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Kiro Build Status

Kiro is a tiny UTF-8 text editor on terminal written in Rust. Kiro was started as a Rust port of awesome minimal text editor kilo and has grown with various extensions & improvements.

main screenshot

It provides basic features as a minimal text editor:

  • Open/Save text files
  • Create new text files and empty text buffer on memory
  • Edit a text (put/delete characters, insert/delete lines, ...)
  • Simple syntax highlighting
  • Simple incremental text search

And Kiro extends kilo to improve editing (please see 'Extended Features' section and 'Implementation' section below for more details):

  • Support editing UTF-8 characters like '🐢' (kilo only supports ASCII characters)
  • Undo/Redo
  • More useful shortcuts (Alt modifier is supported)
  • 24bit colors (true colors) and 256 colors support using gruvbox retro color palette with 16 colors fallback
  • More efficient screen rendering and highlighting (kilo renders entire screen each time)
  • Open multiple files (switch buffers by Ctrl-X/Alt-X)
  • Resizing terminal window supported. Screen size is responsible
  • Highlight more languages (Rust, Go, JavaScript, C++) and items (statements, types, number literals, ...)
  • Automatically closes the message bar at bottom of line
  • Modular implementation for each logics such as parsing key inputs, rendering screen, calculating highlight, modifying text buffer (kilo implements everything in one kilo.c with several global variables)
  • Incremental text search is fixed and improved (kiro only highlights current match and only hits once per line).

Kiro aims to support kinds of xterm terminals on Unix-like systems. For example,, Gnome-Terminal, (hopefully) Windows Terminal on WSL.

I learned various things by making this project following 'Build Your Own Text Editor' guide. Please read 'Implementation' section below to find some interesting topics.


Please install kiro-editor package by building from sources using cargo.

$ cargo install kiro-editor

Note: Please use a Rust stable toolchain as new as possible.

For NetBSD, kiro-editor package is available.



Installing kiro-editor package introduces kiro command in your system.

$ kiro                 # Start with an empty text buffer
$ kiro file1 file2...  # Open files to edit

Please see kiro --help for command usage.

Edit Text

Kiro is a mode-less text editor. Like other famous mode-less text editors such as Nano, Emacs, Gedit or NotePad.exe, you can edit text in terminal window using a keyboard.

And several keys with Ctrl or Alt modifiers are mapped to various features. You don't need to remember all mappings. Please type Ctrl-? to know all mappings in editor.

  • Operations
Mapping Description
Ctrl-? Show all key mappings in editor screen.
Ctrl-Q Quit Kiro. If current text is not saved yet, you need to input Ctrl-Q twice.
Ctrl-S Save current buffer to file. Prompt shows up to enter file name for unnamed buffer.
Ctrl-G Incremental text search.
Ctrl-O Open file or empty buffer.
Ctrl-X Switch to next buffer.
Alt-X Switch to previous buffer.
Ctrl-L Refresh screen.
  • Moving cursor
Mapping Description
Ctrl-P or ↑ Move cursor up.
Ctrl-N or ↓ Move cursor down.
Ctrl-F or β†’ Move cursor right.
Ctrl-B or ← Move cursor left.
Ctrl-A or Alt-← or HOME Move cursor to head of line.
Ctrl-E or Alt-β†’ or END Move cursor to end of line.
Ctrl-[ or Ctrl-V or PAGE DOWN Next page.
Ctrl-] or Alt-V or PAGE UP Previous page.
Alt-F or Ctrl-β†’ Move cursor to next word.
Alt-B or Ctrl-← Move cursor to previous word.
Alt-N or Ctrl-↓ Move cursor to next paragraph.
Alt-P or Ctrl-↑ Move cursor to previous paragraph.
Alt-< Move cursor to top of file.
Alt-> Move cursor to bottom of file.
  • Edit text
Mapping Description
Ctrl-H or BACKSPACE Delete character
Ctrl-D or DELETE Delete next character
Ctrl-W Delete a word
Ctrl-J Delete until head of line
Ctrl-K Delete until end of line
Ctrl-M Insert new line
Ctrl-U Undo last change
Ctrl-R Redo last undo change

Here is some screenshots for basic features.

  • Create a new file

screenshot for creating a new file

  • Incremental text search

screenshot for incremental text search

Extended Features

Support Editing UTF-8 Text

Kiro is a UTF-8 text editor. So you can open/create/insert/delete/search UTF-8 text including double width characters support.

UTF-8 supports

Note that emojis using U+200D (zero width joiner) like 'πŸ‘ͺ' are not supported yet.

Please read 'Support Editing UTF-8 Text' subsection for implementation details.

24-bit colors (true colors) and 256 colors support

Kiro utilizes colors as much as possible looking your terminal supports. It outputs 24-bit colors with gruvbox color scheme falling back to 256 colors or eventually to 16 colors.

  • 24-bit colors

24-bit colors screenshot

  • 256 colors

256 colors screenshot

  • 16 colors

16 colors screenshot

Handle window resize

Terminal notifies a window resize event via SIGWINCH signal. Kiro catches the signal and properly redraws its screen with new window size.

resize window


Kiro supports undo/redo editing (Ctrl-U for undo, Ctrl-R for redo). Max number of history entries is 1000. After exceeding it, oldest entry is removed on adding new change to text.

undo/redo screencast

Please read 'Text editing as sequence of diffs' subsection.


This project was a study to understand how a text editor can be implemented interacting with a terminal application. I learned many things related to interactions between terminal and application and several specs of terminal escape sequences such as VT100 or xterm.

I started from porting an awesome minimal text editor kilo following a guide 'Built Your Own Text Editor'. And then I added several improvements to my implementation.

Here I write topics which were particularly interesting for me.

Efficient Rendering and Highlighting

kilo updates rendering and highlighting each time you input a key. This implementation is great to make implementation simple and it works fine.

However, it is insufficient and I felt some performance issue on editing larger (10000~ lines) C file.

So Kiro improves the implementation to render the screen and to update highlighting only when necessary.

Kiro has a variable dirty_start in Screen struct of It manages from which line rendering should be started.

For example, let's say we have C code bellow:

int main() {

And put ! like printf("hello!\n");.

In the case, first line does not change. So we don't need to update the line. However, Kiro renders the } line also even if the line does not change. This is because modifying text may cause highlight of lines after the line. For example, when deleting " after \n, string literal is not terminated so next line continues string literal highlighting.

Highlighting has the similar characteristic. Though kilo calculates highlighting of entire text buffer each time you input key, actually the lines after bottom of screen are not rendered. For current syntax highlighting, changes to former lines may affect later lines highlighting (e.g. block comments /* */), changes to later lines don't affect former lines highlighting. So Kiro stops calculating highlights at the line of bottom of screen.

UTF-8 Support

kilo only supports ASCII text. Width of ASCII character is fixed to 1 byte. This assumption reduces complexity of implementation of kilo greatly because:

  • every character can be represented as char (almost the same as u8 in Rust)
  • any character in ASCII text can be accessed via byte index in O(1)
  • length of text is the same as number of bytes of the text

So kilo can contain text buffer as simple char * and accesses characters in it via byte index. In addition, display width of all printable ASCII characters is fixed except for 0x09 tab character.

But actually there are more characters in the world defined as Unicode characters. Since I'm Japanese, the characters such as Kanji or Hiragana I'm daily using are not ASCII. And the most major text encoding is UTF-8. So I determined to extend Kiro editor to support UTF-8.

In UTF-8, byte length of character is variable. Any character takes 1~4 bytes (or more in special case). The important point here is that accessing to character in UTF-8 text is not O(1). To access to N-th character or to know length of text, it requires to check characters from head of the text.

Accessing to character in text and getting text length happen frequently while updating text buffer and highlights. So checking them in O(N) for each time is not efficient. To solve this problem, Kiro contains byte indices of each characters in line text as Vec<usize>. These indices are only existing when at least one character in line text is non-ASCII character.

UTF-8 support diagram

In Row struct which represents one text line, indices field (Vec<usize>) is dedicated to store byte indices of each character.

In the first case "Rust is nice", all characters are ASCII so byte index can be used to access to characters in the text. In the case, indices field is an empty (and capacity is set to zero). A Vec instance with zero capacity is guaranteed not to allocate heap memory. So the memory overhead here is 24 bytes of Vec<usize> instance itself (pointer, capacity as usize and length as usize) only.

In the second case "RustπŸ¦€θ‰―γ„", there are some non-ASCII characters so self.indices caches byte indices of each characters. Thanks to this cache, each character can be accessed in O(1) and its text length can be obtained in O(1) as self.indices.len(). Row also contains a rendered text and updates it when internal text buffer is updated by TextBuffer. So self.indices cache is also updated at the same timing efficiently.

Though keeping byte indices in Vec<usize> is quite memory inefficient, the indices are only required when the line text contains non-ASCII characters. In terms of programming code editor, it is relatively rare case, I believe.

Text Editing as Sequence of Diffs

In Kiro editor, every text edit is represented as diff of text. So text editing means applying diffs to current text buffer. Undo is represented as 'unapplying' diffs. Redo is represented as applying diffs again.

One undo is represented as multiple diffs, not one diff. This is because users usually don't want to undo per inserting one character. So diffs each character inserts a character is put together as one undo.

UTF-8 support diagram

At first a user inputs "abc" to text. The input is represented as 3 diffs of each characters and they consist of one undo unit. So inserting "abc" is reverted at once on undo though it is represented as multiple diffs. Then a user backs cursor by one character and delete characters "ab" until head of line. It is represented as one diff. Finally a user adds a new line by ENTER key. Inserting line is represented as two diffs. At first, editor truncates a text after cursor ("c") and then it inserts new line "c" to next line to the cursor. These two diffs consist of one undo unit.

By managing history of text editing with undo units, every text edit can be represented as sequence of diffs. Redo applies diffs in one undo unit to current text buffer. And undo unapplies diffs in one undo unit to current text buffer.

Normal input is also treated as redo internally so that editor doesn't need to handle normal input with separate implementation.

Porting C editor to Rust

Separate one C source into several Rust modules

To simplify and minimize implementation, kilo uses some global variables and local static variables. Editor's state is stored in a global variable E and it is referred everywhere.

While porting the code to Rust, I split kilo.c into some Rust modules for each logics. I removed the global variables and local static variables by moving them to each logic's structs.

  • Exports Editor struct, which manages an editor lifecycle; Runs loop which gets key input, updates a text buffer and highlight then renders screen.
  • Exports TextBuffer struct, which manages an editing text buffer as Vec<Row>. It also contains metadata such as file name and file type of the buffer.
  • Editing text is defined as applying sequence of diffs to text. This module exports an enum EditDiff which represents the diff and logic to apply it to text.
  • Exports Row struct which represents one line of text buffer and contains actual text and rendered text. Since Kiro is dedicated for UTF-8 text editing, internal text buffer is also kept as UTF-8 string. When the internal text buffer is updated by Editor, it automatically updates rendered text also. It may also contain character indices for UTF-8 non-ASCII characters (Please see below 'UTF-8 Support' section).
  • It exports struct History which manages the edit history. The history is represented as sequence of edit diffs. It manages the state of undo/redo and how many changes should happen on one undo/redo operation.
  • Exports StdinRawMode struct and InputSequences iterator. StdinRawMode setups STDIN as raw mode (disable various terminal features such as echo back). InputSequences reads user's key input as byte sequence with timeout and parses it as stream of key sequence. VT100 and xterm escape sequences like \x1b[D for ← key are parsed here.
  • Exports Highlighting struct, which contains highlight information of each character in text buffer. It also manages highlighting in an editor lifecycle. It calculates highlights of characters which is rendered and updates its information.
  • Exports Screen struct, which represents screen rendering. It renders each Row with highlight colors by outputting characters and escape sequences to STDOUT. As described in previous section, it manages efficient rendering. It also manages and renders status bar and message bar located at bottom of screen.
  • Exports StatusBar struct which manages fields displayed in the status bar. It has flag redraw to determine if it should be re-rendered.
  • Exports structs related to user prompt using message bar. This module has logic to run user prompt and text search. Callbacks while prompt is represented as a PromptAction trait.
  • Exports small TermColor enum and Color enum, which represents terminal colors. This module also has logic to detect 24-bit colors and 256 colors support of terminal.
  • Exports small Language enum, which represents file types like C, Rust, Go, JavaScript, C++. It contains logic to detect a file type from file name.
  • Exports SigwinchWatcher struct, which receives SIGWINCH signal and notifies it to Screen. The signal is sent when terminal window size changed. Screen requires the notification for resizing the screen.
  • Exports Error enum and Result<T> type to handle all kinds of error which may occur in Kiro editor.

Error handling and resource clean up

kilo outputs message by perror() and immediately exits on error. It also cleans up STDIN configuration with atexit hook.

Kiro is implemented in Rust. So it utilizes Rust idioms to handle errors with io::Result and ? operator. It reduces codes for error handling so that I could focus on implementing editor logics.

For resource clean up, Rust's Drop crate works greatly in

struct StdinRawMode {
    stdin: io::Stdin,
    // ...

impl StdinRawMode {
    fn new() -> io::Result<StdinRawMode> {
        // Setup terminal raw mode of stdin here
        // ...

impl Drop for StdinRawMode {
    fn drop(&mut self) {
        // Restore original terminal mode of stdin here

impl Deref for StdinRawMode {
    type Target = io::Stdin;
    fn deref(&self) -> &Self::Target {

impl DerefMut for StdinRawMode {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.stdin

The drop() method is called when StdinRawMode instance dies. So user doesn't need to remember the clean up. And StdinRawMode also implements Deref and DerefMut so that it behaves almost as if it were Stdin. By wrapping io::Stdin like this, I could add the ability to enter/leave terminal raw mode to io::Stdin.

Abstract input and output of editor

pub struct Editor<I, W>
    I: Iterator<Item = io::Result<InputSeq>>,
    W: Write,
    // ...

impl<I, W> Editor<I, W>
    I: Iterator<Item = io::Result<InputSeq>>,
    W: Write,
    // Initialize Editor struct with given input and output
    pub fn new(input: I, output: W) -> io::Result<Editor<I, W>> {
        // ...

The input of terminal text editor is a stream of input sequences from terminal which include user's key input and control sequences. The input is represented with Iterator trait of input sequence. Here InputSeq represents one key input or one control sequence.

The output of terminal text editor is also stream of sequences to terminal which include output strings and control sequences. It's done by simply writing to stdout. So it is represented with Write trait.

The benefit of these abstractions are testability of each modules. By creating a dummy struct which implements Iterator<Item = io::Result<InputSeq>>, the input can be easily replaced with dummy input. Since kilo does not have tests, these abstractions are not necessary for it.

struct DummyInput(Vec<InputSeq>);

impl Iterator for DummyInput {
    type Item = io::Result<InputSeq>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.0.is_empty() {
        } else {

// Dummy Ctrl-Q input to editor
let dummy_input = DummyInput(vec![ InputSeq::ctrl(b'q') ]);

And by implementing a small struct which simply discards output, we can ignore the output. It does not need to draw screen in terminal window. And it does not rely on global state (terminal raw mode) so that tests can run in parallel. As the result tests can run faster and terminal window doesn't mess up.

struct Discard;

impl Write for Discard {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {

    fn flush(&mut self) -> io::Result<()> {

By using these mocks the input and output of editor can be tested easily as follows:

fn test_editor() {
    let mut editor = Editor::new(dummy_input, Discard).unwrap();
    for line in editor.lines() {
        // Check lines of the current text buffer

Dependant Crates

This project depends on some small crates. I selected them carefully not to prevent learning how a text editor on terminal works.

  • termios: Safe binding to termios interface provided by OS.
  • term_size: Safe binding to getting terminal window size with ioctl(2).
  • unicode-width: Small library to calculate Unicode character's display width.
  • term: Library for terminfo and terminal colors. This project uses this library only to parse terminfo for 256 colors support.
  • signal-hook: Small wrapper for signal handler to catch SIGWINCH for resize support.
  • getopts: Fairly small library to parse command line arguments. Kiro only has quite simple CLI options so clap is too heavy.


  • Unit tests are not sufficient. More tests should be added
  • Improve scrolling performance (Is terminal scrolling available?)
  • Minimal documentation
  • Text selection and copy from or paste to system clipboard
  • Keeping all highlights (Vec<Highlight>) is not memory efficient. Keep bits only for current screen (rowoff..rowoff+num_rows)
  • Use parser library combine or nom to calculate highlighting. Need some investigation since highlight parser must stop calculating when current line exceeds the bottom line of screen. Also syntect is interesting.

Future Works

  • Use incremental parsing for accurate syntax highlighting
  • Support more systems and terminals
  • Look editor configuration file such as EditorConfig or .vscode VS Code workspace settings
  • Support emojis using U+200D
  • WebAssembly support
  • Mouse support
  • Completion, go to definition and look up using language servers


Benchmarks are done by cargo bench and fuzzing is done by cargo fuzz and libFuzzer.

# Create release build
cargo build --release

# Run tests
cargo test

# Run benchmarks
cargo +nightly bench -- --logfile out.txt && cat out.txt

# Run fuzzing
cargo +nightly fuzz run input_text


This project is distributed under the MIT License.