Docs: Add algorithm diagrams for checkin

doc/development/algorithms/checkin.rst

@@ -0,0 +1,28 @@
+.. spelling: libpretixsync
+
+Check-in algorithms
+===================
+
+When a ticket is scanned at the entrance or exit of an event, we follow a series of steps to determine whether
+the check-in is allowed or not. To understand some of the terms in the following diagrams, you should also check
+out the documentation of the :ref:`ticket redemption API endpoint <rest-checkin-redeem>`.
+
+Server-side
+-----------
+
+The following diagram shows the series of checks executed on the server when a ticket is redeemed through the API.
+Some simplifications have been made, for example the de-duplication mechanism based on the ``nonce`` parameter
+to prevent re-uploads of the same scan is not shown.
+
+.. image:: /images/checkin_online.png
+
+Client-side
+-----------
+
+The process of verifying tickets offline is a little different. There are two different approaches,
+depending on whether we have information about all tickets in the local database. The following diagram shows
+the algorithm as currently implemented in recent versions of `libpretixsync`_.
+
+.. image:: /images/checkin_offline.png
+
+.. _libpretixsync: https://github.com/pretix/libpretixsync