Diff Checker
Line-by-line comparison using the longest-common-subsequence method. Both texts stay in your browser.
How it works
仕組みCompare two blocks of text line by line and see exactly what changed. Paste the original on the left and the revision on the right; the tool computes a longest-common-subsequence diff (the same idea behind git’s diff) and renders a unified view where removed lines are tinted red with a − prefix and added lines green with a +. Plates above the result count added, removed, and unchanged lines at a glance. Both texts stay in your browser, so it’s safe to compare configs, contracts, or code you’d rather not paste into someone else’s server.
The comparison works whole-line: if even one character on a line changes, the old line is marked removed and the new one added. That’s the standard behaviour of line diffs, including git’s. For prose, breaking long paragraphs into shorter lines before comparing makes the output much easier to read.
It’s the largest set of lines the two texts share in the same order; the diff keeps those as unchanged and marks everything else added or removed. This gives the minimal, most human-sensible set of changes rather than flagging everything after the first difference. It’s the classic algorithm behind most diff tools since the 1970s.
Each side is capped at 400 lines to keep the comparison instant on any device; a notice appears if your text is longer. Within that cap the diff recomputes live on every keystroke. For very large files, a desktop tool or git itself is the better fit.