates when there is a category change)

I can use the min(modifydate) to get the 1st occurrence of each new category, but I am having trouble populating the oldvalue and then also getting the duration.

Adding to the complexity, I have to take the last entry of a null category prior to category being populated the first time and populate it with the first occurrence of category, Likewise, I have to populate the first occurrence of null after the last occurrence of a popoulated category. In other words, this simulates populated categories prior to first occurrence and after the last occurrence.

Any help would be great.