Aged Brie
Each day, quality goes up until it hits 50
After sell-by date, quality goes up double
Sulfuras
Sell in date nor quality increase at any point - quality is always 80
Backstage passes
Above 10 days, quality increases by 1
5 < Sellin <= 10 means quality increases by 2
0 < Sellin <= 5 means quality increases by 3
Sellin <= 0 means quality is 0
Normal items
Quality decreases by 1 each day
If sellin is less than or equal to 0, then quality decreases at twice speed