core/src/test/java/org/apache/commons/functor/example/kata/one/SupermarketPricingExample.java

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 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
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 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
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 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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package org.apache.commons.functor.example.kata.one;

import static org.junit.Assert.assertEquals;

import org.apache.commons.functor.Function;
import org.apache.commons.functor.adapter.BinaryFunctionFunction;
import org.apache.commons.functor.core.Identity;
import org.apache.commons.functor.core.comparator.IsGreaterThan;
import org.apache.commons.functor.core.composite.Composite;
import org.apache.commons.functor.core.composite.ConditionalFunction;
import org.apache.commons.functor.core.composite.CompositeBinaryFunction;
import org.junit.Test;

/**
 * Dave Thomas's Kata One asks us to think about how one might
 * implement pricing rules:
 *
 * "Some things in supermarkets have simple prices: this can of
 * beans costs $0.65. Other things have more complex prices.
 * For example:
 *
 * o three for a dollar (so what?s the price if I buy 4, or 5?)
 *
 * o $1.99/pound (so what does 4 ounces cost?)
 *
 * o buy two, get one free (so does the third item have a price?)"
 *
 * Functors provide one approach to this sort of problem, and in
 * this example we'll demonstrate some simple cases.
 *
 * See http://pragprog.com/pragdave/Practices/Kata/KataOne.rdoc,v
 * for more information on this Kata.
 *
 * @version $Revision$ $Date$
 */
public class SupermarketPricingExample {

    // tests
    //----------------------------------------------------------

    /*
     * The simplest form of pricing is simply a constant
     * rate.  In Dave's example, a can of beans costs $0.65,
     * and n cans of beans cost n*0.65.
     *
     * This pricing rule simply multiplies the quantity by
     * a constant, e.g.:
     *   ToMoney.from(Multiply.by(65))
     *
     * This case is so common, we may want to introduce a
     * special Product constructor to wrap up create the
     * functors for us.
     */
    @Test
    public void testConstantPricePerUnit() throws Exception {
        {
            Product beans = new Product(
                "Can of Beans",
                "SKU-0001",
                ToMoney.from(Multiply.by(65)));

            assertEquals(new Money(0*65),beans.getPrice(0));
            assertEquals(new Money(1*65),beans.getPrice(1));
            assertEquals(new Money(2*65),beans.getPrice(2));
            assertEquals(new Money(3*65),beans.getPrice(3));
        }
        // or, using the speical constructor:
        {
            Product beans = new Product(
                "Can of Beans",
                "SKU-0001",
                65);

            assertEquals(new Money(0*65),beans.getPrice(0));
            assertEquals(new Money(1*65),beans.getPrice(1));
            assertEquals(new Money(2*65),beans.getPrice(2));
            assertEquals(new Money(3*65),beans.getPrice(3));
        }
    }

    /*
     * A slighly more complicated example is a bulk
     * discount.  For example, bananas may be
     * $0.33 cents each, or 4 for a dollar ($1.00).
     *
     * This rule is underspecified by itself, there are
     * at least two ways to interpret this pricing rule:
     *
     * a) the cost is $0.33 cents for 3 or fewer, $0.25
     *    for 4 or more
     *
     * or
     *
     * b) the cost is $1.00 for every group of 4, $0.33
     *    each for anything left over
     *
     * although I think in practice, "4 for a dollar"
     * usually means the former and not the latter.
     *
     * We can implement either:
     */
    @Test
    public void testFourForADollar_A() throws Exception {
        Product banana = new Product(
            "Banana",
            "SKU-0002",
            ToMoney.from(
                new ConditionalFunction<Integer, Number>(
                    IsGreaterThan.instance(new Integer(3)),
                    Multiply.by(25),
                    Multiply.by(33))));

        assertEquals(new Money(0*33),banana.getPrice(0));
        assertEquals(new Money(1*33),banana.getPrice(1));
        assertEquals(new Money(2*33),banana.getPrice(2));
        assertEquals(new Money(3*33),banana.getPrice(3));
        assertEquals(new Money(4*25),banana.getPrice(4));
        assertEquals(new Money(5*25),banana.getPrice(5));
        assertEquals(new Money(6*25),banana.getPrice(6));
        assertEquals(new Money(7*25),banana.getPrice(7));
        assertEquals(new Money(8*25),banana.getPrice(8));
    }


    @Test
    public void testFourForADollar_B() throws Exception {
        Product banana = new Product(
            "Banana",
            "SKU-0002",
            ToMoney.from(
                new BinaryFunctionFunction<Integer, Number>(
                    new CompositeBinaryFunction<Integer, Integer, Number>(
                        Add.instance(),
                        Composite.function(
                            Multiply.by(100),
                            Divide.by(4)),
                        Composite.function(
                            Multiply.by(33),
                            Mod.by(4))))));
        assertEquals(new Money(0*33+0*25),banana.getPrice(0));
        assertEquals(new Money(1*33+0*25),banana.getPrice(1));
        assertEquals(new Money(2*33+0*25),banana.getPrice(2));
        assertEquals(new Money(3*33+0*25),banana.getPrice(3));
        assertEquals(new Money(0*33+4*25),banana.getPrice(4));
        assertEquals(new Money(1*33+4*25),banana.getPrice(5));
        assertEquals(new Money(2*33+4*25),banana.getPrice(6));
        assertEquals(new Money(3*33+4*25),banana.getPrice(7));
        assertEquals(new Money(0*33+8*25),banana.getPrice(8));
    }


    /*
     * Another interesting pricing rule is
     * something like "buy 2, get 1 free".
     *
     * This may be implemented using a formula
     * like:
     *   costPerUnit * (quantity - quantity / 2)
     *
     * For example...
     */
    @Test
    public void testBuyTwoGetOneFree_1() throws Exception {
        Product apple = new Product(
            "Apple",
            "SKU-0003",
            ToMoney.from(
                    Composite.function(Multiply.by(40),
                    BinaryFunctionFunction.adapt(new CompositeBinaryFunction<Number, Number, Number>(Subtract.instance(),
                            new Identity<Number>(),
                            Divide.by(3))))));

        assertEquals(new Money(0*40),apple.getPrice(0));
        assertEquals(new Money(1*40),apple.getPrice(1));
        assertEquals(new Money(2*40),apple.getPrice(2));
        assertEquals(new Money(2*40),apple.getPrice(3));
        assertEquals(new Money(3*40),apple.getPrice(4));
        assertEquals(new Money(4*40),apple.getPrice(5));
        assertEquals(new Money(4*40),apple.getPrice(6));
        assertEquals(new Money(5*40),apple.getPrice(7));
        assertEquals(new Money(6*40),apple.getPrice(8));
        assertEquals(new Money(6*40),apple.getPrice(9));
        assertEquals(new Money(7*40),apple.getPrice(10));
    }

    /*
     * ...but our pricing rule is starting to get ugly,
     * and we haven't even considered things
     * something like "buy 3, get 2 free", etc.
     *
     * Perhaps a special Function instance is in
     * order:
     */

    class BuyNGetMFree implements Function<Number, Number> {
       public BuyNGetMFree(int n, int m, int costPerUnit) {
           this.n = n;
           this.m = m;
           this.costPerUnit = costPerUnit;
       }

       public Number evaluate(Number num) {
           int quantity = num.intValue();
           int cost = 0;

           while(quantity >= n) {
               // buy n
               cost += n * costPerUnit;
               quantity -= n;
               // get m (or fewer) free
               quantity -= Math.min(quantity,m);
           }
           // buy less than n
           cost += quantity * costPerUnit;

           return new Integer(cost);
       }

       private int n, m, costPerUnit;
   }

    @Test
    public void testBuyTwoGetOneFree_2() throws Exception {
        Product apple = new Product(
            "Apple",
            "SKU-0003",
            ToMoney.from(new BuyNGetMFree(2,1,40)));

        assertEquals(new Money(0*40),apple.getPrice(0));
        assertEquals(new Money(1*40),apple.getPrice(1));
        assertEquals(new Money(2*40),apple.getPrice(2));
        assertEquals(new Money(2*40),apple.getPrice(3));
        assertEquals(new Money(3*40),apple.getPrice(4));
        assertEquals(new Money(4*40),apple.getPrice(5));
        assertEquals(new Money(4*40),apple.getPrice(6));
        assertEquals(new Money(5*40),apple.getPrice(7));
        assertEquals(new Money(6*40),apple.getPrice(8));
        assertEquals(new Money(6*40),apple.getPrice(9));
        assertEquals(new Money(7*40),apple.getPrice(10));
    }

    @Test
    public void testBuyThreeGetTwoFree() throws Exception {
        Product apple = new Product(
            "Apple",
            "SKU-0003",
            ToMoney.from(new BuyNGetMFree(3,2,40)));

        assertEquals(new Money(0*40),apple.getPrice(0));
        assertEquals(new Money(1*40),apple.getPrice(1));
        assertEquals(new Money(2*40),apple.getPrice(2));
        assertEquals(new Money(3*40),apple.getPrice(3));
        assertEquals(new Money(3*40),apple.getPrice(4));
        assertEquals(new Money(3*40),apple.getPrice(5));
        assertEquals(new Money(4*40),apple.getPrice(6));
        assertEquals(new Money(5*40),apple.getPrice(7));
        assertEquals(new Money(6*40),apple.getPrice(8));
        assertEquals(new Money(6*40),apple.getPrice(9));
        assertEquals(new Money(6*40),apple.getPrice(10));
        assertEquals(new Money(7*40),apple.getPrice(11));
    }

    @Test
    public void testBuyTwoGetFiveFree() throws Exception {
         Product apple = new Product(
             "Apple",
             "SKU-0003",
             ToMoney.from(new BuyNGetMFree(2,5,40)));

         assertEquals(new Money(0*40),apple.getPrice(0));
         assertEquals(new Money(1*40),apple.getPrice(1));
         assertEquals(new Money(2*40),apple.getPrice(2));
         assertEquals(new Money(2*40),apple.getPrice(3));
         assertEquals(new Money(2*40),apple.getPrice(4));
         assertEquals(new Money(2*40),apple.getPrice(5));
         assertEquals(new Money(2*40),apple.getPrice(6));
         assertEquals(new Money(2*40),apple.getPrice(7));
         assertEquals(new Money(3*40),apple.getPrice(8));
         assertEquals(new Money(4*40),apple.getPrice(9));
         assertEquals(new Money(4*40),apple.getPrice(10));
         assertEquals(new Money(4*40),apple.getPrice(11));
         assertEquals(new Money(4*40),apple.getPrice(12));
         assertEquals(new Money(4*40),apple.getPrice(13));
         assertEquals(new Money(4*40),apple.getPrice(14));
         assertEquals(new Money(5*40),apple.getPrice(15));
     }
}