Add rprompt support for right-side prompt display

This adds support for displaying a right-aligned prompt (rprompt)
similar to zsh's RPROMPT feature. The rprompt is displayed at the
right edge of the terminal and automatically hides when the input
line gets too long.

Usage:
  Reline.rprompt = "[%H:%M]"
  Reline.readline("> ")

The rprompt is rendered as part of Reline's normal render cycle,
so it persists during line editing unlike workarounds using
pre_input_hook.

Author: amatsuda

lib/reline.rb

@@ -50,6 +50,7 @@ class Core
       output_modifier_proc
       prompt_proc
       auto_indent_proc
+      rprompt
       pre_input_hook
       dig_perfect_match_proc
     ).each(&method(:attr_reader))
@@ -158,6 +159,10 @@ def dig_perfect_match_proc=(p)
       @dig_perfect_match_proc = p
     end
 
+    def rprompt=(val)
+      @rprompt = val&.encode(encoding)
+    end
+
     DialogProc = Struct.new(:dialog_proc, :context)
     def add_dialog_proc(name_sym, p, context = nil)
       raise ArgumentError unless name_sym.instance_of?(Symbol)
@@ -323,6 +328,7 @@ def readline(prompt = '', add_hist = false)
       line_editor.prompt_proc = prompt_proc
       line_editor.auto_indent_proc = auto_indent_proc
       line_editor.dig_perfect_match_proc = dig_perfect_match_proc
+      line_editor.rprompt = rprompt
 
       pre_input_hook&.call
 

lib/reline/line_editor.rb

@@ -13,6 +13,7 @@ class Reline::LineEditor
   attr_accessor :prompt_proc
   attr_accessor :auto_indent_proc
   attr_accessor :dig_perfect_match_proc
+  attr_accessor :rprompt
 
   VI_MOTIONS = %i{
     ed_prev_char
@@ -476,6 +477,20 @@ def render
       prompt_width = Reline::Unicode.calculate_width(prompt, true)
       [[0, prompt_width, prompt], [prompt_width, Reline::Unicode.calculate_width(line, true), line]]
     end
+
+    # Add rprompt to the first visible line if set and there's room
+    if @rprompt && !@rprompt.empty? && new_lines[0]
+      rprompt_width = Reline::Unicode.calculate_width(@rprompt, true)
+      right_col = screen_width - rprompt_width
+      first_line = new_lines[0]
+      # Calculate the end of the current content (prompt + input)
+      content_end = first_line.sum { |_, width, _| width }
+      # Only show rprompt if there's at least 1 char gap between content and rprompt
+      if right_col > content_end
+        first_line << [right_col, rprompt_width, @rprompt]
+      end
+    end
+
     if @menu_info
       @menu_info.lines(screen_width).each do |item|
         new_lines << [[0, Reline::Unicode.calculate_width(item), item]]

test/reline/test_reline.rb

@@ -19,6 +19,7 @@ def setup
     Reline.auto_indent_proc = nil
     Reline.pre_input_hook = nil
     Reline.dig_perfect_match_proc = nil
+    Reline.rprompt = nil
   end
 
   def teardown
@@ -232,6 +233,21 @@ def test_pre_input_hook
     assert_equal(l, Reline.pre_input_hook)
   end
 
+  def test_rprompt
+    assert_equal(nil, Reline.rprompt)
+
+    Reline.rprompt = "[Time]"
+    assert_equal("[Time]", Reline.rprompt)
+    assert_equal(get_reline_encoding, Reline.rprompt.encoding)
+
+    Reline.rprompt = "[Time]".encode(Encoding::ASCII)
+    assert_equal("[Time]", Reline.rprompt)
+    assert_equal(get_reline_encoding, Reline.rprompt.encoding)
+
+    Reline.rprompt = nil
+    assert_equal(nil, Reline.rprompt)
+  end
+
   def test_dig_perfect_match_proc
     assert_equal(nil, Reline.dig_perfect_match_proc)
 

test/reline/yamatanooroti/multiline_repl

@@ -212,6 +212,9 @@ opt.on('--autocomplete-width-long') {
     }.select{ |c| c.start_with?(target) }
   }
 }
+opt.on('--rprompt VAL') { |v|
+  Reline.rprompt = v
+}
 opt.parse!(ARGV)
 
 begin

test/reline/yamatanooroti/test_rendering.rb

@@ -183,6 +183,35 @@ def test_prompt
       close
     end
 
+    def test_rprompt
+      start_terminal(5, 40, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --rprompt [RPROMPT]}, startup_message: 'Multiline REPL.')
+      assert_screen(<<~EOC)
+        Multiline REPL.
+        prompt>                       [RPROMPT]
+      EOC
+      close
+    end
+
+    def test_rprompt_with_input
+      start_terminal(5, 40, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --rprompt [RPROMPT]}, startup_message: 'Multiline REPL.')
+      write("hello")
+      assert_screen(<<~EOC)
+        Multiline REPL.
+        prompt> hello                 [RPROMPT]
+      EOC
+      close
+    end
+
+    def test_rprompt_hides_when_input_reaches_rprompt
+      start_terminal(5, 40, %W{ruby -I#{@pwd}/lib #{@pwd}/test/reline/yamatanooroti/multiline_repl --rprompt [RPROMPT]}, startup_message: 'Multiline REPL.')
+      write("a" * 30)
+      assert_screen(<<~EOC)
+        Multiline REPL.
+        prompt> aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+      EOC
+      close
+    end
+
     def test_mode_string_emacs
       write_inputrc <<~LINES
         set show-mode-in-prompt on