nat: Introduce a prefix lookup tree (trie) for number rewriting

* 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.

1 files changed, 20 insertions, 0 deletions

diff --git a/openbsc/tests/bsc-nat-trie/bsc_nat_trie_test.ok b/openbsc/tests/bsc-nat-trie/bsc_nat_trie_test.ok
new file mode 100644
index 000000000..4d4cc9949
--- /dev/null
+++ b/openbsc/tests/bsc-nat-trie/bsc_nat_trie_test.ok
@@ -0,0 +1,20 @@
+Testing the trie
+Dumping the internal trie
+1,1
+12,2
+123,3
+1234,4
+12345,5
+123456,6
+1234567,7
+12345678,8
+123456789,9
+1234567890,10
+13,11
+14,12
+15,13
+16,14
+82,16
+823455,15
++49123,17
+Done with the tests.