Add shared specs for the interface of Random::Formatter

* Depend on the instance method `bytes`
* Ensure the output is the same if the input bytes are the same
* Ensure the output is not the same if the input bytes are not the same

Right now, this is only used in the specs for `alphanumeric`, since this
is the only method of Random::Formatter with specs, but this should be
used in every other spec we add to this folder, since the specs have no
dependency on the exact format that is being used.

library/random/formatter/alphanumeric_spec.rb

@@ -1,4 +1,5 @@
 require_relative '../../../spec_helper'
+require_relative 'shared/interface'
 
 ruby_version_is "3.1" do
   require 'random/formatter'
@@ -58,4 +59,6 @@ def @object.bytes(n)
       @object.alphanumeric(1, chars: [to_s, to_s]).should == "[mock to_s]"
     end
   end
+
+  it_behaves_like :random_formatter_interface, :alphanumeric
 end

library/random/formatter/shared/interface.rb

@@ -0,0 +1,33 @@
+require_relative '../../../../spec_helper'
+
+describe :random_formatter_interface, shared: true do
+  it "calls the method #bytes for 4 blocks of 4 bytes" do
+    bytes = mock('bytes')
+    bytes.extend(Random::Formatter)
+    bytes.should_receive(:bytes).with(4).exactly(4).and_return("\x00".b * 4)
+    bytes.send(@method)
+  end
+
+  it "has the same output if the random byte streams are the same" do
+    bytes = Object.new
+    bytes.extend(Random::Formatter)
+    def bytes.bytes(n)
+      "\x00".b * n
+    end
+    bytes.send(@method).should == bytes.send(@method)
+  end
+
+  it "has different output if the random byte streams are not the same" do
+    lhs = Object.new
+    lhs.extend(Random::Formatter)
+    def lhs.bytes(n)
+      "\x00".b * n
+    end
+    rhs = Object.new
+    rhs.extend(Random::Formatter)
+    def rhs.bytes(n)
+      "\x01".b * n
+    end
+    lhs.send(@method).should != rhs.send(@method)
+  end
+end