Text editor implementation as a programming étude

If you’re a programmer yourself and are looking to improve your skills, I would like to propose to generally “deliberately practice” your craft, in the sense “repeatedly attack problems at the edge of you capabilities in an exercise context, not at work”. And, as finding personal project ideas can be quite tricky for some, I would like to propose implementing a text editor as a really good project choice.

Now this proposition makes sense, in my opinion, because of the wide array of real hard problems related to so many different subjects of programming it has to offer. Also, there are good chances it’s quite different from what you work on daily (a majority of programmers are knee deep in the web of mobile applications these days) hence, will feel like a playful, new and exciting project.

Now hold your horses as, implementing a complete text editor that can rival with you current one (even if it’s Notepad or ed(1)) is a really big task. Try to start by setting your eyes on implementing a subpart of a one first.

Now for ideas on sub-parts that you could aim for:

  • A line editor: You know how your shell allows you to press backspace, delete characters to the beginning of the line, move the cursor? Well all of those are really nice thing but are quite novelties, most of there where not present in older shells. So, try your hand at implementing a program that asks for input but implements line editing features. Think of what a REPL does. Try for:
    • Typing characters
    • Backspace
    • Moving with arrows
    • Deleting to beginning/end of line
    • Moving to beginning/end of line
    • Moving word by word backward/forward
    • Entering in “replace mode” (like the insert key on keyboards)
  • Rendering a window tree of files: Try writing a terminal program that renders a tree of windows, each node being one of three types: horizontal split, vertical split, actual file/window. That one will get you thinking about recursing in a tree, caching information about location in files, calculating what is a line, how wide is a char, a tab, a Unicode char, how do you make it fast enough so that render wouldn’t block the editors main loop in a real editor. Try for:
    1. Reading files from disk
    2. Selecting a current position (so you get to implement scrolling)
    3. Creating a window tree so that all files have their own window split
    4. Rendering the window tree with nice window borders
    5. Framing and rendering the currently visible lines of the files
    6. Maybe have a status bar under each files showing stats line number of lines, current line, chars, file rights, file size, ….
    7. Make it fast by only rendering what’s needed when some file changes on disk
  • A file datastructure: Holding a file in memory, a task most editors need to do, is not an easy task. Getting it to be the right balance between: size in memory, insertion speed, deletion speed and interface complexity is a real struggle. The other problem you can attack after you have the basics right is testing operations on a file that is huge, bigger than 100MB and make that use case operations work in a decent time (< 100ms at least). Try exploring the following prior art in the matter:
    • Rope
    • Gap Buffer
    • Circular buffer
    • Red-Black tree
    • or a simple Linked List
    • or try the venerable Array
      Try testing how fast (and what’s the Big O of each?) of the following operations:
    • Inserting 1 character at the start/middle/end
    • Deleting 1 character at the start/middle/end
    • Inserting 10 000 characters at the start/middle/end
    • Deleting 10 000 characters at the start/middle/end
    • Inserting at the start the right after at the end
    • Loading in memory
    • Writing to disk
  • A command set/language: This one is a fun one for language lovers as it involves implementing a set of commands the user can use to edit files. You need to be able to parse an input string into an abstract syntax tree, then interpret it and execute it against a file contents. Here are good examples of editors that implemented a command set:
    • Vi - covers a lot, succinct [1][2][3]
    • Ed - not a visual editing, the grandfather of many others 1
    • Teco - really a language, also inspired a few others [1]
    • Emacs - less on point but think about keyboard keybindings and how natural they are to hit [1]
  • A plugin/configuration language: This one is all about implementing a full blown programming language (parser, interpreter, interface with host implementation language). A lot of toy editor project go without this one as it is a big chunk of work but a really crucial one in all popular editors theses day. You’ll be designing a language with the direct goal of exposing editor features, configuration and allowing the implementation of plugins that can change behaviour and call core editor methods. Take a look at the following languages that are used in popular editors:
    • ELisp
    • VimL
    • Lua
    • Python
    • Guile Scheme
    • Perl
  • A progressive rendering algorithm: This one is a bit smaller and needs quite a few pieces around it to make it work/visual. It consists in writing an algorithm that allows you to start a rerender of the screen following a user’s input but allows for stopping in the middle to handle user input then start back for where we where at the last rerender call making sure to invalidate parts that where just changed by the user’s action. Try reading this book at this point, it’s really one of the only books going in deep about many subject related to text editor implementation:

There a more smaller projects/parts that could be added to this list but at this point I would advise starting small but trying to build a complete editor and adding adding is all of those subparts together. Those first big goal being to be able to write the text editor with the new editor itself. :D

If you are interested in reading the implementation of a few toy editors with rather simple codebases you can look at:

Happy hacking & learning!