Text Compare

How diff algorithms find what changed

This tool uses the same core idea as programming diff tools: the longest common subsequence (LCS). It finds the largest set of lines the two texts share in order, then marks everything outside that set as removed (-) or added (+). This is why a single edited line shows as one removal plus one addition — the algorithm sees an old line that vanished and a new line that appeared.

Line-based comparison is the sweet spot for most real work: it is precise enough to pinpoint changes and coarse enough to stay readable. For prose, putting each sentence on its own line before comparing gives you sentence-level precision.

Practical comparison workflows

Common uses: verifying which requirements changed between two versions of a client brief, checking what a colleague edited in a shared document when track-changes was off, comparing two exported configuration files, and confirming that a pasted backup matches the original. For list reconciliation — which emails are in list A but not list B — sort both lists first (our Alphabetical Order tool) so the diff aligns matching lines instead of flagging ordering differences.