diff --git a/lib/getoptlong.rb b/lib/getoptlong.rb
index d681a42..aaac5cd 100644
--- a/lib/getoptlong.rb
+++ b/lib/getoptlong.rb
@@ -395,6 +395,8 @@ class NeedlessArgument < Error; end
   class MissingArgument  < Error; end
   class InvalidOption    < Error; end
 
+  attr_accessor :argv, :env
+
   #
   # Returns a new \GetoptLong object based on the given +arguments+.
   # See {Options}[#class-GetoptLong-label-Options].
@@ -409,11 +411,11 @@ class InvalidOption    < Error; end
   # - Any option name or alias is not a string.
   # - Any option type is invalid.
   #
-  def initialize(*arguments)
+  def initialize(*arguments, argv: ARGV, env: ENV)
     #
     # Current ordering.
     #
-    if ENV.include?('POSIXLY_CORRECT')
+    if env.include?('POSIXLY_CORRECT')
       @ordering = REQUIRE_ORDER
     else
       @ordering = PERMUTE
@@ -467,6 +469,9 @@ def initialize(*arguments)
     if 0 < arguments.length
       set_options(*arguments)
     end
+
+    self.argv = argv
+    self.env  = env
   end
 
   # Sets the ordering; see {Ordering}[#class-GetoptLong-label-Ordering];
@@ -502,7 +507,7 @@ def ordering=(ordering)
     if !ORDERINGS.include?(ordering)
       raise ArgumentError, "invalid ordering `#{ordering}'"
     end
-    if ordering == PERMUTE && ENV.include?('POSIXLY_CORRECT')
+    if ordering == PERMUTE && env.include?('POSIXLY_CORRECT')
       @ordering = REQUIRE_ORDER
     else
       @ordering = ordering
@@ -614,9 +619,7 @@ def terminate
     raise RuntimeError, "an error has occurred" if @error != nil
 
     @status = STATUS_TERMINATED
-    @non_option_arguments.reverse_each do |argument|
-      ARGV.unshift(argument)
-    end
+    argv.unshift(*@non_option_arguments)
 
     @canonical_names = nil
     @argument_flags = nil
@@ -690,26 +693,26 @@ def get
     #
     if 0 < @rest_singles.length
       argument = '-' + @rest_singles
-    elsif (ARGV.length == 0)
+    elsif argv.empty?
       terminate
       return nil
     elsif @ordering == PERMUTE
-      while 0 < ARGV.length && ARGV[0] !~ /\A-./
-        @non_option_arguments.push(ARGV.shift)
+      while !(argv.empty? || /\A-./.match?(argv.first))
+        @non_option_arguments.push(argv.shift)
       end
-      if ARGV.length == 0
+      if argv.empty?
         terminate
         return nil
       end
-      argument = ARGV.shift
+      argument = argv.shift
     elsif @ordering == REQUIRE_ORDER
-      if (ARGV[0] !~ /\A-./)
+      unless /\A-./.match?(argv.first)
         terminate
         return nil
       end
-      argument = ARGV.shift
+      argument = argv.shift
     else
-      argument = ARGV.shift
+      argument = argv.shift
     end
 
     #
@@ -756,8 +759,8 @@ def get
       if @argument_flags[option_name] == REQUIRED_ARGUMENT
         if argument =~ /=(.*)/m
           option_argument = $1
-        elsif 0 < ARGV.length
-          option_argument = ARGV.shift
+        elsif !argv.empty?
+          option_argument = argv.shift
         else
           set_error(MissingArgument,
                     "option `#{argument}' requires an argument")
@@ -765,8 +768,8 @@ def get
       elsif @argument_flags[option_name] == OPTIONAL_ARGUMENT
         if argument =~ /=(.*)/m
           option_argument = $1
-        elsif 0 < ARGV.length && ARGV[0] !~ /\A-./
-          option_argument = ARGV.shift
+        elsif !(argv.empty? || /\A-./.match?(argv.first))
+          option_argument = argv.shift
         else
           option_argument = ''
         end
@@ -792,8 +795,8 @@ def get
           if 0 < @rest_singles.length
             option_argument = @rest_singles
             @rest_singles = ''
-          elsif 0 < ARGV.length
-            option_argument = ARGV.shift
+          elsif !argv.empty?
+            option_argument = argv.shift
           else
             # 1003.2 specifies the format of this message.
             set_error(MissingArgument, "option requires an argument -- #{ch}")
@@ -802,8 +805,8 @@ def get
           if 0 < @rest_singles.length
             option_argument = @rest_singles
             @rest_singles = ''
-          elsif 0 < ARGV.length && ARGV[0] !~ /\A-./
-            option_argument = ARGV.shift
+          elsif !(argv.empty? || /\A-./.match?(argv.first))
+            option_argument = argv.shift
           else
             option_argument = ''
           end
diff --git a/test/test_getoptlong.rb b/test/test_getoptlong.rb
index 0cd370b..5828585 100644
--- a/test/test_getoptlong.rb
+++ b/test/test_getoptlong.rb
@@ -3,27 +3,31 @@
 
 class TestGetoptLong < Test::Unit::TestCase
 
-  def verify(test_argv, expected_remaining_argv, expected_options)
-    # Save ARGV and replace it with a test ARGV.
-    argv_saved = ARGV.dup
-    ARGV.replace(test_argv)
-    # Define options.
-    opts = GetoptLong.new(
+  def getoptlong_new(argv: [], env: nil, options: nil)
+    options ||= [
       ['--xxx', '-x', '--aaa', '-a', GetoptLong::REQUIRED_ARGUMENT],
       ['--yyy', '-y', '--bbb', '-b', GetoptLong::OPTIONAL_ARGUMENT],
       ['--zzz', '-z', '--ccc', '-c', GetoptLong::NO_ARGUMENT]
-    )
-    opts.quiet = true
+    ]
+    env ||= ENV
+
+    GetoptLong.new(*options, argv:, env:)
+              .tap { |opts| opts.quiet = true }
+  end
+
+  def verify(test_argv, expected_remaining_argv, expected_options, env = nil)
+    # Define options.
+    opts = getoptlong_new(argv: test_argv, env:)
+    yield opts if block_given?
+
     # Gather options.
     actual_options = []
     opts.each do |opt, arg|
       actual_options << "#{opt}: #{arg}"
     end
-    # Save remaining test ARGV and restore original ARGV.
-    actual_remaining_argv = ARGV.dup
-    ARGV.replace(argv_saved)
+
     # Assert.
-    assert_equal(expected_remaining_argv, actual_remaining_argv, 'ARGV')
+    assert_equal(expected_remaining_argv, test_argv, 'ARGV')
     assert_equal(expected_options, actual_options, 'Options')
   end
 
@@ -46,6 +50,18 @@ def test_required_argument
     end
   end
 
+  def test_required_argument_assign
+    expected_options = [
+      '--xxx: arg'
+    ]
+    expected_argv = %w[foo bar]
+    options = %w[--xxx --xx --x --aaa --aa --a]
+    options.each do |option|
+      argv = ['foo', "#{option}=arg", 'bar']
+      verify(argv, expected_argv, expected_options)
+    end
+  end
+
   def test_required_argument_missing
     options = %w[--xxx --xx --x -x --aaa --aa --a -a]
     options.each do |option|
@@ -69,6 +85,18 @@ def test_optional_argument
     end
   end
 
+  def test_optional_argument_assign
+    expected_options = [
+      '--yyy: arg'
+    ]
+    expected_argv = %w[foo bar]
+    options = %w[--yyy --y --y --bbb --bb --b]
+    options.each do |option|
+      argv = ['foo', 'bar', "#{option}=arg"]
+      verify(argv, expected_argv, expected_options)
+    end
+  end
+
   def test_optional_argument_missing
     expected_options = [
       '--yyy: '
@@ -160,4 +188,120 @@ def test_new_with_invalid_flag
     end
   end
 
+  def test_raise_ambiguous_option
+    e = assert_raise(GetoptLong::AmbiguousOption) do
+      options = [
+        ['--xxx', GetoptLong::REQUIRED_ARGUMENT],
+        ['--xxy', GetoptLong::NO_ARGUMENT]
+      ]
+      getoptlong_new(argv: ['--xx'], options:).each { nil }
+    end
+    assert_match(/ambiguous/, e.message)
+  end
+
+  def test_option_prefix
+    assert_nothing_raised do
+      options = [
+        ['--xxx',   GetoptLong::NO_ARGUMENT],
+        ['--xxx-y', GetoptLong::NO_ARGUMENT],
+        ['--xx',    GetoptLong::NO_ARGUMENT]
+      ]
+      getoptlong_new(argv: ['--xx', '--xxx'], options:).each { nil }
+    end
+  end
+
+  def test_raise_needless_argument
+    e = assert_raise(GetoptLong::NeedlessArgument) do
+      options = [['--x', GetoptLong::NO_ARGUMENT]]
+      getoptlong_new(argv: ['--x=z'], options:).each { nil }
+    end
+    assert_match(/doesn't allow an argument/, e.message)
+  end
+
+  def test_raise_too_many_flags
+    e = assert_raise(ArgumentError) do
+      options = [
+        ['-y', GetoptLong::REQUIRED_ARGUMENT, GetoptLong::NO_ARGUMENT]
+      ]
+      getoptlong_new(options:)
+    end
+    assert_match(/too many/, e.message)
+  end
+
+  def test_raise_option_redefined
+    e = assert_raise(ArgumentError) do
+      options = [
+        ['--xxx', '-x', GetoptLong::REQUIRED_ARGUMENT],
+        ['--exclude', '-x', GetoptLong::NO_ARGUMENT]
+      ]
+      getoptlong_new(options:)
+    end
+    assert_match(/redefined/, e.message)
+  end
+
+  def test_set_ordering_raise
+    e = assert_raise(ArgumentError) do
+      getoptlong_new.ordering = 42
+    end
+    assert_match(/invalid ordering/, e.message)
+  end
+
+  def test_raise_unrecognized_option
+    e = assert_raise(GetoptLong::InvalidOption) do
+      getoptlong_new(argv: ['--asdf']).each { nil }
+    end
+    assert_match(/unrecognized option/, e.message)
+  end
+
+  def test_ordering
+    GetoptLong::ORDERINGS.each do |order|
+      argv = ['foo', '--xxx', 'arg', 'bar']
+
+      expected_options, expected_argv =
+        case order
+        when GetoptLong::REQUIRE_ORDER
+          [[], %w[foo --xxx arg bar]]
+        when GetoptLong::PERMUTE
+          [['--xxx: arg'], %w[foo bar]]
+        when GetoptLong::RETURN_IN_ORDER
+          [[': foo', '--xxx: arg', ': bar'], []]
+        end
+
+      verify(argv, expected_argv, expected_options) do |opts|
+        opts.ordering = order
+      end
+    end
+  end
+
+  def test_env_posixly_correct
+    [{}, { 'POSIXLY_CORRECT' => '1' }].each do |order|
+      argv = ['foo', '--xxx', 'arg', 'bar']
+
+      expected_options, expected_argv =
+        case order
+        when {}
+          [['--xxx: arg'], %w[foo bar]]
+        else
+          [[], %w[foo --xxx arg bar]]
+        end
+
+      verify(argv, expected_argv, expected_options, order)
+    end
+  end
+
+  def test_raise_invalid_option_with_single_hyphen
+    options = [
+      ['-x', GetoptLong::NO_ARGUMENT],
+      ['--x', GetoptLong::NO_ARGUMENT]
+    ]
+    argvs = [%w[-x-x foo], %w[-x- foo]]
+
+    argvs.each do |argv|
+      e = assert_raise(GetoptLong::InvalidOption) do
+        getoptlong_new(argv:, options:).each { nil }
+      end
+      assert_match(/invalid option/, e.message)
+    end
+  end
+
 end