Proposal to introduce "completion_filter_proc" configuration option

lib/reline.rb

@@ -47,6 +47,7 @@ class Core
       filename_quote_characters
       special_prefixes
       completion_proc
+      completion_filter_proc
       output_modifier_proc
       prompt_proc
       auto_indent_proc
@@ -134,6 +135,11 @@ def completion_proc=(p)
       @completion_proc = p
     end
 
+    def completion_filter_proc=(p)
+      raise ArgumentError unless p.respond_to?(:call) or p.nil?
+      @completion_filter_proc = p
+    end
+
     def output_modifier_proc=(p)
       raise ArgumentError unless p.respond_to?(:call) or p.nil?
       @output_modifier_proc = p
@@ -318,6 +324,7 @@ def readline(prompt = '', add_hist = false)
         line_editor.multiline_off
       end
       line_editor.completion_proc = completion_proc
+      line_editor.completion_filter_proc = completion_filter_proc
       line_editor.completion_append_character = completion_append_character
       line_editor.output_modifier_proc = output_modifier_proc
       line_editor.prompt_proc = prompt_proc

lib/reline/line_editor.rb

@@ -8,6 +8,7 @@ class Reline::LineEditor
   attr_reader :byte_pointer
   attr_accessor :confirm_multiline_termination_proc
   attr_accessor :completion_proc
+  attr_writer :completion_filter_proc
   attr_accessor :completion_append_character
   attr_accessor :output_modifier_proc
   attr_accessor :prompt_proc
@@ -799,6 +800,14 @@ def editing_mode
     @menu_info = MenuInfo.new(list)
   end
 
+  private def completion_filter_proc
+    @completion_filter_proc || if @config.completion_ignore_case
+      ->(target, candidate) { candidate.downcase.start_with?(target) }
+    else
+      ->(target, candidate) { candidate.start_with?(target) }
+    end
+  end
+
   private def filter_normalize_candidates(target, list)
     target = target.downcase if @config.completion_ignore_case
     list.select do |item|
@@ -810,11 +819,7 @@ def editing_mode
         end
       end
 
-      if @config.completion_ignore_case
-        item.downcase.start_with?(target)
-      else
-        item.start_with?(target)
-      end
+      completion_filter_proc.(target, item)
     end.map do |item|
       item.unicode_normalize
     rescue Encoding::CompatibilityError
@@ -894,7 +899,7 @@ def dialog_proc_scope_completion_journey_data
     list = call_completion_proc(preposing, target, postposing, quote)
     return unless list.is_a?(Array)
 
-    candidates = list.select{ |item| item.start_with?(target) }
+    candidates = list.select(&completion_filter_proc.curry.(target))
     return if candidates.empty?
 
     pre = preposing.split("\n", -1).last || ''

test/reline/test_key_actor_emacs.rb

@@ -1018,6 +1018,30 @@ def test_completion_with_completion_ignore_case
     assert_line_around_cursor('Foo_baz', '')
   end
 
+  def test_completion_with_completion_filter_proc
+    @line_editor.completion_proc = proc { |word|
+      %w{
+        foo_bar
+        foo_baz
+        qux
+      }.map { |i|
+        i.encode(@encoding)
+      }
+    }
+    # Fuzzy matching: characters appear in order but not necessarily contiguous
+    @line_editor.completion_filter_proc = ->(target, candidate) {
+      idx = 0
+      target.each_char.all? { |c| (idx = candidate.index(c, idx)) && (idx += 1) }
+    }
+    input_keys('fb')
+    assert_line_around_cursor('fb', '')
+    input_keys("\C-i")
+    # 'fb' fuzzy matches 'foo_bar' and 'foo_baz' (f...b)
+    assert_line_around_cursor('foo_ba', '')
+    input_keys("\C-i")
+    assert_equal(%w{foo_bar foo_baz}, @line_editor.instance_variable_get(:@menu_info).list)
+  end
+
   def test_completion_in_middle_of_line
     @line_editor.completion_proc = proc { |word|
       %w{

test/reline/test_reline.rb

@@ -15,6 +15,7 @@ def setup
     Reline.send(:test_mode)
     Reline.output_modifier_proc = nil
     Reline.completion_proc = nil
+    Reline.completion_filter_proc = nil
     Reline.prompt_proc = nil
     Reline.auto_indent_proc = nil
     Reline.pre_input_hook = nil
@@ -163,6 +164,28 @@ def test_completion_proc
     assert_equal(dummy, Reline.completion_proc)
   end
 
+  def test_completion_filter_proc
+    assert_equal(nil, Reline.completion_filter_proc)
+
+    dummy_proc = proc {}
+    Reline.completion_filter_proc = dummy_proc
+    assert_equal(dummy_proc, Reline.completion_filter_proc)
+
+    l = lambda {}
+    Reline.completion_filter_proc = l
+    assert_equal(l, Reline.completion_filter_proc)
+
+    assert_raise(ArgumentError) { Reline.completion_filter_proc = 42 }
+    assert_raise(ArgumentError) { Reline.completion_filter_proc = "hoge" }
+
+    dummy = DummyCallbackObject.new
+    Reline.completion_filter_proc = dummy
+    assert_equal(dummy, Reline.completion_filter_proc)
+
+    Reline.completion_filter_proc = nil
+    assert_nil(Reline.completion_filter_proc)
+  end
+
   def test_output_modifier_proc
     assert_equal(nil, Reline.output_modifier_proc)