Diff Viewer

How to Use the Diff Viewer

This diff viewer compares two blocks of text and highlights every difference between them. Paste or type the original text on the left and the modified version on the right, then click Compare. The tool uses the Longest Common Subsequence algorithm to produce an accurate, minimal diff that shows exactly what changed between the two versions.

Added lines appear in green with a plus prefix, deleted lines appear in red with a minus prefix, and unchanged lines display without highlighting. Line numbers from both texts are shown on the left side so you can reference specific locations in each version. The Swap button lets you reverse the order of comparison, and the Clear button resets both fields.

Understanding Diff Output

The diff output format follows the unified diff convention used by version control systems like Git. Each line of output corresponds to one line from either the original or modified text. When a line exists in the original but not in the modified version, it appears as a deletion. When a line exists in the modified version but not the original, it appears as an addition. This format makes it easy to see what was changed, added, or removed at a glance.

Common Use Cases

Developers frequently use diff viewers to review code changes before committing. Writers compare document drafts to track revisions. System administrators diff configuration files to identify unexpected changes. Students compare essay drafts to see how their writing evolved. Any scenario where you need to understand the differences between two versions of text benefits from a visual diff tool.

The LCS Algorithm

The Longest Common Subsequence algorithm is the foundation of most diff tools. It finds the longest sequence of lines that appear in both texts in the same order, though not necessarily consecutively. Lines not in the LCS are differences. This approach produces the smallest possible set of changes needed to transform the original into the modified version, making the output clean and easy to read.

Frequently Asked Questions

What is a diff viewer?

A tool that compares two texts and highlights additions, deletions, and unchanged lines using color coding. Essential for code review, document comparison, and tracking changes.

How does the comparison algorithm work?

It uses the Longest Common Subsequence (LCS) algorithm to find the optimal alignment, then marks lines outside the LCS as additions or deletions.

Can I compare code files?

Yes, this works with any plain text including source code, configs, and documentation. Whitespace and indentation are preserved for accurate comparison.

What do the colors mean?

Green indicates additions (new lines), red indicates deletions (removed lines), and no highlight means the line is unchanged in both texts.

Is there a size limit?

The comparison runs in your browser with no server limit. For best performance, keep comparisons under a few thousand lines.