Choosing coins with least or no change given

Question

I am making a game which consists of coin denominations of $10, $5, $3, and $1. The player may have 0 or more of each type of currency in his inventory with a maximum of 15 coin

Answer 1

I don't know PHP so I've tried it in Java. I hope that is ok as its the algorithm that is important.

My code is as follows:

package stackoverflow.changecalculator;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class ChangeCalculator
{
    List coinsInTil = new ArrayList<>();

    public void setCoinsInTil(List coinsInTil)
    {
        this.coinsInTil = coinsInTil;
    }

    public Map getPaymentDetailsFromCoinsAvailable(final int amountOwed, List inPocketCoins)
    {
        List paid = new ArrayList<>();
        int remaining = amountOwed;

        // Check starting with the largest coin.
        for (Integer coin : inPocketCoins)
            if (remaining > 0 && (remaining - coin) >= 0) {
                    paid.add(coin);
                    remaining = remaining - coin;
            }

        ProcessAlternative processAlternative = new ProcessAlternative(amountOwed, inPocketCoins, paid, remaining).invoke();
        paid = processAlternative.getPaid();
        remaining = processAlternative.getRemaining();

        removeUsedCoinsFromPocket(inPocketCoins, paid);
        int changeOwed = payTheRestWithNonExactAmount(inPocketCoins, paid, remaining);
        List change = calculateChangeOwed(changeOwed);

        Map result = new HashMap<>();
        result.put("paid", paid);
        result.put("change", change);
        return result;
    }

    private void removeUsedCoinsFromPocket(List inPocketCoins, List paid)
    {
        for (int i = 0; i < inPocketCoins.size(); i++) {
            Integer coin = inPocketCoins.get(i);
            if (paid.contains(coin))
                inPocketCoins.remove(i);
        }
    }

    private List calculateChangeOwed(int changeOwed)
    {
        List change = new ArrayList<>();
        if (changeOwed < 0) {
            for (Integer coin : coinsInTil) {
                if (coin + changeOwed == 0) {
                    change.add(coin);
                    changeOwed = changeOwed + coin;
                }
            }
        }
        return change;
    }

    private int payTheRestWithNonExactAmount(List inPocketCoins, List paid, int remaining)
    {
        if (remaining > 0) {
            for (int coin : inPocketCoins) {
                while (remaining > 0) {
                    paid.add(coin);
                    remaining = remaining - coin;
                }
            }
        }
        return remaining;
    }
}

The ProcessAlternative class handles cases where the largest coin doesn't allow us to get a case where there is no change to be returned so we try an alternative.

package stackoverflow.changecalculator;

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

// if any remaining, check if we can pay with smaller coins first.
class ProcessAlternative
{
    private int amountOwed;
    private List inPocketCoins;
    private List paid;
    private int remaining;

    public ProcessAlternative(int amountOwed, List inPocketCoins, List paid, int remaining)
    {
        this.amountOwed = amountOwed;
        this.inPocketCoins = inPocketCoins;
        this.paid = paid;
        this.remaining = remaining;
    }

    public List getPaid()
    {
        return paid;
    }

    public int getRemaining()
    {
        return remaining;
    }

    public ProcessAlternative invoke()
    {
        List alternative = new ArrayList<>();
        int altRemaining = amountOwed;
        if (remaining > 0) {
            for (Integer coin : inPocketCoins)
                if (altRemaining > 0 && factorsOfAmountOwed(amountOwed).contains(coin)) {
                    alternative.add(coin);
                    altRemaining = altRemaining - coin;
                }
            // if alternative doesn't require change, use it.
            if (altRemaining == 0) {
                paid = alternative;
                remaining = altRemaining;
            }
        }
        return this;
    }

    private ArrayList factorsOfAmountOwed(int num)
    {
        ArrayList aux = new ArrayList<>();
        for (int i = 1; i <= num / 2; i++)
            if ((num % i) == 0)
                aux.add(i);
        return aux;
    }
}

I worked in it by doing a test for example 1, then for example 2, and lastly moved on to example 3. The process alternative bit was added here and the alternative for the original test coins returned 0 change required so I updated to the amount input to 15 instead of 12 so it would calculate the change required.

Tests are as follows:

package stackoverflow.changecalculator;

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

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;

public class ChangeCalculatorTest
{
    public static final int FIFTY_PENCE = 0;
    public static final int TWENTY_PENCE = 1;
    public static final int TEN_PENCE = 2;
    public static final int FIVE_PENCE = 3;
    public static final int TWO_PENCE = 4;
    public static final int PENNY = 5;

    public ChangeCalculator calculator;

    @Before
    public void setUp() throws Exception
    {
        calculator = new ChangeCalculator();
        List inTil = new ArrayList<>();
        inTil.add(FIFTY_PENCE);
        inTil.add(TWENTY_PENCE);
        inTil.add(TEN_PENCE);
        inTil.add(FIVE_PENCE);
        inTil.add(TWO_PENCE);
        inTil.add(PENNY);
        calculator.setCoinsInTil(inTil);
    }

    @Test
    public void whenHaveExactAmount_thenNoChange() throws Exception
    {
        // $5, $3, $3, $3, $1, $1, $1, $1
        List inPocket = new ArrayList<>();
        inPocket.add(5);
        inPocket.add(3);
        inPocket.add(3);
        inPocket.add(3);
        inPocket.add(1);
        inPocket.add(1);
        inPocket.add(1);
        inPocket.add(1);

        Map result = calculator.getPaymentDetailsFromCoinsAvailable(12, inPocket);

        List change = result.get("change");
        assertTrue(change.size() == 0);
        List paid = result.get("paid");
        List expected = new ArrayList<>();
        expected.add(5);
        expected.add(3);
        expected.add(3);
        expected.add(1);
        assertEquals(expected, paid);
    }

    @Test
    public void whenDoNotHaveExactAmount_thenChangeReturned() throws Exception {
        // $5, $3, $3, $3, $3
        List inPocket = new ArrayList<>();
        inPocket.add(5);
        inPocket.add(3);
        inPocket.add(3);
        inPocket.add(3);
        inPocket.add(3);

        Map result = calculator.getPaymentDetailsFromCoinsAvailable(15, inPocket);

        List change = result.get("change");
        Object actual = change.get(0);
        assertEquals(2, actual);
        List paid = result.get("paid");
        List expected = new ArrayList<>();
        expected.add(5);
        expected.add(3);
        expected.add(3);
        expected.add(3);
        expected.add(3);
        assertEquals(expected, paid);
    }

    @Test
    public void whenWeHaveExactAmountButItDoesNotIncludeBiggestCoin_thenPayWithSmallerCoins() throws Exception {
        // $5, $3, $3, $3
        List inPocket = new ArrayList<>();
        inPocket.add(5);
        inPocket.add(3);
        inPocket.add(3);
        inPocket.add(3);

        Map result = calculator.getPaymentDetailsFromCoinsAvailable(6, inPocket);

        List change = result.get("change");
        assertTrue(change.size() == 0);
        List paid = result.get("paid");
        List expected = new ArrayList<>();
        expected.add(3);
        expected.add(3);
        assertEquals(expected, paid);
    }
}

The tests are not the cleanest yet but they are all passing thus far. I may go back and add some more test cases later to see if I can break it but don't have time right now.