A first stab at rebasing

lib/snapshot.rb

@@ -1,5 +1,4 @@
 require_relative 'db'
-require 'yajl'
 
 class Snapshot
   def self.last
@@ -11,10 +10,31 @@ class Snapshot
   end
 
   def to_base(base)
+    rebased_rates = rates
+
+    unless base == 'EUR'
+      base_rate = rebased_rates
+        .update('EUR' => 1.0)
+        .delete base
+
+      rebased_rates.each do |iso_code, rate|
+        new_rate = rate / base_rate
+        rebased_rates[iso_code] =
+          case new_rate
+          when new_rate > 100
+            new_rate.round 2
+          when new_rate > 10
+            new_rate.round 3
+          else
+            new_rate.round 4
+          end
+      end
+    end
+
     {
       base:  base,
       date:  @date,
-      rates: rates
+      rates: rebased_rates
     }
   end
 

spec/rebasing_spec.rb

@@ -0,0 +1,29 @@
+require_relative 'helper'
+require 'snapshot'
+
+describe 'when rebased to a new base' do
+  let(:base)     { 'USD' }
+  let(:snapshot) { Snapshot.new 'a date' }
+  let(:rates)    { { 'USD' => 1.2781 } }
+  let(:rebased) do
+    snapshot.stub :rates, rates do
+      snapshot.to_base base
+    end
+  end
+
+  it 'resets base' do
+    rebased[:base].must_equal base
+  end
+
+  it 'adds former base to rates' do
+    rebased[:rates].keys.must_include 'EUR'
+  end
+
+  it 'removes new base from rates' do
+    rebased[:rates].keys.wont_include base
+  end
+
+  it 'rebases rates' do
+    rebased[:rates]['EUR'].must_equal 0.7824
+  end
+end