Exercise: Open/Closed
This exercise puts the Open/Closed Principle into code.
Scenario
A pricing engine receives a list of ["apply", type, base, param] commands and returns one output line per command. Three discount types are supported: percent reduces the base price by a percentage, flat subtracts a fixed amount, and none leaves the price unchanged. An unrecognized type produces an error line. All prices and parameters are integers.
Commands
| Command | Behavior | Output |
|---|---|---|
["apply", "percent", base, param] | Deduct (base * param) // 100 using integer division | The resulting price as a string |
["apply", "flat", base, param] | Deduct param from base, floored at 0 | The resulting price as a string |
["apply", "none", base, param] | Return base unchanged | The resulting price as a string |
["apply", unknown, base, param] | Unrecognized type | "Unknown discount: <type>" |
Example
Input
6 apply percent 200 10 apply flat 200 30 apply none 200 0 apply flat 5 20 apply percent 150 25 apply mystery 100 5
Output
180 170 200 0 113 Unknown discount: mystery
Explanation
10% off 200 is 20, leaving 180. Flat 30 off 200 leaves 170. No discount leaves 200. Flat 20 off 5 would be negative, so it floors at 0. 25% of 150 is 37 (integer division), leaving 113. The type `mystery` is not in the registry, so it produces an error.
Your task
Create the three discount classes used by the registry below the Discount abstract class in the editor: PercentDiscount, FlatDiscount, and NoDiscount, each subclassing Discount and implementing apply. Do not modify Discount, the DISCOUNT_REGISTRY, or run_discounts.