Diff Checker

A diff checker (also called a text comparison tool or diff viewer) compares two blocks of text and highlights the differences between them. It identifies which lines were added, which were removed, and which remain unchanged.

Diff checking is fundamental to software development — every version control system (Git, SVN, Mercurial) relies on diff algorithms to track changes. But diffs are equally useful outside of code: comparing contracts, reviewing document revisions, verifying configuration changes, or checking data transformations.

The most common approach to computing a diff is the Longest Common Subsequence (LCS) algorithm. Given two sequences of lines, LCS finds the longest ordered sequence of lines that appears in both inputs. Lines not in the LCS are either additions (present only in the new text) or deletions (present only in the old text).

More advanced algorithms like Myers' diff algorithm optimize this process for performance, producing minimal edit scripts that represent the smallest set of changes needed to transform one text into another. This tool uses an LCS-based approach that balances clarity and speed for typical text comparison tasks.

Code review is the most common use case — comparing a file before and after changes to understand exactly what was modified. Configuration auditing is another: comparing server configs, environment files, or infrastructure-as-code templates to catch unintended changes.

Writers and editors use diff tools to track revisions in manuscripts and documentation. Data engineers compare CSV exports or JSON payloads to verify data pipeline outputs. Security teams diff configuration files to detect unauthorized modifications.

In modern development, diff checking is at the heart of code review. Platforms like GitHub, GitLab, and Bitbucket display pull request diffs so reviewers can see exactly what changed. Understanding how to read a diff — recognizing additions, deletions, and context lines — is an essential developer skill.

This online diff checker is useful when you need a quick comparison without setting up a local repo — for example, comparing a snippet from documentation against your implementation, or checking two versions of a config file shared via email or chat.