Add tailnew: follow the newest log file in a directory

tailnew tails the most recently started file in a directory and switches
to each new one as it appears, for watching harnesses such as boxmaker's
run-plan.sh that write one log per task. A file counts as new when its
name is new, when the name points at a different file, or when it has
shrunk (a rerun truncating its old log).

Three front ends share one follower: plain tail -f style output, a
ratatui terminal view with scrollback, and an egui window with a folder
dialog and a Recursive checkbox. The Makefile provides build, gate and a
per-user install that also installs a desktop entry for the window.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -1 +1,4 @@
 A monorepo for tools interacting with LLMs.
+
+- `retrieval/`: arxiv retrieval layer (semantic and keyword search).
+- `tailnew/`: follows the newest log file in a directory (CLI, terminal UI, egui window).

tailnew/.gitignore

@@ -0,0 +1 @@
+/target

tailnew/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "tailnew"
+version = "0.1.0"
+edition = "2024"
+description = "Follow the newest file in a directory, switching to each new file as it appears"
+
+[dependencies]
+clap = { version = "4.6", features = ["derive"] }
+eframe = { version = "0.32", optional = true }
+rfd = { version = "0.15", optional = true }
+ratatui = "0.29"
+unicode-width = "0.2"
+
+[features]
+default = ["gui"]
+# The desktop window (--gui) and its folder dialog. Build with --no-default-features for a terminal-only binary.
+gui = ["dep:eframe", "dep:rfd"]
+
+[dev-dependencies]
+tempfile = "3"

tailnew/Makefile

@@ -0,0 +1,31 @@
+# Per-user install: the binary and the desktop entry for the window.
+
+PREFIX  ?= $(HOME)/.local
+BINDIR  := $(PREFIX)/bin
+APPDIR  := $(or $(XDG_DATA_HOME),$(HOME)/.local/share)/applications
+
+.PHONY: build gate install uninstall
+
+build:
+	cargo build --release
+
+# Formatting, lints with and without the window, tests, and the desktop entry. Offline once
+# dependencies are fetched.
+gate:
+	cargo fmt --check
+	cargo clippy --all-targets -- -D warnings
+	cargo clippy --all-targets --no-default-features -- -D warnings
+	cargo test
+	desktop-file-validate dist/tailnew.desktop
+	@echo "gate: ok"
+
+# dist/tailnew.desktop carries an @BINDIR@ placeholder so it points wherever PREFIX put the binary.
+install: build
+	install -Dm755 target/release/tailnew $(BINDIR)/tailnew
+	install -d $(APPDIR)
+	sed 's|@BINDIR@|$(BINDIR)|' dist/tailnew.desktop > $(APPDIR)/tailnew.desktop
+	-update-desktop-database $(APPDIR)
+
+uninstall:
+	rm -f $(BINDIR)/tailnew $(APPDIR)/tailnew.desktop
+	-update-desktop-database $(APPDIR)

tailnew/README.md

@@ -0,0 +1,40 @@
+# tailnew
+
+Follows the newest file in a directory, like `tail -f`, and switches to each new file as it
+appears. Built for watching harnesses such as boxmaker's `tools/run-plan.sh`, which write one log
+per task.
+
+    make install                       # ~/.local/bin/tailnew and a desktop entry for the window
+    tailnew .state/runs/M2a            # plain output, `==> file <==` banner on each switch
+    tailnew -t -r .state/runs          # full-screen terminal view, every milestone's directory
+    tailnew --gui -r .state/runs       # desktop window (egui)
+    tailnew --gui                      # desktop window, asks for the folder first
+
+`make gate` checks formatting, runs clippy with and without the window, runs the tests, and
+validates the desktop entry. `PREFIX=/usr/local make install` installs elsewhere;
+`cargo build --no-default-features` builds without the window.
+
+A file is new when its name has not been seen before, when the name now points at a different
+file, or when it has shrunk (a rerun that truncated its old log with `>`). The directory is polled
+(`-i MS`, default 250) and need not exist yet. Hidden files are ignored; `-g '*.log'` narrows the
+match further.
+
+Keys in the full-screen terminal view (`-t`):
+
+| key                     | action                                             |
+|-------------------------|----------------------------------------------------|
+| ↑ ↓ / k j               | scroll a line                                      |
+| PgUp PgDn / b space     | scroll a page (ctrl-u / ctrl-d: half a page)       |
+| g / G                   | top / back to following the tail                   |
+| [ ]                     | older / newer file (pins the view to it)           |
+| p                       | pin to the current file; again to unpin and jump to the newest |
+| L                       | unpin and jump to the newest file                  |
+| w                       | toggle line wrapping                               |
+| f                       | toggle the file list                               |
+| q / esc                 | quit                                               |
+
+The window (`--gui`) has a **Folder…** button (the desktop's folder dialog, also `o`) and a
+**Recursive** checkbox, buttons for the same file controls, a clickable file list, and the keys
+`[` `]` `p` `L` `f`, `G`/End to jump to the end, and `q`/Esc to quit. Started without a folder,
+as from the launcher, it opens the folder dialog straight away. It scrolls with the mouse,
+sticks to the bottom while you are there, and does not wrap long lines (scroll sideways instead).

tailnew/dist/tailnew.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=tailnew
+GenericName=Log Follower
+Comment=Follow the newest log file in a folder
+Exec=@BINDIR@/tailnew --gui %f
+Icon=utilities-log-viewer
+Terminal=false
+Categories=Development;
+Keywords=log;tail;follow;
+StartupWMClass=tailnew

tailnew/src/cli.rs

@@ -0,0 +1,47 @@
+//! Plain `tail -f` style output: bytes go to stdout as they arrive, with a `==> file <==` banner
+//! whenever the followed file changes.
+
+use std::io::{self, Write};
+use std::thread;
+use std::time::Duration;
+
+use crate::watch::{Event, Follower};
+
+pub fn run(mut follower: Follower, interval: Duration, quiet: bool) -> io::Result<()> {
+    let mut out = io::stdout().lock();
+    let mut at_line_start = true;
+    let mut first_banner = true;
+    let mut announced_wait = false;
+    loop {
+        let events = follower.poll()?;
+        if follower.current_path().is_none() && !announced_wait {
+            eprintln!("tailnew: waiting for files in {}", follower.dir().display());
+            announced_wait = true;
+        }
+        for event in events {
+            let banner = match &event {
+                Event::Switched(path) => format!("==> {} <==", path.display()),
+                Event::Restarted(path) => format!("==> {} (restarted) <==", path.display()),
+                Event::Data(bytes) => {
+                    out.write_all(bytes)?;
+                    at_line_start = bytes.ends_with(b"\n");
+                    continue;
+                }
+            };
+            if quiet {
+                continue;
+            }
+            if !at_line_start {
+                writeln!(out)?;
+            }
+            if !first_banner {
+                writeln!(out)?;
+            }
+            writeln!(out, "{banner}")?;
+            first_banner = false;
+            at_line_start = true;
+        }
+        out.flush()?;
+        thread::sleep(interval);
+    }
+}

tailnew/src/gui.rs

@@ -0,0 +1,311 @@
+//! Desktop window (egui): a header with the folder and file controls, an optional file list, the
+//! log itself, and a status line.
+
+use std::io;
+use std::path::PathBuf;
+use std::sync::mpsc::{self, Receiver, TryRecvError};
+use std::thread;
+use std::time::{Duration, Instant};
+
+use eframe::egui::{
+    self, Color32, Key, Label, RichText, ScrollArea, TextStyle, TextWrapMode, ViewportCommand,
+};
+
+use crate::view::View;
+use crate::watch::{Follower, Options, Start};
+
+/// Follows `opts.dir`, or with `chosen` false, opens the folder dialog first.
+pub fn run(
+    opts: Options,
+    chosen: bool,
+    lines: usize,
+    interval: Duration,
+    scrollback: usize,
+) -> io::Result<()> {
+    let options = eframe::NativeOptions {
+        viewport: egui::ViewportBuilder::default()
+            .with_title("tailnew")
+            .with_app_id("tailnew")
+            .with_inner_size([1100.0, 750.0]),
+        ..Default::default()
+    };
+    let recursive = opts.recursive;
+    let mut gui = Gui {
+        view: View::new(Follower::new(opts, Start::LastLines(lines)), scrollback),
+        chosen,
+        recursive,
+        picking: None,
+        lines,
+        scrollback,
+        interval,
+        next_poll: Instant::now(),
+        show_files: true,
+        seen_generation: 0,
+        to_end: false,
+    };
+    eframe::run_native(
+        "tailnew",
+        options,
+        Box::new(move |cc| {
+            if !gui.chosen {
+                gui.pick(&cc.egui_ctx);
+            }
+            Ok(Box::new(gui))
+        }),
+    )
+    .map_err(|e| io::Error::other(e.to_string()))
+}
+
+struct Gui {
+    view: View,
+    /// False until a folder has been chosen, when started without one; nothing is polled till then.
+    chosen: bool,
+    /// The Recursive checkbox; applied by rebuilding the view.
+    recursive: bool,
+    /// The folder dialog runs on its own thread so the window keeps drawing; its answer lands here.
+    picking: Option<Receiver<Option<PathBuf>>>,
+    lines: usize,
+    scrollback: usize,
+    interval: Duration,
+    next_poll: Instant,
+    show_files: bool,
+    seen_generation: u64,
+    /// Scroll to the end on the next frame; the scroll area then sticks there as lines arrive.
+    to_end: bool,
+}
+
+impl eframe::App for Gui {
+    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
+        self.check_pick();
+        if self.chosen && Instant::now() >= self.next_poll {
+            self.view.poll();
+            self.next_poll = Instant::now() + self.interval;
+        }
+        ctx.request_repaint_after(self.next_poll.saturating_duration_since(Instant::now()));
+
+        self.keys(ctx);
+        if self.view.generation() != self.seen_generation {
+            self.seen_generation = self.view.generation();
+            self.to_end = true;
+        }
+
+        egui::TopBottomPanel::top("header").show(ctx, |ui| self.header(ui));
+        egui::TopBottomPanel::bottom("status").show(ctx, |ui| self.status(ui));
+        if self.show_files {
+            egui::SidePanel::left("files")
+                .resizable(true)
+                .default_width(220.0)
+                .show(ctx, |ui| self.files(ui));
+        }
+        egui::CentralPanel::default().show(ctx, |ui| self.log(ui));
+    }
+}
+
+impl Gui {
+    /// Opens the folder dialog, starting from the current folder.
+    fn pick(&mut self, ctx: &egui::Context) {
+        if self.picking.is_some() {
+            return;
+        }
+        let start = std::fs::canonicalize(self.view.follower.dir()).ok();
+        let (tx, rx) = mpsc::channel();
+        let ctx = ctx.clone();
+        thread::spawn(move || {
+            let mut dialog = rfd::FileDialog::new().set_title("Folder to watch");
+            if let Some(start) = start {
+                dialog = dialog.set_directory(start);
+            }
+            let _ = tx.send(dialog.pick_folder());
+            ctx.request_repaint();
+        });
+        self.picking = Some(rx);
+    }
+
+    fn check_pick(&mut self) {
+        let Some(rx) = &self.picking else { return };
+        match rx.try_recv() {
+            Ok(Some(dir)) => {
+                self.picking = None;
+                self.watch(dir, self.recursive);
+            }
+            Ok(None) | Err(TryRecvError::Disconnected) => self.picking = None,
+            Err(TryRecvError::Empty) => {}
+        }
+    }
+
+    /// Starts over on `dir` with a fresh view.
+    fn watch(&mut self, dir: PathBuf, recursive: bool) {
+        let opts = Options {
+            dir,
+            recursive,
+            ..self.view.follower.options().clone()
+        };
+        self.view = View::new(
+            Follower::new(opts, Start::LastLines(self.lines)),
+            self.scrollback,
+        );
+        self.chosen = true;
+        self.recursive = recursive;
+        self.seen_generation = 0;
+        self.next_poll = Instant::now();
+    }
+
+    fn keys(&mut self, ctx: &egui::Context) {
+        let pressed = |key| ctx.input(|i| i.key_pressed(key));
+        if pressed(Key::Q) || pressed(Key::Escape) {
+            ctx.send_viewport_cmd(ViewportCommand::Close);
+        }
+        if pressed(Key::OpenBracket) {
+            self.view.step(-1);
+        }
+        if pressed(Key::CloseBracket) {
+            self.view.step(1);
+        }
+        if pressed(Key::P) {
+            self.view.toggle_pin();
+        }
+        if pressed(Key::L) {
+            self.view.go_latest();
+        }
+        if pressed(Key::F) {
+            self.show_files = !self.show_files;
+        }
+        if pressed(Key::O) {
+            self.pick(ctx);
+        }
+        if pressed(Key::End) || pressed(Key::G) {
+            self.to_end = true;
+        }
+    }
+
+    fn header(&mut self, ui: &mut egui::Ui) {
+        ui.horizontal(|ui| {
+            let button = egui::Button::new("📁 Folder…");
+            if ui
+                .add_enabled(self.picking.is_none(), button)
+                .on_hover_text("Choose the folder to watch (o)")
+                .clicked()
+            {
+                self.pick(ui.ctx());
+            }
+            let mut recursive = self.recursive;
+            if ui
+                .checkbox(&mut recursive, "Recursive")
+                .on_hover_text("Also watch subfolders")
+                .changed()
+            {
+                if self.chosen {
+                    let dir = self.view.follower.dir().to_path_buf();
+                    self.watch(dir, recursive);
+                } else {
+                    self.recursive = recursive;
+                }
+            }
+            ui.separator();
+            if !self.chosen {
+                ui.weak("no folder chosen");
+                return;
+            }
+            let dir = self.view.follower.dir().display().to_string();
+            ui.weak(dir);
+            // The folder is already shown, so the title's own "waiting for files in …" would repeat it.
+            if self.view.follower.current_path().is_some() {
+                ui.label(RichText::new(self.view.title()).strong().monospace());
+            } else {
+                ui.weak("waiting for files");
+            }
+            if self.view.follower.pinned() {
+                ui.label(
+                    RichText::new("PINNED")
+                        .strong()
+                        .color(Color32::from_rgb(230, 180, 40)),
+                );
+            }
+            ui.with_layout(egui::Layout::right_to_left(egui::Align::Center), |ui| {
+                if ui
+                    .button("Newest")
+                    .on_hover_text("Unpin and follow the newest file (L)")
+                    .clicked()
+                {
+                    self.view.go_latest();
+                }
+                let mut pinned = self.view.follower.pinned();
+                if ui
+                    .toggle_value(&mut pinned, "Pin")
+                    .on_hover_text("Stay on this file when new ones appear (p)")
+                    .clicked()
+                {
+                    self.view.toggle_pin();
+                }
+                if ui.button("Newer ▶").on_hover_text("]").clicked() {
+                    self.view.step(1);
+                }
+                if ui.button("◀ Older").on_hover_text("[").clicked() {
+                    self.view.step(-1);
+                }
+                ui.toggle_value(&mut self.show_files, "Files")
+                    .on_hover_text("Show the file list (f)");
+            });
+        });
+    }
+
+    fn status(&self, ui: &mut egui::Ui) {
+        ui.horizontal(|ui| {
+            if let Some(e) = self.view.error() {
+                ui.colored_label(ui.visuals().error_fg_color, e);
+            } else if let Some(status) = self.view.status() {
+                ui.weak(status);
+            }
+            ui.with_layout(egui::Layout::right_to_left(egui::Align::Center), |ui| {
+                ui.weak(format!("{} lines", self.view.total()));
+            });
+        });
+    }
+
+    fn files(&mut self, ui: &mut egui::Ui) {
+        ui.strong("Files, newest first");
+        ui.separator();
+        let mut open = None;
+        ScrollArea::vertical().auto_shrink(false).show(ui, |ui| {
+            let current = self.view.follower.current_path();
+            for e in self.view.follower.entries().iter().rev() {
+                let here = Some(e.path.as_path()) == current;
+                let name = RichText::new(self.view.relative(&e.path)).monospace();
+                if ui.selectable_label(here, name).clicked() && !here {
+                    open = Some(e.path.clone());
+                }
+            }
+        });
+        if let Some(path) = open {
+            self.view.open_pinned(&path);
+        }
+    }
+
+    fn log(&mut self, ui: &mut egui::Ui) {
+        if !self.chosen {
+            ui.centered_and_justified(|ui| {
+                let button = egui::Button::new("📁 Choose a folder to watch…");
+                if ui.add_enabled(self.picking.is_none(), button).clicked() {
+                    self.pick(ui.ctx());
+                }
+            });
+            return;
+        }
+        ui.spacing_mut().item_spacing.y = 0.0;
+        let row_height = ui.text_style_height(&TextStyle::Monospace);
+        let total = self.view.total();
+        let mut area = ScrollArea::both().auto_shrink(false).stick_to_bottom(true);
+        if std::mem::take(&mut self.to_end) {
+            // The whole content height; the scroll area clamps it to the last page. Not f32::MAX:
+            // show_rows turns the offset into a row index, which would overflow.
+            area = area.vertical_scroll_offset(total as f32 * row_height);
+        }
+        // Only the visible rows are laid out, so a long scrollback costs nothing per frame.
+        area.show_rows(ui, row_height, total, |ui, rows| {
+            for i in rows {
+                let text = RichText::new(self.view.line(i)).monospace();
+                ui.add(Label::new(text).wrap_mode(TextWrapMode::Extend));
+            }
+        });
+    }
+}

tailnew/src/main.rs

@@ -0,0 +1,104 @@
+mod cli;
+#[cfg(feature = "gui")]
+mod gui;
+mod tui;
+mod view;
+mod watch;
+
+use std::io;
+use std::path::PathBuf;
+use std::process::ExitCode;
+use std::time::Duration;
+
+use clap::Parser;
+
+use watch::{Follower, Options, Start};
+
+/// Follow the newest file in a directory, switching to each new file as it appears.
+///
+/// A file is new when its name has not been seen before, when the name now points at a different
+/// file, or when it has shrunk (a rerun that truncated its old log). Hidden files are ignored.
+#[derive(Parser)]
+#[command(version)]
+struct Args {
+    /// Directory to watch; it need not exist yet [default: ".", or a folder dialog with --gui].
+    dir: Option<PathBuf>,
+
+    /// Full-screen terminal view with scrollback instead of plain output.
+    #[arg(short, long)]
+    tui: bool,
+
+    /// Desktop window instead of plain output. Its folder dialog changes the directory.
+    #[cfg(feature = "gui")]
+    #[arg(long, conflicts_with = "tui")]
+    gui: bool,
+
+    /// Also watch subdirectories (for example `.state/runs` for every milestone).
+    #[arg(short, long)]
+    recursive: bool,
+
+    /// Only follow files whose names match, with `*` and `?` wildcards, such as '*.log'.
+    #[arg(short, long, value_name = "PATTERN")]
+    glob: Option<String>,
+
+    /// Lines of the newest file to show at startup [default: 10, or the scrollback with --tui/--gui].
+    #[arg(short = 'n', long, value_name = "N")]
+    lines: Option<usize>,
+
+    /// How often to check for new files and data, in milliseconds.
+    #[arg(short, long, value_name = "MS", default_value_t = 250)]
+    interval: u64,
+
+    /// Leave out the `==> file <==` banners in plain output.
+    #[arg(short, long)]
+    quiet: bool,
+
+    /// Lines kept for scrolling back in the full-screen view and the window.
+    #[arg(long, value_name = "N", default_value_t = 50_000)]
+    scrollback: usize,
+}
+
+fn main() -> ExitCode {
+    let args = Args::parse();
+    #[cfg(feature = "gui")]
+    let chosen = args.dir.is_some();
+    let opts = Options {
+        dir: args.dir.unwrap_or_else(|| PathBuf::from(".")),
+        recursive: args.recursive,
+        glob: args.glob,
+    };
+    let interval = Duration::from_millis(args.interval.max(10));
+    #[cfg(feature = "gui")]
+    if args.gui {
+        let lines = args.lines.unwrap_or(args.scrollback);
+        return finish(gui::run(opts, chosen, lines, interval, args.scrollback));
+    }
+    let result = if args.tui {
+        let lines = args.lines.unwrap_or(args.scrollback);
+        tui::run(
+            Follower::new(opts, Start::LastLines(lines)),
+            interval,
+            args.scrollback,
+        )
+    } else {
+        let lines = args.lines.unwrap_or(10);
+        cli::run(
+            Follower::new(opts, Start::LastLines(lines)),
+            interval,
+            args.quiet,
+        )
+    };
+    finish(result)
+}
+
+fn finish(result: io::Result<()>) -> ExitCode {
+    match result {
+        Ok(()) => ExitCode::SUCCESS,
+        // `tailnew | head` should end quietly.
+        Err(e) if e.kind() == io::ErrorKind::BrokenPipe => ExitCode::SUCCESS,
+        Err(e) => {
+            eprintln!("tailnew: {e}");
+            ExitCode::FAILURE
+        }
+    }
+}

tailnew/src/tui.rs

@@ -0,0 +1,282 @@
+//! Full-screen view: a header naming the followed file, the log itself with scrollback, an
+//! optional list of the files in the directory, and a footer with the keys.
+
+use std::io;
+use std::time::{Duration, Instant};
+
+use ratatui::crossterm::event::{
+    self, Event as TermEvent, KeyCode, KeyEvent, KeyEventKind, KeyModifiers,
+};
+use ratatui::layout::{Constraint, Layout, Rect};
+use ratatui::style::{Modifier, Style, Stylize};
+use ratatui::text::{Line, Span};
+use ratatui::widgets::{Block, Borders, List, ListItem, Paragraph};
+use ratatui::{DefaultTerminal, Frame};
+use unicode_width::UnicodeWidthChar;
+
+use crate::view::View;
+use crate::watch::Follower;
+
+const HELP: &str = "q quit  ↑↓ PgUp PgDn g G scroll  [ ] older/newer  p pin  w wrap  f files";
+
+pub fn run(follower: Follower, interval: Duration, scrollback: usize) -> io::Result<()> {
+    let mut terminal = ratatui::init();
+    let result = App::new(follower, scrollback).event_loop(&mut terminal, interval);
+    ratatui::restore();
+    result
+}
+
+struct App {
+    view: View,
+    /// How many logical lines the view is scrolled up from the bottom; 0 follows the tail.
+    scroll: usize,
+    wrap: bool,
+    show_files: bool,
+    page: usize,
+    seen_generation: u64,
+    seen_appended: u64,
+}
+
+impl App {
+    fn new(follower: Follower, scrollback: usize) -> Self {
+        App {
+            view: View::new(follower, scrollback),
+            scroll: 0,
+            wrap: true,
+            show_files: false,
+            page: 20,
+            seen_generation: 0,
+            seen_appended: 0,
+        }
+    }
+
+    fn event_loop(&mut self, terminal: &mut DefaultTerminal, interval: Duration) -> io::Result<()> {
+        let mut next_poll = Instant::now();
+        loop {
+            if Instant::now() >= next_poll {
+                self.view.poll();
+                next_poll = Instant::now() + interval;
+            }
+            self.sync_scroll();
+            terminal.draw(|frame| self.render(frame))?;
+            let timeout = next_poll.saturating_duration_since(Instant::now());
+            if event::poll(timeout)?
+                && let TermEvent::Key(key) = event::read()?
+                && key.kind == KeyEventKind::Press
+                && self.key(key)
+            {
+                return Ok(());
+            }
+        }
+    }
+
+    /// Follows the tail after the lines start over, and otherwise keeps a scrolled-back view
+    /// still while the log grows beneath it.
+    fn sync_scroll(&mut self) {
+        if self.view.generation() != self.seen_generation {
+            self.scroll = 0;
+        } else if self.scroll > 0 {
+            self.scroll += (self.view.appended() - self.seen_appended) as usize;
+        }
+        self.seen_generation = self.view.generation();
+        self.seen_appended = self.view.appended();
+        self.scroll = self.scroll.min(self.view.total().saturating_sub(1));
+    }
+
+    /// Handles a key; returns true to quit.
+    fn key(&mut self, key: KeyEvent) -> bool {
+        let ctrl = key.modifiers.contains(KeyModifiers::CONTROL);
+        let total = self.view.total();
+        let max = total.saturating_sub(1);
+        match key.code {
+            KeyCode::Char('q') | KeyCode::Esc => return true,
+            KeyCode::Char('c') if ctrl => return true,
+            KeyCode::Up | KeyCode::Char('k') => self.scroll = (self.scroll + 1).min(max),
+            KeyCode::Down | KeyCode::Char('j') => self.scroll = self.scroll.saturating_sub(1),
+            KeyCode::PageUp | KeyCode::Char('b') => {
+                self.scroll = (self.scroll + self.page).min(max)
+            }
+            KeyCode::PageDown | KeyCode::Char(' ') => {
+                self.scroll = self.scroll.saturating_sub(self.page)
+            }
+            KeyCode::Char('u') if ctrl => self.scroll = (self.scroll + self.page / 2).min(max),
+            KeyCode::Char('d') if ctrl => self.scroll = self.scroll.saturating_sub(self.page / 2),
+            KeyCode::Home | KeyCode::Char('g') => self.scroll = total.saturating_sub(self.page),
+            KeyCode::End | KeyCode::Char('G') => self.scroll = 0,
+            KeyCode::Char('w') => self.wrap = !self.wrap,
+            KeyCode::Char('f') => self.show_files = !self.show_files,
+            KeyCode::Char('[') => self.view.step(-1),
+            KeyCode::Char(']') => self.view.step(1),
+            KeyCode::Char('p') => self.view.toggle_pin(),
+            KeyCode::Char('L') => self.view.go_latest(),
+            _ => {}
+        }
+        false
+    }
+
+    fn render(&mut self, frame: &mut Frame) {
+        let [header, body, footer] = Layout::vertical([
+            Constraint::Length(1),
+            Constraint::Min(1),
+            Constraint::Length(1),
+        ])
+        .areas(frame.area());
+        self.render_header(frame, header);
+        self.render_footer(frame, footer);
+
+        let log = if self.show_files {
+            let [files, log] =
+                Layout::horizontal([Constraint::Percentage(25), Constraint::Min(20)]).areas(body);
+            self.render_files(frame, files);
+            log
+        } else {
+            body
+        };
+        self.page = (log.height as usize).max(1);
+        let total = self.view.total();
+        let end = total - self.scroll.min(total);
+        let rows = visible_rows(
+            end,
+            log.height as usize,
+            log.width as usize,
+            self.wrap,
+            |i| self.view.line(i).into_owned(),
+        );
+        let text: Vec<Line> = rows.into_iter().map(Line::from).collect();
+        frame.render_widget(Paragraph::new(text), log);
+    }
+
+    fn render_header(&self, frame: &mut Frame, area: Rect) {
+        let name = self.view.title();
+        let mut spans = vec![
+            Span::raw(" tailnew ").bold(),
+            Span::raw("│ "),
+            Span::raw(name).bold(),
+        ];
+        if self.view.follower.pinned() {
+            spans.push(Span::raw("  PINNED").yellow().bold());
+        }
+        if self.scroll > 0 {
+            spans.push(
+                Span::raw(format!("  SCROLLED -{}", self.scroll))
+                    .cyan()
+                    .bold(),
+            );
+        } else {
+            spans.push(Span::raw("  FOLLOWING").green());
+        }
+        let style = Style::new().add_modifier(Modifier::REVERSED);
+        frame.render_widget(Paragraph::new(Line::from(spans)).style(style), area);
+    }
+
+    fn render_footer(&self, frame: &mut Frame, area: Rect) {
+        let status = if let Some(e) = self.view.error() {
+            Span::raw(format!(" {e} ")).red().bold()
+        } else if let Some(status) = self.view.status() {
+            Span::raw(format!(" {status} ")).dim()
+        } else {
+            Span::raw("")
+        };
+        let [help, right] = Layout::horizontal([
+            Constraint::Min(0),
+            Constraint::Length(status.width() as u16),
+        ])
+        .areas(area);
+        frame.render_widget(Paragraph::new(Span::raw(format!(" {HELP}")).dim()), help);
+        frame.render_widget(Paragraph::new(status), right);
+    }
+
+    fn render_files(&self, frame: &mut Frame, area: Rect) {
+        let current = self.view.follower.current_path();
+        let items: Vec<ListItem> = self
+            .view
+            .follower
+            .entries()
+            .iter()
+            .rev()
+            .map(|e| {
+                let name = self.view.relative(&e.path);
+                if Some(e.path.as_path()) == current {
+                    ListItem::new(format!("▸ {name}")).bold()
+                } else {
+                    ListItem::new(format!("  {name}"))
+                }
+            })
+            .collect();
+        let block = Block::new()
+            .borders(Borders::RIGHT)
+            .title(" files, newest first ");
+        frame.render_widget(List::new(items).block(block), area);
+    }
+}
+
+/// The screen rows that show logical lines `..end`, bottom-aligned in `height` rows of `width`
+/// columns. With `wrap`, long lines take several rows; without it, the terminal clips them.
+fn visible_rows(
+    end: usize,
+    height: usize,
+    width: usize,
+    wrap: bool,
+    line: impl Fn(usize) -> String,
+) -> Vec<String> {
+    let mut rows: Vec<String> = Vec::new();
+    let mut i = end;
+    while i > 0 && rows.len() < height {
+        i -= 1;
+        let text = line(i);
+        let mut pieces = if wrap {
+            wrap_line(&text, width)
+        } else {
+            vec![text]
+        };
+        pieces.reverse();
+        rows.extend(pieces);
+    }
+    rows.truncate(height);
+    rows.reverse();
+    rows
+}
+
+fn wrap_line(text: &str, width: usize) -> Vec<String> {
+    let width = width.max(1);
+    let mut out = vec![String::new()];
+    let mut used = 0;
+    for c in text.chars() {
+        let w = c.width().unwrap_or(0);
+        if used + w > width && used > 0 {
+            out.push(String::new());
+            used = 0;
+        }
+        out.last_mut().unwrap().push(c);
+        used += w;
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn wrap_line_splits_at_width() {
+        assert_eq!(wrap_line("abcdefg", 3), vec!["abc", "def", "g"]);
+        assert_eq!(wrap_line("", 3), vec![""]);
+        assert_eq!(wrap_line("日本語", 4), vec!["日本", "語"]);
+    }
+
+    #[test]
+    fn visible_rows_are_bottom_aligned() {
+        let lines = ["one", "two", "three", "a long line here"];
+        let get = |i: usize| lines[i].to_string();
+        assert_eq!(
+            visible_rows(4, 2, 80, true, get),
+            vec!["three", "a long line here"]
+        );
+        assert_eq!(
+            visible_rows(4, 3, 6, true, get),
+            vec!["a long", " line ", "here"]
+        );
+        assert_eq!(visible_rows(3, 2, 80, true, get), vec!["two", "three"]);
+        assert_eq!(visible_rows(2, 5, 80, false, get), vec!["one", "two"]);
+    }
+}

tailnew/src/view.rs

@@ -0,0 +1,280 @@
+//! State shared by the interactive front ends: the lines of the followed file, moving between
+//! files, pinning, and the status line. Scrolling belongs to each front end.
+
+use std::borrow::Cow;
+use std::collections::VecDeque;
+use std::path::Path;
+use std::time::{Duration, Instant};
+
+use crate::watch::{Event, Follower, Start};
+
+pub struct View {
+    pub follower: Follower,
+    lines: VecDeque<String>,
+    /// The bytes after the last newline, shown as a live last line.
+    partial: Vec<u8>,
+    scrollback: usize,
+    /// Bumped whenever the lines start over (a new file, or the current one restarted).
+    generation: u64,
+    /// Complete lines added since the lines last started over, including any since dropped.
+    appended: u64,
+    since: Option<Instant>,
+    note: &'static str,
+    error: Option<String>,
+}
+
+impl View {
+    pub fn new(follower: Follower, scrollback: usize) -> Self {
+        View {
+            follower,
+            lines: VecDeque::new(),
+            partial: Vec::new(),
+            scrollback: scrollback.max(1),
+            generation: 0,
+            appended: 0,
+            since: None,
+            note: "",
+            error: None,
+        }
+    }
+
+    pub fn generation(&self) -> u64 {
+        self.generation
+    }
+
+    pub fn appended(&self) -> u64 {
+        self.appended
+    }
+
+    pub fn poll(&mut self) {
+        match self.follower.poll() {
+            Ok(events) => {
+                self.error = None;
+                for event in events {
+                    self.apply(event);
+                }
+            }
+            Err(e) => self.error = Some(e.to_string()),
+        }
+    }
+
+    fn apply(&mut self, event: Event) {
+        match event {
+            Event::Switched(_) | Event::Restarted(_) => {
+                self.lines.clear();
+                self.partial.clear();
+                self.generation += 1;
+                self.appended = 0;
+                self.since = Some(Instant::now());
+                self.note = if matches!(event, Event::Restarted(_)) {
+                    "restarted"
+                } else {
+                    "opened"
+                };
+            }
+            Event::Data(bytes) => {
+                self.partial.extend_from_slice(&bytes);
+                let Some(last) = self.partial.iter().rposition(|&b| b == b'\n') else {
+                    return;
+                };
+                let rest = self.partial.split_off(last + 1);
+                let complete = std::mem::replace(&mut self.partial, rest);
+                for raw in complete[..complete.len() - 1].split(|&b| b == b'\n') {
+                    self.lines
+                        .push_back(clean_line(&String::from_utf8_lossy(raw)));
+                    if self.lines.len() > self.scrollback {
+                        self.lines.pop_front();
+                    }
+                    self.appended += 1;
+                }
+            }
+        }
+    }
+
+    /// Lines available to show, counting a partial last line.
+    pub fn total(&self) -> usize {
+        self.lines.len() + usize::from(!self.partial.is_empty())
+    }
+
+    pub fn line(&self, i: usize) -> Cow<'_, str> {
+        match self.lines.get(i) {
+            Some(line) => Cow::Borrowed(line),
+            None => Cow::Owned(clean_line(&String::from_utf8_lossy(&self.partial))),
+        }
+    }
+
+    /// Opens the file `delta` places newer (positive) or older (negative) than the current one,
+    /// and pins it so that the next new file does not pull the view away.
+    pub fn step(&mut self, delta: isize) {
+        let entries = self.follower.entries();
+        if entries.is_empty() {
+            return;
+        }
+        let here = self
+            .follower
+            .current_path()
+            .and_then(|p| entries.iter().position(|e| e.path == p))
+            .unwrap_or(entries.len() - 1);
+        let to = (here as isize + delta).clamp(0, entries.len() as isize - 1) as usize;
+        if to != here {
+            let path = entries[to].path.clone();
+            self.open_pinned(&path);
+        }
+    }
+
+    /// Pins the current file, or if already pinned, unpins and returns to the newest.
+    pub fn toggle_pin(&mut self) {
+        if self.follower.pinned() {
+            self.go_latest();
+        } else {
+            self.follower.set_pinned(true);
+        }
+    }
+
+    /// Unpins and returns to the newest file.
+    pub fn go_latest(&mut self) {
+        self.follower.set_pinned(false);
+        let newest = self.follower.entries().last().map(|e| e.path.clone());
+        if let Some(path) = newest
+            && self.follower.current_path() != Some(path.as_path())
+        {
+            self.open(&path);
+        }
+    }
+
+    /// Opens `path` and pins it.
+    pub fn open_pinned(&mut self, path: &Path) {
+        self.follower.set_pinned(true);
+        self.open(path);
+    }
+
+    fn open(&mut self, path: &Path) {
+        if let Err(e) = self.follower.open(path, Start::LastLines(self.scrollback)) {
+            self.error = Some(format!("{}: {e}", path.display()));
+            return;
+        }
+        self.poll();
+    }
+
+    /// `path` relative to the watched directory, for display.
+    pub fn relative(&self, path: &Path) -> String {
+        path.strip_prefix(self.follower.dir())
+            .unwrap_or(path)
+            .display()
+            .to_string()
+    }
+
+    /// The followed file, or what is being waited for.
+    pub fn title(&self) -> String {
+        match self.follower.current_path() {
+            Some(p) => self.relative(p),
+            None => format!("waiting for files in {}", self.follower.dir().display()),
+        }
+    }
+
+    pub fn error(&self) -> Option<&str> {
+        self.error.as_deref()
+    }
+
+    /// Such as "opened 2m05s ago".
+    pub fn status(&self) -> Option<String> {
+        self.since
+            .map(|since| format!("{} {} ago", self.note, elapsed(since.elapsed())))
+    }
+}
+
+pub fn elapsed(d: Duration) -> String {
+    let s = d.as_secs();
+    match s {
+        0..60 => format!("{s}s"),
+        60..3600 => format!("{}m{:02}s", s / 60, s % 60),
+        _ => format!("{}h{:02}m", s / 3600, s % 3600 / 60),
+    }
+}
+
+/// Makes one line of log output safe to draw: drops ANSI escape sequences and other control
+/// characters, keeps only what follows the last carriage return (as a terminal would show a
+/// progress line), and expands tabs.
+pub fn clean_line(raw: &str) -> String {
+    let raw = raw.strip_suffix('\r').unwrap_or(raw);
+    let raw = raw.rsplit('\r').next().unwrap_or(raw);
+    let mut out = String::with_capacity(raw.len());
+    let mut chars = raw.chars().peekable();
+    while let Some(c) = chars.next() {
+        match c {
+            '\x1b' => match chars.next() {
+                // CSI: parameters and intermediates, then a final byte in @..~.
+                Some('[') => {
+                    for c in chars.by_ref() {
+                        if ('@'..='~').contains(&c) {
+                            break;
+                        }
+                    }
+                }
+                // OSC: runs to BEL or ST (ESC \).
+                Some(']') => {
+                    while let Some(c) = chars.next() {
+                        if c == '\x07' || (c == '\x1b' && chars.next_if_eq(&'\\').is_some()) {
+                            break;
+                        }
+                    }
+                }
+                _ => {}
+            },
+            '\t' => {
+                let col = out.chars().count();
+                out.extend(std::iter::repeat_n(' ', 8 - col % 8));
+            }
+            c if c.is_control() => {}
+            c => out.push(c),
+        }
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::watch::Options;
+    use std::fs;
+    use std::path::PathBuf;
+
+    fn view(dir: PathBuf, scrollback: usize) -> View {
+        let opts = Options {
+            dir,
+            recursive: false,
+            glob: None,
+        };
+        View::new(Follower::new(opts, Start::Beginning), scrollback)
+    }
+
+    #[test]
+    fn splits_lines_keeps_partial_and_caps_scrollback() {
+        let tmp = tempfile::tempdir().unwrap();
+        fs::write(tmp.path().join("a.log"), "one\ntwo\nthree\nfou").unwrap();
+        let mut v = view(tmp.path().to_path_buf(), 2);
+        v.poll();
+        assert_eq!(v.generation(), 1);
+        assert_eq!(v.appended(), 3);
+        let shown: Vec<_> = (0..v.total()).map(|i| v.line(i).into_owned()).collect();
+        assert_eq!(shown, ["two", "three", "fou"]);
+    }
+
+    #[test]
+    fn clean_line_strips_escapes_and_handles_carriage_returns() {
+        assert_eq!(clean_line("\x1b[1;32mok\x1b[0m done"), "ok done");
+        assert_eq!(clean_line("\x1b]0;title\x07text"), "text");
+        assert_eq!(clean_line("\x1b]8;;http://x\x1b\\link"), "link");
+        assert_eq!(clean_line("10%\r50%\r100%"), "100%");
+        assert_eq!(clean_line("dos line\r"), "dos line");
+        assert_eq!(clean_line("a\tb"), "a       b");
+        assert_eq!(clean_line("bell\x07"), "bell");
+    }
+
+    #[test]
+    fn elapsed_formats() {
+        assert_eq!(elapsed(Duration::from_secs(5)), "5s");
+        assert_eq!(elapsed(Duration::from_secs(125)), "2m05s");
+        assert_eq!(elapsed(Duration::from_secs(3720)), "1h02m");
+    }
+}

tailnew/src/watch.rs

@@ -0,0 +1,568 @@
+//! Finds the newest file in a directory and follows it, switching whenever a new file appears.
+//!
+//! The directory is polled rather than watched with inotify: it is one directory, the cost is a
+//! `readdir` per interval, and polling behaves the same on every filesystem.
+//!
+//! A file counts as new when its path has not been seen before, when the path now names a
+//! different inode, or when it has shrunk since the last scan. The last case catches a harness
+//! that reruns a task and truncates the old log with `>` rather than creating a fresh file.
+
+use std::collections::{HashMap, HashSet};
+use std::fs::{self, File, Metadata};
+use std::io::{self, Read, Seek, SeekFrom};
+use std::mem;
+use std::os::unix::fs::MetadataExt;
+use std::path::{Path, PathBuf};
+use std::time::SystemTime;
+
+/// The most bytes handed out by one poll, so a huge backlog cannot stall a front end.
+const MAX_READ: u64 = 4 << 20;
+
+#[derive(Clone)]
+pub struct Options {
+    pub dir: PathBuf,
+    pub recursive: bool,
+    /// Matched against the file name only, with `*` and `?` wildcards.
+    pub glob: Option<String>,
+}
+
+/// Where to begin reading a file that is being opened.
+#[derive(Clone, Copy, Debug)]
+pub enum Start {
+    Beginning,
+    LastLines(usize),
+}
+
+#[derive(Debug, PartialEq)]
+pub enum Event {
+    /// Now following this file; any data that follows belongs to it.
+    Switched(PathBuf),
+    /// The followed file was truncated or replaced; its data starts over.
+    Restarted(PathBuf),
+    Data(Vec<u8>),
+}
+
+#[derive(Clone, Debug)]
+pub struct Entry {
+    pub path: PathBuf,
+    /// When the file's current contents began: its creation time, or when it was seen to restart.
+    pub started: SystemTime,
+    id: (u64, u64),
+    size: u64,
+}
+
+struct Current {
+    path: PathBuf,
+    id: (u64, u64),
+    file: File,
+    offset: u64,
+}
+
+pub struct Follower {
+    opts: Options,
+    initial: Start,
+    /// What the previous scan saw; `None` until the first scan.
+    known: Option<HashMap<PathBuf, Entry>>,
+    /// Oldest first.
+    entries: Vec<Entry>,
+    current: Option<Current>,
+    pinned: bool,
+    pending: Vec<Event>,
+}
+
+impl Follower {
+    /// `initial` is where to begin in the file that is newest at startup. Files that appear later
+    /// are always read from the beginning.
+    pub fn new(opts: Options, initial: Start) -> Self {
+        Follower {
+            opts,
+            initial,
+            known: None,
+            entries: Vec::new(),
+            current: None,
+            pinned: false,
+            pending: Vec::new(),
+        }
+    }
+
+    pub fn dir(&self) -> &Path {
+        &self.opts.dir
+    }
+
+    #[cfg_attr(not(feature = "gui"), allow(dead_code))]
+    pub fn options(&self) -> &Options {
+        &self.opts
+    }
+
+    /// Files seen by the last scan, oldest first.
+    pub fn entries(&self) -> &[Entry] {
+        &self.entries
+    }
+
+    pub fn current_path(&self) -> Option<&Path> {
+        self.current.as_ref().map(|c| c.path.as_path())
+    }
+
+    pub fn pinned(&self) -> bool {
+        self.pinned
+    }
+
+    /// While pinned, new files do not take over from the current one.
+    pub fn set_pinned(&mut self, pinned: bool) {
+        self.pinned = pinned;
+    }
+
+    /// Follows `path` from now on. The `Switched` event arrives with the next poll.
+    pub fn open(&mut self, path: &Path, start: Start) -> io::Result<()> {
+        let mut file = File::open(path)?;
+        let meta = file.metadata()?;
+        let offset = match start {
+            Start::Beginning => 0,
+            Start::LastLines(n) => tail_offset(&mut file, meta.len(), n)?,
+        };
+        self.current = Some(Current {
+            path: path.to_path_buf(),
+            id: file_id(&meta),
+            file,
+            offset,
+        });
+        self.pending.push(Event::Switched(path.to_path_buf()));
+        Ok(())
+    }
+
+    /// Rescans the directory, switches files if a new one appeared, and returns what happened
+    /// along with any data written since the last poll.
+    pub fn poll(&mut self) -> io::Result<Vec<Event>> {
+        let mut entries = scan(&self.opts)?;
+        let mut fresh = HashSet::new();
+        if let Some(known) = &self.known {
+            let now = SystemTime::now();
+            for e in &mut entries {
+                match known.get(&e.path) {
+                    Some(k) if k.id == e.id && e.size >= k.size => {
+                        e.started = k.started;
+                        continue;
+                    }
+                    // Truncated in place keeps its creation time, so date the restart ourselves.
+                    Some(k) if k.id == e.id => e.started = now,
+                    // New, or replaced by another file: its creation time is right.
+                    _ => {}
+                }
+                fresh.insert(e.path.clone());
+            }
+            entries.sort_by(|a, b| (a.started, &a.path).cmp(&(b.started, &b.path)));
+        }
+        let target = match &self.known {
+            None => entries.last().map(|e| (e.path.clone(), self.initial)),
+            Some(_) => entries
+                .iter()
+                .rev()
+                .find(|e| fresh.contains(&e.path))
+                .map(|e| (e.path.clone(), Start::Beginning)),
+        };
+        self.known = Some(
+            entries
+                .iter()
+                .map(|e| (e.path.clone(), e.clone()))
+                .collect(),
+        );
+        self.entries = entries;
+
+        if let Some((path, start)) = target {
+            // A restart of the current file is handled when it is read, below.
+            let is_current = self.current_path() == Some(path.as_path());
+            if !is_current && (!self.pinned || self.current.is_none()) {
+                match self.open(&path, start) {
+                    Ok(()) => {}
+                    // Deleted between the scan and the open; the next scan will sort it out.
+                    Err(e) if e.kind() == io::ErrorKind::NotFound => {}
+                    Err(e) => return Err(e),
+                }
+            }
+        }
+
+        let mut events = mem::take(&mut self.pending);
+        self.read_current(&mut events)?;
+        Ok(events)
+    }
+
+    fn read_current(&mut self, events: &mut Vec<Event>) -> io::Result<()> {
+        let Some(cur) = self.current.as_mut() else {
+            return Ok(());
+        };
+        if let Ok(meta) = fs::metadata(&cur.path)
+            && file_id(&meta) != cur.id
+            && let Ok(file) = File::open(&cur.path)
+        {
+            cur.id = file_id(&file.metadata()?);
+            cur.file = file;
+            cur.offset = 0;
+            events.push(Event::Restarted(cur.path.clone()));
+        }
+        let len = cur.file.metadata()?.len();
+        if len < cur.offset {
+            cur.offset = 0;
+            events.push(Event::Restarted(cur.path.clone()));
+        }
+        if len > cur.offset {
+            cur.file.seek(SeekFrom::Start(cur.offset))?;
+            let mut buf = Vec::new();
+            (&cur.file)
+                .take((len - cur.offset).min(MAX_READ))
+                .read_to_end(&mut buf)?;
+            cur.offset += buf.len() as u64;
+            if !buf.is_empty() {
+                events.push(Event::Data(buf));
+            }
+        }
+        Ok(())
+    }
+}
+
+fn file_id(meta: &Metadata) -> (u64, u64) {
+    (meta.dev(), meta.ino())
+}
+
+/// Lists the matching regular files under `opts.dir`, oldest first. A missing directory is
+/// treated as empty so that the follower can be started before the harness creates it.
+fn scan(opts: &Options) -> io::Result<Vec<Entry>> {
+    let mut out = Vec::new();
+    match walk(&opts.dir, opts, &mut out) {
+        Err(e) if e.kind() == io::ErrorKind::NotFound => {}
+        other => other?,
+    }
+    out.sort_by(|a, b| (a.started, &a.path).cmp(&(b.started, &b.path)));
+    Ok(out)
+}
+
+fn walk(dir: &Path, opts: &Options, out: &mut Vec<Entry>) -> io::Result<()> {
+    for dirent in fs::read_dir(dir)? {
+        let Ok(dirent) = dirent else { continue };
+        let name = dirent.file_name();
+        let name = name.to_string_lossy();
+        if name.starts_with('.') {
+            continue;
+        }
+        let path = dirent.path();
+        let Ok(kind) = dirent.file_type() else {
+            continue;
+        };
+        if kind.is_dir() {
+            if opts.recursive {
+                // Unreadable or vanished subdirectories are skipped, not fatal.
+                let _ = walk(&path, opts, out);
+            }
+            continue;
+        }
+        if let Some(glob) = &opts.glob
+            && !wildcard(glob.as_bytes(), name.as_bytes())
+        {
+            continue;
+        }
+        // Follows symlinks, so a `latest.log` link to a real file is still followed.
+        let Ok(meta) = fs::metadata(&path) else {
+            continue;
+        };
+        if !meta.is_file() {
+            continue;
+        }
+        let created = meta
+            .created()
+            .or_else(|_| meta.modified())
+            .unwrap_or(SystemTime::UNIX_EPOCH);
+        out.push(Entry {
+            path,
+            started: created,
+            id: file_id(&meta),
+            size: meta.len(),
+        });
+    }
+    Ok(())
+}
+
+/// Matches `s` against `pat`, where `*` matches any run of bytes and `?` any single byte.
+pub fn wildcard(pat: &[u8], s: &[u8]) -> bool {
+    let (mut p, mut i) = (0, 0);
+    let mut star: Option<(usize, usize)> = None;
+    while i < s.len() {
+        if p < pat.len() && (pat[p] == b'?' || pat[p] == s[i]) {
+            p += 1;
+            i += 1;
+        } else if p < pat.len() && pat[p] == b'*' {
+            star = Some((p, i));
+            p += 1;
+        } else if let Some((sp, si)) = star {
+            p = sp + 1;
+            i = si + 1;
+            star = Some((sp, si + 1));
+        } else {
+            return false;
+        }
+    }
+    pat[p..].iter().all(|&c| c == b'*')
+}
+
+/// The offset at which the last `n` lines of a file of length `len` begin. A final newline does
+/// not count as starting an empty last line.
+fn tail_offset(file: &mut File, len: u64, n: usize) -> io::Result<u64> {
+    if n == 0 {
+        return Ok(len);
+    }
+    let mut buf = vec![0u8; 64 * 1024];
+    let mut pos = len;
+    let mut seen = 0;
+    while pos > 0 {
+        let chunk = pos.min(buf.len() as u64);
+        pos -= chunk;
+        file.seek(SeekFrom::Start(pos))?;
+        file.read_exact(&mut buf[..chunk as usize])?;
+        for i in (0..chunk as usize).rev() {
+            let at = pos + i as u64;
+            if buf[i] == b'\n' && at != len - 1 {
+                seen += 1;
+                if seen == n {
+                    return Ok(at + 1);
+                }
+            }
+        }
+    }
+    Ok(0)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs::OpenOptions;
+    use std::io::Write;
+    use std::thread;
+    use std::time::Duration;
+
+    fn follower(dir: &Path, initial: Start) -> Follower {
+        Follower::new(
+            Options {
+                dir: dir.to_path_buf(),
+                recursive: false,
+                glob: None,
+            },
+            initial,
+        )
+    }
+
+    /// Creation times need to differ for "newest" to mean anything.
+    fn write_new(path: &Path, text: &str) {
+        thread::sleep(Duration::from_millis(15));
+        fs::write(path, text).unwrap();
+    }
+
+    fn append(path: &Path, text: &str) {
+        let mut f = OpenOptions::new().append(true).open(path).unwrap();
+        f.write_all(text.as_bytes()).unwrap();
+    }
+
+    fn data(events: &[Event]) -> String {
+        events
+            .iter()
+            .filter_map(|e| match e {
+                Event::Data(d) => Some(String::from_utf8_lossy(d).into_owned()),
+                _ => None,
+            })
+            .collect()
+    }
+
+    #[test]
+    fn starts_on_newest_file_with_last_lines() {
+        let tmp = tempfile::tempdir().unwrap();
+        write_new(&tmp.path().join("00-a.log"), "old\n");
+        write_new(&tmp.path().join("01-b.log"), "1\n2\n3\n4\n");
+        let mut f = follower(tmp.path(), Start::LastLines(2));
+        let ev = f.poll().unwrap();
+        assert_eq!(ev[0], Event::Switched(tmp.path().join("01-b.log")));
+        assert_eq!(data(&ev), "3\n4\n");
+    }
+
+    #[test]
+    fn follows_appended_data() {
+        let tmp = tempfile::tempdir().unwrap();
+        let p = tmp.path().join("a.log");
+        write_new(&p, "one\n");
+        let mut f = follower(tmp.path(), Start::Beginning);
+        f.poll().unwrap();
+        assert!(f.poll().unwrap().is_empty());
+        append(&p, "two\npart");
+        assert_eq!(data(&f.poll().unwrap()), "two\npart");
+    }
+
+    #[test]
+    fn switches_to_a_new_file_from_its_beginning() {
+        let tmp = tempfile::tempdir().unwrap();
+        write_new(&tmp.path().join("00.log"), "a\n");
+        let mut f = follower(tmp.path(), Start::LastLines(0));
+        f.poll().unwrap();
+        let p = tmp.path().join("01.log");
+        write_new(&p, "first\nsecond\n");
+        let ev = f.poll().unwrap();
+        assert_eq!(ev[0], Event::Switched(p));
+        assert_eq!(data(&ev), "first\nsecond\n");
+    }
+
+    #[test]
+    fn waits_for_a_directory_that_does_not_exist_yet() {
+        let tmp = tempfile::tempdir().unwrap();
+        let dir = tmp.path().join("runs/M2a");
+        let mut f = follower(&dir, Start::LastLines(10));
+        assert!(f.poll().unwrap().is_empty());
+        fs::create_dir_all(&dir).unwrap();
+        write_new(&dir.join("00.log"), "hello\n");
+        let ev = f.poll().unwrap();
+        assert_eq!(ev[0], Event::Switched(dir.join("00.log")));
+        assert_eq!(data(&ev), "hello\n");
+    }
+
+    #[test]
+    fn restarts_when_the_current_file_is_truncated() {
+        let tmp = tempfile::tempdir().unwrap();
+        let p = tmp.path().join("a.log");
+        write_new(&p, "a long first run\n");
+        let mut f = follower(tmp.path(), Start::Beginning);
+        f.poll().unwrap();
+        fs::write(&p, "rerun\n").unwrap();
+        let ev = f.poll().unwrap();
+        assert_eq!(ev[0], Event::Restarted(p));
+        assert_eq!(data(&ev), "rerun\n");
+    }
+
+    #[test]
+    fn restarts_when_the_current_file_is_replaced() {
+        let tmp = tempfile::tempdir().unwrap();
+        let p = tmp.path().join("a.log");
+        write_new(&p, "one\n");
+        let mut f = follower(tmp.path(), Start::Beginning);
+        f.poll().unwrap();
+        let q = tmp.path().join(".tmp");
+        fs::write(&q, "a whole new file\n").unwrap();
+        fs::rename(&q, &p).unwrap();
+        let ev = f.poll().unwrap();
+        assert_eq!(ev[0], Event::Restarted(p));
+        assert_eq!(data(&ev), "a whole new file\n");
+    }
+
+    #[test]
+    fn switches_to_an_older_file_that_is_rewritten() {
+        let tmp = tempfile::tempdir().unwrap();
+        let old = tmp.path().join("00.log");
+        write_new(&old, "a long first run of task zero\n");
+        write_new(&tmp.path().join("01.log"), "b\n");
+        let mut f = follower(tmp.path(), Start::Beginning);
+        f.poll().unwrap();
+        fs::write(&old, "rerun\n").unwrap();
+        let ev = f.poll().unwrap();
+        assert_eq!(ev[0], Event::Switched(old.clone()));
+        assert_eq!(data(&ev), "rerun\n");
+        // It now ranks as the newest file, and keeps that rank on later scans.
+        assert_eq!(f.entries().last().unwrap().path, old);
+        append(&old, "more\n");
+        f.poll().unwrap();
+        assert_eq!(f.entries().last().unwrap().path, old);
+    }
+
+    #[test]
+    fn appends_to_other_files_do_not_switch() {
+        let tmp = tempfile::tempdir().unwrap();
+        let old = tmp.path().join("00.log");
+        write_new(&old, "a\n");
+        write_new(&tmp.path().join("01.log"), "b\n");
+        let mut f = follower(tmp.path(), Start::Beginning);
+        f.poll().unwrap();
+        append(&old, "more\n");
+        assert!(f.poll().unwrap().is_empty());
+    }
+
+    #[test]
+    fn pinned_stays_put() {
+        let tmp = tempfile::tempdir().unwrap();
+        let a = tmp.path().join("00.log");
+        write_new(&a, "a\n");
+        let mut f = follower(tmp.path(), Start::Beginning);
+        f.poll().unwrap();
+        f.set_pinned(true);
+        write_new(&tmp.path().join("01.log"), "b\n");
+        append(&a, "more\n");
+        assert_eq!(f.poll().unwrap(), vec![Event::Data(b"more\n".to_vec())]);
+        assert_eq!(f.entries().len(), 2);
+    }
+
+    #[test]
+    fn open_switches_by_hand() {
+        let tmp = tempfile::tempdir().unwrap();
+        let a = tmp.path().join("00.log");
+        write_new(&a, "x\ny\n");
+        write_new(&tmp.path().join("01.log"), "b\n");
+        let mut f = follower(tmp.path(), Start::Beginning);
+        f.poll().unwrap();
+        f.open(&a, Start::LastLines(1)).unwrap();
+        let ev = f.poll().unwrap();
+        assert_eq!(ev[0], Event::Switched(a));
+        assert_eq!(data(&ev), "y\n");
+    }
+
+    #[test]
+    fn filters_by_glob_skips_hidden_and_recurses_on_request() {
+        let tmp = tempfile::tempdir().unwrap();
+        write_new(&tmp.path().join("a.log"), "log\n");
+        write_new(&tmp.path().join(".hidden.log"), "hidden\n");
+        write_new(&tmp.path().join("b.txt"), "txt\n");
+        let mut f = Follower::new(
+            Options {
+                dir: tmp.path().to_path_buf(),
+                recursive: false,
+                glob: Some("*.log".into()),
+            },
+            Start::Beginning,
+        );
+        assert_eq!(data(&f.poll().unwrap()), "log\n");
+
+        let sub = tmp.path().join("M2b");
+        fs::create_dir(&sub).unwrap();
+        write_new(&sub.join("00.log"), "nested\n");
+        assert!(f.poll().unwrap().is_empty());
+
+        let mut r = Follower::new(
+            Options {
+                dir: tmp.path().to_path_buf(),
+                recursive: true,
+                glob: Some("*.log".into()),
+            },
+            Start::Beginning,
+        );
+        assert_eq!(data(&r.poll().unwrap()), "nested\n");
+    }
+
+    #[test]
+    fn wildcard_matching() {
+        assert!(wildcard(b"*.log", b"04-task.log"));
+        assert!(!wildcard(b"*.log", b"04-task.log.swp"));
+        assert!(wildcard(b"??-*", b"04-task.log"));
+        assert!(!wildcard(b"??-*", b"4-task.log"));
+        assert!(wildcard(b"*", b""));
+        assert!(wildcard(b"a*b*c", b"aXbYbZc"));
+        assert!(!wildcard(b"a*b*c", b"aXbYbZ"));
+    }
+
+    #[test]
+    fn tail_offset_counts_lines_from_the_end() {
+        let tmp = tempfile::tempdir().unwrap();
+        let p = tmp.path().join("f");
+        let check = |text: &str, n: usize, want: &str| {
+            fs::write(&p, text).unwrap();
+            let mut file = File::open(&p).unwrap();
+            let off = tail_offset(&mut file, text.len() as u64, n).unwrap();
+            assert_eq!(&text[off as usize..], want, "text {text:?} n {n}");
+        };
+        check("a\nb\nc\n", 2, "b\nc\n");
+        check("a\nb\nc", 2, "b\nc");
+        check("a\nb\nc\n", 10, "a\nb\nc\n");
+        check("a\nb\nc\n", 0, "");
+        check("", 3, "");
+        check("\n\n", 1, "\n");
+    }
+}