Building a basic text editor in Rust

Two years ago, I came across this excellent tutorial written by snaptoken. It walks through the process of building a small text editor in C. I don’t really enjoy following along with code tutorials verbatim, so I thought it would be a fun challenge to use snaptoken’s tutorial as a general reference and implement the text editor in Rust instead. I effectively treated the tutorial like a product manager and came away from each chapter with a new list of features to implement:

Chapter 2

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• Enable raw mode. Write each stdin char to stdout. Ctrl indicates a carriage return. q indicates that the user wants to quit. Ctrl-# combinations (e.g., Ctrl-Z) should be ignored.

Chapter 3

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• Clear the screen on startup, shutdown, and on tick.
• Draw a tilde border on the left-hand side of the screen.
• Render a cursor in the first position of the first line.
• Replace println!() with write!()s to a BufWriter.
• Display a welcome message at the bottom of the screen.
• Allow the user to move their cursor using arrow keys. Cursor info should be stored in global state. Don’t allow the cursor to move offscreen.
• Page Up and Page Down can snap the cursor to the top and bottom of the screen.
• Home and End can snap the cursor to the beginning and end of the current line.

Chapter 4

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• If run with a filename argument, read the file into memory.
• Enable vertical scrolling through a long text file. Enable horizontal scrolling on long lines. (Basically, allow users to scroll offscreen if the text allows it.)
• Snap to the next line if the user scrolls past the end of a line. Snap to the previous line if the user scrolls left at the beginning of a line.
• Update Page Up and Page Down to scroll up and down the entire page.
• Display a status bar that includes the filename, # of lines in the file, and current line number. If the file has no name, display [No Name].
• Render key binding hints in the status bar (e.g., Ctrl-Q = quit). Display on startup and hide 5 seconds after the user presses a key.

Chapter 5

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• Allow saving to disk via Ctrl-S.
• Track “dirtiness” in global state. The buffer is “dirty” if it has been modified since opening or saving the file.
• Require Ctrl-Q 3x to confirm quitting with unsaved changes.
• Implement Backspace. Backspacing at the beginning of a line should wrap the line onto the previous line. If there is no previous line, do nothing.
• Implement Delete. Deleting at the end of a line should wrap onto the next line. If there is no next line, do nothing.
• Implement Enter. This can insert new lines or split a line into two lines.
• Prompt the user to input a filename when saving a new file.
• Allow Backspace in prompt input.

Chapter 6

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• Implement search with Ctrl-F.
  • The user inputs their query into the prompt bar. On each keypress, the cursor snaps to a string that contains their query substring.
  • When search is cancelled with Escape, restore the cursor to its original position.
  • Users can browse through substring matches using arrow keys. Left/Up to move to the previous match, Right/Down to move to the next match.

Chapter 7

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• Implement basic C syntax highlighting.
  • Digits are red. Search results are blue. Strings are magenta. Single-line and multi-line comments are cyan. Types are green. Keywords are yellow.
  • Syntax highlighting should only appear if the open filename ends with .c.
  • Search result highlighting should overwrite other highlighting. Original highlighting should be restored when search is cancelled.

Crates I used

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• termion: This looked like the simplest way to enable raw mode without C interop. (Avoiding C interop was my only self-imposed rule for this project; after all, I wanted to have fun 😋) Cursor rendering and text rendering also met all my requirements and was simple enough to use.
• termsize: I used this to get terminal window size on startup. (I never actually updated my implementation to handle mid-session window resizes…)
• unicode_segmentation: I used this to split strings on grapheme cluster boundaries instead of character boundaries. This ensures that a character with a diacritic like ä is kept together as a single element instead of split apart (also allows the text editor to support other exotic Unicode input like emoji, Cyrillic, etc.)

My experience

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I had a heck of a time with this project overall. I dropped it twice: once in 2022 because I was intimidated by Chapter 6, and another time in mid 2023 because I was intimidated by Chapter 7 and sick of looking at my years old over-engineered code.

It didn’t take long for my code to diverge significantly from snaptoken’s code. It started with, “oh, well I can’t really do that in Rust, so I have to do it this other way instead,” and ended with, “yeahhh, my design is nothing like that, I have to do this differently”.

The final structure that I ended up with is mind-boggling, to be honest:

📂src
 ┣ 📂backend
 ┃ ┣ cursor.rs
 ┃ ┣ mod.rs
 ┃ ┣ operations.rs
 ┃ ┗ prompt.rs
 ┣ 📂data
 ┃ ┣ enums.rs
 ┃ ┣ mod.rs
 ┃ ┣ payload.rs
 ┃ ┗ textrow.rs
 ┣ 📂gfx
 ┃ ┣ controller.rs
 ┃ ┣ mod.rs
 ┃ ┗ render.rs
 ┣ input.rs
 ┣ main.rs
 ┗ utils.rs

The RenderController owns a CursorHandler and an OperationsHandler. The OperationsHandler is a middle layer between the RenderController, RenderDriver, and PromptProcessor. Lol?

This object-oriented approach must have been veryyy satisfying for me to build out in 2022, but it quickly became a rat king as I had to wrestle with issues like: “how do I pass data from the PromptProcessor to its parent’s parent RenderController!?!?” If implementing a text editor is truly a gateway drug into kernel development, then I overdosed.

Kilo is considered a simple project because it can be implemented with ~1000 lines of code. Running tokei on my repo tells a grim tale:

===================================================================
 Language            Files        Lines         Code     Comments
===================================================================
 C                       1           28           18            4
 Markdown                1           13            0           10
 Rust                   14         1897         1410          299
 TOML                    1           12            9            1
===================================================================
 Total                  17         1950         1437          314
===================================================================

You’re invited to gawk at my repo for yourself:

Over-abstraction aside, I had a lot of fun implementing all of these features. It’s easy to dream up a bunch of potential extensions to Kilo. I’ve had enough of text editor development for the time being 🙂 but I was tempted to implement automated testing. It was tedious to get through Chapter 6 + 7 when large code changes required me to manually check for regressions in all previously implemented features. Someday, I might try compiling my code to WASM or rebuilding with xterm-js-rs so I can get the app web-embeddable.