Paste two versions of a text and see exactly which lines were added, removed or changed, down to the word.
When two versions of a document look almost the same, the eye is bad at spotting what moved. A diff checker reads both versions and reports the smallest set of edits that turns the first into the second: lines that were added, lines that were removed, and lines that were edited in place. Inside an edited line it also marks the exact words that differ, so a changed price, a swapped date or a missing comma stands out instead of hiding in a paragraph.
Typical uses: checking what a colleague changed in a config file before you deploy it, comparing two drafts of a contract clause, confirming that a copied list of GST numbers matches the original, reviewing a SQL query someone tweaked, or finding the one line that differs between a working and a broken .env file.
The comparison uses the Myers diff algorithm, published by Eugene Myers in 1986 and still the default in git. Picture a grid with the old lines along one side and the new lines along the other. Every path from the top left corner to the bottom right is a way of editing one text into the other: a step right deletes a line, a step down inserts one, and a free diagonal step means the lines match. Myers searches outward one edit at a time, so the first path to reach the corner is guaranteed to use the fewest insertions and deletions. This page uses the linear space version, which works from both corners at once and splits the problem at the point where they meet, so memory stays small even for long files. Lines are first turned into numbers, so comparing long lines costs no more than comparing short ones. The word level view runs the same algorithm again on the words of each changed pair.
Load the sample, a weekly delivery schedule. The checker reports one changed line, one removed line and one added line: Tuesday moved from 9 am to 1 pm to 10 am to 2 pm, with only the two numbers marked; the Wednesday closed line was removed; and a new Saturday slot was added at the end. The four lines that did not change are counted as unchanged.
The two texts together can hold up to 20,000 lines. Beyond that the page stops and explains, because a very different pair of large files can keep a browser busy for a long time; split the file or use a command line tool for those. A line that was moved elsewhere shows as one removal and one addition, which is how line diffs work. Word marking is skipped on extremely long single lines. Everything runs inside your browser tab, and Ur Next Door never receives either text, so it is safe for private code, contracts and customer lists.
