Add `--strip-ansi=auto` option

When using `auto`, escape sequences will be stripped unless printing
plain text.

6 files changed, 107 insertions, 18 deletions

diff --git a/doc/long-help.txt b/doc/long-help.txt
index 93f56968..d9cdce39 100644
--- a/doc/long-help.txt
+++ b/doc/long-help.txt
@@ -123,8 +123,9 @@ Options:
           Set the maximum number of consecutive empty lines to be printed.
 
       --strip-ansi <when>
-          Specify when to strip ANSI escape sequences from the input. Possible values: always,
-          *never*.
+          Specify when to strip ANSI escape sequences from the input. The automatic mode will remove
+          escape sequences unless the syntax highlighting language is plain text. Possible values:
+          auto, always, *never*.
 
       --style <components>
           Configure which elements (line numbers, file headers, grid borders, Git modifications, ..)
diff --git a/src/bin/bat/app.rs b/src/bin/bat/app.rs
index 62ffdd6d..0d2600b2 100644
--- a/src/bin/bat/app.rs
+++ b/src/bin/bat/app.rs
@@ -250,6 +250,7 @@ impl App {
             {
                 Some("never") => StripAnsiMode::Never,
                 Some("always") => StripAnsiMode::Always,
+                Some("auto") => StripAnsiMode::Auto,
                 _ => unreachable!("other values for --strip-ansi are not allowed"),
             },
             theme: self
diff --git a/src/bin/bat/clap_app.rs b/src/bin/bat/clap_app.rs
index 32c7c077..e70b1a5b 100644
--- a/src/bin/bat/clap_app.rs
+++ b/src/bin/bat/clap_app.rs
@@ -407,11 +407,13 @@ pub fn build_app(interactive_output: bool) -> Command {
                 .long("strip-ansi")
                 .overrides_with("strip-ansi")
                 .value_name("when")
-                .value_parser(["always", "never"])
+                .value_parser(["auto", "always", "never"])
                 .default_value("never")
                 .hide_default_value(true)
-                .help("Strip colors from the input (always, *never*)")
-                .long_help("Specify when to strip ANSI escape sequences from the input. Possible values: always, *never*.")
+                .help("Strip colors from the input (auto, always, *never*)")
+                .long_help("Specify when to strip ANSI escape sequences from the input. \
+                The automatic mode will remove escape sequences unless the syntax highlighting \
+                language is plain text. Possible values: auto, always, *never*.")
                 .hide_short_help(true)
         )
         .arg(
diff --git a/src/preprocessor.rs b/src/preprocessor.rs
index 707946f9..dc2aa66e 100644
--- a/src/preprocessor.rs
+++ b/src/preprocessor.rs
@@ -154,6 +154,7 @@ pub enum StripAnsiMode {
     #[default]
     Never,
     Always,
+    Auto,
 }
 
 #[test]
diff --git a/src/printer.rs b/src/printer.rs
index d76e6e0a..e9bea3fd 100644
--- a/src/printer.rs
+++ b/src/printer.rs
@@ -268,26 +268,40 @@ impl<'a> InteractivePrinter<'a> {
             .content_type
             .map_or(false, |c| c.is_binary() && !config.show_nonprintable);
 
-        let highlighter_from_set = if is_printing_binary || !config.colored_output {
-            None
-        } else {
-            // Determine the type of syntax for highlighting
-            let syntax_in_set =
-                match assets.get_syntax(config.language, input, &config.syntax_mapping) {
-                    Ok(syntax_in_set) => syntax_in_set,
-                    Err(Error::UndetectedSyntax(_)) => assets
-                        .find_syntax_by_name("Plain Text")?
-                        .expect("A plain text syntax is available"),
-                    Err(e) => return Err(e),
-                };
+        let needs_to_match_syntax = !is_printing_binary
+            && (config.colored_output || config.strip_ansi == StripAnsiMode::Auto);
 
-            Some(HighlighterFromSet::new(syntax_in_set, theme))
+        let (is_plain_text, highlighter_from_set) = if needs_to_match_syntax {
+            // Determine the type of syntax for highlighting
+            const PLAIN_TEXT_SYNTAX: &str = "Plain Text";
+            match assets.get_syntax(config.language, input, &config.syntax_mapping) {
+                Ok(syntax_in_set) => (
+                    syntax_in_set.syntax.name == PLAIN_TEXT_SYNTAX,
+                    Some(HighlighterFromSet::new(syntax_in_set, theme)),
+                ),
+
+                Err(Error::UndetectedSyntax(_)) => (
+                    true,
+                    Some(
+                        assets
+                            .find_syntax_by_name(PLAIN_TEXT_SYNTAX)?
+                            .map(|s| HighlighterFromSet::new(s, theme))
+                            .expect("A plain text syntax is available"),
+                    ),
+                ),
+
+                Err(e) => return Err(e),
+            }
+        } else {
+            (false, None)
         };
 
         // Determine when to strip ANSI sequences
         let strip_ansi = match config.strip_ansi {
             _ if config.show_nonprintable => false,
             StripAnsiMode::Always => true,
+            StripAnsiMode::Auto if is_plain_text => false, // Plain text may already contain escape sequences.
+            StripAnsiMode::Auto => true,
             _ => false,
         };
 
diff --git a/tests/integration_tests.rs b/tests/integration_tests.rs
index bc86cb9b..d6009361 100644
--- a/tests/integration_tests.rs
+++ b/tests/integration_tests.rs
@@ -2740,3 +2740,73 @@ fn strip_ansi_does_not_strip_when_show_nonprintable() {
 
     assert!(output.contains("␛"))
 }
+
+#[test]
+fn strip_ansi_auto_strips_ansi_when_detected_syntax_by_filename() {
+    bat()
+        .arg("--style=plain")
+        .arg("--decorations=always")
+        .arg("--color=never")
+        .arg("--strip-ansi=auto")
+        .arg("--file-name=test.rs")
+        .write_stdin("fn \x1B[33mYellow\x1B[m() -> () {}")
+        .assert()
+        .success()
+        .stdout("fn Yellow() -> () {}");
+}
+
+#[test]
+fn strip_ansi_auto_strips_ansi_when_provided_syntax_by_option() {
+    bat()
+        .arg("--style=plain")
+        .arg("--decorations=always")
+        .arg("--color=never")
+        .arg("--strip-ansi=auto")
+        .arg("--language=rust")
+        .write_stdin("fn \x1B[33mYellow\x1B[m() -> () {}")
+        .assert()
+        .success()
+        .stdout("fn Yellow() -> () {}");
+}
+
+#[test]
+fn strip_ansi_auto_does_not_strip_when_plain_text_by_filename() {
+    let output = String::from_utf8(
+        bat()
+            .arg("--style=plain")
+            .arg("--decorations=always")
+            .arg("--color=never")
+            .arg("--strip-ansi=auto")
+            .arg("--file-name=ansi.txt")
+            .write_stdin("\x1B[33mYellow\x1B[m")
+            .assert()
+            .success()
+            .get_output()
+            .stdout
+            .clone(),
+    )
+    .expect("valid utf8");
+
+    assert!(output.contains("\x1B[33mYellow"))
+}
+
+#[test]
+fn strip_ansi_auto_does_not_strip_ansi_when_plain_text_by_option() {
+    let output = String::from_utf8(
+        bat()
+            .arg("--style=plain")
+            .arg("--decorations=always")
+            .arg("--color=never")
+            .arg("--strip-ansi=auto")
+            .arg("--language=txt")
+            .write_stdin("\x1B[33mYellow\x1B[m")
+            .assert()
+            .success()
+            .get_output()
+            .stdout
+            .clone(),
+    )
+    .expect("valid utf8");
+
+    assert!(output.contains("\x1B[33mYellow"))
+}