Basic sales tax is applicable at a rate of 10% on all goods, except books, food, and medical products that are exempt. Import duty is an additional sales tax applicable on all imported goods at a rate of 5%, with no exemptions.
When I purchase items I receive a receipt which lists the name of all the items and their price (including tax), finishing with the total cost of the items, and the total amounts of sales taxes paid. The rounding rules for sales tax are that for a tax rate of n%, a shelf price of p contains (np/100 rounded up to the nearest 0.05) amount of sales tax.
Write an application that prints out the receipt details for these shopping baskets...
INPUT:
Input 1:
1 book at 12.49
1 music CD at 14.99
1 chocolate bar at 0.85
Input 2:
1 imported box of chocolates at 10.00
1 imported bottle of perfume at 47.50
Input 3:
1 imported bottle of perfume at 27.99
1 bottle of perfume at 18.99
1 packet of headache pills at 9.75
1 box of imported chocolates at 11.25
OUTPUT
Output 1:
1 book : 12.49
1 music CD: 16.49
1 chocolate bar: 0.85
Sales Taxes: 1.50
Total: 29.83
Output 2:
1 imported box of chocolates: 10.50
1 imported bottle of perfume: 54.65
Sales Taxes: 7.65
Total: 65.15
Output 3:
1 imported bottle of perfume: 32.19
1 bottle of perfume: 20.89
1 packet of headache pills: 9.75
1 imported box of chocolates: 11.85
Sales Taxes: 6.70
Total: 74.68
or
git clone https://github.com/antoniolarosa/SalesTaxesProblem.git
cd SalesTaxesProblem
dotnet publish -c Release
go to the path:
\SalesTaxesProblem\SalesTaxes\bin\Release\netcoreapp2.0\publish\
There is a Inputs folder with 3 txt files.
The appsettings.json has the following default configuration for input files:
"input": {
"folder": "Inputs",
"fileName" : "Input1.txt"
}change the fileName to test the input you prefer.
dotnet SalesTaxes.dll
go to the path:
\SalesTaxesProblem\SalesTaxes.Fixture
dotnet test
- Input file is correct and valid.
- 1 product has one and only one category.
- The price in the input file is related with the single product. For example
1 book at 12.49, 12.49 is the price of one book. If in input I want to write 2 books I will update that string in this way2 book at 12.49
The core of this software is the engine for tax calculations. There are a lot of responsabilities to handle, so I'll explain how I split them in different classes.
I add to a target product the logic of tax calculation at runtime with the Decorator pattern. So a concrete decorator has the responsability to handle business logic for tax calculation.
I create Tax base class for handling 2 other responsabilities:
- Logic for tax applicability
- Instantiate the right concrete decorator
public abstract class Tax
{
public string Description { get; set; }
public abstract bool IsApplicableFor(IProduct product);
public abstract ProductDecorator GetTaxableProductDecorator(IProduct product);
}As an example this is an extension of Tax:
public class ImportedTax : Tax
{
public decimal Rate { get; set; }
public override bool IsApplicableFor(IProduct product)
{
if (product == null) throw new ArgumentNullException(nameof(product));
return product.IsImported;
}
public override ProductDecorator GetTaxableProductDecorator(IProduct product)
{
if (product == null) throw new ArgumentNullException(nameof(product));
return new FlatTaxDecorator(product, Description, Rate);
}
}Then I need a TaxCalculator which orchestrates everything:
public IList<TaxedProduct> ApplyTaxes(ShoppingBasket shoppingBasket)
{
if (shoppingBasket == null) throw new ArgumentNullException(nameof(shoppingBasket));
IList<TaxedProduct> taxedProducts = new List<TaxedProduct>();
for (var i = 0; i < shoppingBasket.Products.Count; i++)
{
IProduct product = shoppingBasket.Products[i];
foreach (Tax taxRule in _taxRules)
{
if (taxRule.IsApplicableFor(product))
{
ProductDecorator productDecorator = taxRule.GetTaxableProductDecorator(product);
product = productDecorator;
}
}
decimal grossAmount = product.CalculateGrossAmount();
taxedProducts.Add(new TaxedProduct(product, grossAmount));
}
return taxedProducts;
}Running time analysis of this method is O(number of taxes * number of products in shopping cart), which is a linear running time considering that the number of taxes is constant:
O(constant * number of products) = O(number of products)
I want to allow the possibility to update parts of the software without the need to do a new deploy. That's why some of the information is configurable in appsettings.json.
Now I can handle a lot of use cases. For example:
-
What if I want to change a tax rate?
- I can update the
appsettings.json. No deploy needed.
- I can update the
-
What if I want to remove a category from excluded categories in
Basic Sales Tax?- I can update the appsettings.json. No deploy needed.
-
What if I want to add a category?
- I can't just update the
appsettings.jsonbecause every cagegory is mapped to aCategoryType. Each category is not a string but an enumeration, something more than a string due to its business value.
- I can't just update the
-
What if I want to add a new tax of the same type (same applicability logic, same concrete decoretor, different description and parameters) to an existing one? For example a new flat tax.
- I can update the
appsettings.json. No deploy needed.
- I can update the
-
What if I want to insert a new tax?
- add a new section in appsettings.json/taxes
- update the
TaxSettingsclass - create a new class that inherits from
Tax - if necessary, create a new concrete decorator
The only class that I update is TaxSettings. I don't touch the tax calculator engine. I just create new classes.
In this way I'm following the open/closed principle.