* It is a trie. The max depth of the trie is the length of the
longest prefix. The lookup is O(lookuped_prefix), but as the prefix
length is limited, the lookup time is constant.
* Each node can hold the entire prefix, has place for the rewrite
rule with up to three digits.
* A trie with 20k entries will take about 3MB ram.
* Filling the trie 100 times takes ~800ms on my i7 laptop
* 10.000.000 lookups take 315ms.. (for the same prefix).
* 93/99 lines are tested, 6/6 functions are tested, 49 of 54 branches
are tested. Only memory allocation failures are not covered
* A late addition is to handle the '+' sign and to increase the number
of chars in the rewrite prefix. The timing/line coverage has not
been updated after this change.
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