MeshTerm/meshterm/ui/tui/fastrender.py

"""Write an already-composed frame straight to the terminal, one changed row at a time.

MeshTerm composes every frame itself: :func:`~meshterm.ui.tui.frame.compose_base` returns
a finished ANSI string, exactly one line per terminal row, already sliced to the viewport
and already folded to the platform's glyph set. prompt_toolkit's own renderer then takes
that string and does a full round trip with it — parses the ANSI back into style/text
fragments, writes every one of the ~1400 cells into a fresh grid of ``Char`` objects,
diffs that grid against the previous frame's grid, and re-emits ANSI for the cells that
moved.

That round trip is the single largest cost in a keystroke on the PicoCalc, and it is paid
in full on *every* paint — a measured 70-125 ms whether the frame changed by one row, by
all of them, or not at all (the 2 s header tick recomposes an identical frame and still
pays it). Since we already know the frame as rows of text, the same pixels can be reached
by comparing this frame's rows against the last frame's rows and rewriting only the ones
that differ: ~0.1 ms, and a smaller write besides.

The bypass is deliberately narrow. It runs only when the frame is a plain full-screen
composition — one base screen, no floating dialog and no busy overlay — because those are
laid out by prompt_toolkit's float containers rather than by us, and compositing them here
would mean re-implementing that placement. Anything else falls straight through to the
stock renderer, which stays the authority: :meth:`FastRenderer.render` is an accelerator
for the common case, never a second implementation of the whole layout engine.

Two details make the row diff safe rather than merely fast:

* **A row is rewritten whole, from column 0.** The terminal's own cursor advance re-lays
  the line, so a glyph it draws narrower than we measured cannot leave the rest of the row
  shifted — the same guarantee :meth:`~meshterm.ui.tui.session.TuiSession._emit` relies on,
  reached here for free rather than by a scrub pass.
* **Each row is prefixed with a reset and followed by an erase-to-end-of-line**, so no
  style leaks in from the row above and a row that got shorter leaves no tail behind.
"""

from __future__ import annotations

import os
from typing import Callable, Optional

from prompt_toolkit.renderer import Renderer


def enabled() -> bool:
    """Whether the direct row writer is active.

    On by default — it is 2-3x on every navigation keystroke and its output is verified
    byte-identical against the stock renderer by reading the console back. Set
    ``MESHTERM_FASTRENDER=0`` to fall back to prompt_toolkit's renderer, which is the
    escape hatch if a terminal ever disagrees.
    """
    return os.environ.get("MESHTERM_FASTRENDER", "1") != "0"


class FastRenderer(Renderer):
    """A :class:`~prompt_toolkit.renderer.Renderer` that short-circuits simple frames.

    Args:
        frame_source: Returns the composed full-screen frame as one ANSI string (rows
            joined by newlines), or ``None`` when this paint is not a plain base frame
            and must go through prompt_toolkit's layout instead.
    """

    def __init__(self, *args, frame_source: Callable[[], Optional[str]], **kwargs) -> None:
        super().__init__(*args, **kwargs)
        self._frame_source = frame_source
        self._prev_rows: Optional[list[str]] = None
        self._prev_size: Optional[tuple[int, int]] = None
        #: Paints answered here vs handed to the stock renderer — read by the bench.
        self.fast_paints = 0
        self.slow_paints = 0

    def reset(self, _scroll: bool = False, leave_alternate_screen: bool = True) -> None:
        """Forget the last frame along with prompt_toolkit's own render state."""
        self._prev_rows = None
        super().reset(_scroll=_scroll, leave_alternate_screen=leave_alternate_screen)

    def erase(self, leave_alternate_screen: bool = True) -> None:
        """The screen is being cleared out from under us — the next paint must be full."""
        self._prev_rows = None
        super().erase(leave_alternate_screen=leave_alternate_screen)

    def render(self, app, layout, is_done: bool = False) -> None:  # noqa: ANN001
        """Paint the frame, taking the row-diff path when this frame is a plain one."""
        text = None if is_done else self._frame_source()
        if text is None:
            # A dialog, the busy overlay, or the closing paint: prompt_toolkit lays those
            # out, so it must also own the diff — its grid is the only record of what is
            # on screen, and it is about to be rebuilt from scratch anyway.
            self._prev_rows = None
            self.slow_paints += 1
            super().render(app, layout, is_done)
            return

        output = self.output
        if self.full_screen and not self._in_alternate_screen:
            self._in_alternate_screen = True
            output.enter_alternate_screen()
        if not self._bracketed_paste_enabled:
            output.enable_bracketed_paste()
            self._bracketed_paste_enabled = True
        if not self._cursor_key_mode_reset:
            output.reset_cursor_key_mode()
            self._cursor_key_mode_reset = True

        size = output.get_size()
        dims = (size.rows, size.columns)
        rows = text.split("\n")[: size.rows]

        # A resize invalidates both records of the screen: ours and prompt_toolkit's.
        full = self._prev_rows is None or self._prev_size != dims
        if full:
            self._last_screen = None
            output.hide_cursor()
            output.reset_attributes()
            output.erase_screen()
            output.cursor_goto(0, 0)

        write = output.write_raw
        prev = self._prev_rows
        last = -2
        for i, row in enumerate(rows):
            if not full and i < len(prev) and prev[i] == row:
                continue
            # Stepping down a row with CRLF returns the terminal to a true column 0
            # whatever happened on the row above, and costs three bytes less than a
            # cursor address; only a real jump pays for one.
            write("\r\n" if i == last + 1 else f"\x1b[{i + 1};1H")
            # Erase *before* drawing, never after. A row that fills the terminal exactly
            # leaves the cursor in the last column with wrap pending, and an erase-to-end
            # there wipes the very character just written (the row's last cell went
            # missing on the PicoCalc's 53-column console). Clearing first also drops the
            # tail of a row that got shorter, which is what the erase was for.
            write("\x1b[0m\x1b[K")
            write(row)
            last = i
        # Park the cursor out of the text. The app hides it, but a terminal that ignores
        # that should not leave it blinking in the middle of a row.
        write(f"\x1b[{len(rows)};1H")
        output.flush()

        self._prev_rows = rows
        self._prev_size = dims
        self._last_size = size
        # prompt_toolkit's grid no longer describes the screen — if a later paint falls
        # back to it, it must redraw everything rather than diff against a stale record.
        self._last_screen = None
        self.fast_paints += 1