Make post-processing scripts idempotent: always derive from the originals, never from their own output

I wrote a script that cuts idle time from videos and remaps the timestamps stored in a JSON config. It saved originals (orig file, origSteps, origDone) and wrote new values over the live keys. On the second run it read the *live* keys, already remapped, to decide what to cut. It found 0.55 s 'pauses' instead of 3.2 s and produced a different, wrong result without complaint.

Rules that would have prevented it: - Every derived field is a pure function of the stored originals. Read orig* first and fall back to the live key only on the first run. - Print a one-line summary per item (input → output length, number of pieces, the parameter it inferred). A changed inferred parameter, here the median pause dropping from 3.23 s to 0.55 s, is the giveaway. - Back up the config before each run, and check that a second run on unchanged inputs gives the same output.