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Niels Lohmann
ec93161a40
When a parser callback rejects a value, the placeholder stored for it has to
be removed from its parent again. remove_discarded_value() found it by
scanning the parent from the beginning, so filtering a container cost one
scan per rejected member - quadratic in the number of members of a single
container.
A rejected value can only ever be the one most recently added to its parent:
the last element of an array, or the placeholder key() stored under the
current key in an object. Record that key alongside the existing
key_keep_stack, and for a container record it again alongside ref_stack so
end_object()/end_array() can find it in the parent. Removal is then O(1) for
an array and O(log n) for an object, and finding nothing there means nothing
was stored, so there is nothing to remove.
The key for a container is read before handle_value() may consume it, so it
is also correct when the callback rejects the container at its start event
and it never reaches its parent at all.
Discarding half the members of one object, before -> after:
members value rejected container rejected at start
16 000 392 ms -> 3.7 ms 803 ms -> 8.2 ms
64 000 6238 ms -> 14.6 ms 12651 ms -> 32.2 ms
128 000 25339 ms -> 30.6 ms
Results are unchanged: 48 000 randomized documents parsed under 12 different
filtering callbacks - covering duplicate keys, empty keys, rejected keys and
containers rejected at both their start and end events - produce byte
identical output before and after.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>