[ruby/reline] Fallback to 256color if COLORTERM != truecolor

(https://github.com/ruby/reline/pull/604)

* Fallback to 256color if COLORTERM != truecolor

* Add Reline::Face.force_truecolor to force truecolor without COLORTERM env

https://github.com/ruby/reline/commit/090e1e4df0

3 files changed, 108 insertions, 4 deletions

diff --git a/doc/reline/face.md b/doc/reline/face.md
index cf3bb69440..1fa916123b 100644
--- a/doc/reline/face.md
+++ b/doc/reline/face.md
@@ -102,7 +102,10 @@ irb(main):001:0> Reline::Face.configs
    :scrollbar=>{:foreground=>:white, :background=>:cyan, :escape_sequence=>"\e[0m\e[37;46m"}}}
 ```
 
-## Backlog
-
-- Support for 256-color terminal emulator. Fallback hex color code such as "#FF1020" to 256 colors
+## 256-Color and TrueColor
 
+Reline will automatically detect if your terminal emulator supports truecolor with `ENV['COLORTERM] in 'truecolor' | '24bit'`. When this env is not set, Reline will fallback to 256-color.
+If your terminal emulator supports truecolor but does not set COLORTERM env, add this line to `.irbrc`.
+```ruby
+Reline::Face.force_truecolor
+```
diff --git a/lib/reline/face.rb b/lib/reline/face.rb
index b78f3b1ca5..e18ec957e8 100644
--- a/lib/reline/face.rb
+++ b/lib/reline/face.rb
@@ -74,6 +74,13 @@ class Reline::Face
       @definition[name] = values
     end
 
+    def reconfigure
+      @definition.each_value do |values|
+        values.delete(:escape_sequence)
+        values[:escape_sequence] = format_to_sgr(values.to_a).freeze
+      end
+    end
+
     def [](name)
       @definition.dig(name, :escape_sequence) or raise ArgumentError, "unknown face: #{name}"
     end
@@ -82,6 +89,14 @@ class Reline::Face
 
     def sgr_rgb(key, value)
       return nil unless rgb_expression?(value)
+      if Reline::Face.truecolor?
+        sgr_rgb_truecolor(key, value)
+      else
+        sgr_rgb_256color(key, value)
+      end
+    end
+
+    def sgr_rgb_truecolor(key, value)
       case key
       when :foreground
         "38;2;"
@@ -90,6 +105,24 @@ class Reline::Face
       end + value[1, 6].scan(/../).map(&:hex).join(";")
     end
 
+    def sgr_rgb_256color(key, value)
+      # 256 colors are
+      # 0..15: standard colors, hight intensity colors
+      # 16..232: 216 colors (R, G, B each 6 steps)
+      # 233..255: grayscale colors (24 steps)
+      # This methods converts rgb_expression to 216 colors
+      rgb = value[1, 6].scan(/../).map(&:hex)
+      # Color steps are [0, 95, 135, 175, 215, 255]
+      r, g, b = rgb.map { |v| v <= 95 ? v / 48 : (v - 35) / 40 }
+      color = (16 + 36 * r + 6 * g + b)
+      case key
+      when :foreground
+        "38;5;#{color}"
+      when :background
+        "48;5;#{color}"
+      end
+    end
+
     def format_to_sgr(ordered_values)
       sgr = "\e[" + ordered_values.map do |key_value|
         key, value = key_value
@@ -124,6 +157,15 @@ class Reline::Face
 
   private_constant :SGR_PARAMETERS, :Config
 
+  def self.truecolor?
+    @force_truecolor || %w[truecolor 24bit].include?(ENV['COLORTERM'])
+  end
+
+  def self.force_truecolor
+    @force_truecolor = true
+    @configs&.each_value(&:reconfigure)
+  end
+
   def self.[](name)
     @configs[name]
   end
diff --git a/test/reline/test_face.rb b/test/reline/test_face.rb
index 14da4f6d65..8fa2be8fa4 100644
--- a/test/reline/test_face.rb
+++ b/test/reline/test_face.rb
@@ -5,12 +5,19 @@ require_relative 'helper'
 class Reline::Face::Test < Reline::TestCase
   RESET_SGR = "\e[0m"
 
+  def setup
+    @colorterm_backup = ENV['COLORTERM']
+    ENV['COLORTERM'] = 'truecolor'
+  end
+
   def teardown
     Reline::Face.reset_to_initial_configs
+    ENV['COLORTERM'] = @colorterm_backup
   end
 
   class WithInsufficientSetupTest < self
     def setup
+      super
       Reline::Face.config(:my_insufficient_config) do |face|
       end
       @face = Reline::Face[:my_insufficient_config]
@@ -37,6 +44,7 @@ class Reline::Face::Test < Reline::TestCase
 
   class WithSetupTest < self
     def setup
+      super
       Reline::Face.config(:my_config) do |face|
         face.define :default, foreground: :blue
         face.define :enhanced, foreground: "#FF1020", background: :black, style: [:bold, :underlined]
@@ -148,9 +156,15 @@ class Reline::Face::Test < Reline::TestCase
 
   class ConfigTest < self
     def setup
+      super
       @config = Reline::Face.const_get(:Config).new(:my_config) { }
     end
 
+    def teardown
+      super
+      Reline::Face.instance_variable_set(:@force_truecolor, nil)
+    end
+
     def test_rgb?
       assert_equal true, @config.send(:rgb_expression?, "#FFFFFF")
     end
@@ -190,9 +204,54 @@ class Reline::Face::Test < Reline::TestCase
       )
     end
 
-    def test_sgr_rgb
+    def test_truecolor
+      ENV['COLORTERM'] = 'truecolor'
+      assert_equal true, Reline::Face.truecolor?
+      ENV['COLORTERM'] = '24bit'
+      assert_equal true, Reline::Face.truecolor?
+      ENV['COLORTERM'] = nil
+      assert_equal false, Reline::Face.truecolor?
+      Reline::Face.force_truecolor
+      assert_equal true, Reline::Face.truecolor?
+    end
+
+    def test_sgr_rgb_truecolor
+      ENV['COLORTERM'] = 'truecolor'
       assert_equal "38;2;255;255;255", @config.send(:sgr_rgb, :foreground, "#ffffff")
       assert_equal "48;2;18;52;86", @config.send(:sgr_rgb, :background, "#123456")
     end
+
+    def test_sgr_rgb_256color
+      ENV['COLORTERM'] = nil
+      assert_equal '38;5;231', @config.send(:sgr_rgb, :foreground, '#ffffff')
+      assert_equal '48;5;16', @config.send(:sgr_rgb, :background, '#000000')
+      # Color steps are [0x00, 0x5f, 0x87, 0xaf, 0xd7, 0xff]
+      assert_equal '38;5;24', @config.send(:sgr_rgb, :foreground, '#005f87')
+      assert_equal '38;5;67', @config.send(:sgr_rgb, :foreground, '#5f87af')
+      assert_equal '48;5;110', @config.send(:sgr_rgb, :background, '#87afd7')
+      assert_equal '48;5;153', @config.send(:sgr_rgb, :background, '#afd7ff')
+      # Boundary values are [0x30, 0x73, 0x9b, 0xc3, 0xeb]
+      assert_equal '38;5;24', @config.send(:sgr_rgb, :foreground, '#2f729a')
+      assert_equal '38;5;67', @config.send(:sgr_rgb, :foreground, '#30739b')
+      assert_equal '48;5;110', @config.send(:sgr_rgb, :background, '#9ac2ea')
+      assert_equal '48;5;153', @config.send(:sgr_rgb, :background, '#9bc3eb')
+    end
+
+    def test_force_truecolor_reconfigure
+      ENV['COLORTERM'] = nil
+
+      Reline::Face.config(:my_config) do |face|
+        face.define :default, foreground: '#005f87'
+        face.define :enhanced, background: '#afd7ff'
+      end
+
+      assert_equal "\e[0m\e[38;5;24m", Reline::Face[:my_config][:default]
+      assert_equal "\e[0m\e[48;5;153m", Reline::Face[:my_config][:enhanced]
+
+      Reline::Face.force_truecolor
+
+      assert_equal "\e[0m\e[38;2;0;95;135m", Reline::Face[:my_config][:default]
+      assert_equal "\e[0m\e[48;2;175;215;255m", Reline::Face[:my_config][:enhanced]
+    end
   end
 end