README and COPYING import for hat tries.

Change-Id: Ibb5c289cbd04a3affb57dcf31d3d7bcf6b4b79b0

src/common/hattrie/COPYING

+Copyright (C) 2011 by Daniel C. Jones <dcjones@cs.washington.edu>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN

src/common/hattrie/README.md

+
+Hat-Trie
+========
+
+This a ANSI C99 implementation of the HAT-trie data structure of Askitis and
+Sinha, an extremely efficient (space and time) modern variant of tries.
+
+The version implemented here maps arrays of bytes to words (i.e., unsigned
+longs), which can be used to store counts, pointers, etc, or not used at all if
+you simply want to maintain a set of unique strings.
+
+For details see,
+
+  1. Askitis, N., & Sinha, R. (2007). HAT-trie: a cache-conscious trie-based data
+     structure for strings. Proceedings of the thirtieth Australasian conference on
+     Computer science-Volume 62 (pp. 97–105). Australian Computer Society, Inc.
+
+  2. Askitis, N., & Zobel, J. (2005). Cache-conscious collision resolution in
+     string hash tables. String Processing and Information Retrieval (pp.
+     91–102). Springer.
+
+
+Installation
+------------
+
+    git clone git@github.com:dcjones/hat-trie.git
+    cd hat-trie
+    autoreconf -i
+    ./configure
+    make install
+
+To use the library, include `hat-trie.h` and link using `lhat-trie`.