Calculating Discounts

Today we found an interesting bug in the software when discounts are applied to the transaction.

Here is an example.

Variables
  ItemPrice = 50.00
  Discount  = 14.49%

Calculate Discount Value (DisValue)
  DisValue  = ItemPrice * (Discount/100)
  DisValue  = 50.00 * (14.49/100)
  DisValue  = 50.00 * 0.1449
  DisValue  = 7.245

Calculate NewPrice
  NewPrice  = ItemPrice - DisValue
  NewPrice  = 50.00 - 7.245
  NewPrice  = 42.755

At this point we need to round the values, becuase prices have two decimal places. There are two options, we can either round the values up or down.

Round Up Values
  NewPrice = 42.76
  DisValue =  7.25
  Total    = 50.01 (1p over)

Round Down Values
  NewPrice = 42.75
  DisValue =  7.24
  Total    = 49.99 (1p under)

As you can see, when we add up the rounded totals, they do not match the original item price. So we need find a way to solve this problem.