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crane-yuan's solution

to Book Store in the Java Track

Published at Mar 06 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

You can run all the tests for an exercise by entering

$ gradle test

in your terminal.

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);
    }
}
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class BookStore {

    static final int PRICE = 8;
    static final double[] RATIOS = {-1, 1, 0.95, 0.9, 0.8, 0.75};

    public double calculateBasketCost(List<Integer> books) {
        double total = 0.0;
        List<Integer> cart = new ArrayList<>(books);
        int cartCount = cart.size();
        List<Integer> groups = new ArrayList<>();

        while (cartCount > 0) {
            // find max group of distinct books
            List<Integer> distinct = cart.stream()
                                         .distinct()
                                         .collect(Collectors.toList());
            int c = distinct.size();

            // if we find a group of 3 and we have previously
            // found a group of 5, remove the 5 group and add
            // two groups of 4 as it is a better deal.
            if (c == 3 && groups.contains(5)) {
                groups.remove(Integer.valueOf(5));
                groups.addAll(Arrays.asList(4, 4));
            } else {
                groups.add(c);
            }

            // remove the distinct books that we found from the cart.
            for (Integer d : distinct) {
                cart.remove(d);
            }

            // update the cart count
            cartCount = cart.size();
        }

        for (Integer group : groups) {
            total += group * PRICE * RATIOS[group];
        }
        return total;
    }
}

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