Text Diff Checker

Diffing is a matching problem before it is a display problem. The Text Diff Checker builds a longest-common-subsequence table across the two versions, then walks it backwards to decide which lines are shared, which were added and which were removed. The payoff is a pairing that survives insertion: add a paragraph at the top of B and everything below it still lines up against A, instead of every remaining line reporting as changed.

The result is four columns wide, a line number and its content for each side. Removed lines sit tinted red on the left with the right cell blank, added lines sit green on the right with the left cell blank, and unchanged lines carry both numbers. A tally of added, removed and unchanged runs above the grid.

Comparison is line-level rather than word-level, so altering one character marks the whole line removed and re-added instead of highlighting the character itself. Both versions stay in the page, which matters when the thing being compared is a contract or a production config.

Walkthrough: Diffing Two Config Files

1. Paste the old version into Original Text (A) and the new one into Modified Text (B). The Compare → button stays disabled until at least one pane holds something.
2. Press Compare →. The editing view gives way to the diff, and the bar above reads something like 3 added, 1 removed, 146 unchanged.
3. Scroll inside the diff. It caps at 500 pixels of height with the column headers pinned, so # and Original (A) stay visible through a long file.
4. Read the gutter numbers rather than the row positions. A removed line carries a number on the left only, which is how you locate it in the original.
5. Press ← Edit to go back. It empties both panes as well as returning you to the editing view, so copy anything you still need before pressing it.

What Happens When a Line Moves

• A moved line reads as one removal plus one addition. The algorithm has no notion of relocation, so a block cut from the top and pasted at the bottom appears twice, once red and once green.
• Changing a single character marks the entire line both removed and added, because matching is exact equality across the whole line.
• Whitespace counts. A line differing only by a trailing space, or by a tab against 4 spaces, is a changed line here. That is usually right in a config file and rarely right in prose.
• Blank lines take part like any other line, so deleting one produces a red row with nothing visible in it.
• Comparing a file against an empty pane marks every line added or removed, which doubles as a quick way to number the lines of a single file.

How Big a File This Can Diff

The table holds one number per pair of lines, so cost grows with the product of the two line counts rather than their sum. Two 500-line files is 250,000 cells and returns at once. Two 2,000-line files is 4 million cells, still workable but visibly slower. Two 10,000-line files is 100 million cells and will lock the tab up, so split the file or reach for a command-line diff at that size. Nothing is uploaded and no ceiling is enforced, which means your browser is the only thing standing between you and that 100-million-cell table.