[PATCH 3/3] Solving scalability issue: for chain list "name" searching.

Solving scalability issue: for chain list "name" searching.
Functions: iptcc_find_label(), iptc_is_chain().

Testing if a chain exist, requires a linearly walk of linked list with
chain-names (doing a strcmp(3) in each step). Giving a worst-case
runtime of O(n) where n is the number of chains.

Why is this important to fix?! If only called once, this should not be
a big concern, even-though the string compares are expensive.

The performance issue arise with many chains for example; when using
"iptables-restore", or when listing all "iptables -nL" rules, or when
using CPAN IPTables::libiptc.

Having 50k chains, the rule listing, with the command:
 "./iptables -nL > /dev/null",
Without patch it takes approximately 5 minutes,
With the patch it takes 0.5 seconds.

Listing without patch:
 real    4m49.426s
 user    4m37.993s
 sys     0m0.280s

Listing with patch:
 real    0m0.558s
 user    0m0.484s
 sys     0m0.064s

How is it solved?!

The issue is solved introducing a new data structure, that allow us to
do binary search of chain names. Thus, reducing the worst-case runtime
to O(log n).

Being more specific:

 The new data structure is called "chain index", which is an array with
 pointers into the chain list, with CHAIN_INDEX_BUCKET_LEN spacing.
 This facilitates the ability to speedup chain list searching, by find
 a more optimal starting points when searching the linked list.

 The runtime complexity is actually also affected by this "bucket" size
 concept. Thus, O(log(n/k) + k) where k is CHAIN_INDEX_BUCKET_LEN.

 A nice property of the chain index, is that the "bucket" list
 length is max CHAIN_INDEX_BUCKET_LEN (when just build, inserts will
 change this). Oppose to hashing, where the "bucket" list length can
 vary a lot.

Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>

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