vterm/

term.rs

//! The terminal emulator: bytes in, a live [`ansi::Screen`] out.
//!
//! [`Term`] holds a primary and an alternate [`ansi::Screen`] (the active one
//! owns the cursor — there is no second copy to keep in sync), the
//! current SGR [`ansi::Style`], a scroll region, tab stops, a saved-cursor
//! slot, and the flags a VT100 tracks (autowrap, cursor visibility, and the
//! pending-wrap latch at the right edge). [`Term::feed`] runs the child's
//! output through an internal [`ansi::Parser`] and applies each token.
//!
//! Coordinates are 0-based internally; CSI parameters are 1-based on the wire
//! and converted on the way in. Where VT defaults a missing parameter to 1
//! (cursor motion, CUP), we treat a 0 from the parser as that default; where
//! it defaults to 0 (erase/SGR selectors), 0 is used directly.

use ansi::{Cell, Cursor, Event, Parser, Screen, Style, Utf8Decoder};

/// A minimal VT100/ECMA-48 terminal emulator over an [`ansi::Screen`].
pub struct Term {
    /// The primary (normal) screen buffer.
    primary: Screen,
    /// The alternate screen buffer, used while an app enters alt-screen mode
    /// (`?1049h`/`?47h`/`?1047h`). Kept the same size as `primary`.
    alt: Screen,
    /// True while the alternate buffer is active.
    in_alt: bool,
    /// Current SGR style applied to newly written cells.
    style: Style,
    /// Scroll region, 0-based inclusive `[top, bottom]` (DECSTBM).
    scroll_top: usize,
    scroll_bottom: usize,
    /// Saved cursor state (DECSC/`ESC 7` and CSI `s`): position and style.
    saved: Option<(Cursor, Style)>,
    /// Tab stops: `tabs[c]` true means a tab stop at column `c`.
    tabs: Vec<bool>,
    /// Pending-wrap latch: set after writing the last column so the *next*
    /// printable wraps to the next line first (DECAWM deferred-wrap).
    wrap_pending: bool,
    /// Autowrap mode (DECAWM). Default on.
    autowrap: bool,
    /// Cursor visibility (DECTCEM, `?25h`/`?25l`). Tracked for the host; the
    /// screen cursor position is always synced regardless.
    cursor_visible: bool,
    /// The parser driving `feed`; one per stream so mid-sequence chunk splits
    /// are handled by `ansi`.
    parser: Parser,
    /// Decodes text runs, which `ansi::Event` hands over as raw bytes, and
    /// carries a character split across chunks. Allocates nothing.
    decoder: Utf8Decoder,
    /// Set to `true` by `feed` (non-empty bytes) and `resize`; reset and
    /// returned by `take_dirty`. Lets callers skip a composite when nothing
    /// has changed since the last rendered frame.
    dirty: bool,
    /// Synchronized-output depth (DEC private mode 2026, `?2026h`/`?2026l`).
    /// While an app is inside a synchronized update it is redrawing the frame;
    /// a compositor that samples the screen mid-update sees a half-drawn frame
    /// (leftover/overlapping text). While `in_sync`, [`Term::screen`] hands back
    /// `sync_frame`, a clone of the last *complete* frame captured when the
    /// update opened, so external readers only ever see whole frames. Writes
    /// still land on the live buffer; the new frame is revealed on `?2026l`.
    in_sync: bool,
    /// The last complete frame, snapshotted when a synchronized update opens.
    /// `Some` exactly while `in_sync`; `None` otherwise.
    sync_frame: Option<Screen>,
    /// Scratch for [`Term::print_text`]'s blit, kept so a run of text costs no
    /// allocation. Only ever used within one call.
    blit: Vec<Cell>,
}

impl Term {
    /// A blank emulator of `rows x cols` (both clamped to at least 1), cursor
    /// at the origin, full-height scroll region, default tab stops every 8
    /// columns, autowrap on, cursor visible.
    pub fn new(rows: usize, cols: usize) -> Self {
        let rows = rows.max(1);
        let cols = cols.max(1);
        Term {
            primary: Screen::new(rows, cols),
            alt: Screen::new(rows, cols),
            in_alt: false,
            style: Style::default(),
            scroll_top: 0,
            scroll_bottom: rows - 1,
            saved: None,
            tabs: default_tabs(cols),
            wrap_pending: false,
            autowrap: true,
            cursor_visible: true,
            parser: Parser::new(),
            decoder: Utf8Decoder::new(),
            dirty: false,
            in_sync: false,
            sync_frame: None,
            blit: Vec::with_capacity(cols),
        }
    }

    /// Feed a chunk of child output. Chunks may split at any byte boundary
    /// (mid-escape, mid-UTF-8); the internal parser holds partial state across
    /// calls, so the final screen is independent of how bytes were chunked.
    pub fn feed(&mut self, bytes: &[u8]) {
        if bytes.is_empty() {
            return;
        }
        self.dirty = true;
        // The parser and decoder are lifted out for the call so the callback
        // can borrow the rest of `self`; both are restored before returning.
        let mut parser = std::mem::take(&mut self.parser);
        let mut decoder = std::mem::take(&mut self.decoder);
        parser.feed_with(bytes, |event| self.apply(event, &mut decoder));
        self.parser = parser;
        self.decoder = decoder;
    }

    /// Returns `true` if any bytes have been fed (or a resize has occurred)
    /// since the last call to `take_dirty`, and resets the flag. Call once
    /// per frame decision to avoid redundant compositing.
    pub fn take_dirty(&mut self) -> bool {
        std::mem::replace(&mut self.dirty, false)
    }

    /// The active buffer (alternate while in alt-screen, else primary). Its
    /// cursor is the emulator's cursor, so a compositor / `Screen::diff` parks
    /// the terminal cursor correctly.
    pub fn screen(&self) -> &Screen {
        // The active buffer owns the cursor, so it is always current. While a
        // synchronized update (`?2026h`) is open we hand back the last complete
        // frame instead of the live, half-drawn buffer (double-buffering).
        match &self.sync_frame {
            Some(frame) => frame,
            None => self.active(),
        }
    }

    /// True while the child is inside a synchronized update (DEC mode 2026):
    /// it has emitted `?2026h` and not yet the matching `?2026l`. During this
    /// window [`Term::screen`] returns the last complete frame. Exposed so a
    /// host can also gate its own compositing on it if desired.
    pub fn in_sync(&self) -> bool {
        self.in_sync
    }

    /// Resize to `rows x cols` (clamped to at least 1). For 0.1 this rebuilds
    /// fresh buffers of the new size (the caller repaints); the cursor and
    /// scroll region are clamped, tab stops rebuilt, and pending wrap cleared.
    pub fn resize(&mut self, rows: usize, cols: usize) {
        let rows = rows.max(1);
        let cols = cols.max(1);
        let cursor = self.cursor();
        let old_primary = std::mem::replace(&mut self.primary, Screen::new(rows, cols));
        let old_alt = std::mem::replace(&mut self.alt, Screen::new(rows, cols));
        copy_top_left(&old_primary, &mut self.primary);
        copy_top_left(&old_alt, &mut self.alt);
        self.scroll_top = self.scroll_top.min(rows - 1);
        self.scroll_bottom = rows - 1;
        // The fresh active buffer takes the cursor, clamped onto the new size.
        self.active_mut().set_cursor(cursor);
        self.tabs = default_tabs(cols);
        self.wrap_pending = false;
        // A resize rebuilds the buffers; any held sync frame is now the wrong
        // size and stale. Drop it and reveal the freshly-sized live buffer.
        self.in_sync = false;
        self.sync_frame = None;
        self.dirty = true;
    }

    // --- internals --------------------------------------------------------

    fn active(&self) -> &Screen {
        if self.in_alt {
            &self.alt
        } else {
            &self.primary
        }
    }

    fn active_mut(&mut self) -> &mut Screen {
        if self.in_alt {
            &mut self.alt
        } else {
            &mut self.primary
        }
    }

    fn rows(&self) -> usize {
        self.active().rows()
    }

    fn cols(&self) -> usize {
        self.active().cols()
    }

    /// The emulator cursor: the active buffer's.
    fn cursor(&self) -> Cursor {
        self.active().cursor()
    }

    fn set_row(&mut self, row: usize) {
        let col = self.cursor().col;
        self.active_mut().set_cursor(Cursor { row, col });
    }

    fn set_col(&mut self, col: usize) {
        let row = self.cursor().row;
        self.active_mut().set_cursor(Cursor { row, col });
    }

    fn apply(&mut self, event: Event<'_>, decoder: &mut Utf8Decoder) {
        if let Event::Text(bytes) = event {
            decoder.decode(bytes, |s| self.print_text(s));
            return;
        }
        // Anything but text ends a run, so a half-finished character can never
        // be completed: render it as U+FFFD, exactly as `ansi::Parser::feed`
        // would have.
        decoder.flush_incomplete(|s| self.print_text(s));
        match event {
            Event::Control(b) => self.control(b),
            Event::Csi {
                private,
                params,
                final_byte,
                ..
            } => self.csi(private.map(char::from), params, char::from(final_byte)),
            Event::Esc { final_byte, .. } => self.esc(final_byte),
            // Titles, DCS/SOS/PM/APC strings: nothing to render.
            Event::Text(_) | Event::Osc(_) | Event::Other { .. } => {}
        }
    }

    /// Write a run of text at the cursor.
    ///
    /// Terminal output is overwhelmingly plain ASCII, and going through
    /// [`Term::print`] for each character re-reads the cursor, re-borrows the
    /// active buffer, looks up a width and bounds-checks a cell — per character.
    /// So a run of ordinary single-width ASCII that fits on the current row is
    /// blitted in one [`Screen::copy_cells`] with the cursor moved once.
    ///
    /// The fast path is deliberately narrow, and anything it does not cover
    /// falls through to `print` unchanged: no wrap is pending, the characters
    /// are printable ASCII (never control, never wide, never zero-width), and
    /// the run stops one column short of the edge so the pending-wrap latch is
    /// still decided by `print`.
    fn print_text(&mut self, s: &str) {
        let mut rest = s;
        while !rest.is_empty() {
            let n = self.plain_run(rest);
            if n == 0 {
                // Not the fast path: one character the careful way.
                let ch = rest.chars().next().expect("non-empty");
                self.print(ch);
                rest = &rest[ch.len_utf8()..];
                continue;
            }
            let style = self.style;
            self.blit.clear();
            self.blit.extend(
                rest.as_bytes()[..n]
                    .iter()
                    .map(|&b| Cell::new(b as char, style)),
            );
            let screen = if self.in_alt {
                &mut self.alt
            } else {
                &mut self.primary
            };
            let Cursor { row, col } = screen.cursor();
            screen.copy_cells(row, col, &self.blit);
            screen.set_cursor(Cursor { row, col: col + n });
            rest = &rest[n..];
        }
    }

    /// How many leading bytes of `s` the fast path in [`Term::print_text`] can
    /// blit: printable ASCII, stopping before the last column of the row.
    fn plain_run(&self, s: &str) -> usize {
        if self.wrap_pending {
            return 0;
        }
        let screen = if self.in_alt {
            &self.alt
        } else {
            &self.primary
        };
        let col = screen.cursor().col;
        // Leave the final column to `print`, which owns the pending-wrap latch.
        let room = screen.cols().saturating_sub(col + 1);
        s.as_bytes()
            .iter()
            .take(room)
            .take_while(|&&b| (0x20..0x7f).contains(&b))
            .count()
    }

    /// Write one printable character at the cursor, honoring autowrap via the
    /// pending-wrap latch and East Asian double-width via [`uwidth`].
    ///
    /// A width-2 glyph (CJK, fullwidth, emoji) occupies two columns: its left
    /// half is a [`Cell::wide`] carrying the character, its right half a
    /// [`Cell::continuation`] carrying no glyph. The cursor advances by two. A
    /// wide glyph that would straddle the right edge is not split — the whole
    /// glyph wraps to the next line (autowrap on) or is dropped (autowrap off),
    /// matching real terminals. Zero-width characters (combining marks, joiners,
    /// variation selectors) are dropped: this grid stores one character per
    /// cell and cannot compose them, and dropping avoids the column drift that
    /// giving them a cell of their own would cause.
    fn print(&mut self, ch: char) {
        let w = uwidth::char_width(ch) as usize;

        // Zero-width: no cell, no cursor movement (see doc comment).
        if w == 0 {
            return;
        }

        if self.wrap_pending && self.autowrap {
            // The previous write filled the last column; wrap now.
            self.set_col(0);
            self.line_feed();
            self.wrap_pending = false;
        }

        // A double-width glyph needs two columns; if only one remains it cannot
        // be split. Wrap the whole glyph to the next line (or drop it when
        // autowrap is off, rather than overwrite half a cell at the edge).
        if w == 2 && self.cursor().col + 1 >= self.cols() {
            if !self.autowrap {
                return;
            }
            self.set_col(0);
            self.line_feed();
            self.wrap_pending = false;
        }

        // The per-character hot path borrows the active buffer once for the
        // cells and the cursor together (field-disjoint, so `self.wrap_pending`
        // stays writable below).
        let (autowrap, style) = (self.autowrap, self.style);
        let screen = if self.in_alt {
            &mut self.alt
        } else {
            &mut self.primary
        };
        let cols = screen.cols();
        let Cursor { row: r, col: c } = screen.cursor();
        if w == 2 {
            screen.set(r, c, Cell::wide(ch, style));
            screen.set(r, c + 1, Cell::continuation(style));
        } else {
            screen.set(r, c, Cell::new(ch, style));
        }
        // Advance by the glyph's width. If that lands at or past the last
        // column, stay at the last column and latch a pending wrap (deferred to
        // the next printable), matching real terminals and the `ansi` diff's
        // pending-wrap comment. A width-1 glyph advances exactly as before.
        let at_edge = c + w >= cols;
        let next = if at_edge { cols - 1 } else { c + w };
        screen.set_cursor(Cursor { row: r, col: next });
        if at_edge && autowrap {
            self.wrap_pending = true;
        }
    }

    fn control(&mut self, b: u8) {
        match b {
            b'\r' => {
                self.set_col(0);
                self.wrap_pending = false;
            }
            b'\n' | 0x0b | 0x0c => {
                // LF, VT, FF: move down one line, scrolling at the margin.
                self.line_feed();
                self.wrap_pending = false;
            }
            0x08 => {
                // BS: cursor left one column, clamped at column 0.
                if self.cursor().col > 0 {
                    self.set_col(self.cursor().col - 1);
                }
                self.wrap_pending = false;
            }
            b'\t' => {
                self.set_col(self.next_tab_stop());
                self.wrap_pending = false;
            }
            _ => {} // BEL and other C0 controls: nothing to render.
        }
    }

    /// Move the cursor down one line; if it is at the bottom margin, scroll the
    /// scroll region up by one instead.
    fn line_feed(&mut self) {
        if self.cursor().row == self.scroll_bottom {
            self.scroll_up(1);
        } else if self.cursor().row + 1 < self.rows() {
            self.set_row(self.cursor().row + 1);
        }
    }

    /// Next tab stop strictly right of the cursor, or the last column.
    fn next_tab_stop(&self) -> usize {
        let cols = self.cols();
        let mut c = self.cursor().col + 1;
        while c < cols {
            if self.tabs.get(c).copied().unwrap_or(false) {
                return c;
            }
            c += 1;
        }
        cols - 1
    }

    // --- CSI dispatch -----------------------------------------------------

    fn csi(&mut self, private: Option<char>, params: &[u16], final_byte: char) {
        // Private-marker sequences (`?...`) are the mode toggles.
        if private == Some('?') {
            match final_byte {
                'h' => self.set_mode(params, true),
                'l' => self.set_mode(params, false),
                _ => {} // other private CSI: ignore quietly
            }
            return;
        }
        if private.is_some() {
            return; // `>`/`<`/`=` device sequences: ignore quietly
        }

        // Helpers: VT motion params default to 1 (0 from the parser means the
        // param was absent → use 1). Erase/line selectors default to 0.
        let n1 = |i: usize| -> usize { at1(params, i) };

        match final_byte {
            // Cursor absolute position: CUP / HVP (row;col, 1-based).
            'H' | 'f' => {
                let r = n1(0).saturating_sub(1);
                let c = n1(1).saturating_sub(1);
                self.move_to(r, c);
            }
            'A' => self.move_up(n1(0)),
            'B' => self.move_down(n1(0)),
            'C' => self.move_right(n1(0)),
            'D' => self.move_left(n1(0)),
            'E' => {
                // CNL: cursor next line, column 0.
                self.move_down(n1(0));
                self.set_col(0);
                self.wrap_pending = false;
            }
            'F' => {
                // CPL: cursor previous line, column 0.
                self.move_up(n1(0));
                self.set_col(0);
                self.wrap_pending = false;
            }
            'G' => {
                // CHA: cursor to absolute column (1-based).
                let c = n1(0).saturating_sub(1);
                self.set_col(c.min(self.cols() - 1));
                self.wrap_pending = false;
            }
            'd' => {
                // VPA: cursor to absolute row (1-based).
                let r = n1(0).saturating_sub(1);
                self.set_row(r.min(self.rows() - 1));
                self.wrap_pending = false;
            }
            'J' => self.erase_display(at0(params, 0)),
            'K' => self.erase_line(at0(params, 0)),
            'X' => self.erase_chars(n1(0)),
            'm' => self.style.apply_sgr(params),
            'r' => self.set_scroll_region(params),
            'L' => self.insert_lines(n1(0)),
            'M' => self.delete_lines(n1(0)),
            'S' => self.scroll_up(n1(0)),
            'T' => self.scroll_down(n1(0)),
            '@' => self.insert_chars(n1(0)),
            'P' => self.delete_chars(n1(0)),
            's' => self.save_cursor(),
            'u' => self.restore_cursor(),
            _ => {} // unimplemented CSI: ignore quietly
        }
    }

    fn set_mode(&mut self, params: &[u16], set: bool) {
        for &p in params {
            match p {
                25 => self.cursor_visible = set,
                7 => self.autowrap = set, // DECAWM
                47 | 1047 | 1049 => self.set_alt(set),
                2026 => self.set_sync(set), // synchronized output
                _ => {}                     // other private modes: ignore quietly
            }
        }
    }

    fn set_alt(&mut self, enter: bool) {
        // One logical cursor across both buffers: it moves with the switch.
        let cursor = self.cursor();
        if enter && !self.in_alt {
            // Entering: switch to a cleared alternate buffer.
            self.alt = Screen::new(self.rows(), self.cols());
            self.in_alt = true;
        } else if !enter && self.in_alt {
            // Leaving: restore the primary buffer as-is.
            self.in_alt = false;
        }
        self.active_mut().set_cursor(cursor);
        self.wrap_pending = false;
    }

    /// Enter/leave a synchronized update (DEC mode 2026). On open we snapshot
    /// the current active buffer as the last complete frame and serve that from
    /// `screen()` until close, so a mid-redraw frame is never exposed. Nested
    /// opens keep the first (good) snapshot rather than capturing a half-drawn
    /// one. On close we drop the snapshot, revealing the now-complete buffer.
    fn set_sync(&mut self, open: bool) {
        if open {
            if !self.in_sync {
                self.sync_frame = Some(self.active().clone());
                self.in_sync = true;
            }
        } else {
            self.in_sync = false;
            self.sync_frame = None;
        }
    }

    // --- cursor motion (clamped) -----------------------------------------

    fn move_to(&mut self, r: usize, c: usize) {
        self.active_mut().set_cursor(Cursor { row: r, col: c });
        self.wrap_pending = false;
    }

    fn move_up(&mut self, n: usize) {
        self.set_row(self.cursor().row.saturating_sub(n));
        self.wrap_pending = false;
    }

    fn move_down(&mut self, n: usize) {
        self.set_row((self.cursor().row + n).min(self.rows() - 1));
        self.wrap_pending = false;
    }

    fn move_right(&mut self, n: usize) {
        self.set_col((self.cursor().col + n).min(self.cols() - 1));
        self.wrap_pending = false;
    }

    fn move_left(&mut self, n: usize) {
        self.set_col(self.cursor().col.saturating_sub(n));
        self.wrap_pending = false;
    }

    // --- erase ------------------------------------------------------------

    /// ED: 0 = cursor to end of screen, 1 = start of screen to cursor, 2 = all.
    fn erase_display(&mut self, mode: u16) {
        let (rows, cols) = (self.rows(), self.cols());
        let Cursor { row: cr, col: cc } = self.cursor();
        let blank = Cell::default();
        match mode {
            0 => {
                for c in cc..cols {
                    self.active_mut().set(cr, c, blank);
                }
                for r in (cr + 1)..rows {
                    for c in 0..cols {
                        self.active_mut().set(r, c, blank);
                    }
                }
            }
            1 => {
                for r in 0..cr {
                    for c in 0..cols {
                        self.active_mut().set(r, c, blank);
                    }
                }
                for c in 0..=cc {
                    self.active_mut().set(cr, c, blank);
                }
            }
            2 => {
                for r in 0..rows {
                    for c in 0..cols {
                        self.active_mut().set(r, c, blank);
                    }
                }
            }
            _ => {}
        }
    }

    /// EL: 0 = cursor to end of line, 1 = start of line to cursor, 2 = whole line.
    fn erase_line(&mut self, mode: u16) {
        let cols = self.cols();
        let Cursor { row: cr, col: cc } = self.cursor();
        let blank = Cell::default();
        let range = match mode {
            0 => cc..cols,
            1 => 0..(cc + 1).min(cols),
            2 => 0..cols,
            _ => return,
        };
        for c in range {
            self.active_mut().set(cr, c, blank);
        }
    }

    /// ECH: erase (blank) `n` cells from the cursor rightward, **without** moving
    /// the cursor or shifting the rest of the line (unlike DCH, which closes the
    /// gap). This is a common way to clear a run of cells (e.g. a status line or
    /// trailing content), so dropping it leaves stale text behind. Clamped to the
    /// row's right edge.
    fn erase_chars(&mut self, n: usize) {
        let cols = self.cols();
        let Cursor { row: cr, col: cc } = self.cursor();
        let end = (cc + n.max(1)).min(cols);
        let blank = Cell::default();
        for c in cc..end {
            self.active_mut().set(cr, c, blank);
        }
        self.wrap_pending = false;
    }

    // --- scroll region + line/char insert/delete --------------------------

    /// DECSTBM: set scroll region `top;bottom` (1-based, clamped); resets the
    /// cursor to the origin. An empty/`0` bottom means the last row.
    fn set_scroll_region(&mut self, params: &[u16]) {
        let rows = self.rows();
        let top = at1(params, 0).saturating_sub(1);
        let bottom = match at0(params, 1) {
            0 => rows - 1,
            b => (b as usize - 1).min(rows - 1),
        };
        // Ignore an inverted/empty region, as VT100 does.
        if top < bottom {
            self.scroll_top = top;
            self.scroll_bottom = bottom;
        }
        self.active_mut().set_cursor(Cursor::default());
        self.wrap_pending = false;
    }

    /// Scroll the scroll region up by `n` lines: top lines lost, blanks in at
    /// the bottom margin.
    ///
    /// One block move in the screen ([`Screen::scroll_rows_up`]), not a copy per
    /// cell: this runs on every line of output, and the per-cell loop held the
    /// emulator to ~11 MB/s on a 40x160 grid.
    fn scroll_up(&mut self, n: usize) {
        let (top, bottom) = (self.scroll_top, self.scroll_bottom);
        self.active_mut()
            .scroll_rows_up(top, bottom, n, Cell::default());
    }

    /// Scroll the scroll region down by `n` lines: bottom lines lost, blanks in
    /// at the top margin.
    fn scroll_down(&mut self, n: usize) {
        let (top, bottom) = (self.scroll_top, self.scroll_bottom);
        self.active_mut()
            .scroll_rows_down(top, bottom, n, Cell::default());
    }

    /// IL: insert `n` blank lines at the cursor row, within the scroll region;
    /// lines below shift down and fall off the bottom margin. No-op outside the
    /// region.
    fn insert_lines(&mut self, n: usize) {
        if self.cursor().row < self.scroll_top || self.cursor().row > self.scroll_bottom {
            return;
        }
        let saved_top = self.scroll_top;
        self.scroll_top = self.cursor().row;
        self.scroll_down(n);
        self.scroll_top = saved_top;
        self.set_col(0);
        self.wrap_pending = false;
    }

    /// DL: delete `n` lines at the cursor row, within the scroll region; lines
    /// below shift up and blanks fill the bottom margin. No-op outside region.
    fn delete_lines(&mut self, n: usize) {
        if self.cursor().row < self.scroll_top || self.cursor().row > self.scroll_bottom {
            return;
        }
        let saved_top = self.scroll_top;
        self.scroll_top = self.cursor().row;
        self.scroll_up(n);
        self.scroll_top = saved_top;
        self.set_col(0);
        self.wrap_pending = false;
    }

    /// ICH: insert `n` blank cells at the cursor, shifting the rest of the line
    /// right off the edge.
    fn insert_chars(&mut self, n: usize) {
        let cols = self.cols();
        let Cursor { row: r, col: start } = self.cursor();
        let n = n.min(cols - start);
        for c in (start..cols).rev() {
            let cell = if c >= start + n {
                self.active().cell(r, c - n)
            } else {
                Cell::default()
            };
            self.active_mut().set(r, c, cell);
        }
        self.wrap_pending = false;
    }

    /// DCH: delete `n` cells at the cursor, shifting the rest of the line left
    /// and filling the tail with blanks.
    fn delete_chars(&mut self, n: usize) {
        let cols = self.cols();
        let Cursor { row: r, col: start } = self.cursor();
        let n = n.min(cols - start);
        for c in start..cols {
            let cell = if c + n < cols {
                self.active().cell(r, c + n)
            } else {
                Cell::default()
            };
            self.active_mut().set(r, c, cell);
        }
        self.wrap_pending = false;
    }

    // --- save / restore + ESC --------------------------------------------

    fn save_cursor(&mut self) {
        self.saved = Some((self.cursor(), self.style));
    }

    fn restore_cursor(&mut self) {
        if let Some((cursor, style)) = self.saved {
            self.active_mut().set_cursor(cursor);
            self.style = style;
        }
        self.wrap_pending = false;
    }

    fn esc(&mut self, final_byte: u8) {
        match final_byte {
            b'7' => self.save_cursor(),    // DECSC
            b'8' => self.restore_cursor(), // DECRC
            _ => {}                        // charset selection etc.: ignore quietly
        }
    }
}

/// Tab stops every 8 columns (VT default), with a stop at column 0.
fn default_tabs(cols: usize) -> Vec<bool> {
    (0..cols).map(|c| c % 8 == 0).collect()
}

/// Read a param defaulting a missing/zero value to 1 (VT motion default).
fn at1(params: &[u16], i: usize) -> usize {
    match params.get(i).copied().unwrap_or(0) {
        0 => 1,
        v => v as usize,
    }
}

/// Read a param defaulting a missing value to 0 (erase/selector default).
fn at0(params: &[u16], i: usize) -> u16 {
    params.get(i).copied().unwrap_or(0)
}

/// Copy the overlapping top-left region of `src` into `dst` (for resize).
fn copy_top_left(src: &Screen, dst: &mut Screen) {
    let rows = src.rows().min(dst.rows());
    let cols = src.cols().min(dst.cols());
    for r in 0..rows {
        for c in 0..cols {
            dst.set(r, c, src.cell(r, c));
        }
    }
}