Language struct + header reading

src/main.rs

@@ -16,6 +16,71 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with Saul. If not, see <https://www.gnu.org/licenses/>.
 
+use std::{
+    borrow::Cow,
+    io::{self, BufRead},
+};
+
+/// Configuration of a particular language. Affects parsing and output.
+pub struct Language {
+    /// A string that appears before all commented lines.
+    pub comment: Cow<'static, str>,
+}
+
+impl Language {
+    /// The default Rust language config.
+    pub const RUST: Self = Self {
+        comment: Cow::Borrowed("// "),
+    };
+
+    /// Extracts a header (as a list of lines) from an input in this language.
+    ///
+    /// Don't reply on the state of the reader being consistent after this function
+    /// is called.
+    pub fn read_header(&self, f: &mut impl BufRead) -> io::Result<Vec<String>> {
+        let mut header = Vec::new();
+        let mut is_first_line = true;
+
+        for line in f.lines() {
+            let line = line?;
+
+            if is_first_line && line.starts_with("#!") {
+                is_first_line = false;
+                continue;
+            }
+
+            is_first_line = false;
+
+            let Some(content) = line.strip_prefix(self.comment.as_ref()) else {
+                break;
+            };
+
+            header.push(content.trim().to_string());
+        }
+
+        Ok(header)
+    }
+}
+
 fn main() {
     println!("Hello, world!");
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn header() {
+        let src = b"// header content\n// header content 2\nbody";
+        let lines = Language::RUST.read_header(&mut src.as_slice()).unwrap();
+        assert_eq!(lines, vec!["header content", "header content 2"]);
+    }
+
+    #[test]
+    fn skip_header_shebang() {
+        let src = b"#!/bin/sh\n// header content\nbody goes here";
+        let lines = Language::RUST.read_header(&mut src.as_slice()).unwrap();
+        assert_eq!(lines, vec!["header content"]);
+    }
+}