Exact reverse from history

The most reliable way to reverse a document is to use the exact run that produced it. Piixie keeps the mappings of every run in its history; exact reverse simply runs them backwards.

How Piixie recognizes a file

When you drop files into the staging area, Piixie computes a fingerprint of each and checks it against the outputs it has produced. If a file matches a past run, it knows precisely which run, and offers to reverse it:

expediente-clinico-david-romero.pdf was found in history (anonymized from expediente-clinico-marcos-patel.pdf on 11 Jun 2026). Deanonymize it instead?

Choose Deanonymize, and if several files matched, tick Apply this answer to the remaining matched files to handle them all the same way.

What it does

For a run that turned Marcos Patel → David Romero Gil, 1029384 → 84913366, and so on, exact reverse:

Takes the run’s stored mapping list.
Inverts each pair (replacement → original).
Applies them to the document, longest matches first so nothing is partially rewritten.

Anonymized:  Paciente David Romero Gil (NHC 84913366) atendido por la Dra. Lucía Sáez Marín.
                              ↓  exact reverse
Restored:    Paciente Marcos Patel (NHC 1029384) atendido por la Dra. Ruiz.

Because it uses the run’s actual mappings, the restoration is exact — every value goes back to precisely what it was.

Local, free, no model

Exact reverse is pure substitution from data already on your machine. It doesn’t call a model, doesn’t touch the network, and is instant. (Contrast with dictionary reverse, which uses the model to find candidates when there’s no run record.)

When it can’t be exact

A couple of cases push you to dictionary reverse instead:

The file was edited elsewhere. If an external tool reflowed or rewrote the document, its fingerprint no longer matches the stored output, so Piixie can’t tie it to a run. It’ll offer to try a dictionary instead.
No history record. The file was produced on another machine, or its history was cleared.

Ambiguous mappings

Exact reverse needs each fake value to map back to a single original. If a run mapped two different originals to the same replacement (which can happen with aggressive settings), the reverse is ambiguous — Piixie can’t know which original a given fake should become — and it tells you so rather than guessing. In practice this is rare with synthetic mode, where distinct people get distinct fakes.

After reversing

The restored file is written like any output, and the reversal is recorded so you can trace it. Keep the round trip in mind: the restored document carries real PII again, so it’s back inside your trust boundary.