Changed the byte encoding to use short constant format in some cases,

and added a few test cases

.rspec

@@ -0,0 +1,2 @@
+--color
+--format progress

lib/ev3/core_extensions.rb

@@ -1,12 +1,23 @@
 module EV3
   module CoreExtensions
-    refine Integer do
-      BYTE_CONVERSION = {1 => 'c', 2 => 'v', 4 => 'V'}
+    BYTE_CONVERSION = {1 => 'c', 2 => 'v', 4 => 'V'}
+    BYTES_FOLLOWING_ENCODING = {1 => 0b01, 2 => 0b10, 4 => 0b11}
 
+    refine Integer do
       # Convert to EV3 variable data
       # see http://python-ev3.org/parameterencoding.html#subpar
+      # TODO: Add 1 and 2 byte encoding (http://python-ev3.org/parameterencoding.html)
       def to_ev3_data
-        [0b1000_0011] + self.to_little_endian_byte_array(4)
+        case self
+        when 0..31
+          self
+        when -32..-1
+          # Short negative constant
+          0b0011_1111 & self.to_little_endian_byte_array(1).first
+        else
+          # 4-byte variable
+          [0b1000_0000 | BYTES_FOLLOWING_ENCODING[4]] + self.to_little_endian_byte_array(4)
+        end
       end
 
       # Convert the number to an array of little endian bytes

ruby-ev3.gemspec

@@ -22,5 +22,6 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler", "~> 1.5"
   spec.add_development_dependency "rake"
+  spec.add_development_dependency "rspec"
   spec.add_development_dependency "yard"
 end

spec/ev3/core_extensions_spec.rb

@@ -0,0 +1,50 @@
+require 'rspec'
+require 'ev3'
+
+describe EV3::CoreExtensions do
+  using EV3::CoreExtensions
+
+  describe "#to_ev3_data" do
+    def short_negative(number)
+      0b0011_1111 & [number].pack('c').bytes.first
+    end
+
+    def long_variable_number(number, byte_count)
+      bytes_to_follow = EV3::CoreExtensions::BYTES_FOLLOWING_ENCODING[byte_count]
+      byte_conversion = EV3::CoreExtensions::BYTE_CONVERSION[byte_count]
+      [0b1000_0000 | bytes_to_follow] + [number].pack(byte_conversion).bytes
+    end
+
+    subject { number.to_ev3_data }
+
+    context "when a small constant" do
+      let(:number) { 0 }
+      it { should eql(number) }
+    end
+
+    context "when a small constant" do
+      let(:number) { 31 }
+      it { should eql(number) }
+    end
+
+    context "when slightly negative" do
+      let(:number) { -1 }
+      it { should eql(short_negative(number)) }
+    end
+
+    context "when slightly negative" do
+      let(:number) { -32 }
+      it { should eql(short_negative(number)) }
+    end
+
+    context "when large" do
+      let(:number) { 100_000 }
+      it { should eql(long_variable_number(number, 4)) }
+    end
+
+    context "when a large negative" do
+      let(:number) { -100_000 }
+      it { should eql(long_variable_number(number, 4)) }
+    end
+  end
+end

spec/spec_helper.rb

@@ -0,0 +1,17 @@
+# This file was generated by the `rspec --init` command. Conventionally, all
+# specs live under a `spec` directory, which RSpec adds to the `$LOAD_PATH`.
+# Require this file using `require "spec_helper"` to ensure that it is only
+# loaded once.
+#
+# See http://rubydoc.info/gems/rspec-core/RSpec/Core/Configuration
+RSpec.configure do |config|
+  config.treat_symbols_as_metadata_keys_with_true_values = true
+  config.run_all_when_everything_filtered = true
+  config.filter_run :focus
+
+  # Run specs in random order to surface order dependencies. If you find an
+  # order dependency and want to debug it, you can fix the order by providing
+  # the seed, which is printed after each run.
+  #     --seed 1234
+  config.order = 'random'
+end