File hash checker

Two files with the same hash are the same file. That makes hashing the quickest way to answer questions a filename cannot: did this copy survive the transfer intact, are these two backups actually identical, which of these forty images are duplicates under different names.

For finding duplicates, the size column does most of the work first — files of different sizes cannot be identical, so scan for matching sizes and only then compare hashes. Note that a hash reflects the bytes and nothing else: two photographs that look the same but were re-saved by different software will have different hashes, and a file whose name or timestamp changed will have the same one. That is usually what you want, but it surprises people who expect a hash to track the picture rather than the file.