# beer-protocols

The terminal-protocol building blocks beer is built on: byte codecs, the
terminfo capability table, character-set translation, SGR parsing, and the
keyboard/mouse wire encoders. Everything here is a pure function or a plain
data type, with no terminal state, so each protocol detail can be read and
tested on its own. beer feeds these into its `vte`-driven dispatcher and its
grid model.

This README doubles as the reference for **which escape sequences, OSC
commands, and POSIX behaviours beer actually implements** - the parts a user
notices. Sequences not listed here are parsed and ignored rather than passed
through. Notation: `ESC` is `0x1b`, `CSI` is `ESC [`, `OSC` is `ESC ]`, `DCS`
is `ESC P`, `ST` is the string terminator `ESC \` (a `BEL` is also accepted to
end an OSC). `Ps` is a numeric parameter, `Pt` a text parameter.

## Crate layout

| Module | What it covers |
| --- | --- |
| `codec` | base64 (OSC 52), hex (XTGETTCAP names), `file://` URI percent-decoding (OSC 7) |
| `caps` | terminfo capabilities answered over XTGETTCAP |
| `charset` | G0/G1 designation and DEC special-graphics line drawing |
| `sgr` | the multi-parameter SGR colour and underline forms |
| `key` | legacy xterm/VT and kitty keyboard-protocol key encoding |
| `mouse` | X10 / UTF-8 / SGR mouse-report encoding |
| `style` | the enums an SGR/DECSET stream selects |

---

## C0 control characters

| Byte | Name | Effect |
| --- | --- | --- |
| `0x07` | BEL | rings the bell (visual flash / command / urgency, per config) |
| `0x08` | BS | backspace |
| `0x09` | HT | tab to next stop |
| `0x0A`-`0x0C` | LF/VT/FF | line feed |
| `0x0D` | CR | carriage return |
| `0x0E` | SO | invoke G1 (shift out) |
| `0x0F` | SI | invoke G0 (shift in) |

## Cursor movement and editing (CSI)

| Sequence | Name | Effect |
| --- | --- | --- |
| `CSI Ps A` | CUU | cursor up |
| `CSI Ps B` / `CSI Ps e` | CUD | cursor down |
| `CSI Ps C` / `CSI Ps a` | CUF | cursor forward |
| `CSI Ps D` | CUB | cursor back |
| `CSI Ps E` | CNL | cursor down, to column 1 |
| `CSI Ps F` | CPL | cursor up, to column 1 |
| `CSI Ps G` / `CSI Ps \`` | CHA | move to column |
| `CSI Ps d` | VPA | move to row |
| `CSI Ps ; Ps H` / `f` | CUP/HVP | move to row;col |
| `CSI Ps J` | ED | erase in display (0 below, 1 above, 2 all) |
| `CSI Ps K` | EL | erase in line (0 right, 1 left, 2 all) |
| `CSI Ps @` | ICH | insert blank characters |
| `CSI Ps P` | DCH | delete characters |
| `CSI Ps L` | IL | insert lines |
| `CSI Ps M` | DL | delete lines |
| `CSI Ps X` | ECH | erase characters |
| `CSI Ps S` | SU | scroll up |
| `CSI Ps T` | SD | scroll down |
| `CSI Ps ; Ps r` | DECSTBM | set scroll region (top;bottom) |
| `CSI Ps g` | TBC | clear tab stop (0) or all tabs (3) |
| `CSI s` / `CSI u` | SCOSC/SCORC | save / restore cursor |

## ESC (non-CSI)

| Sequence | Name | Effect |
| --- | --- | --- |
| `ESC D` | IND | line feed |
| `ESC M` | RI | reverse index |
| `ESC E` | NEL | next line |
| `ESC 7` / `ESC 8` | DECSC/DECRC | save / restore cursor |
| `ESC H` | HTS | set tab stop |
| `ESC c` | RIS | full reset |
| `ESC ( c` / `ESC ) c` | SCS | designate G0 / G1 charset (`0` = DEC special graphics, `B` = ASCII) |

## Graphic rendition (SGR, `CSI Ps m`)

Bold (1), dim (2), italic (3), underline (4), blink (5/6), reverse (7), hidden
(8), strike (9), and their resets (22-29). Overline (53) and its reset (55).
Underline styles via `4:x` and SGR 21: single, double, curly, dotted, dashed.

Colours: the 16 ANSI colours (30-37, 90-97 foreground; 40-47, 100-107
background) and default (39/49). Extended colour for foreground (38),
background (48), and underline (58/59), in both the legacy semicolon form
(`38;5;n`, `38;2;r;g;b`) and the colon-subparameter form (`38:5:n`,
`38:2:r:g:b`, with an ignored colour-space id). 256-colour and 24-bit truecolor
are fully supported.

## Modes (DECSET/DECRST `CSI [?] Ps h` / `l`)

| Mode | Name | Effect |
| --- | --- | --- |
| `4` (ANSI) | IRM | insert/replace |
| `?1` | DECCKM | application cursor keys |
| `?6` | DECOM | origin mode |
| `?7` | DECAWM | autowrap |
| `?25` | DECTCEM | cursor visibility |
| `?9` | - | X10 mouse reporting |
| `?1000` | - | normal mouse (press/release) |
| `?1002` | - | button-event mouse (drag) |
| `?1003` | - | any-event mouse (all motion) |
| `?1004` | - | focus in/out reporting |
| `?1005` | - | UTF-8 mouse coordinates |
| `?1006` | - | SGR mouse coordinates |
| `?47` / `?1047` | - | alternate screen |
| `?1049` | - | alternate screen + save/restore cursor |
| `?2004` | - | bracketed paste |
| `?2026` | - | synchronized output |

Mode state is reportable with **DECRQM** (`CSI [?] Ps $ p`), which replies
`CSI [?] Ps ; state $ y` (1 set, 2 reset, 0 unrecognized).

## Cursor style (DECSCUSR, `CSI Ps SP q`)

`0`/`1` blinking block, `2` steady block, `3` blinking underline, `4` steady
underline, `5` blinking bar, `6` steady bar. The configured default applies
until an application overrides it.

## Device reports

| Sequence | Name | Reply |
| --- | --- | --- |
| `CSI c` | DA1 | `CSI ?62;22c` (VT220 + ANSI colour) |
| `CSI > c` | DA2 | `CSI >0;276;0c` |
| `CSI = c` | DA3 | `DCS !\|00000000 ST` |
| `CSI 5 n` | DSR | `CSI 0n` (terminal OK) |
| `CSI 6 n` | CPR | `CSI row;col R` (cursor position) |
| `CSI > q` | XTVERSION | `DCS >\|beer(version) ST` |
| `DCS + q <names> ST` | XTGETTCAP | per name, `DCS 1 + r name=value ST` or `DCS 0 + r name ST` |

XTGETTCAP answers `TN` (terminal name `beer`), `Co`/`colors` (256), and `RGB`
(`8/8/8`, i.e. truecolor).

## Window title

`OSC 0 ; Pt` and `OSC 2 ; Pt` set the title. The title stack (`CSI 22 t` push,
`CSI 23 t` pop) is supported, so full-screen programs can save and restore it.

## OSC commands

| Command | Effect |
| --- | --- |
| `OSC 0` / `OSC 2` | set window title |
| `OSC 4 ; idx ; spec` | set / query palette entry (`?` queries) |
| `OSC 104 [; idx ...]` | reset palette (all, or listed entries) |
| `OSC 10` / `OSC 11` | set / query default foreground / background |
| `OSC 110` / `OSC 111` | reset foreground / background |
| `OSC 12` / `OSC 112` | set / reset cursor colour |
| `OSC 17` / `OSC 19` | set / query selection background / foreground |
| `OSC 7 ; file://host/path` | report working directory (used for new windows) |
| `OSC 8 ; params ; URI` | hyperlink (empty URI ends it) |
| `OSC 9 ; Pt` | desktop notification (iTerm2 style) |
| `OSC 777 ; notify ; title ; body` | desktop notification (rxvt style) |
| `OSC 99 ; metadata ; body` | desktop notification (kitty style, single-chunk) |
| `OSC 52 ; target ; data` | clipboard set (base64) or query (`?`) |
| `OSC 133 ; A/B/C/D` | shell-integration prompt marks |

Colour specs follow the X11 forms `#rrggbb` and `rgb:rr/gg/bb` (1-4 hex digits
per channel). Colour queries reply in the `rgb:rrrr/gggg/bbbb` form.

### Clipboard (OSC 52)

`OSC 52 ; c ; <base64>` sets the clipboard, `; p ;` the primary selection;
`; c ; ?` queries. For privacy, a query is answered from the text beer itself
last placed there, **not** the live system clipboard - so a remote program
cannot read what another application copied.

### Shell integration (OSC 133 / OSC 7)

Prompt marks `A` (prompt start), `B` (command start), `C` (output start), and
`D` (command end) drive prompt-jumping and "pipe last command output". `OSC 7`
tracks the working directory so a new window opens in the same place.

## Keyboard

The legacy xterm/VT encoding covers cursor keys (with DECCKM application mode),
the editing keypad (Insert/Delete/PageUp/PageDown), F1-F12, and the
xterm modifier parameter (`CSI 1 ; m <letter>`), with Alt sending an ESC
(meta) prefix.

The **kitty keyboard protocol** (`CSI > flags u` push, `CSI < flags u` pop,
`CSI = flags ; mode u` set, `CSI ? u` query) is implemented with the
disambiguate, report-event-types, report-alternate-keys, report-all-keys, and
report-associated-text enhancement flags. Keys are encoded as `CSI
code[:shifted] ; mod[:event] [; text] u`, with a 32-deep push/pop stack. The
common keys (Enter, Tab, Backspace) keep their legacy bytes when unmodified so
a plain shell stays usable.

## Mouse

Reports are framed in the legacy byte form (`CSI M Cb Cx Cy`), the UTF-8
coordinate form (DECSET 1005), or the SGR form (`CSI < Cb ; Cx ; Cy M/m`,
DECSET 1006). Shift/Alt/Ctrl modifier bits and the motion bit are encoded;
legacy releases collapse to button code 3. Reporting level is chosen by the
application via the mouse modes above; with reporting off the pointer drives
local selection and scrollback.

## POSIX / terminal behaviour

- A real PTY pair (`rustix` `openpt`/`grantpt`/`unlockpt`), with the child's
  `TERM` set to the configured value (default `beer`) and the window size kept
  in sync via `TIOCSWINSZ` (`SIGWINCH` reaches the child).
- The child's exit status is propagated as beer's own exit code.
- Alternate screen, scroll regions, autowrap and reflow on resize, and a
  scrollback buffer.
- Bracketed paste (DECSET 2004) and synchronized output (DECSET 2026) so
  applications can paste safely and update atomically.