Support 256-color terminal environments

Fixes #110

With this commit, delta enters "24 bit color mode" iff either of the
following are true:

1. The --24-bit-color command line option value is "always"

2. The --24-bit-color command line option value is "auto" and the
   COLORTERM environment variable is set to "truecolor" or "24bit".
   See
   https://gist.github.com/XVilka/8346728#true-color-detection
   https://github.com/sharkdp/bat/blob/7779d9f6221b3e98c43a43ceb5596ba285fdf4f8/src/bin/bat/app.rs#L29-L33

Otherwise, delta enters "8-bit color mode".

In "24 bit color mode", delta will

1. Emit 24-bit RGB color shell escape sequences that will only be
   displayed correctly by a terminal application that supports 24 bit
   colors.

2. Select default background colors that will only be displayed
   correctly by a terminal application that supports 24 bit colors.

In "8-bit color mode", delta will

1. Emit color shell escape sequences specifying the entry in the ANSI
   256 color palette that is closest (according to the ansi_colours
   library) to the requested color.

2. Select default background colors that will work well in a terminal
   application that supports 8-bit color but not 24-bit color.

Cargo.toml

@@ -16,6 +16,7 @@ name = "delta"
 path = "src/main.rs"
 
 [dependencies]
+ansi_colours = "1.0.1"
 ansi_term = "0.11.0"
 atty = "0.2.13"
 box_drawing = "0.1.2"

src/cli.rs

@@ -8,6 +8,7 @@ use structopt::StructOpt;
 use crate::bat::assets::HighlightingAssets;
 use crate::bat::output::PagingMode;
 use crate::config;
+use crate::env;
 use crate::style;
 
 #[derive(StructOpt, Clone, Debug)]
@@ -155,6 +156,14 @@ pub struct Opt {
     #[structopt(long = "max-line-distance", default_value = "0.3")]
     pub max_line_distance: f64,
 
+    /// Whether to emit 24-bit ("true color") RGB color codes. Options are auto, always, and never.
+    /// "auto" means that delta will emit 24-bit color codes iff the environment variable COLORTERM
+    /// has the value "truecolor" or "24bit". If your terminal application (the application you use
+    /// to enter commands at a shell prompt) supports 24 bit colors, then it probably already sets
+    /// this environment variable, in which case you don't need to do anything.
+    #[structopt(long = "24-bit-color", default_value = "auto")]
+    pub true_color: String,
+
     /// Whether to use a pager when displaying output. Options are: auto, always, and never. The
     /// default pager is `less`: this can be altered by setting the environment variables BAT_PAGER
     /// or PAGER (BAT_PAGER has priority).
@@ -257,12 +266,32 @@ pub fn process_command_line_arguments<'a>(
         }
     };
 
+    let true_color = match opt.true_color.as_ref() {
+        "always" => true,
+        "never" => false,
+        "auto" => is_truecolor_terminal(),
+        _ => {
+            eprintln!(
+                "Invalid value for --24-bit-color option: {} (valid values are \"always\", \"never\", and \"auto\")",
+                opt.true_color
+            );
+            process::exit(1);
+        }
+    };
+
     config::get_config(
         opt,
         &assets.syntax_set,
         &assets.theme_set,
+        true_color,
         available_terminal_width,
         background_color_width,
         paging_mode,
     )
 }
+
+fn is_truecolor_terminal() -> bool {
+    env::get_env_var("COLORTERM")
+        .map(|colorterm| colorterm == "truecolor" || colorterm == "24bit")
+        .unwrap_or(false)
+}

src/config.rs

@@ -4,11 +4,11 @@ use std::str::FromStr;
 use syntect::highlighting::{Color, Style, StyleModifier, Theme, ThemeSet};
 use syntect::parsing::SyntaxSet;
 
+use crate::bat::output::PagingMode;
 use crate::cli;
 use crate::env;
 use crate::paint;
 use crate::style;
-use crate::bat::output::{PagingMode};
 
 pub struct Config<'a> {
     pub theme: Option<&'a Theme>,
@@ -22,6 +22,7 @@ pub struct Config<'a> {
     pub hunk_color: Color,
     pub syntax_set: &'a SyntaxSet,
     pub terminal_width: usize,
+    pub true_color: bool,
     pub width: Option<usize>,
     pub tab_width: usize,
     pub opt: &'a cli::Opt,
@@ -34,6 +35,7 @@ pub fn get_config<'a>(
     opt: &'a cli::Opt,
     syntax_set: &'a SyntaxSet,
     theme_set: &'a ThemeSet,
+    true_color: bool,
     terminal_width: usize,
     width: Option<usize>,
     paging_mode: PagingMode,
@@ -53,11 +55,9 @@ pub fn get_config<'a>(
     };
 
     let minus_style_modifier = StyleModifier {
-        background: Some(color_from_arg_or_mode_default(
+        background: Some(color_from_arg(
             opt.minus_color.as_ref(),
-            is_light_mode,
-            style::LIGHT_THEME_MINUS_COLOR,
-            style::DARK_THEME_MINUS_COLOR,
+            style::get_minus_color_default(is_light_mode, true_color),
         )),
         foreground: if opt.highlight_removed {
             None
@@ -68,11 +68,9 @@ pub fn get_config<'a>(
     };
 
     let minus_emph_style_modifier = StyleModifier {
-        background: Some(color_from_arg_or_mode_default(
+        background: Some(color_from_arg(
             opt.minus_emph_color.as_ref(),
-            is_light_mode,
-            style::LIGHT_THEME_MINUS_EMPH_COLOR,
-            style::DARK_THEME_MINUS_EMPH_COLOR,
+            style::get_minus_emph_color_default(is_light_mode, true_color),
         )),
         foreground: if opt.highlight_removed {
             None
@@ -83,22 +81,18 @@ pub fn get_config<'a>(
     };
 
     let plus_style_modifier = StyleModifier {
-        background: Some(color_from_arg_or_mode_default(
+        background: Some(color_from_arg(
             opt.plus_color.as_ref(),
-            is_light_mode,
-            style::LIGHT_THEME_PLUS_COLOR,
-            style::DARK_THEME_PLUS_COLOR,
+            style::get_plus_color_default(is_light_mode, true_color),
         )),
         foreground: None,
         font_style: None,
     };
 
     let plus_emph_style_modifier = StyleModifier {
-        background: Some(color_from_arg_or_mode_default(
+        background: Some(color_from_arg(
             opt.plus_emph_color.as_ref(),
-            is_light_mode,
-            style::LIGHT_THEME_PLUS_EMPH_COLOR,
-            style::DARK_THEME_PLUS_EMPH_COLOR,
+            style::get_plus_emph_color_default(is_light_mode, true_color),
         )),
         foreground: None,
         font_style: None,
@@ -114,6 +108,7 @@ pub fn get_config<'a>(
         commit_color: color_from_rgb_or_ansi_code(&opt.commit_color),
         file_color: color_from_rgb_or_ansi_code(&opt.file_color),
         hunk_color: color_from_rgb_or_ansi_code(&opt.hunk_color),
+        true_color,
         terminal_width,
         width,
         tab_width: opt.tab_width,
@@ -192,20 +187,9 @@ fn color_from_rgb_or_ansi_code(s: &str) -> Color {
     }
 }
 
-fn color_from_arg_or_mode_default(
-    arg: Option<&String>,
-    is_light_mode: bool,
-    light_theme_default: Color,
-    dark_theme_default: Color,
-) -> Color {
+fn color_from_arg(arg: Option<&String>, default: Color) -> Color {
     match arg {
         Some(string) => color_from_rgb_or_ansi_code(&string),
-        None => {
-            if is_light_mode {
-                light_theme_default
-            } else {
-                dark_theme_default
-            }
-        }
+        None => default,
     }
 }

src/delta.rs

@@ -204,6 +204,7 @@ fn handle_commit_meta_header_line(
         config.terminal_width,
         config.commit_color,
         true,
+        config.true_color,
     )?;
     Ok(())
 }
@@ -234,10 +235,11 @@ fn handle_generic_file_meta_header_line(
     writeln!(painter.writer)?;
     draw_fn(
         painter.writer,
-        &paint::paint_text_foreground(line, config.file_color),
+        &paint::paint_text_foreground(line, config.file_color, config.true_color),
         config.terminal_width,
         config.file_color,
         false,
+        config.true_color,
     )?;
     Ok(())
 }
@@ -279,13 +281,14 @@ fn handle_hunk_meta_line(
             config.terminal_width,
             config.hunk_color,
             false,
+            config.true_color,
         )?;
         painter.output_buffer.clear();
     }
     writeln!(
         painter.writer,
         "\n{}",
-        paint::paint_text_foreground(line_number, config.hunk_color)
+        paint::paint_text_foreground(line_number, config.hunk_color, config.true_color)
     )?;
     Ok(())
 }
@@ -431,6 +434,7 @@ mod tests {
 
     #[test]
     fn test_theme_selection() {
+        #[derive(PartialEq)]
         enum Mode {
             Light,
             Dark,
@@ -496,6 +500,7 @@ mod tests {
             } else {
                 env::set_var("BAT_THEME", bat_theme_env_var);
             }
+            let is_true_color = true;
             let mut options = get_command_line_options();
             options.theme = theme_option;
             match mode_option {
@@ -521,31 +526,19 @@ mod tests {
             }
             assert_eq!(
                 config.minus_style_modifier.background.unwrap(),
-                match expected_mode {
-                    Mode::Light => style::LIGHT_THEME_MINUS_COLOR,
-                    Mode::Dark => style::DARK_THEME_MINUS_COLOR,
-                }
+                style::get_minus_color_default(expected_mode == Mode::Light, is_true_color)
             );
             assert_eq!(
                 config.minus_emph_style_modifier.background.unwrap(),
-                match expected_mode {
-                    Mode::Light => style::LIGHT_THEME_MINUS_EMPH_COLOR,
-                    Mode::Dark => style::DARK_THEME_MINUS_EMPH_COLOR,
-                }
+                style::get_minus_emph_color_default(expected_mode == Mode::Light, is_true_color)
             );
             assert_eq!(
                 config.plus_style_modifier.background.unwrap(),
-                match expected_mode {
-                    Mode::Light => style::LIGHT_THEME_PLUS_COLOR,
-                    Mode::Dark => style::DARK_THEME_PLUS_COLOR,
-                }
+                style::get_plus_color_default(expected_mode == Mode::Light, is_true_color)
             );
             assert_eq!(
                 config.plus_emph_style_modifier.background.unwrap(),
-                match expected_mode {
-                    Mode::Light => style::LIGHT_THEME_PLUS_EMPH_COLOR,
-                    Mode::Dark => style::DARK_THEME_PLUS_EMPH_COLOR,
-                }
+                style::get_plus_emph_color_default(expected_mode == Mode::Light, is_true_color)
             );
         }
     }
@@ -580,7 +573,7 @@ mod tests {
     fn paint_text(input: &str, style_modifier: StyleModifier, config: &Config) -> String {
         let mut output = String::new();
         let style = config.no_style.apply(style_modifier);
-        paint::paint_text(&input, style, &mut output);
+        paint::paint_text(&input, style, &mut output, config.true_color);
         output
     }
 
@@ -623,6 +616,7 @@ mod tests {
             file_color: "Blue".to_string(),
             hunk_style: cli::SectionStyle::Box,
             hunk_color: "blue".to_string(),
+            true_color: "always".to_string(),
             width: Some("variable".to_string()),
             paging_mode: "auto".to_string(),
             tab_width: 4,