It all started when I was recently reading the excellent Project Oberon, where in chapter 5 a data structure for managing text is introduced. I found this rather appealing and wanted to see how it works in practice.

After some time I decided that besides just having fun hacking around I might as well build something which could (at least in the long run) replace my current editor of choice: vim.

This should be accomplished by a reasonable amount of clean (your mileage may vary), modern and legacy free C code. Certainly not an old, 500'000 lines long #ifdef cluttered mess which tries to run on all broken systems ever envisioned by mankind.

Admittedly vim has a lot of functionally, most of which I don't use. I therefore set out with the following main goals:

• Unicode aware

• handle arbitrary files including

  • large ones e.g. >500M SQL dumps or CSV exports
  • single line ones e.g. minified JavaScript
  • binary ones e.g. ELF files

• unlimited undo/redo support, the possibility to revert to any earlier/later state

• regex search (and replace)

• multiple file/window support

• syntax highlighting

The goal could thus be summarized as "80% of vim's features (in other words the useful ones) implemented in roughly 1% of the code".

Finally and most importantly it is fun! Writing a text editor presents some interesting challenges and design decisions, some of which are explained below.

The core of this editor is a persistent data structure called a piece table which supports all modifications in O(m), where m is the number of non-consecutive editing operations. This bound could be further improved to O(log m) by use of a balanced search tree, however the additional complexity doesn't seem to be worth it, for now.

The actual data is stored in buffers which are strictly append only. There exist two types of buffers, one fixed-sized holding the original file content and multiple append-only ones storing the modifications.

A text, i.e. a sequence of bytes, is represented as a double linked list of pieces each with a pointer into a buffer and an associated length. Pieces are never deleted but instead always kept around for redo/undo support. A span is a range of pieces, consisting of a start and end piece. Changes to the text are always performed by swapping out an existing, possibly empty, span with a new one.

An empty document is represented by two special sentinel pieces which always exist:

/-+ --> +-\
| |     | |
\-+ <-- +-/
 #1     #2

Loading a file from disk is as simple as mmap(2)-ing it into a buffer, creating a corresponding piece and adding it to the double linked list. Hence loading a file is a constant time operation i.e. independent of the actual file size (assuming the operating system uses demand paging).

/-+ --> +-----------------+ --> +-\
| |     | I am an editor! |     | |
\-+ <-- +-----------------+ <-- +-/
 #1             #3              #2

Inserting a junk of data amounts to appending the new content to a modification buffer. Followed by the creation of new pieces. An insertion in the middle of an existing piece requires the creation of 3 new pieces. Two of them hold references to the text before respectively after the insertion point. While the third one points to the newly added text.

/-+ --> +---------------+ --> +----------------+ --> +--+ --> +-\
| |     | I am an editor|     |which sucks less|     |! |     | |
\-+ <-- +---------------+ <-- +----------------+ <-- +--+ <-- +-/
 #1            #4                   #5                #6      #2

       modification buffer content: "which sucks less"

During this insertion operation the old span [3,3] has been replaced by the new span [4,6]. Notice that the pieces in the old span were not changed, therefore still point to their predecessors/successors, and can thus be swapped back in.

If the insertion point happens to be at a piece boundary, the old span is empty, and the new span only consists of the newly allocated piece.

Similarly a delete operation splits the pieces at appropriate places.

/-+ --> +-----+ --> +--+ --> +-\
| |     | I am|     |! |     | |
\-+ <-- +-----+ <-- +--+ <-- +-/
 #1       #7         #6      #2

Where the old span [4,5] got replaced by the new span [7,7]. The underlying buffers remain unchanged.

Notice that the common case of appending text to a given piece is fast since, the new data is simply appended to the buffer and the piece length is increased accordingly. In order to keep the number of pieces down, the least recently edited piece is cached and changes to it are done in place (this is the only time buffers are modified in a non-append only way). As a consequence they can not be undone.

Since the buffers are append only and the spans/pieces are never destroyed undo/redo functionality is implemented by swapping the required spans/pieces back in.

As illustrated above, each change to the text is recorded by an old and a new span. An action consists of multiple changes which logically belong to each other and should thus also be reverted together. For example a search and replace operation is one action with possibly many changes all over the text.

The text states can be marked by means of a snapshotting operation. Snapshotting saves a new node to the history graph and creates a fresh Action to which future changes will be appended until the next snapshot.

Actions make up the nodes of a connected digraph, each representing a state of the file at some time during the current editing session. The edges of the digraph represent state transitions that are supported by the editor. The edges are implemented as four Action pointers (prev, next, earlier, and later).

The editor operations that execute the four aforementioned transitions are undo, redo,earlier, and later, respectively. Undo and redo behave in the traditional manner, changing the state one Action at a time. Earlier and later, however, traverse the states in chronological order, which may occasionally involve undoing and redoing many Actions at once.

The main advantage of the piece chain as described above is that all operations are performed independent of the file size but instead linear in the number of pieces i.e. editing operations. The original file buffer never changes which means the mmap(2) can be performed read only which makes optimal use of the operating system's virtual memory / paging system.

The maximum editable file size is limited by the amount of memory a process is allowed to map into its virtual address space, this shouldn't be a problem in practice. The whole process assumes that the file can be used as is. In particular the editor assumes all input and the file itself is encoded as UTF-8. Supporting other encodings would require conversion using iconv(3) or similar upon loading and saving the document.

Similarly the editor has to cope with the fact that lines can be terminated either by \n or \r\n. There is no conversion to a line based structure in place. Instead the whole text is exposed as a sequence of bytes. All addressing happens by means of zero based byte offsets from the start of the file.

The main disadvantage of the piece chain data structure is that the text is not stored contiguous in memory which makes seeking around somewhat harder. This also implies that standard library calls like the regex(3) functions can not be used as is. However this is the case for all but the most simple data structures used in text editors.

This section contains some ideas for further architectural changes.

In principle it would be nice to follow a similar client/server approach as sam/samterm i.e. having the main editor as a server and each window as a separate client process with communication over a unix domain socket.

That way window management would be taken care of by dwm or dvtm and the different client processes would still share common cut/paste registers etc.

Currently the editor copies the whole text to a contiguous memory block and then uses the standard regex functions from libc. Clearly this is not a satisfactory solution for large files.

The long term solution is to write our own regular expression engine or modify an existing one to make use of the iterator API. This would allow efficient search without having to double memory consumption.

The used regex engine should use a non-backtracking algorithm. Useful resources include:

• Russ Cox's regex page
• TRE as used by musl which uses a parallel TNFA matcher
• Plan9's regex library which has its root in Rob Pike's sam text editor
• RE2 C++ regex library

The editor core is written in a library like fashion which should make it possible to write multiple frontends with possibly different user interfaces/paradigms.

The default, and currently only, interface is a vim clone called vis. The following section gives a quick overview over various vim features and their current support in vis.

d   (delete)
c   (change)
y   (yank)
p   (put)
>   (shift-right)
<   (shift-left),
J   (join)
~   (swap case)
gu  (make lowercase)
gU  (make uppercase)

Operators can be forced to work line wise by specifying V.

h        (char left)
l        (char right)
j        (line down)
k        (line up)
gj       (display line down)
gk       (display line up)
0        (start of line)
^        (first non-blank of line)
g_       (last non-blank of line)
$        (end of line)
%        (match bracket)
b        (previous start of a word)
B        (previous start of a WORD)
w        (next start of a word)
W        (next start of a WORD)
e        (next end of a word)
E        (next end of a WORD)
ge       (previous end of a word)
gE       (previous end of a WORD)
{        (previous paragraph)
}        (next paragraph)
(        (previous sentence)
)        (next sentence)
[[       (previous start of C-like function)
[]       (previous end of C-like function)
][       (next start of C-like function)
]]       (next end of C-like function)
gg       (begin of file)
g0       (begin of display line)
gm       (middle of display line)
g$       (end of display line)
G        (goto line or end of file)
|        (goto column)
n        (repeat last search forward)
N        (repeat last search backwards)
*        (search word under cursor forwards)
#        (search word under cursor backwards)
f{char}  (to next occurrence of char to the right)
t{char}  (till before next occurrence of char to the right)
F{char}  (to next occurrence of char to the left)
T{char}  (till before next occurrence of char to the left)
;        (repeat last to/till movement)
,        (repeat last to/till movement but in opposite direction)
/{text}  (to next match of text in forward direction)
?{text}  (to next match of text in backward direction)

An empty line is currently neither a word nor a WORD.

The semantics of a paragraph and a sentence is also not always 100% the same as in vim.

Some of these commands do not work as in vim when prefixed with a digit i.e. a multiplier. As an example in vim 3$ moves to the end of the 3rd line down. However vis treats it as a move to the end of current line which is repeated 3 times where the last two have no effect.

All of the following text objects are implemented in an inner variant (prefixed with i) and a normal variant (prefixed with a):

w  word
W  WORD
s  sentence
p  paragraph
[,], (,), {,}, <,>, ", ', `         block enclosed by these symbols

For sentence and paragraph there is no difference between the inner and normal variants.

Additionally the following text objects, which are not part of stock vim are also supported:

ae      entire file content
ie      entire file content except for leading and trailing empty lines
af      C-like function definition including immeadiately preceding comments
if      C-like function definition only function body
al      current line
il      current line without leading and trailing white spaces

At the moment there exists a more or less functional insert, replace and visual mode (in both line and character wise variants).

Visual block mode is not implemented and there exists no immediate plan to do so. Instead vis has built in support for multiple cursors.

vis supports multiple cursors with immediate visual feedback (unlike in the visual block mode of vim where for example inserts only become visible upon exit). There always exists one primary cursor, additional ones can be created as needed.

To manipulate multiple cursors use in normal mode:

CTRL-K   create a new cursor on the line above
CTRL-J   create a new cursor on the line below
CTRL-P   remove least recently added cursor
CTRL-N   select word the cursor is currently over, switch to visual mode
CTRL-A   try to align all cursor on the same column
ESC      if a selection is active, clear it.
         Otherwise dispose all but the primary cursor.

Visual mode was enhanced to recognize:

I        create a cursor at the start of every selected line
A        create a cursor at the end of every selected line
CTRL-N   create new cursor and select next word matching current selection
CTRL-X   clear (skip) current selection, but select next matching word
CTRL-P   remove least recently added cursor

[a-z] general purpose marks
<     start of the last selected visual area in current buffer
>     end of the last selected visual area in current buffer

No marks across files are supported. Marks are not preserved over editing sessions.

Only the 26 lower case registers [a-z] and 1 additional default register is supported.

The text is currently snapshotted whenever an operator is completed as well as when insert or replace mode is left. Additionally a snapshot is also taken if in insert or replace mode a certain idle time elapses.

Another idea is to snapshot based on the distance between two consecutive editing operations (as they are likely unrelated and thus should be individually reversible).

Besides the regular undo functionality, the key bindings g+ and g- traverse the history in chronological order. Further more the :earlier and :later commands provide means to restore the text to an arbitrary state.

The repeat command . works for all operators and is able to repeat the last insertion or replacement.

[a-z] are recoginized macro names, q starts a recording, @ plays it back. @@ refers to the least recently recorded macro.

At the :-command prompt only the following commands are recognized, any valid unique prefix can be used:

:nnn        go to line nnn
:bdelete    close all windows which display the same file as the current one
:edit       replace current file with a new one or reload it from disk
:open       open a new window
:qall       close all windows, exit editor
:quit       close currently focused window
:read       insert content of another file at current cursor position
:split      split window horizontally
:vsplit     split window vertically
:new        open an empty window, arrange horizontally
:vnew       open an empty window, arrange vertically
:wq         write changes then close window
:xit        like :wq but write only when changes have been made
:write      write current buffer content to file
:saveas     save file under another name
:substitute search and replace currently implemented in terms of `sed(1)`
:!          filter range through external command
:earlier    revert to older text state
:later      revert to newer text state 
:set        set the options below

 tabwidth   [1-8]

   set display width of a tab and number of spaces to use if
   expandtab is enabled

 expandtab  (yes|no)

   whether typed in tabs should be expanded to tabwidth spaces

 autoindent (yes|no)

   replicate spaces and tabs at the beginning of the line when
   starting a new line.

 number         (yes|no)
 relativenumber (yes|no)

   whether absolute or relative line numbers are printed alongside
   the file content

 syntax      name

   use syntax definition given (e.g. "c") or disable syntax
   highlighting if no such definition exists (e.g :set syntax off)

 show        newlines=[1|0] tabs=[1|0] spaces=[0|1]

   show/hide special white space replacement symbols

Each command can be prefixed with a range made up of a start and an end position as in start,end. Valid position specifiers are:

.          start of the current line
+n and -n  start of the line relative to the current line
'm         position of mark m
/pattern/  first match after current position

If only a start position without a command is given then the cursor is moved to that position. Additionally the following ranges are predefined:

%          the whole file, equivalent to 1,$
*          the current selection, equivalent to '<,'>

History support, tab completion and wildcard expansion are other worthwhile features. However implementing them inside the editor feels wrong.

Tabs can optionally be expaned to a configurable number of spaces. The first line ending in the file determines what will be inserted upon a line break (defaults to \n).

A per window, file local jump list (navigate with CTRL+O and CTRL+I) and change list (navigate with g; and g,) is supported. The jump list is implemented as a fixed sized ring buffer.

The mouse is currently not used at all.

Potentially interesting features:

• code completion: this should be done as an external process. I will have to take a look at the tools from the llvm / clang project. Maybe dvtm's terminal emulation support could be reused to display an slmenu inside the editor at the cursor position?

• something similar to vim's quick fix functionality

• text folding

• runtime configurable key bindings

Some of the features of vim which will not be implemented:

• tabs / multiple workspaces / advanced window management
• file and directory browser
• support for file archives (tar, zip, ...)
• support for network protocols (ftp, http, ssh ...)
• encryption
• compression
• GUIs (neither x11, motif, gtk, win32 ...) although the codebase should make it easy to add them
• VimL
• plugins (certainly not vimscript, if anything it should be lua based)
• right-to-left text
• ex mode (if you need a stream editor use ssam(1)
• diff mode
• vimgrep
• internal spell checker
• compile time configurable features / #ifdef mess

At this point it might be best to fetch the code, edit some scratch file, notice an odd behavior or missing functionality, write and submit a patch for it, then iterate.

Additional test cases either for the [low level text manipulation routines] (https://github.com/martanne/vis/tree/test/test/text) or as [commands for the vis frontend] (https://github.com/martanne/vis/tree/test/test/vis) would be highly appreciated.

WARNING: There are probably still some bugs left which could corrupt your unsaved changes. Use at your own risk. At this point I suggest to only edit non-critical files which are under version control and thus easily recoverable!

A quick overview over the code structure to get you started:

Hope this gets the interested people started.

Feel free to ask questions if something is unclear! There are still a lot of bugs left to fix, but by now I'm fairly sure that the general concept should work.

As always, comments and patches welcome!

Cheers, Marc