cumulus/DESIGN.md

cumulus: design

This is the design approved on 2026-10-01, after six rounds of questions. It's the reference while cumulus is being built: when a decision changes, this file changes with it.

What cumulus is

A keyboard-driven brainstorming app for the terminal, in Rust with ratatui. The aim is "the idea node graph to end them all": quick idea dumps, mind maps, whiteboard-style planning and linked notes, in one tool. It runs on Windows and Linux, with vim keys first and full mouse support.

Scope

v1

• The canvas: nodes, branches, cross-links with every style, folding and focus, zoom, the K card, the mouse.
• The Outline and Note views, splits and the document bar.
• The key engine: medium vim, which-key, help and status-bar hints.
• Visual, move and connect modes; text editing with vim normal mode.
• The command line, with completion and history.
• Portals, with the jump list, backlinks and breadcrumb.
• Rapid entry and the Inbox.
• RON documents with autosave, reload, backups and undo; file names that follow the root's title.
• The startup rules and cumulus new.
• TOML config, key and theme files, with named styles and styles set by tags.

v1.1: the Board view · the Search picker across documents · :tidy and align · branch out · export (Markdown, plain text) and import (indented lists).

Later: peeking into portals and pulling a document back in · branches that keep themselves tidy · the Links map · backlinks inside a document · picking single nodes in visual mode · Alt+hjkl · opening a body in $EDITOR · persistent undo · sessions · operator + motion and macros · a tutorial document · checkbox, link and date fields · Graphviz and Mermaid export · a Settings view · vim tab pages.

Not planned unless asked: f hint jumps · a minimap · straight (braille) lines.

Decisions

Documents and structure

• Each document has one main root, and the document's name is the root's title. Loose top-level nodes float around it.
• A tree plus cross-links: each node has at most one parent, and cross-links join any two nodes, at most one link per pair.
• A node holds a title (one line, never empty), a body, tags and a color. A new node left empty disappears.
• A portal node shows another document's root, live (its title, color and node count), and leads there. Portals always land on the root.
• Backlinks are stored in the target document and synced on save, so they travel with the files.
• Files live anywhere: there's no workspace folder. Portal paths are relative, with / on every OS.
• Each file carries a random uid, so "where you left off" survives renames and a portal can tell when its file was replaced by another document.
• File names follow the root's title, and a rename fixes the portals pointing at the file. A folder's default document keeps its name (ideas.cum).

Canvas

• Free placement: a new node appears next to its parent, and nothing moves unless you move it. Moving a node brings its branch along.
• Folding: each branch is Open, Auto (open while the cursor is inside) or Closed (a jump inside opens it until you leave). New branches start Open; a focus toggle makes Open branches act like Auto.
• Zoom levels: Normal (boxes), Far (one line per node: the same width, half the height) and Overview (colored dots: a quarter of the width, a third of the height). Far keeps the width because one-line titles collided when distances were halved both ways (changed during stage 2). Body text never shows on the canvas: K pops it up, and the Note view shows all of it.
• Right-angle lines with proper junctions (├ ┬ ┼); lines pass under nodes.
• A node's color goes on its border. Each node can pick its border, and the theme's named styles and tag rules can set color and border together.
• Connections can set arrowheads (off, end, start, both), a line (solid, dashed, dotted, thick, double), a color (a palette color or the node's own) and a label, or use a named style. Branches start solid without arrowheads, cross-links as dashed arrows.

Keys

• Medium vim: counts, operators with node targets, registers (with "+ for the system clipboard), marks (capital marks across documents), . to repeat.
• Tab and Ctrl-i jump forward, as in vim, so a adds a child.
• dd deletes a node with its branch; x deletes just the node, and its children move up.
• Rapid entry is what happens after o, O or a: Enter starts the next sibling, Tab / Shift-Tab indent and outdent it, Enter on an empty line stops. Esc switches to vim motions inside the text; a second Esc leaves.
• C (connect mode) edits the connection to another node. Aimed at your own parent, it styles or cuts the branch line; cutting makes the node loose.
• M (move mode) moves the branch; M again switches to just the node.
• gh gj gk gl move through the tree in every view; plain hjkl follow each view's layout (on the canvas: the nearest node that way).
• Folds: za zo zc zR zM, plus zf (Auto) and zi (focus).
• q stays free for macros; quit with :q, :qa or ZZ.
• Which-key works like skyshift's; the leader menu pops up at once. The status bar shows key hints, and ? opens searchable help.
• Keys can be changed in keys.toml, in vim notation.

Files and saving

• Documents are RON files ending in .cum. Configs are TOML: config.toml, keys.toml and themes/<name>.toml.
• Autosave about a second after a change (autosave = false for vim-style :w). A file changed outside cumulus is reloaded; if you also had unsaved changes, cumulus asks which to keep. Daily backups go to the data folder.
• Undo per document, while cumulus runs: one step per command, per text-editing session, and per node in rapid entry.
• Each document reopens where you left it (cursor, zoom, scroll).

Screen

• Vim-style splits; panes on the same document share one cursor.
• Open documents are buffers, listed in a bar at the top; the status bar is at the bottom.

Project

• A library plus a thin main.rs, so build stages can run as examples.
• Rust pinned to 1.90.0; nothing needs nightly.

Data model (model/: pure data, no UI, no files)

// Ids and units: newtypes, so they can't be mixed up.
NodeId(u32)                      // unique inside a document, never reused while it's open
BufferId(u32)                    // an open document, like vim's buffer numbers (:ls, :b 3)
PaneId(u32)                      // a split
DocUid(u64)                      // stored in the file, survives renames
CanvasPos { x: i32, y: i32 }     // cells at Normal zoom, may be negative
ScreenPos { col: u16, row: u16 } // a terminal cell
Camera { origin: CanvasPos, zoom: Zoom }   // converts between the two
enum Zoom { Normal, Far, Overview }

// Text, checked once when it comes in.
Title    // one line, never empty
Body     // any text
Tag      // lowercase, no spaces, stored without '#'
Label    // a connection label: one short line
ColorName, StyleName   // names looked up in the theme

// A node and its place in the tree.
struct Node  { content: Content, tags: Tags, look: NodeLook, pos: CanvasPos, fold: Fold }
enum Content { Idea(Idea), Portal(Portal) }
enum Fold    { Open, Auto, Closed }
enum Place   { Root, Loose, Child { parent: NodeId, branch: LineLook } }
struct NodeLook { color: Option<ColorName>, border: Option<Border>, style: Option<StyleName> }
enum Border  { Rounded, Square, Double, Thick, Dashed, Hidden }

// Cross-links. Branch lines use the same LineLook.
struct Link     { from: NodeId, to: NodeId, label: Option<Label>, look: LineLook }
struct LineLook { arrows: Option<Arrows>, line: Option<Stroke>, color: Option<LineColor>, style: Option<StyleName> }
enum Arrows     { Off, End, Start, Both }
enum Stroke     { Solid, Dashed, Dotted, Thick, Double }
enum LineColor  { Named(ColorName), FollowNode }

// Between documents.
struct Portal   { path: LinkPath, uid: DocUid, title: Title }  // title: kept in case the file goes missing
struct Backlink { from: LinkPath, uid: DocUid, node: NodeId }

// The document. Its fields are private, so only its methods can change it.
struct Document { uid, root: NodeId, entries: BTreeMap<NodeId, Entry>, loose: Vec<NodeId>,
                  links: BTreeMap<LinkKey, Link>, backlinks: Vec<Backlink>, next_id }
struct Entry    { node: Node, place: Place, children: Vec<NodeId> }

The Option fields in NodeLook and LineLook are overrides. A look is built up in layers: the theme default, then the tag style, then the named style, then the node's own fields. Each layer only states what it changes.

Rules the Document methods guarantee

• There is exactly one root, and it can't be deleted or given a parent.
• Each node has at most one parent, and there are no cycles: a move checks ancestors first.
• Children keep their order, and the file lists nodes in outline order.
• A link never joins a node to itself, both of its ends always exist, and deleting a node removes its links.

Undo. Every change runs as a transaction: doc.edit(|e| { e.add(…)?; e.update(…)?; Ok(()) }). The transaction records the old value of everything it touches. If a step fails, it rolls back; if it succeeds, that record becomes one undo step. Undo writes the old values back, redo the new ones. No command needs its own "reverse" code, and the records are plain data, ready for a persistent undo file later.

Folding is one pure function of the document, the cursor and the focus toggle, so every view folds the same way.

Portals. A cache holds each target's root title, color and node count, refreshed when files change. When a root's title changes, the next save renames the file, then fixes the portal paths in every document listed in its backlinks, and the backlink paths in every document it points to.

Screen and views

 cumulus  Garden plans [+] │ Kitchen │ Reading list
┌ Outline ──────────────┐┌ Canvas ───────────────────────────────────────────┐
│ ▾ Garden plans        ││                      ┌──────┐                     │
│   ├ Soil              ││                   ┌──┤ Soil │                     │
│   ├ [Raised beds]     ││                   │  └──────┘                     │
│   │ └ Cedar           ││ ╭──────────────╮  │  ┏━━━━━━━━━━━━━┓    ┌───────┐ │
│   └ Watering          ││ │ Garden plans ├──┼──┨ Raised beds ┠────┤ Cedar │ │
│ ◦ ask about compost   ││ ╰──────────────╯  │  ┗━━━━━━━━━━━━━┛    └───────┘ │
│                       ││                   │  ┌──────────┐                 │
│                       ││                   └──┤ Watering │                 │
│                       ││                      └──────────┘                 │
└───────────────────────┘└───────────────────────────────────────────────────┘
 NORMAL │ Canvas │ Garden plans › Raised beds                               2g

View	What it's for
Canvas	The 2D map: free placement, 3 zoom levels, right-angle lines and the mouse.
Outline	Reorganizing and reading: an indented tree with fold markers, +N for hidden nodes and » for portals, with loose nodes listed at the end.
Note	Writing: one node's title, tags, color and body (with vim motions inside), plus its links in and out and its children. It follows the cursor.
Board (v1.1)	Sorting: columns by tag or color. Moving a card to another column changes its tag.

• Pickers: Documents in v1, Search in v1.1.
• Popups: which-key (bottom right, as in skyshift), ? help, the K card, the right-click menu, confirmations, and the command line with its completion list.
• Top bar: the open documents. Bottom bar: the mode, the view, the breadcrumb (or a message), and the keys typed so far (2g above).

Mouse (on the canvas; other views get click, double-click and scroll): click selects, double-click edits the title, dragging a node moves it with its branch, dragging empty space pans, scrolling pans, Ctrl+drag selects a rectangle, Ctrl+click adds or removes one node, right-click opens a menu built from the same actions as the keys. Zoom is on the + and - keys.

Modes and keys

enum Mode {
    Normal,
    Insert(TextEdit),     // typing a title or body; rapid entry lives here
    TextNormal(TextEdit), // vim motions inside that text
    Visual(Visual),       // anchor + Rectangle or Branches
    Move(Grab),           // a node or a branch being moved
    Connect(Connect),     // the source node and the link being styled
    Command(CommandLine), // `:` with completion
}

Each mode has its own key tables. Which-key, ? help and keys.toml all read those same tables. The engine handles counts, registers, operator + target (d + ab), and keys that take a character after them (ma, fx inside text).

Group	Keys
Move (every view)	hjkl or arrows · gh gj gk gl tree moves · w b next / previous in reading order · gg root · G last · / n N search · Ctrl-o Ctrl-i (Tab) jump back / forward · m{a-z} '{a-z} marks (A–Z across documents) · Enter or gf through a portal · gd follow a cross-link
View	zz center · Ctrl-e Ctrl-y zh zl pan · Ctrl-d Ctrl-u half a screen · + - zoom · za zo zc zR zM folds · zf Auto · zi focus · K body card
Create and edit	o O sibling below / above · a child · i A Enter edit the title · I edit from its start · cc S retype
Operators	dd or dab delete branch · dib delete children · x delete just the node · yy yab yib yank · p P paste after / before · ]p paste as child · >> << indent / outdent · "a–"z and "+ registers · . repeat · u Ctrl-r undo / redo
Modes	v V visual · M move · C connect · : command · Esc back to normal
Leader <Space>	c color · s style · B border · t tags · p new portal · i inbox · b documents · v then c/o/n for canvas / outline / note · w which-key on/off · ? help
Panes, documents	Ctrl-w v Ctrl-w s split · Ctrl-w hjkl focus · Ctrl-w c close · Ctrl-w o only · Ctrl-w = even sizes · gt gT ]b [b next / previous document · ZZ close pane
Typing	Ctrl-w delete word · Ctrl-u delete line · arrows, Home, End · Enter (title: done or next sibling; body: new line) · Tab Shift-Tab (new node: child / back out) · Esc to vim motions
Inside text	h j k l w b e 0 ^ $ f t x r J o O p P u Ctrl-r . · d c y with motions and iw aw i" a" i( a( · i a I A back to typing
Move mode	hjkl 1 cell · HJKL 5 cells · M branch ↔ node · Enter or Esc done
Connect mode	move to the target as usual · Enter link · d remove · > attach as child · <Space> style menu (s named, a arrows, l line, c color, t label) · Esc cancel
Visual	d y > < M C and <Space>c <Space>s <Space>t act on the whole selection · o swaps ends

Files

Documents (.cum, RON). A backlinks: [...] list appears once another document has a portal pointing here.

// cumulus document. Hand edits are fine: cumulus reloads it when it changes.
(
    format: 1,
    uid: "7f3c9a1e52b04d18",
    root: 1,
    nodes: [
        (
            id: 1,
            title: "Garden plans",
            pos: (0, 4),
            color: "green",
        ),
        (
            id: 2,
            parent: 1,
            title: "Raised beds",
            pos: (22, 4),
            tags: "#todo",
            fold: Auto,
            body: "Cedar, three of them.
40 cm high.",
        ),
        (
            id: 3,
            parent: 1,
            portal: (
                path: "kitchen.cum",
                uid: "c01d5e7aa0f3b912",
                title: "Kitchen",
            ),
            pos: (22, 8),
        ),
        (
            id: 4,
            title: "ask about compost",
            pos: (0, 14),
        ),
    ],
    links: [
        (
            from: 2,
            to: 4,
            label: "needs",
            line: Dashed,
            arrows: End,
        ),
    ],
)

Config (TOML) lives in %APPDATA%\cumulus (~/.config/cumulus on Linux); cumulus --paths prints the folders. :config writes these files with a comment on every option, and cumulus reloads them when they change.

# config.toml
theme = "default"            # a file in the themes folder
default_document = "ideas"   # what `cumulus` looks for at startup
autosave = true
mouse = true
which_key = true
which_key_delay = "300ms"
max_node_width = 30
glyphs = "unicode"           # "ascii" for fonts without box-drawing characters

# keys.toml
leader = "<Space>"
[normal]
"<Space>x" = "delete-branch"
[canvas]
"H" = "previous-document"

# themes/default.toml
accent = "cyan"
[palette]                    # terminal names, 0-255, or "#rrggbb"
red = 167
orange = 208
green = 108                  # … yellow, teal, blue, purple, pink, brown, gray
[modes]
insert = "green"
visual = "magenta"
[styles.node.question]
color = "yellow"
border = "dashed"
[styles.link.blocks]
color = "red"
line = "dashed"
arrows = "end"
label = "blocks"
[tags]
question = "question"        # a #question node gets the "question" style

Data folder: where you left off in each document (keyed by uid), command history, capital marks, recent documents, daily backups, and lock files. If a second cumulus opens a document that's already open, it opens it read-only.

Command line

• cumulus opens ./ideas.cum, or ~/ideas.cum if the current folder doesn't have one.
• cumulus garden.cum opens that file; cumulus some/folder opens that folder's default document.
• cumulus new creates ./ideas.cum, and cumulus new "Garden plans" creates ./garden-plans.cum. If the file already exists, cumulus opens it and says so.
• cumulus --paths, --version and --help.

Module layout and flow

cumulus/
├─ Cargo.toml  rust-toolchain.toml  build.rs  rustfmt.toml  .cargo/config.toml
├─ README.md  DESIGN.md  .gitignore  .gitattributes
├─ examples/       runnable checkpoints during the build (canvas.rs, …)
└─ src/
   ├─ main.rs      command-line arguments, then app::run
   ├─ lib.rs       the module list
   ├─ model/       ids, text, geometry, look, node, portal, link, document, edit, history, fold
   ├─ input/       key (vim notation), command (Command enum + names), bindings (default tables),
   │               keymap (+ keys.toml), engine (counts, registers, operators), which_key, mouse
   ├─ command/     registry, parse, complete, history
   ├─ view/        mod.rs (the View trait), canvas/ (camera, boxes, lines, route, render, hit,
   │               navigate, placement), outline, note, board
   ├─ ui/          text_editor (+ vim normal), which_key, command_line, status_bar, document_bar,
   │               card, menu, picker, help, theme, glyphs
   ├─ config/      paths, settings, keys_file, theme_file (TOML)
   ├─ storage/     document_file (RON), atomic save, backups, lock, state, watch (change polling)
   └─ app/         App, mode, action, buffers, panes (split tree), jumps, marks, registers,
                   portals (cache, backlinks, renames), update, render,
                   terminal (setup, teardown, panic hook)

Flow: a terminal event comes in → input turns the key or mouse event into a Command → the focused view turns that command into Actions → App::update applies them (the only place data changes) → the screen is drawn from the state.

trait View {   // expected shape; stage 5 settles it from what stages 2–4 needed
    fn kind(&self) -> ViewKind;
    fn keys(&self, mode: &Mode) -> &[KeyTable];                  // its tables, most specific first
    fn command(&mut self, command: Command, ctx: &Ctx) -> Vec<Action>;
    fn mouse(&mut self, gesture: Gesture, ctx: &Ctx) -> Vec<Action>;
    fn render(&self, frame: &mut Frame, area: Rect, ctx: &Ctx);  // reads state, draws, changes nothing
}

Build order

Hardest parts first, so they shape a simple interface that every view shares and the core ends up thin. The model comes first because every hard part reads it.

#	Stage	What you can run
0	Skeleton: Cargo.toml, toolchain, build.rs, rustfmt, aliases, README, DESIGN.md	cumulus --version
1	Model: types, document rules, transactions and undo, folding, RON round trip	cargo test
2	Canvas engine: camera, boxes, a line layer with junctions, right-angle routing, arrows and labels, zoom levels, hit-testing, nearest-node moves, placement	cargo run --example canvas
3	Key engine: notation, tables, counts, registers, operator + target, keys that take a character, which-key, keys.toml	the example gets real keys and which-key
4	Command line: registry, parser, completion (popup + ghost text), history	: in the example
5	The shared View interface, Action and Mode, pulled out of stages 2–4	the example runs through it
6	Text editor: typing, plus vim normal mode inside text	editing titles in the example
7	Outline and Note views; status bar, document bar, help, K card, right-click menu, Documents picker	the example with splits
8	App core: main loop, terminal setup and panic hook, buffers, splits, modes, jumps, marks, registers, files (load, autosave, reload, backups, locks, where you left off), config/theme/keys, portals, backlinks and renames, startup rules	cumulus: v1
9	v1.1: Board, Search picker, :tidy and align, branch out, export/import

Each stage ends with cargo build, cargo lint and cargo test passing, and one commit.

Crates and tooling

• Crates: ratatui 0.30 (with the crossterm 0.29 it ships with) · serde, ron 0.12 and toml 1 · directories 6 · thiserror 2 and anyhow · arboard 3 (with Wayland support) · unicode-width and unicode-segmentation. No testing crates. Each crate is added in the stage that first needs it.
• Toolchain: rust-toolchain.toml pins 1.90.0 with clippy, rustfmt and the musl target.
• build.rs: embeds the git commit for cumulus --version.
• Windows first: only key Press and Repeat events count; a panic hook restores the terminal (raw mode, the screen, the mouse); paths come from directories.
• Same as skyshift: edition 2024, rustfmt at 110 columns, cargo lint and cargo linux, the static C runtime on Windows, LF line endings, unsafe forbidden.
• Doc comments on public types and functions, written where they explain something. (The missing_docs lint was dropped: it demands a comment on every field and enum variant, which is the comment noise we don't want.)

Tests

• Keys: notation, sequences, counts, registers, operator + target, overrides, and no binding hiding another.
• Command line: parsing, abbreviations, completion ranking, history.
• Model: tree rules, undo/redo round trips, folding.
• Canvas: camera conversions, routing and junctions (small text snapshots), hit-testing, nearest-node moves.
• Files: RON round trip, load errors, config parsing, and backlink and rename fix-ups, in temporary folders.