Awesome

Atto Emacs

The smallest functional Emacs in less than 2000 lines of C.

Atto Emacs is inspired by MicroEmacs, Nano, Pico and my earlier project known as Perfect Emacs [1].

A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away. -- <cite>Antoine de Saint-Exupery</cite>

Goals of Atto Emacs

• Be the smallest fuctional Emacs in less than 2000 lines of C.
• Provide a rich level of functionality in the smallest amount of code
• Be easy to understand without extensive study (to encourage further experimentation).

In Defining Atto as the lowest functional Emacs I have had to consider the essential feature set that makes Emacs, 'Emacs'. I have defined this point as a basic Emacs command set and key bindings; the ability to edit multiple files (buffers), and switch between them; edit the buffers in multiple windows, cut, copy and paste; forward and reverse searching, a replace function, basic syntax highlighting and UTF8 support. The proviso being that all this will fit in less than 2000 lines of C.

Why the name Atto?

The small Emacs naming scheme appears to use sub-unit prefixes in descending order with each further reduction of functionality. Atto means 10 to the power of minus 18. Logically Femto (10^-15) comes after Pico (10^-12). However choosing Atto allows for the potential for Femto to be an Atto based Emacs with a scripting language.

• As of November 2017 Atto's big brother Femto can be found here

Derivation

Atto is based on the public domain code of Anthony Howe's editor (commonly known as Anthony's Editor or AE, [2]). Rather than representing a file as a linked list of lines, the AE Editor uses the concept of a Buffer-Gap [4,5,6]. A Buffer-Gap editor stores the file in a single piece of contiguous memory with some extra unused space known as the buffer gap. On character insertion and deletion the gap is first moved to the current point. A character deletion then extends the gap by moving the gap pointer back by 1 OR the gap is reduced by 1 when a character is inserted. The Buffer-Gap technique is elegant and significantly reduces the amount of code required to load a file, modify it and redraw the display. The proof of this is seen when you consider that Atto supports almost the same command set that Pico supports, but Pico requires almost 17 times the amount of code.

Comparisons with Other Emacs Implementations

Editor         Binary   BinSize     KLOC  Files

atto           atto       33002     1.9k     10
pEmacs         pe         59465     5.7K     16
Esatz-Emacs    ee         59050     5.7K     14
GNOME          GNOME      55922     9.8k     13
Zile           zile      257360    11.7k     48
Mg             mg        585313    16.5K     50
uEmacs/Pk      em        147546    17.5K     34
Pico           pico      438534    24.0k     29
Nano           nano      192008    24.8K     17
jove           jove      248824    34.7k     94
Qemacs         qe        379968    36.9k     59
ue3.10         uemacs    171664    52.4K     16
GNUEmacs       emacs   14632920   358.0k    186

Atto Key Bindings

C-A   begining-of-line
C-B   backward-character
C-D   delete-char
C-E   end-of-line
C-F   forward Character
C-G	  Abort (at prompts)
C-H   backspace
C-I   handle-tab
C-J   newline
C-K   kill-to-eol
C-L   refresh display
C-M   Carrage Return
C-N   next line
C-P   previous line
C-R   search-backwards
C-S	  search-forwards
C-U   Undo
C-V   Page Down
C-W   Kill Region (Cut)
C-X   CTRL-X command prefix
C-Y   Yank (Paste)

M-<   Start of file
M->   End of file
M-v   Page Up
M-f   Forward Word
M-b   Backwards Word
M-g   goto-line
M-r   Search and Replace
M-w   copy-region

C-<spacebar> Set mark at current position.

^X^C  Exit. Any unsaved files will require confirmation.
^X^F  Find file; read into a new buffer created from filename.
^X^S  Save current buffer to disk, using the buffer's filename as the name of
^X^W  Write current buffer to disk. Type in a new filename at the prompt to
^Xi   Insert file at point
^X=   Show Character at position
^X^N  next-buffer
^Xn   next-buffer
^Xk   kill-buffer
^X1   delete-other-windows
^X2   split-window
^Xo   other-window

Home  Beginning-of-line
End   End-of-line
Del   Delete character under cursor
Ins   Toggle Overwrite Mode
Left  Move left
Right Move point right
Up    Move to the previous line
Down  Move to the next line
Backspace delete character on the left
Ctrl+Up      beginning of file
Ctrl+Down    end of file
Ctrk+Left    Page Down
Ctrl+Right   Page Up

Copying and moving

C-<spacebar> Set mark at current position
^W   Delete region
^Y   Yank back kill buffer at cursor
M-w  Copy Region

A region is defined as the area between this mark and the current cursor position. The kill buffer is the text which has been most recently deleted or copied.

Generally, the procedure for copying or moving text is:

1. Mark out region using M-<spacebar> at the beginning and move the cursor to the end.
2. Delete it (with ^W) or copy it (with M-W) into the kill buffer.
3. Move the cursor to the desired location and yank it back (with ^Y).

Searching

C-S or C-R enters the search prompt, where you type the search string
BACKSPACE - will reduce the search string, any other character will extend it
C-S at the search prompt will search forward, will wrap at end of the buffer
C-R at the search prompt will search backwards, will wrap at start of the buffer
ESC will escape from the search prompt and return to the point of the match
C-G abort the search and return to point before the search started

Building on Ubuntu

When building on Ubuntu you will need to install the libcurses dev package.

$ sudo apt-get install apt-file
$ apt-file update

now search for which package would have curses.h and install it:

$ apt-file search curses.h
libncurses5-dev: /usr/include/curses.h
$ sudo apt-get install libncurses5-dev

Future Enhancements and Collaboration

The priority now is bug fixes and keeping the code count below 2000 lines. Bug fixes and optimisations are welcome especially if they deliver code reductions which make space for further fixes.

Multiple Windows or Not?

Atto supports multiple windows ! This was the hardest part of the project to get working reliably.

The lack of multiple windows would have been quickly noticed as it is a very visible feature of the Emacs user interface. It is very useful to be able to look at some code in one window whilst editing another section of the same file (or a different file) in another window. As more than one window can access the same buffer the current point now has now to be associated with the window structure and updated back to the buffer structure whenever any gap or display code is called that accesses the point location. The strategy I used in the end was to treat the buffer as the master and update the window structure with copies of the critical values (point, page, epage, cursor row & col) after each display update of that window. This is because the display code does the calculations necessary to reframe the screen when the point scrolls up off the screen or below the screen. Getting everything to work correctly when displaying the same buffer in more that one winow was a real challenge and took arpund 15-20 hours to get it working.

A multi-window display issue (specifically evident in a buffer-gap editor) was resolved in Atto 1.6. This is where you are editing a file that is displayed in more than one window. Say you are in window1 and delete 3 lines, the current point in the other windows (if the point is below the point in window1) have to be adjusted to take into account that their relative positions in the buffer have now shifted up. We do this by tracking the size of the text in the buffer before and after each command. At the start of the display function we can work out the difference and adjust the other windows when they are updated. This mechanism works well even when inserting a text file that means that the gap has to be re-allocated.

Known Issues

• Goto-line will fail to go to the very last line. This is a special case that could easily be fixed.

Copying

Atto code is released to the public domain. hughbarney AT gmail.com 2017

Acknowledgements

Ed Davies for bringing Athony's Editor to my attention
Anthony Howe for his original codebase
Matt Fielding (Magnetic Realms) for providing fixes for multi-byte / wide characters, delete, backspace and cursor position
The Infinnovation team for bug fixes to complete.c

References

[1] Perfect Emacs - https://github.com/hughbarney/pEmacs
[2] Anthony's Editor - https://github.com/hughbarney/Anthony-s-Editor
[3] MG - https://github.com/rzalamena/mg
[4] Jonathan Payne, Buffer-Gap: http://ned.rubyforge.org/doc/buffer-gap.txt
[5] Anthony Howe,  http://ned.rubyforge.org/doc/editor-101.txt
[6] Anthony Howe, http://ned.rubyforge.org/doc/editor-102.txt