The Terminal

1. Introduction.

A minimal, yet modular X terminal configurable at compile time.
Modules are always explicitly compiled together and communicate through
C macros and/or inline functions with predefined names.
A module implements a part of the desired functionality of a terminal (from now
on, called PARTS). Each part might have multiple possible implementations, but
in one configuration, only one is active.

Each desired capability described in this document has a priority, which is one
of the following:
    MANDATORY       Absolutely necessary for even experimenting.
    IMPORTANT       Necessary for the terminal to be usable.
    DESIRABLE       Would be nice to have this capability.
    OPTIONAL        When everything else works.

2. Parts.

2.1. IO

This part reads the output of a process and sends keystrokes back - thus
communicates with the outside world. For a "tabbed" terminal (handling
multiple BUFFERs) more than one IO session can be active.
Gets information on keystrokes from DISPLAY.

Implementations:

    IO_STDIO        MANDATORY
    Reads from the standard input of the terminal process and writes to
    standard output. For testing. One session only.

    IO_PTY          IMPORTANT
    Opens a PTY and drives a terminal session as a real terminal should.

    IO_LIB          DESIRABLE
    Exports IO functionality as library functions, so the terminal can be
    compiled as a shared library and driven with arbitrary data. (For example:
    for a quake-like game console.)

    IO_NETWORK      OPTIONAL
    With this, the terminal may directly function as a telnet or SSH client
    (would be a handy, small replacement for PuTTY on Windows).

2.2. EMULATION

This part interprets the data read through the IO part, processes the terminal
control sequences, and drives the BUFFER.

Implementations:

    EMU_DUMB        MANDATORY
    A minimal emulator that understands just a small subset of the usual
    control sequences (such as a linefeed).

    EMU_LINUX       IMPORTANT
    An emulator that behaves like the linux console (as described in
    CONSOLE_CODES(4)).

    EMU_VT100/et al.    OPTIONAL
    Other emulation behaviors for historical/fun purposes.

2.3. BUFFER

Data structure and logic maintaining the display state (the text in the
terminal, as well as cursor position, current colors, etc).

Implementations:

    BUF_SIMPLE      MANDATORY
    Maintains a single, 80x25 buffer.

    BUF_REAL        IMPORTANT
    Handles multiple buffers (with one selected as current). Has scrollback
    memory.

2.4. DISPLAY

Displays the state got from BUFFER visually to the user; reads keyboard and
other user interface events and handles/dispatches them as necessary
(interprets them as terminal control hotkeys, or sends them to IO).

Implementations:

    DSP_X11_SIMPLE  MANDATORY
    Basic display for X-window. Opens a 80x25, unsizable window. Text is drawn
    using a fixed-width X font.

    DSP_X11         IMPORTANT
    Final display for X-window. Supports window resizing, fixed X and console
    fonts. Supports xterm mouse click events.

    DSP_SDL         DESIRABLE
    Display to an SDL surface. Used when the terminal is compiled as a shared
    library: exports functions for control. Draws text using console fonts or a
    given font surface (a bitmap of X*W x Y*H size, where W/H are character
    dimensions, with Y rows and X columns of characters in the bitmap; this way
    handles a 8-bit character set).

    DSP_WINDOWS     OPTIONAL
    Native windows display (probably used with IO_NETWORK).

3. Configuration.

Some possible configuration options of the parts/modules follow.



BUFFER: scrollback [IMPORTANT] {CFG_BUFFER_SCROLLBACK}
    Size of maximum scrollback memory for a buffer (given in lines).

BUFFER: enable tabs [IMPORTANT] {CFG_BUFFER_TABS}
    So one can disable tab support.

BUFFER: enable tabbar [DESIRABLE] {CFG_BUFFER_TABBAR}
    Enables or disables tab bar on the top row of the terminal window.



DISPLAY: font [IMPORTANT] {CFG_DISPLAY_NATIVE_FONT; CFG_DISPLAY_VGA_FONT}
    Selects the font used when rendering text. Might be a standard X11 font
    string, or the file of a console font.
    This option would also exist for DSP_WINDOWS (might or might not support a
    linux console font file); and DSP_SDL (with support for a font surface).

DISPLAY: colors [IMPORTANT] {CFG_DISPLAY_COLOR1...CFG_DISPLAY_COLOR16}
    RGB color values for the 16 terminal colors.

DISPLAY: cursor shape [DESIRABLE] {CFG_DISPLAY_CURSOR_SHAPE}
    Controls the shape of the active and inactive cursors. Possible options
    may be: CURS_BLOCK (for xterm like block cursor), CURS_COLUMN (column 
		on the left side of the character block, as in gvim), CURS_VGA (bar on
		the bottom of the character block), CURS_EMPTYBLOCK (for xterm like 
		unfocused empty block cursor)

DISPLAY: cursor color [DESIRABLE] {CFG_DISPLAY_CURSOR_COLOR_BG, CFG_DISPLAY_CURSOR_COLOR_FG}
    Color for the active and inactive cursor.

DISPLAY: cursor blink [DESIRABLE] {CFG_DISPLAY_CURSOR_BLINK_PERIOD, CFG_DISPLAY_CURSOR_BLINK_ONTIME}
		If PERIOD is not 0, the cursor is blinking. The period of the blink is
		PERIOD ms and the cursor is visible for ONTIME percentage.

DISPLAY: reverse mode [DESIRABLE] {CFG_DISPLAY_REVERSE}
    Reverses colors (black-on-white mode).

DISPLAY: lock size [DESIRABLE] {CFG_DISPLAY_LOCK_SIZE}
    Locks window size at a predefined value (e.g. 80x25).



4. Requirements

Software that ABSOLUTELY HAS TO RUN, priority 1:

- aptitude

Other software that might be useful if it ran:

- mc
- screen
- vim
- irssi