Reduce mixed currency money

src/bank/index.spec.ts

@@ -3,6 +3,13 @@ import Expression, { Sum } from '../expression';
 import Money from '../money';
 
 describe('Bank', () => {
+  describe('rates', () => {
+    it('should return 1 if currencies are the same', () => {
+      const bank: Bank = new Bank();
+      expect(bank.rate('USD', 'USD')).toEqual(1);
+    });
+  });
+
   describe('reduce', () => {
     describe('sum', () => {
       it('should reduce a sum expression', () => {
@@ -13,6 +20,19 @@ describe('Bank', () => {
 
         expect(result).toEqual(Money.dollar(7));
       });
+
+      it('should reduce Money', () => {
+        const bank: Bank = new Bank();
+        const result: Money = bank.reduce(Money.dollar(1), 'USD');
+        expect(result).toEqual(Money.dollar(1));
+      });
+
+      it('should reduce mixed Money currencies', () => {
+        const bank: Bank = new Bank();
+        bank.addRate('CHF', 'USD', 2);
+        const result: Money = bank.reduce(Money.franc(2), 'USD');
+        expect(result).toEqual(Money.dollar(1));
+      });
     });
   });
 });

src/bank/index.ts

@@ -1,9 +1,23 @@
 import Money from '../money';
-import Expression, { Sum } from '../expression';
+import Expression from '../expression';
 
 export default class Bank {
-  reduce(to: Expression, currency: string): Money {
-    const sum: Sum = to as Sum;
-    return sum.reduce(currency);
+  private _rates: Object;
+
+  constructor() {
+    this._rates = {};
+  }
+
+  reduce(source: Expression, to: string): Money {
+    return source.reduce(this, to);
+  }
+
+  rate(from: string, to: string): number {
+    if (from === to) return 1;
+    return this._rates[`${from}-${to}`];
+  }
+
+  addRate(from: string, to: string, rate: number): void {
+    this._rates[`${from}-${to}`] = rate;
   }
 }

src/expression/index.ts

@@ -1,6 +1,9 @@
+import Bank from '../bank';
 import Money from '../money';
 
-export default interface Expression {}
+export default interface Expression {
+  reduce(bank: Bank, to: string): Money;
+}
 
 export class Sum implements Expression {
   private _augend: Money;
@@ -19,7 +22,7 @@ export class Sum implements Expression {
     return this._addend;
   }
 
-  reduce(to: string): Money {
+  reduce(bank: Bank, to: string): Money {
     const amount: number = this.augend.amount + this.addend.amount;
     return new Money(amount, to);
   }

src/money/index.ts

@@ -1,6 +1,7 @@
+import Bank from '../bank';
 import Expression, { Sum } from '../expression';
 
-export default class Money {
+export default class Money implements Expression {
   private _amount: number;
   private _currency: string;
 
@@ -34,6 +35,11 @@ export default class Money {
     return new Sum(this, addend);
   }
 
+  reduce(bank: Bank, to: string): Money {
+    const rate: number = bank.rate(this.currency, to);
+    return new Money(this.amount / rate, to);
+  }
+
   get currency(): string {
     return this._currency;
   }