fastsheet

A Lotus 1-2-3-inspired desktop spreadsheet with native Excel file compatibility (.xlsx and .xls) and a keyboard-first UI. Built on Tauri 2 + SvelteKit + Rust + IronCalc.

Status: early development, but xlsx and xls round-trip on real-world templates including macros and array-formula UDFs.

Install (Windows)

Grab the latest release from Releases:

• fastsheet_x.y.z_x64-setup.exe — NSIS installer (recommended; installs to your user profile, no admin required, adds a Start Menu entry).
• fastsheet_x.y.z_x64_en-US.msi — same thing as an MSI for group-policy-friendly deployments.

On first launch Windows SmartScreen may show a "Windows protected your PC" prompt — fastsheet builds aren't yet code-signed. Click More info → Run anyway.

Requirements: Windows 10 1809+ or Windows 11. The WebView2 runtime ships with both, so no extra install needed.

First-time users: read QUICKSTART.md for a keyboard-shortcut reference, the full / menu tree (with implemented vs not-yet-implemented marked), and a walkthrough of the formula-trace popup.

Why

Modern spreadsheet UIs are built around a mouse. fastsheet is built around the keyboard:

• Lotus-style menu tree — / opens the menu, single-letter keys navigate. No clicks required for any operation.
• Address-bar goto — F5 and a sheet-qualified address jumps anywhere in the workbook instantly.
• Modal hints in the menu bar — when an operation takes over the keyboard (e.g. range pick for copy/move), the keys you can press are shown where you'd expect them.
• Arrow-key responsiveness as a first-class goal — virtualized rendering, rAF-coalesced movement, overlay-based selection. Stays smooth on 100k-cell workbooks.
• Formula-trace popup (/T T) — walks a cell's full dependency chain across sheets and named ranges so you can see why a formula evaluates to what it does. F2 edit-mode highlights every reference the formula points at, including named ranges.

Stack

• UI: SvelteKit (SPA mode via adapter-static) + TypeScript
• Shell: Tauri 2 (native WebView2 on Windows, small binary, fast cold-start)
• Calc engine: IronCalc 0.7.x, vendored under vendor/ with additions for pseudo-spill UDFs and a couple of evaluation-consistency fixes
• xls reader / writer: hand-rolled BIFF8 over calamine + cfb — see src-tauri/src/xls_load.rs and xls_save.rs. Macros / VBA storages are captured on load and replayed on save so Excel users keep working files.
• xlsx: read via IronCalc; saves go through an in-place byte-patcher that preserves charts / pivots / drawings / VBA when the original is available

Build from source (Windows)

git clone https://github.com/hendorog/fastsheet
cd fastsheet
npm install
npm run tauri build

Output: src-tauri/target/release/bundle/nsis/*.exe and src-tauri/target/release/bundle/msi/*.msi.

For dev, npm run tauri dev launches a hot-reload build pointed at http://localhost:1420.

Headless probes

Three CLI tools live under src-tauri/src/bin/ for validating behaviour against real spreadsheet files without launching the GUI:

cd src-tauri

# Load a file, dump styles / col widths / hidden ranges.
cargo run --release --bin probe -- ../fixtures/<file>.xlsx

# Round-trip an .xls file through save → reload, report cell-by-cell diff.
cargo run --release --bin save_probe -- <file>.xls

# Walk the dependency tree of a specific cell — the same trace the
# /T T menu produces, but printed to stdout.
cargo run --release --bin trace_probe -- <file>.xls 0:4:7

save_probe accepts diagnostic env vars:

• FASTSHEET_PROBE_FULL_DIFF=1 — group remaining diffs by category
• FASTSHEET_PROBE_COMPARE=sheet:row:col[,...] — dump cell variants side-by-side
• FASTSHEET_PROBE_CELL=sheet:row:col — print the parsed Node tree
  • formatted value for a specific cell

Tests

cd src-tauri && cargo test

21 unit tests for the BIFF8 writer plus integration tests exercising the .xls round-trip path (including VBA / macro preservation). Real- file round-trip tests opt-in via env vars:

FASTSHEET_RT_FILE=/path/to/file.xls FASTSHEET_RT_THRESHOLD=5 \
  cargo test --test xls_roundtrip rt_external_fixture

Repo layout

src/                 SvelteKit frontend
  routes/+page.svelte  Top-level page (keyboard dispatch, layout)
  lib/                 Components + utilities
    FormulaTrace.svelte  /T T popup
    Grid.svelte          virtualized cell grid
    Navigator.svelte     /F R file picker
    menu.ts              Lotus-style menu tree
src-tauri/           Rust backend + Tauri shell
  src/
    workbook.rs        Open / new / save / backup commands
    cells.rs           Cell read / write / layout / trace commands
    trace.rs           Formula dependency walker
    xls_load.rs        .xls loader (calamine + custom BIFF scanner)
    xls_save.rs        .xls writer (from-scratch BIFF8 emit)
    xls_preserve.rs    VBA / macro storage capture+replay
    xlsx_load.rs       .xlsx loader (IronCalc + preprocessing)
    xlsx_save.rs       .xlsx in-place byte patcher
    ...
  tests/               Integration tests
  bin/                 Headless probes
vendor/              Vendored IronCalc with fastsheet additions
.github/workflows/   CI: release.yml builds Windows artefacts on tag push
fixtures/            Generated test workbooks
scripts/             Maintainer scripts (Linux/WSL dev workflow)

Development (WSL → Windows)

The project's primary author develops in WSL and cross-builds to Windows via cargo-xwin. If you're on Linux/macOS and want to do the same:

# Build + install to %USERPROFILE%\Tools\fastsheet\ + drop a Start
# Menu shortcut. Idempotent — re-run after every change.
scripts/install-windows.sh

This runs:

npx tauri build --runner cargo-xwin --target x86_64-pc-windows-msvc --no-bundle

under the hood. See scripts/install-windows.sh for the install details (path resolution, MOTW stripping, shortcut creation).

FASTSHEET_PROFILE_LOAD=1 enables phase timing for load / evaluate / boot — written to fastsheet_profile.log next to the running .exe.

License

MIT.

IronCalc is vendored under vendor/ (crates ironcalc and ironcalc_base, version 0.7.x) and is licensed under MIT OR Apache-2.0. See https://github.com/ironcalc/IronCalc.