Convert between CUSIPs and ISINs (#86)

README.md

@@ -116,6 +116,7 @@ isin.valid?                # => true
 isin.valid_format?         # => true
 isin.restore!              # => 'US5949181045'
 isin.calculate_check_digit # => 5
+isin.to_cusip              # => #<SecId::CUSIP>
 ```
 
 ### SecId::CUSIP full example
@@ -137,6 +138,7 @@ cusip.valid?                # => true
 cusip.valid_format?         # => true
 cusip.restore!              # => '594918104'
 cusip.calculate_check_digit # => 4
+cusip.to_isin('US')         # => #<SecId::ISIN>
 ```
 
 ### SecId::SEDOL full example

lib/sec_id/cusip.rb

@@ -26,6 +26,15 @@ def calculate_check_digit
       raise InvalidFormatError, "CUSIP '#{full_number}' is invalid and check-digit cannot be calculated!"
     end
 
+    def to_isin(country_code)
+      ISIN::CGS_COUNTRY_CODES.include?(country_code) \
+        or raise(InvalidFormatError, "'#{country_code}' is not a CGS country code!")
+      restore!
+      isin = ISIN.new(country_code + full_number)
+      isin.restore!
+      isin
+    end
+
     private
 
     # https://en.wikipedia.org/wiki/Luhn_algorithm

lib/sec_id/isin.rb

@@ -26,6 +26,21 @@ def calculate_check_digit
       raise InvalidFormatError, "ISIN '#{full_number}' is invalid and check-digit cannot be calculated!"
     end
 
+    CGS_COUNTRY_CODES = %w[
+      US CA KY BM VI VG UM TT SR GS SX VC MF LC KN BL PR PH PW
+      MP FM YT MH HT GY GU GD DM CW BQ BZ BS AW AG AI AS AN
+    ].freeze
+
+    # CUSIP Global Services
+    def cgs?
+      CGS_COUNTRY_CODES.include?(country_code)
+    end
+
+    def to_cusip
+      cgs? or raise(InvalidFormatError, "'#{country_code}' is not a CGS country code!")
+      CUSIP.new(nsin)
+    end
+
     private
 
     # https://en.wikipedia.org/wiki/Luhn_algorithm

spec/sec_id/cusip_spec.rb

@@ -71,6 +71,24 @@
     end
   end
 
+  describe '#to_isin' do
+    context 'when CGS country code' do
+      let(:cusip_number) { '02153X108' }
+
+      it 'returns an ISIN' do
+        expect(cusip.to_isin('VI')).to be_a(SecId::ISIN)
+      end
+    end
+
+    context 'when non-CGS country code' do
+      let(:cusip_number) { '00B296YR7' }
+
+      it 'raises an error' do
+        expect { cusip.to_isin('IE') }.to raise_error(SecId::InvalidFormatError)
+      end
+    end
+  end
+
   describe '.restore!' do
     context 'when CUSIP is incorrect' do
       it 'raises an error' do

spec/sec_id/isin_spec.rb

@@ -74,6 +74,26 @@
     end
   end
 
+  describe '#to_cusip' do
+    context 'when CGS country code' do
+      let(:isin_number) { 'VI02153X1080' }
+
+      it 'returns a CUSIP' do
+        expect(isin.cgs?).to be(true)
+        expect(isin.to_cusip).to be_a(SecId::CUSIP)
+      end
+    end
+
+    context 'when non-CGS country code' do
+      let(:isin_number) { 'IE00B296YR77' }
+
+      it 'raises an error' do
+        expect(isin.cgs?).to be(false)
+        expect { isin.to_cusip }.to raise_error(SecId::InvalidFormatError)
+      end
+    end
+  end
+
   describe '.valid?' do
     context 'when ISIN is incorrect' do
       it 'returns false' do