Ranking by Percentage of Contribution to Category

Okay you all were kind enough to help me out yesterday and I appreciate that, so today I want to see if anyone can help with this thought.

I have a 30,000 line spreadsheet that contains approximately 150 different grocery product categories. I am wanting a formula that will rank each item, within each category, by it's percentage of contribution to the total category units sold. I have attached a simple example of what I am looking for, I hope it makes sense. Please note that the spreadsheet has no breaks that would separate the categories, so the formula will have to be able to know when one category stops and another begins.

Thanks so much for your help!!