Snapshot Comparison

Snapshot testing utility for regression detection with integrated diff output.

Overview

Snapshot Comparison (snapcmp) catches regressions by comparing current output against stored "golden" snapshots. It integrates with output-diffing-utility for rich, semantic diffs of text, JSON, and binary data.

First Tool Using Library Composition

This is the first Tuulbelt tool to use another Tuulbelt tool as a library dependency (not CLI). It imports output-diffing-utility via Cargo path dependency, demonstrating PRINCIPLES.md Exception 2: Tuulbelt tools can compose while maintaining zero external dependencies.

Status: Production Ready (v0.1.0)

Language: Rust

Repository: tuulbelt/snapshot-comparison

Features

Snapshot Testing

Store expected output and automatically detect regressions. Create, check, and update snapshots with simple commands.

Integrated Semantic Diffs

Uses output-diffing-utility for intelligent comparisons:

• Text: Unified diff with context lines
• JSON: Semantic comparison showing field changes
• Binary: Hex dump with byte-level differences

CLI-First Design

Works with any language via stdin/stdout. Pipe output from any program for snapshot testing.

CLI & Library

Use as a command-line tool for shell scripts or integrate as a Rust library.

Zero Runtime Dependencies

Uses only Rust standard library plus output-diffing-utility (another zero-dep Tuulbelt tool).

Quick Start

# Clone the repository
git clone https://github.com/tuulbelt/snapshot-comparison.git
cd snapshot-comparison

# Build
cargo build --release

# Install globally
cargo install --path .

# Create a snapshot
my-program | snapcmp create my-test

# Check against snapshot
my-program | snapcmp check my-test

Demo

See the tool in action:

▶ View interactive recording on asciinema.org

Try it online:

Use Cases

API Response Testing

curl -s https://api.example.com/users | snapcmp create users-api
curl -s https://api.example.com/users | snapcmp check users-api

Build Verification

./build.sh | snapcmp create build-output
./build.sh | snapcmp check build-output

Configuration Validation

cat config.json | snapcmp create -t json valid-config
cat config.json | snapcmp check valid-config

Why Snapshot Comparison?

Unlike Jest snapshots or insta:

1. Zero dependencies - Pure Rust, no runtime dependencies
2. Integrated diffs - Rich semantic diffs included
3. Format-aware - Understands text, JSON, and binary
4. CLI-first - Works with any language

Documentation

• Getting Started - Installation and setup
• CLI Usage - Command-line reference
• Library Usage - Rust API guide
• Examples - Real-world usage patterns
• API Reference - Complete API documentation

Related Tools

• Output Diffing Utility - Semantic diff engine (used internally)
• Test Flakiness Detector - Detect unreliable tests