[ruby/irb] Refactor eval history

(https://github.com/ruby/irb/pull/623)

* Rename `ext/history.rb` to `ext/eval_history.rb`

To confusion with `lib/irb/history.rb`

* Add eval_history tests

* Rename eval_history's History to EvalHistory to avoid confusion

lib/irb.rb

@@ -154,7 +154,7 @@ require_relative "irb/easter-egg"
 #
 #     IRB.conf[:EVAL_HISTORY] = <number>
 #
-# See IRB::Context#eval_history= and History class. The history of command
+# See IRB::Context#eval_history= and EvalHistory class. The history of command
 # results is not permanently saved in any file.
 #
 # == Customizing the IRB Prompt

lib/irb/ext/eval_history.rb

@@ -40,14 +40,14 @@ module IRB # :nodoc:
     #
     # If +no+ is +nil+, execution result history isn't used (default).
     #
-    # History values are available via <code>__</code> variable, see
+    # EvalHistory values are available via <code>__</code> variable, see
-    # IRB::History.
+    # IRB::EvalHistory.
     def eval_history=(no)
       if no
         if defined?(@eval_history) && @eval_history
           @eval_history_values.size(no)
         else
-          @eval_history_values = History.new(no)
+          @eval_history_values = EvalHistory.new(no)
           IRB.conf[:__TMP__EHV__] = @eval_history_values
           @workspace.evaluate("__ = IRB.conf[:__TMP__EHV__]")
           IRB.conf.delete(:__TMP_EHV__)
@@ -89,7 +89,7 @@ module IRB # :nodoc:
   #    __[1]
   #    # => 10
   #
-  class History
+  class EvalHistory
 
     def initialize(size = 16)  # :nodoc:
       @size = size

lib/irb/extend-command.rb

@@ -316,7 +316,7 @@ module IRB # :nodoc:
     CE = ContextExtender # :nodoc:
 
     @EXTEND_COMMANDS = [
-      [:eval_history=, "ext/history.rb"],
+      [:eval_history=, "ext/eval_history.rb"],
       [:use_tracer=, "ext/tracer.rb"],
       [:use_loader=, "ext/use-loader.rb"],
     ]

test/irb/test_eval_history.rb

@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+require "irb"
+
+require_relative "helper"
+
+module TestIRB
+  class EvalHistoryTest < TestCase
+    def setup
+      save_encodings
+      IRB.instance_variable_get(:@CONF).clear
+    end
+
+    def teardown
+      restore_encodings
+    end
+
+    def execute_lines(*lines, conf: {}, main: self, irb_path: nil)
+      IRB.init_config(nil)
+      IRB.conf[:VERBOSE] = false
+      IRB.conf[:PROMPT_MODE] = :SIMPLE
+      IRB.conf.merge!(conf)
+      input = TestInputMethod.new(lines)
+      irb = IRB::Irb.new(IRB::WorkSpace.new(main), input)
+      irb.context.return_format = "=> %s\n"
+      irb.context.irb_path = irb_path if irb_path
+      IRB.conf[:MAIN_CONTEXT] = irb.context
+      capture_output do
+        irb.eval_input
+      end
+    end
+
+    def test_eval_history_is_diabled_by_default
+      out, err = execute_lines(
+        "a = 1",
+        "__"
+      )
+
+      assert_empty(err)
+      assert_match(/undefined local variable or method `__'/, out)
+    end
+
+    def test_eval_history_can_be_retrieved_with_double_underscore
+      out, err = execute_lines(
+        "a = 1",
+        "__",
+        conf: { EVAL_HISTORY: 5 }
+      )
+
+      assert_empty(err)
+      assert_match("=> 1\n" + "=> 1 1\n", out)
+    end
+
+    def test_eval_history_respects_given_limit
+      out, err = execute_lines(
+        "'foo'\n",
+        "'bar'\n",
+        "'baz'\n",
+        "'xyz'\n",
+        "__",
+        conf: { EVAL_HISTORY: 4 }
+      )
+
+      assert_empty(err)
+      # Because eval_history injects `__` into the history AND decide to ignore it, we only get <limit> - 1 results
+      assert_match("2 \"bar\"\n" + "3 \"baz\"\n" + "4 \"xyz\"\n", out)
+    end
+  end
+end