✦ FSM Engine

A visual, interactive toolkit for building and simulating Finite State Machines.

Design, connect, and explore DFAs, NFAs, and free-form state machines — all from your browser.

Try it Live →

Table of Contents

• Overview
• Features
• Architecture
• Tech Stack
• Getting Started
• Project Structure
• Engine API
• Contributing
• Roadmap
• License

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Overview

FSM Engine is an open-source, web-based tool for creating, editing, and visualizing Finite State Machines. It pairs a powerful, framework-agnostic TypeScript engine (@fsm/engine) with a rich, canvas-driven Svelte 5 frontend — giving you both a programmatic API and a visual editor in a single monorepo.

Whether you're a student learning automata theory, an educator preparing a lecture, or an engineer prototyping state-driven logic, FSM Engine gets you from concept to diagram in seconds.

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Features

🎨 Interactive Canvas Editor

• Infinite canvas with smooth zoom & pan (scroll / pinch / trackpad)
• Drag-to-reposition states anywhere on the canvas
• Light & dark theme with toggleable grid overlay (light and dark theme switching will be available soon)

⚡ Multiple Interaction Modes

Mode	Description
Add	Click the canvas to place a new state
Connect	Click two states to draw a directed transition (self-loops supported)
Remove	Single-click to delete any state or transition
Zoom	Zoom in / out with buttons or scroll wheel

🔧 State & Transition Customization

• Rename states and set custom fill colors via a right-click properties panel
• Mark any state as Start, Intermediate, or End
• Edit transition labels with inline controls

🗂️ Project Management

• New / Open / Save projects in a portable .fsm (JSON) format
• Export to PNG at 1×, 2×, or 3× resolution
• Project metadata (name, author, creation date) fully editable from the UI

✨ Auto Layout

• One-click Dagre-powered graph layout that smoothly animates nodes into an optimal arrangement

🧠 Headless Engine (@fsm/engine)

• Pure TypeScript, zero-DOM — use it in Node.js, Bun, Deno, or any JS runtime
• Supports Free-form, DFA, and NFA engine types (More coming soon)
• DFA mode enforces deterministic constraints and provides string validation with path tracing
• Efficient ID recycling via min-heap for states and transitions
• Full save / load serialization out of the box

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Architecture

FSM Engine is structured as a Bun workspace monorepo with a clear separation between the computational core and the visual layer:

┌─────────────────────────────────────────────────────────┐
│                       Monorepo Root                     │
│                   (bun workspaces)                      │
├──────────────┬──────────────────────┬───────────────────┤
│              │                      │                   │
│  packages/   │     apps/web         │    apps/docs      │
│   engine     │   (Svelte 5 + Konva) │  (Astro Starlight)│
│              │                      │                   │
│  ┌─────────┐ │  ┌────────────────┐  │  ┌─────────────┐  │
│  │FSMEngine│◄┼──│  store.svelte  │  │  │  Starlight  │  │
│  │  DFA    │ │  │  (Project)     │  │  │  Docs Site  │  │
│  │ types   │ │  │                │  │  │             │  │
│  └─────────┘ │  └───────┬────────┘  │  └─────────────┘  │
│              │          │           │                   │
│              │    ┌─────▼─────┐     │                   │
│              │    │  Editor   │     │                   │
│              │    │  Canvas   │     │                   │
│              │    │  (Konva)  │     │                   │
│              │    └───────────┘     │                   │
└──────────────┴──────────────────────┴───────────────────┘

Key design decisions:

• The engine package has zero UI dependencies — it can be consumed independently as a library.
• The web app uses Svelte 5 runes ($state, $effect, $derived) with SvelteMap for fine-grained reactive state management.
• Canvas rendering is powered by Konva via svelte-konva, giving hardware-accelerated 2D drawing.
• The engine's internal Map<number, State> store is shared by reference with the Svelte frontend's reactive SvelteMap, keeping both layers synchronized with minimal overhead.

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Tech Stack

Layer	Technology
Runtime	Bun
Language	TypeScript 5 (strict mode)
Frontend Framework	Svelte 5 (Runes mode)
Canvas	Konva + svelte-konva
Styling	Tailwind CSS 4 + shadcn-svelte
Graph Layout	@dagrejs/dagre
Icons	@lucide/svelte
Bundler	Rolldown Vite
Documentation	Astro + Starlight
Linting	ESLint 9 + Prettier
Git Hooks	Husky + lint-staged

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Getting Started

Prerequisites

• Bun ≥ 1.3 (used as both package manager and runtime)

Installation

# Clone the repository
git clone https://github.com/karthik-saiharsh/fsm-engine.git
cd fsm-engine

# Install all workspace dependencies
bun install

Development

# Start the web editor (apps/web)
cd apps/web
bun run dev

# Start the documentation site (apps/docs)
cd apps/docs
bun run dev

Build for Production

cd apps/web
bun run build
bun run preview    # Preview the production build locally

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Project Structure

fsm-engine/
├── packages/
│   └── engine/                  # @fsm/engine — headless FSM library
│       ├── FSMEngine.ts         # Base engine: states, transitions, save/load
│       ├── DFA.ts               # DFA subclass: alphabet enforcement, string validation
│       ├── index.ts             # Public API barrel export
│       └── utils/
│           └── types.ts         # Shared types: State, Transition, EngineTypes
│
├── apps/
│   ├── web/                     # Svelte 5 visual editor
│   │   ├── src/
│   │   │   ├── App.svelte       # Root component: Launch screen ↔ Editor
│   │   │   ├── app.css          # Design tokens (oklch), Geist font, Tailwind
│   │   │   └── lib/
│   │   │       ├── brain/       # State management layer
│   │   │       │   ├── store.svelte.ts   # Project singleton (engine + UI state)
│   │   │       │   ├── extras.svelte.ts  # Secondary reactive stores
│   │   │       │   └── types.ts          # Frontend-specific types
│   │   │       └── components/  # UI components
│   │   │           ├── Editor.svelte     # Konva canvas + rendering loop
│   │   │           ├── Dock.svelte       # Bottom toolbar (Add/Remove/Connect/Zoom)
│   │   │           ├── Launch.svelte     # Welcome / project picker screen
│   │   │           ├── Alert.svelte      # Notification overlay
│   │   │           ├── editor/           # TopBar, MachinePicker
│   │   │           ├── popus/            # NodeCustomizer, TransitionCustomizer, etc.
│   │   │           ├── generic/          # Window, ScreenSizeFallback
│   │   │           └── ui/              # shadcn-svelte primitives
│   │   ├── index.html
│   │   ├── vite.config.ts
│   │   └── svelte.config.js
│   │
│   └── docs/                    # Astro Starlight documentation site
│       ├── astro.config.mjs
│       └── src/content/docs/
│
├── package.json                 # Workspace root (bun workspaces)
├── tsconfig.json                # Shared TypeScript config
├── eslint.config.mts            # ESLint flat config
├── .prettierrc                  # Prettier formatting rules
└── .husky/pre-commit            # Pre-commit lint hook

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Engine API

The @fsm/engine package exposes two main classes that can be used independently of the UI:

FSMEngine (Free-form)

import { FSMEngine } from "@fsm/engine";

const fsm = new FSMEngine("MyStateMachine");

// Add states (returns a numeric reference ID)
const s0 = fsm.addState("q0");
const s1 = fsm.addState("q1");
const s2 = fsm.addState("q2");

// Mark state types
fsm.setStart(s0);
fsm.setEnd(s2);

// Add transitions
fsm.addTransition(s0, s1, "a");
fsm.addTransition(s1, s2, "b");
fsm.addTransition(s1, s1, "a");  // self-loop

// Query the machine
fsm.searchStates("q1");               // → [1]
fsm.searchTransition(s0, s1);         // → [0]
fsm.getTransitionsOn("a");            // → [0, 2]

// Serialize / Deserialize
const snapshot = fsm.saveProject();
fsm.loadProject(snapshot);

DFA (Deterministic Finite Automaton)

import { DFA } from "@fsm/engine";

const dfa = new DFA("BinaryStrings");

// Define language alphabet
dfa.addAlphabets("0", "1");

// Build states
const q0 = dfa.addState("q0");
const q1 = dfa.addState("q1");
dfa.setStartState(q0);
dfa.endState.add(q1);

// Add deterministic transitions
dfa.addTransition(q0, q1, "1");
dfa.addTransition(q0, q0, "0");
dfa.addTransition(q1, q1, "0");
dfa.addTransition(q1, q0, "1");

// Validate strings
dfa.validateString("1010");
// → { path: [0, 1, 1, 0], accepted: true }

DFA enforcement: Adding a duplicate transition on the same alphabet from the same state will throw an error, guaranteeing determinism at the API level.

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Contributing

Contributions are welcome and greatly appreciated! Here's how to get involved:

1. Fork the repository
2. Create a feature branch (git checkout -b feat/amazing-feature)
3. Commit your changes (git commit -m 'feat: add amazing feature')
4. Push to your branch (git push origin feat/amazing-feature)
5. Open a Pull Request

Guidelines

• Run bun run lint before submitting to ensure code quality
• Discuss larger architectural changes in an issue first
• Follow Conventional Commits for commit messages
• Pre-commit hooks (Husky + lint-staged) will auto-lint staged files

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Roadmap

• Interactive canvas editor with zoom & pan
• Add / Remove / Connect states & transitions
• State types: Start, Intermediate, End
• Project export / import (.fsm JSON format)
• Export canvas as PNG (1×, 2×, 3×)
• Auto layout (Dagre)
• Dark / Light theme
• DFA string validation with path tracing
• Transition table generation & display
• NFA → DFA subset construction
• DFA minimization
• Undo / Redo
• Validation & error hints (unreachable states, dead states)
• Regular expression → NFA conversion
• Keyboard shortcuts & accessibility improvements
• Collaborative editing

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License

This project is licensed under the GNU General Public License v3.0.

Copyright (C) 2026 Illindala Karthik Saiharsh

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

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Built with ♥ by Karthik Saiharsh and contributors

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