Added Faker::Code.isbn

README.md

@@ -79,6 +79,15 @@ Faker::Business.credit_card_type #=> "visa"
 ```
 
 
+### Faker::Code
+
+```crystal
+
+Faker::Code.isbn #=> 640354399-7
+
+```
+
+
 ### Faker::Commerce
 
 ```crystal

spec/code_spec.cr

@@ -0,0 +1,17 @@
+require "./spec_helper"
+
+describe Faker::Code do
+  it "base 10 isbn" do
+    Faker::Code.isbn.match(/^\d{9}-[\d|X]$/).should_not eq nil
+  end
+
+  it "base 13 isbn" do
+    Faker::Code.isbn(13).match(/^\d{12}-\d$/).should_not eq nil
+  end
+
+  it "should return deterministic results when seeded" do
+    Faker.seed 123456
+    Faker::Code.isbn.should eq "394314441-0"
+    Faker::Code.isbn(13).should eq "205982563728-9"
+  end
+end

src/faker/code.cr

@@ -0,0 +1,29 @@
+module Faker
+  class Code < Base
+    def self.isbn(base : Int32 = 10)
+      case base
+      when 10 then generate_base10_isbn
+      when 13 then generate_base13_isbn
+      else raise ArgumentError.new("base must be 10 or 13")
+      end
+    end
+
+    private def self.generate_base10_isbn
+      values = Faker.regexify(/\d{9}/)
+      remainder = sum(values) { |value, index| (index + 1) * value.to_i } % 11
+      values += "-#{remainder == 10 ? 'X' : remainder}"
+    end
+
+    private def self.generate_base13_isbn
+      values = Faker.regexify(/\d{12}/)
+      remainder = sum(values) { |value, index| index.even? ? value.to_i : value.to_i * 3 } % 10
+      values += "-#{(10 - remainder) % 10}"
+    end
+
+    private def self.sum(values)
+      values.split(//).each_with_index.reduce(0) do |sum, (value, index)|
+        sum + yield(value, index)
+      end
+    end
+  end
+end