Implement Converter

The API is fairly simple in order to get straight to the goal, convert
an amount from a currency to another.
I used the proposal from #25 which is similar to the google currency
tool.

lib/converter.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require 'bigdecimal'
+require 'quote'
+
+class Converter
+  attr_reader :to, :from
+
+  def initialize(params = {})
+    @amount = params[:amount]
+    @from   = params[:from]
+    @to     = params[:to]
+  end
+
+  # Converts an amount into a new currency
+  # @param quote [Quote] the quote object
+  # @return amount [BigDecimal] the converted amount
+  def convert(quote = latest_quote)
+    return amount if quote.base == to
+    amount * quote.rates.fetch(to) { raise Quote::Invalid, 'Invalid to' }
+  end
+
+  def amount
+    BigDecimal(@amount.to_s)
+  end
+
+  private
+
+  def latest_quote
+    @latest_quote ||= Quote.new(base: from)
+  end
+end

spec/converter_spec.rb

@@ -0,0 +1,45 @@
+require_relative 'helper'
+require 'converter'
+
+describe Converter do
+  let(:to) { 'USD' }
+  let(:from) { 'EUR' }
+  let(:amount) { '100.0' }
+
+  let(:converter) { Converter.new to: to, from: from, amount: amount }
+  let(:quote) { Quote.new }
+
+  describe '#amount' do
+    it 'casts to BigDecimal' do
+      converter.amount.must_be_kind_of BigDecimal
+    end
+  end
+
+  describe '#convert' do
+    it 'should return the amount' do
+      quote.stub :base, to do
+        converter.convert(quote).must_equal BigDecimal(amount)
+      end
+    end
+
+    it 'should use the rates to convert' do
+      quote.stub :rates, Hash(to => 1.2) do
+        converter.convert(quote).must_equal BigDecimal('120.0')
+      end
+    end
+
+    it 'should raise with invalid currency' do
+      quote.stub :rates, Hash.new do
+        -> { converter.convert(quote) }.must_raise Quote::Invalid, 'Invalid to'
+      end
+    end
+
+    it 'should use latest quote as default' do
+      quote.stub :rates, Hash(to => 1.3) do
+        converter.stub :latest_quote, quote do
+          converter.convert.must_equal 130.0
+        end
+      end
+    end
+  end
+end