gridcraft/docs/architecture.md

Architecture

Last reviewed: 2026-10-10 · Last updated: 2026-10-24 · Change: minor (engine building blocks for multi-command actions) · Target: Microsoft Excel (Microsoft 365)

How GridCraft is built today. Rules for contributors are in AGENTS.md.

Workspace and layering

An engine-first Cargo workspace. Layering is enforced by cargo xtask layers; nothing below L6 depends on egui, eframe, winit or rfd, so the UI crate is swappable. Line counts are Rust lines including tests, 2026-10-10.

Layer	Crate	Lines	Job
L0	core	~1.3k	Cell and range addresses (A1, R1C1, sheet-qualified; 1,048,576 × 16,384), Value, CellError, Excel coercion, 1900/1904 date serials, typed-input parsing
L0	numfmt	~2.1k	Excel number-format codes (sections, conditions, colours, dates, elapsed time, fractions, scientific, fill), General, TEXT(); en-US only
L1	formula	~1.7k	Tokenizer, parser, AST, printer (A1 and R1C1), reference adjustment for copy, fill and structural edits
L1	functions	~13.4k	The function library: FnSpec registry (name, arity, category, scalar args, signature, description) and pure implementations; automatic array lifting
L2	model	~2.2k	Workbook, sheets, copy-on-write cell store (64-row bands behind Arc), interned styles, names, tables, CF, validation, comments, charts, shapes, images, sparklines, print settings, protection
L2	calc	~2.9k	Evaluator (references, names, tables, LET/LAMBDA, special forms like OFFSET/INDIRECT/SUBTOTAL) and dependency-graph recalculation with dynamic arrays, spills and cycle detection; single-threaded
L2	xlsx	~10.7k	XLSX read/write (transitional and strict read), CSV/TSV, XLSB and ODS data import, PivotTable caches, DrawingML charts and drawings; byte slices only (WASM-safe); lenient reader that reports warnings
L2	pdf	~1k	Small PDF 1.7 writer (14 standard fonts, vectors, images) for print and export
L3	chart	~2.9k	Toolkit-free chart layout to drawing primitives, a rasterizer, sparklines
L5	engine	~17.7k	Session: open workbooks, selection, history (COW snapshots), clipboard, fill, sort, filter, PivotTables, what-if, print, file I/O (io.rs), and the command registry (cmd/*, 313 commands) with the Excel ribbon catalog (catalog.rs)
L6	ui-egui	~11.6k	Excel-style UI (title bar, QAT, ribbon, formula bar, virtualised grid, editor, tabs, status bar, dialogs, task panes, chart view) and the control channel
L6	mcp	~1.4k	MCP server over stdio (hand-written JSON-RPC), headless or bridged to a running app
L7	apps/gridcraft, apps/gridcraft-cli, apps/gridcraft-web	~2.2k	Desktop (eframe/wgpu), CLI (info, convert, eval, cat, run, commands, functions, mcp, send), web (WASM via trunk)
—	xtask	—	ci, parity, assets, layers, wasm, version, stats

Data flow

1. Every action is a command. The UI, CLI, control channel and MCP call Session::execute(id, params). Commands are CommandSpec { id, label, menu, shortcut, params, enabled, run, journal, undoable }; ids follow Excel's ribbon (home.bold, insert.chart, data.sortAscending) plus primitives (cell.set, selection.set, range.setValues). Session::execute catches escaped panics and restores the workbook, its selection and its undo and redo history as they were before the command (never-crash guard).
2. Edits change the model through copy-on-write bands; undo keeps the previous snapshot. Undo retains at most 100 steps and trims older steps when its estimated retained cell storage exceeds 128 MiB. The newest step remains undoable even if that step alone exceeds the budget. The estimate is a history policy, not a measure or hard cap of process memory.
3. Recalc marks dependents of changed cells dirty through the dependency graph (with range nodes for range dependents), evaluates in topological order, spills dynamic arrays and reports #SPILL!/cycles. Volatile functions are always dirty.
4. Rendering: the egui grid reads display values (number formats applied in engine/display.rs) for the visible window only; charts render through crates/chart primitives.
5. Files: engine/io.rs sniffs content (XLSX, XLSB, ODS) before trusting the extension and dispatches to crates/xlsx; imports that lose data return warnings shown to the user.

Engine building blocks for multi-command actions

Actions made of many commands (macros, a debugger that runs one step per call) use these parts of Session and the engine:

• Undo groups. Session::grouped(label, f) runs f as one undo step and rolls every change back if f fails; group_begin, group_end and group_rollback do the same across calls. While a group is open on a document, its commands add no undo steps of their own and Undo and Redo are disabled there. Groups nest; the outermost one makes the step. A command that opens and closes its own groups still makes one undo step for itself, covering its edits inside and outside the groups. A command that ends a group open before it leaves the group its step and makes one of its own for what it changes after the end; a group a command opens and leaves open starts where the command did, but only when the command succeeds: a command that fails rolls back every group it opened, as group_rollback does. Session::abort_groups(doc), and the command automate.stopAction on the active workbook, roll back the groups left open on a document (a debugger the person abandons, a caller that dropped its token), so Undo works there again. The history limit is applied when the outermost command finishes; until then the steps made inside it are bounded on their own.
• Recording buffer (record.rs). Session::start_recording collects each successful top-level command with the document, sheet and selection it ran in, without secret parameters. Commands run by other commands are not recorded, nor is a menu-style call that only opened a dialog (the dialog's confirming call is the step), and the buffer is independent of the journal (Automate › Record Actions still records through the journal).
• Document trust. DocState::trust is Local for workbooks made in the session and FromFile for workbooks opened from a file, and for workbooks that received a paste from one that writes formulas or validation: Paste, Paste Special › All, All Except Borders, Formulas, Formulas and Number Formats or Validation, and any linked paste other than column widths or notes. Values, formats, column widths and notes carry nothing active. Active content in a FromFile workbook asks before it runs. This is provenance, which no decision changes; DocState::scripts_trusted is the separate per-session consent for stored automate.* scripts (automate.trustScripts).
• Workbook data in hidden names (cmd/stash.rs). An item is split over hidden workbook-scope names _gc_<key>_0, _gc_<key>_1, … of string literals within Excel's 255- and 8,192-character limits, so it survives an xlsx round trip. Characters XML can't carry (controls other than tab, line feed and carriage return, U+FFFE, U+FFFF) are stored as the noncharacter U+FDD0 and four hex digits, as is U+FDD0 itself, and read back as they were. The older single-name _sc_<key> items are read as well.

Agent control

• Desktop control channel: gridcraft --control 7979, JSON lines on localhost (engine.execute, ui.screenshot, pointer and keyboard input…), see control-protocol.md.
• MCP: gridcraft-cli mcp, headless or --connect to a running app, see mcp.md.
• CLI: cli.md.

Platforms and builds

macOS universal, Windows x64/x86/arm64, Linux x86_64/aarch64/riscv64 (AppImage, Flatpak, deb, rpm, tarball), FreeBSD, and web (WASM; WebGPU or WebGL2). Release workflows in .github/workflows/release.yml; see releasing.md.

Known architectural gaps

• No threading in calc (see hardware-parity.md).
• No string catalog or text shaping (see localization-parity.md).
• XLSX reader builds the whole workbook in memory; no streaming (#175).
• Unmodelled XLSX parts are not carried through a save.

Revision history

Date	Change	Summary
2026-10-24	minor	Engine building blocks for multi-command actions: undo groups, recording buffer, document trust, chunked hidden-name storage
2026-10-14	minor	Document Undo's retained-cell-storage budget and latest-step guarantee
2026-10-10	major	Created from the code on main: crates, layers, data flow, agent control, known gaps