# PayBillsContext2.java

    /*
     * Copyright (c) 2010, Rickard Öberg. All Rights Reserved.
     *
     * Licensed under the Apache License, Version 2.0 (the "License");
     * you may not use this file except in compliance with the License.
     * You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
     * Unless required by applicable law or agreed to in writing, software
     * 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
     * limitations under the License.
     *
     */

    package org.qi4j.dci.moneytransfer.context;

    import java.util.ArrayList;
    import java.util.List;
    import org.qi4j.api.injection.scope.Structure;
    import org.qi4j.api.unitofwork.UnitOfWorkFactory;
    import org.qi4j.dci.moneytransfer.domain.data.BalanceData;

    /**
     * Context for paying bills from an account to a list of creditor accounts.
     * <p>
     * Roles are defined within the context.
     * A RoleMap lists what Roles an entity can play.
     * </p>
     */
    public class PayBillsContext2
    {
        @Structure
        UnitOfWorkFactory uowf;

        private SourceAccountRole sourceAccount = new SourceAccountRole();

        public PayBillsContext2 bind( BalanceData account )
        {
            sourceAccount.bind( account );
            return this;
        }

        public void payBills()
            throws Exception
        {
            sourceAccount.payBills();
        }

        /**
         * The SourceAccountRole orchestrates the Pay Bills use case interactions.
         * <p>
         * Code matches the use case text carefully (see references below).
         * </p>
         * <p>
         * Pay Bills use case scenario:
         * </p>
         * <p>
         * 1) Bank finds creditors (could be a use case scenario in itself)
         * </p>
         * <p>
         * 2) Bank calculates the amount owed to creditors
         * </p>
         * <p>
         * 3) Bank verifies sufficient funds
         * </p>
         * <p>
         * 4) Bank transfer money to each creditor
         * </p>
         * <p>
         * Algorithm (steps to implement the scenario):
         * </p>
         * <p>
         * <p>
         * 1a) Source Account finds list of creditors
         * </p>
         * <p>
         * 2a) Source Account loops creditors to find the sum owed
         * </p>
         * <p>
         * 3a) Source Account verifies that its current balance is greater than the sum owed, and throws an exception if not
         * </p>
         * <p>
         * 4a) Source Account loops creditors
         * <p>
         * 4b) Make a MoneyTransfer of the amount owed to each creditor
         * </p>
         */
        class SourceAccountRole
            extends Role<BalanceData>
        {
            void payBills()
                throws IllegalArgumentException
            {
                List<BalanceData> creditors = getCreditors();                                             // 1a

                Integer sumOwed = getSumOwedTo( creditors );                                              // 2a

                if( self.getBalance() - sumOwed < 0 )                                                      // 3a
                {
                    throw new IllegalArgumentException( "Insufficient funds to pay bills." );
                }

                final TransferMoneyContext2 transferMoney = new TransferMoneyContext2();
                for( BalanceData creditor : creditors )                                                    // 4a
                {
                    if( creditor.getBalance() < 0 )
                    {
                        final Integer amountOwed = -creditor.getBalance();

                        // Bind nested context and execute enactment
                        transferMoney.bind( self, creditor ).transfer( amountOwed );                      // 4b
                    }
                }
            }

            // Internal helper methods to make the code above more readable / comparable to the algorithm text

            private List<BalanceData> getCreditors()
            {
                // Creditor retrieval could be a use case in itself...
                List<BalanceData> creditors = new ArrayList<BalanceData>();
                creditors.add( uowf.currentUnitOfWork().get( BalanceData.class, "BakerAccount" ) );
                creditors.add( uowf.currentUnitOfWork().get( BalanceData.class, "ButcherAccount" ) );
                return creditors;
            }

            private Integer getSumOwedTo( List<BalanceData> creditors )
            {
                Integer sumOwed = 0;
                for( BalanceData creditor : creditors )
                {
                    sumOwed += creditor.getBalance() < 0 ? -creditor.getBalance() : 0;
                }

                return sumOwed;
            }
        }
    }