# josephfash11's solution

## to Book Store in the C# Track

Published at Mar 22 2019 · 0 comments
Instructions
Test suite
Solution

To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts on multiple book purchases.

One copy of any of the five books costs \$8.

If, however, you buy two different books, you get a 5% discount on those two books.

If you buy 3 different books, you get a 10% discount.

If you buy 4 different books, you get a 20% discount.

If you buy all 5, you get a 25% discount.

Note: that if you buy four books, of which 3 are different titles, you get a 10% discount on the 3 that form part of a set, but the fourth book still costs \$8.

Your mission is to write a piece of code to calculate the price of any conceivable shopping basket (containing only books of the same series), giving as big a discount as possible.

For example, how much does this basket of books cost?

• 2 copies of the first book
• 2 copies of the second book
• 2 copies of the third book
• 1 copy of the fourth book
• 1 copy of the fifth book

One way of grouping these 8 books is:

• 1 group of 5 --> 25% discount (1st,2nd,3rd,4th,5th)
• +1 group of 3 --> 10% discount (1st,2nd,3rd)

This would give a total of:

• 5 books at a 25% discount
• +3 books at a 10% discount

Resulting in:

• 5 x (8 - 2.00) == 5 x 6.00 == \$30.00
• +3 x (8 - 0.80) == 3 x 7.20 == \$21.60

For a total of \$51.60

However, a different way to group these 8 books is:

• 1 group of 4 books --> 20% discount (1st,2nd,3rd,4th)
• +1 group of 4 books --> 20% discount (1st,2nd,3rd,5th)

This would give a total of:

• 4 books at a 20% discount
• +4 books at a 20% discount

Resulting in:

• 4 x (8 - 1.60) == 4 x 6.40 == \$25.60
• +4 x (8 - 1.60) == 4 x 6.40 == \$25.60

For a total of \$51.20

And \$51.20 is the price with the biggest discount.

## Running the tests

To run the tests, run the command `dotnet test` from within the exercise directory.

Initially, only the first test will be enabled. This is to encourage you to solve the exercise one step at a time. Once you get the first test passing, remove the `Skip` property from the next test and work on getting that test passing. Once none of the tests are skipped and they are all passing, you can submit your solution using `exercism submit BookStore.cs`

## Further information

For more detailed information about the C# track, including how to get help if you're having trouble, please visit the exercism.io C# language page.

## Source

Inspired by the harry potter kata from Cyber-Dojo. http://cyber-dojo.org

### BookStoreTest.cs

``````// This file was auto-generated based on version 1.4.0 of the canonical data.

using System;
using Xunit;

public class BookStoreTest
{
[Fact]
public void Only_a_single_book()
{
var basket = new[] { 1 };
}

[Fact(Skip = "Remove to run test")]
public void Two_of_the_same_book()
{
var basket = new[] { 2, 2 };
}

[Fact(Skip = "Remove to run test")]
{
}

[Fact(Skip = "Remove to run test")]
public void Two_different_books()
{
var basket = new[] { 1, 2 };
}

[Fact(Skip = "Remove to run test")]
public void Three_different_books()
{
var basket = new[] { 1, 2, 3 };
}

[Fact(Skip = "Remove to run test")]
public void Four_different_books()
{
var basket = new[] { 1, 2, 3, 4 };
}

[Fact(Skip = "Remove to run test")]
public void Five_different_books()
{
var basket = new[] { 1, 2, 3, 4, 5 };
}

[Fact(Skip = "Remove to run test")]
public void Two_groups_of_four_is_cheaper_than_group_of_five_plus_group_of_three()
{
var basket = new[] { 1, 1, 2, 2, 3, 3, 4, 5 };
}

[Fact(Skip = "Remove to run test")]
public void Two_groups_of_four_is_cheaper_than_groups_of_five_and_three()
{
var basket = new[] { 1, 1, 2, 3, 4, 4, 5, 5 };
}

[Fact(Skip = "Remove to run test")]
public void Group_of_four_plus_group_of_two_is_cheaper_than_two_groups_of_three()
{
var basket = new[] { 1, 1, 2, 2, 3, 4 };
}

[Fact(Skip = "Remove to run test")]
public void Two_each_of_first_4_books_and_1_copy_each_of_rest()
{
var basket = new[] { 1, 1, 2, 2, 3, 3, 4, 4, 5 };
}

[Fact(Skip = "Remove to run test")]
public void Two_copies_of_each_book()
{
var basket = new[] { 1, 1, 2, 2, 3, 3, 4, 4, 5, 5 };
}

[Fact(Skip = "Remove to run test")]
public void Three_copies_of_first_book_and_2_each_of_remaining()
{
var basket = new[] { 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1 };
}

[Fact(Skip = "Remove to run test")]
public void Three_each_of_first_2_books_and_2_each_of_remaining_books()
{
var basket = new[] { 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1, 2 };
}

[Fact(Skip = "Remove to run test")]
public void Four_groups_of_four_are_cheaper_than_two_groups_each_of_five_and_three()
{
var basket = new[] { 1, 1, 2, 2, 3, 3, 4, 5, 1, 1, 2, 2, 3, 3, 4, 5 };
}
}``````
``````﻿using System;
using System.Collections.Generic;
using System.Linq;

public static class BookStore
{
private static readonly decimal[] Discounts = new[] { 1m, 0.95m, 0.90m, 0.80m, 0.75m };
private static readonly decimal BasicPrice = 8;

private static decimal GroupPrice(int groupSize) => groupSize * BasicPrice * Discounts[groupSize - 1];

public static decimal Total(IEnumerable<int> books)
{
books = books.GroupBy(i => i).OrderByDescending(g => g.Count()).SelectMany(g => g);
}

private static decimal BasketTotal(IEnumerable<int> books, decimal cumulativePrice)
{
if (!books.Any())
return cumulativePrice;

var groups = books.Distinct();
var cheapestPrice = Decimal.MaxValue;

for (int i = 0; i < groups.Count(); i++)
{
var currentGroups = groups.Take(i + 1);
var currentBooks = new List<int>(books);
foreach (var book in currentGroups)
{
currentBooks.Remove(book);
}
var groupPrice = GroupPrice(i + 1);
var currentPrice = BasketTotal(currentBooks, cumulativePrice + groupPrice);

if (currentPrice < cheapestPrice)
cheapestPrice = currentPrice;
}

return cheapestPrice;
}

}``````