T40: review round 1: a notification names the session as the output stands at its place: the tap follows OSC 0 and 2, the title stack's CSI 22 t and 23 t and a full reset as alacritty does, and sends each title in order, since alacritty's own title events come after the tap's; one write and split writes post the same

src/term/titles.rs

@@ -0,0 +1,80 @@
+/// The titles alacritty saves before it drops the oldest.
+const SAVED: usize = 4096;
+
+/// A program's title as alacritty keeps it, read in the output's order: OSC 0 and 2 set it, CSI 22 t saves it, CSI 23 t brings it back, a full reset empties both.
+#[derive(Default)]
+pub(crate) struct Titles {
+    /// Empty while the program names none.
+    title: String,
+    saved: Vec<String>,
+}
+
+impl Titles {
+    /// Sets the title an OSC 0 or 2 body past its number names: its UTF-8 parameters joined by `;` and trimmed, as alacritty joins them.
+    pub fn set(&mut self, rest: &[u8]) -> String {
+        let parts: Vec<&str> = rest
+            .split(|&byte| byte == b';')
+            .filter_map(|part| std::str::from_utf8(part).ok())
+            .collect();
+        self.title = parts.join(";").trim().to_owned();
+        self.title.clone()
+    }
+
+    /// Saves the title: CSI 22 t.
+    pub fn push(&mut self) {
+        if self.saved.len() == SAVED {
+            self.saved.remove(0);
+        }
+        self.saved.push(self.title.clone());
+    }
+
+    /// Brings the last title saved back: CSI 23 t; none when none was saved.
+    pub fn pop(&mut self) -> Option<String> {
+        let title = self.saved.pop()?;
+        self.title.clone_from(&title);
+        Some(title)
+    }
+
+    /// A full reset empties the title and those saved.
+    pub fn reset(&mut self) {
+        self.title.clear();
+        self.saved.clear();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn a_title_reads_as_alacritty_joins_it() {
+        let mut titles = Titles::default();
+        assert_eq!(titles.set(b"  a;b  "), "a;b");
+        assert_eq!(
+            titles.set(b"a;\xff;b"),
+            "a;b",
+            "a part not in UTF-8 passes by"
+        );
+        assert_eq!(titles.set(b""), "");
+    }
+
+    #[test]
+    fn a_title_saved_comes_back_and_a_reset_empties_both() {
+        let mut titles = Titles::default();
+        assert_eq!(titles.pop(), None, "none saved");
+        titles.set(b"shell");
+        titles.push();
+        titles.set(b"vim");
+        assert_eq!(titles.pop().as_deref(), Some("shell"));
+        assert_eq!(titles.pop(), None);
+        titles.push();
+        titles.reset();
+        assert_eq!(titles.pop(), None, "a reset empties those saved");
+        for at in 0..=SAVED {
+            titles.set(at.to_string().as_bytes());
+            titles.push();
+        }
+        assert_eq!(titles.saved.len(), SAVED);
+        assert_eq!(titles.saved[0], "1", "the oldest went");
+    }
+}