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Mkerrigan's solution

to Book Store in the Java Track

Published at Jul 03 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.

Setup

Go through the setup instructions for Java to install the necessary dependencies:

https://exercism.io/tracks/java/installation

Running the tests

You can run all the tests for an exercise by entering the following in your terminal:

$ gradle test

Use gradlew.bat if you're on Windows

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by removing the @Ignore("Remove to run test") annotation.

Source

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

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

BookStoreTest.java

import static org.junit.Assert.assertEquals;

import java.util.Collections;
import java.util.List;
import java.util.Arrays;

import org.junit.Before;
import org.junit.Ignore;
import org.junit.Test;

public class BookStoreTest {

    // This is sufficient accuracy since we're handling currency values, which should be equal
    // to within 2 decimal places.
    private static final double EQUALITY_TOLERANCE = 0.001;

    private BookStore bookStore;

    @Before
    public void setUp() {
        bookStore = new BookStore();
    }

    @Test
    public void onlyASingleBook() {
        List<Integer> books = Collections.singletonList(1);
        assertEquals(8.00, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void twoOfSameBook() {
        List<Integer> books = Arrays.asList(2, 2);
        assertEquals(16.00, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void emptyBasket() {
        List<Integer> books = Collections.emptyList();
        assertEquals(0.00, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void twoDifferentBooks() {
        List<Integer> books = Arrays.asList(1, 2);
        assertEquals(15.20, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void threeDifferentBooks() {
        List<Integer> books = Arrays.asList(1, 2, 3);
        assertEquals(21.60, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void fourDifferentBooks() {
        List<Integer> books = Arrays.asList(1, 2, 3, 4);
        assertEquals(25.60, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void fiveDifferentBooks() {
        List<Integer> books = Arrays.asList(1, 2, 3, 4, 5);
        assertEquals(30.00, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void twoGroupsOfFourIsCheaperThanGroupOfFivePlusGroupOfThree() {
        List<Integer> books = Arrays.asList(1, 1, 2, 2, 3, 3, 4, 5);
        assertEquals(51.20, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void twoGroupsOfFourIsCheaperThanGroupsOfFiveAndThree() {
        List<Integer> books = Arrays.asList(1, 1, 2, 3, 4, 4, 5, 5);
        assertEquals(51.20, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void groupOfFourPlusGroupOfTwoIsCheaperThanTwoGroupsOfThree() {
        List<Integer> books = Arrays.asList(1, 1, 2, 2, 3, 4);
        assertEquals(40.80, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void twoEachOfFirst4BooksAnd1CopyEachOfRest() {
        List<Integer> books = Arrays.asList(1, 1, 2, 2, 3, 3, 4, 4, 5);
        assertEquals(55.60, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void twoCopiesOfEachBook() {
        List<Integer> books = Arrays.asList(1, 1, 2, 2, 3, 3, 4, 4, 5, 5);
        assertEquals(60.00, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void threeCopiesOfFirstBookAnd2EachOfRemaining() {
        List<Integer> books = Arrays.asList(1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1);
        assertEquals(68.00, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void threeEachOFirst2BooksAnd2EachOfRemainingBooks() {
        List<Integer> books = Arrays.asList(1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1, 2);
        assertEquals(75.20, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }

    @Ignore("Remove to run test")
    @Test
    public void fourGroupsOfFourAreCheaperThanTwoGroupsEachOfFiveAndThree() {
        List<Integer> books = Arrays.asList(1, 1, 2, 2, 3, 3, 4, 5, 1, 1, 2, 2, 3, 3, 4, 5);
        assertEquals(102.4, bookStore.calculateBasketCost(books), EQUALITY_TOLERANCE);
    }
}

src/main/java/BookSeries.java

import java.util.HashSet;
import java.util.Set;

public class BookSeries implements Comparable<BookSeries> {
    private static final double BOOK_PRICE = 8.000;
    private Set booksInSeries = new HashSet<Integer>();
    private int maxSeriesSize;

    public BookSeries(int size){
        maxSeriesSize = size;
    }

    public boolean addBookToSeries(int bookNum){
        if (booksInSeries.size() < maxSeriesSize){
            return booksInSeries.add(bookNum);
        }
        return false;
    }

    public double getSeriesPrice(){
        double discount;
        switch (booksInSeries.size()){
            case 0:
                discount = 0.000;
                break;
            case 1:
                discount = 0.000;
                break;
            case 2:
                discount = 0.050;
                break;
            case 3:
                discount = 0.100;
                break;
            case 4:
                discount = 0.200;
                break;
            case 5:
                discount = 0.250;
                break;
            default:
                discount = 0.250;
        }
        return (1.000d - discount) * booksInSeries.size() * BOOK_PRICE;
    }

    @Override
    public int compareTo(BookSeries otherSeries) {
        if (this.booksInSeries.size() == otherSeries.booksInSeries.size())
            return 0;
        else if (this.booksInSeries.size() > otherSeries.booksInSeries.size()) {
            return 1;
        }else {
            return -1;
        }
    }
}

src/main/java/BookStore.java

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class BookStore {
    public double calculateBasketCost(List<Integer> books) {
        double lowestPrice = Double.MAX_VALUE;
        System.out.println("SIZE 4 Greedy ******");
        if (calculatePriceAtSeriesSize(books, 4, true) < lowestPrice) {
            lowestPrice = calculatePriceAtSeriesSize(books, 4, true);
        }
        System.out.println("SIZE 5 Greedy ******");
        if (calculatePriceAtSeriesSize(books, 5, true) < lowestPrice) {
            lowestPrice = calculatePriceAtSeriesSize(books, 5, true);
        }
        System.out.println("SIZE 4 Not Greedy ******");
        if (calculatePriceAtSeriesSize(books, 4, false) < lowestPrice) {
            lowestPrice = calculatePriceAtSeriesSize(books, 4, false);
        }
        System.out.println("SIZE 5 Not Greedy ******");
        if (calculatePriceAtSeriesSize(books, 5, false) < lowestPrice) {
            lowestPrice = calculatePriceAtSeriesSize(books, 5, false);
        }

        return lowestPrice;
    }

    private double calculatePriceAtSeriesSize(List<Integer> books, int size, boolean isGreedy) {
        int sortOrder = isGreedy ? -1 : 1;
        List<BookSeries> seriesList = new ArrayList<>();
        seriesList.add(new BookSeries(size));
        for (Integer book : books) {
            boolean wasAddedToSeries = false;
            for (BookSeries bookSeries : seriesList) {
                if (bookSeries.addBookToSeries(book)) {
                    System.out.printf("Added Book %d to series copy %d \n", book, bookSeries.hashCode());
                    seriesList.sort((o1, o2) -> sortOrder);
                    wasAddedToSeries = true;
                    break;
                }
            }
            if (!wasAddedToSeries) {
                BookSeries series = new BookSeries(size);
                series.addBookToSeries(book);
                seriesList.add(series);
                seriesList.sort((o1, o2) -> sortOrder);
                System.out.printf("Added Book %d to series copy %d \n", book, series.hashCode());
            }

        }
        return priceSeriesList(seriesList);
    }

    private double priceSeriesList(List<BookSeries> seriesList) {
        double totalCost = 0.00;
        for (BookSeries series : seriesList) {
            totalCost += series.getSeriesPrice();
        }
        return totalCost;
    }
}

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