Text Diff Checker

What a diff actually is

A diff compares two versions of some text and reports what was added, what was removed, and what stayed the same. It is the same idea behind the change views you see in code review, version control, and document history.

Line-level LCS in plain English

This tool treats each line as a single unit and finds the Longest Common Subsequence (LCS) of lines that appear in the same order in both versions. Those shared lines are the "unchanged" backbone. Anything in the original but not in that backbone is a deletion, and anything in the changed text but not in the backbone is an addition.

The LCS is computed with a dynamic-programming table sized by the number of lines on each side, then a backtrack walks that table to produce an ordered list of equal, added, and removed operations. This tends to give cleaner, more intuitive results than a naive line-by-line comparison, which falls apart as soon as one line is inserted or removed near the top.

Unified vs side-by-side

A unified view (what this tool renders) shows one combined column: removed lines are marked with a leading minus and a red tint, added lines with a leading plus and a green tint, and unchanged lines sit neutral with a leading space. A side-by-side view instead places the two versions in parallel columns. Unified tends to read well for small, scattered edits; side-by-side can be easier for large rewrites.

Practical uses

• Spot what changed between two versions of a config file or environment file.
• Compare two log captures to isolate the lines that differ between runs.
• Review an edit to an article, email, or contract before sending it.
• Check whether two snippets that look identical really are, down to whitespace.

Everything here runs locally in your browser. The two inputs are never sent anywhere and nothing is stored.