The prefix tree introduced for linear unflatten() kept a std::map per prefix,
copied each token into it and looked every token up a second time while
creating the values. Replace the per-prefix maps by one open-addressing table
keyed by (parent, token) that refers to the parsed tokens, skip the lookups for
the prefix shared with the previous key (flattened objects are mostly sorted),
and derive the prefix numbers of the second pass from parent links instead of
lookups.
Time and memory stay linear in the pointer depth, and the result is still
independent of the iteration order of the flattened object.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
6e051c3bfb