simplify MBC ROM spec and make have reader for ROM ram

lib/waterfoul/mbc/rom.rb

@@ -3,15 +3,17 @@ module Waterfoul
     class ROM
       EXTERNAL_RAM_SIZE = 0x1FFF
 
+      attr_reader :ram
+
       def initialize(program)
-        @external_ram = Array.new EXTERNAL_RAM_SIZE, 0
+        @ram = Array.new EXTERNAL_RAM_SIZE, 0
         @program = program
       end
 
       def [](i)
         case i
         when 0xA000...0xC000
-          @external_ram[i - 0xA000]
+          @ram[i - 0xA000]
         else
           @program[i]
         end
@@ -20,7 +22,7 @@ module Waterfoul
       def []=(i,v)
         case i
         when 0xA000...0xC000
-          @external_ram[i - 0xA000] = v
+          @ram[i - 0xA000] = v
         end
       end
     end

spec/mbc/rom_spec.rb

@@ -2,39 +2,20 @@ require 'spec_helper'
 
 describe Waterfoul::MBC::ROM do
 
-  let(:game_program) { double :game_program }
-  let(:eram) { double :eram }
-
-  subject { Waterfoul::MBC::ROM.new game_program, eram }
+  let(:game_program) { Array.new 65_536, 0 }
+  subject { Waterfoul::MBC::ROM.new game_program }
 
   describe '#[]' do
-    let(:addr) { 0xA1B }
-    it 'reads byte at given addr' do
-      expect(game_program).to receive(:[]).with(addr)
-      subject[addr]
-    end
-
-    context 'reading from external address space' do
-      let(:addr) { 0xA1FB }
-      it 'reads byte from external memory' do
-        expect(eram).to receive(:[]).with(addr)
-        subject[addr]
-      end
-
-      context 'writing to addr 0x123' do
-        let(:addr) { 0x1 }
-        it 'raises an error writng to forbidden space' do
-          allow(eram).to receive(:[]=)
-          expect { subject[addr] = 5 }.to raise_error
-        end
-      end
+    before { game_program[0x151] = 0x51 }
+    it 'reads byte at address from the game program' do
+      expect(subject[0x151]).to eq 0x51
     end
   end
 
   describe '#[]=' do
-    it 'writes byte to external ram with 0xA000 offset' do
-      expect(eram).to receive(:[]=).with(0x1BF, 5)
-      subject[0xA1BF] = 5
+    it 'stores byte in external ram' do
+      subject[0xA001] = 0x8F
+      expect(subject.ram[0x1]).to eq 0x8F
     end
   end
 end