A text diff (short for "difference") is a comparison between two pieces of text that highlights what has been added, removed, or changed. The concept originates from the Unix diff utility created in the early 1970s, and it remains one of the most fundamental tools in software development. At its core, most diff algorithms use the Longest Common Subsequence (LCS) method to find the maximum set of lines that appear in both texts in the same order. Lines present only in the original are marked as deletions, lines present only in the modified version are marked as additions, and shared lines provide context for understanding where changes occurred.
The Longest Common Subsequence algorithm works by building a dynamic programming matrix that compares every line of the original text against every line of the modified text. By backtracking through this matrix, the algorithm reconstructs the optimal alignment that minimizes the total number of changes. This approach produces clean, readable diffs that group related changes together rather than showing confusing interleaved additions and deletions. More advanced implementations like the Myers diff algorithm optimize performance for large files, but the LCS-based approach used here provides clear, accurate results for typical text comparisons.
Text diff tools are indispensable for code review, helping developers see exactly what changed between two versions of a file. They are used to compare configuration files before and after updates, verify that automated transformations produced the expected output, review document revisions, and audit changes to legal or policy documents. This browser-based diff tool provides a color-coded line-by-line comparison with addition and deletion counts, making it easy to spot changes at a glance. Your text is compared entirely on your device, so confidential documents and source code remain secure.