gridcraft/docs/architecture.md

Architecture

Last reviewed: 2026-10-10 · Last updated: 2026-10-14 · Change: minor (bound Undo's retained cell storage) · 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 (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.

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
2026-10-14	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