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144 lines
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144 lines
5.7 KiB
Plaintext
This directory contains several hash-map implementations, similar in
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API to SGI's hash_map class, but with different performance
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characteristics. sparse_hash_map uses very little space overhead, 1-2
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bits per entry. dense_hash_map is very fast, particularly on lookup.
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(sparse_hash_set and dense_hash_set are the set versions of these
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routines.) On the other hand, these classes have requirements that
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may not make them appropriate for all applications.
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All these implementation use a hashtable with internal quadratic
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probing. This method is space-efficient -- there is no pointer
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overhead -- and time-efficient for good hash functions.
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COMPILING
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---------
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To compile test applications with these classes, run ./configure
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followed by make. To install these header files on your system, run
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'make install'. (On Windows, the instructions are different; see
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README_windows.txt.) See INSTALL for more details.
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This code should work on any modern C++ system. It has been tested on
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Linux (Ubuntu, Fedora, RedHat, Debian), Solaris 10 x86, FreeBSD 6.0,
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OS X 10.3 and 10.4, and Windows under both VC++7 and VC++8.
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USING
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-----
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See the html files in the doc directory for small example programs
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that use these classes. It's enough to just include the header file:
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#include <sparsehash/sparse_hash_map> // or sparse_hash_set, dense_hash_map, ...
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google::sparse_hash_set<int, int> number_mapper;
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and use the class the way you would other hash-map implementations.
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(Though see "API" below for caveats.)
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By default (you can change it via a flag to ./configure), these hash
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implementations are defined in the google namespace.
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API
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---
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The API for sparse_hash_map, dense_hash_map, sparse_hash_set, and
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dense_hash_set, are a superset of the API of SGI's hash_map class.
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See doc/sparse_hash_map.html, et al., for more information about the
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API.
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The usage of these classes differ from SGI's hash_map, and other
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hashtable implementations, in the following major ways:
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1) dense_hash_map requires you to set aside one key value as the
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'empty bucket' value, set via the set_empty_key() method. This
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*MUST* be called before you can use the dense_hash_map. It is
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illegal to insert any elements into a dense_hash_map whose key is
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equal to the empty-key.
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2) For both dense_hash_map and sparse_hash_map, if you wish to delete
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elements from the hashtable, you must set aside a key value as the
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'deleted bucket' value, set via the set_deleted_key() method. If
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your hash-map is insert-only, there is no need to call this
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method. If you call set_deleted_key(), it is illegal to insert any
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elements into a dense_hash_map or sparse_hash_map whose key is
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equal to the deleted-key.
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3) These hash-map implementation support I/O. See below.
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There are also some smaller differences:
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1) The constructor takes an optional argument that specifies the
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number of elements you expect to insert into the hashtable. This
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differs from SGI's hash_map implementation, which takes an optional
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number of buckets.
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2) erase() does not immediately reclaim memory. As a consequence,
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erase() does not invalidate any iterators, making loops like this
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correct:
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for (it = ht.begin(); it != ht.end(); ++it)
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if (...) ht.erase(it);
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As another consequence, a series of erase() calls can leave your
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hashtable using more memory than it needs to. The hashtable will
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automatically compact at the next call to insert(), but to
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manually compact a hashtable, you can call
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ht.resize(0)
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I/O
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---
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In addition to the normal hash-map operations, sparse_hash_map can
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read and write hashtables to disk. (dense_hash_map also has the API,
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but it has not yet been implemented, and writes will always fail.)
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In the simplest case, writing a hashtable is as easy as calling two
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methods on the hashtable:
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ht.write_metadata(fp);
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ht.write_nopointer_data(fp);
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Reading in this data is equally simple:
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google::sparse_hash_map<...> ht;
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ht.read_metadata(fp);
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ht.read_nopointer_data(fp);
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The above is sufficient if the key and value do not contain any
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pointers: they are basic C types or agglomorations of basic C types.
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If the key and/or value do contain pointers, you can still store the
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hashtable by replacing write_nopointer_data() with a custom writing
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routine. See sparse_hash_map.html et al. for more information.
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SPARSETABLE
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-----------
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In addition to the hash-map and hash-set classes, this package also
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provides sparsetable.h, an array implementation that uses space
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proportional to the number of elements in the array, rather than the
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maximum element index. It uses very little space overhead: 1 bit per
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entry. See doc/sparsetable.html for the API.
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RESOURCE USAGE
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--------------
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* sparse_hash_map has memory overhead of about 2 bits per hash-map
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entry.
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* dense_hash_map has a factor of 2-3 memory overhead: if your
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hashtable data takes X bytes, dense_hash_map will use 3X-4X memory
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total.
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Hashtables tend to double in size when resizing, creating an
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additional 50% space overhead. dense_hash_map does in fact have a
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significant "high water mark" memory use requirement.
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sparse_hash_map, however, is written to need very little space
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overhead when resizing: only a few bits per hashtable entry.
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PERFORMANCE
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-----------
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You can compile and run the included file time_hash_map.cc to examine
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the performance of sparse_hash_map, dense_hash_map, and your native
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hash_map implementation on your system. One test against the
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SGI hash_map implementation gave the following timing information for
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a simple find() call:
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SGI hash_map: 22 ns
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dense_hash_map: 13 ns
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sparse_hash_map: 117 ns
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SGI map: 113 ns
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See doc/performance.html for more detailed charts on resource usage
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and performance data.
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---
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16 March 2005
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(Last updated: 12 September 2010)
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