--- /dev/null
+// -*- C++ -*-
+
+// Copyright (C) 2005, 2006, 2009 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the terms
+// of the GNU General Public License as published by the Free Software
+// Foundation; either version 3, or (at your option) any later
+// version.
+
+// This library is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+// General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+
+// Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL.
+
+// Permission to use, copy, modify, sell, and distribute this software
+// is hereby granted without fee, provided that the above copyright
+// notice appears in all copies, and that both that copyright notice
+// and this permission notice appear in supporting documentation. None
+// of the above authors, nor IBM Haifa Research Laboratories, make any
+// representation about the suitability of this software for any
+// purpose. It is provided "as is" without express or implied
+// warranty.
+
+/**
+ * @file ranged_hash_example.cpp
+ * A basic example showing how to write a ranged-hash functor.
+ */
+
+/**
+ * In some cases it is beneficial to write a hash function which determines
+ * hash values based on the size of the container object.
+ * The example shows an example of a string-hashing function which
+ * uses a fast method for hashing strings when the number of strings
+ * in the container object is small, and a slower but more careful method
+ * for hashing strings when the number of strings in the container object
+ * is large.
+ */
+
+#include <functional>
+#include <cassert>
+#include <string>
+#include <ext/pb_ds/assoc_container.hpp>
+#include <ext/pb_ds/hash_policy.hpp>
+
+using namespace std;
+using namespace __gnu_pbds;
+
+/**
+ * A (somewhat simplistic) ranged-hash function for strings.
+ * It uses the size of the container object to determine
+ * the hashing method. For smaller sizes it uses a simple hash function;
+ * for larger sizes it uses a more complicated hash function.
+ */
+class simple_string_ranged_hash_fn
+ : public unary_function<string, size_t>
+{
+public:
+ typedef size_t size_type;
+
+ simple_string_ranged_hash_fn() : m_container_size(0) { }
+
+ // Called to notify that the size has changed.
+ void
+ notify_resized(size_t size)
+ { m_container_size = size; }
+
+ // Called for hashing a string into a size_t in a given range.
+ size_t
+ operator()(const string& r_string)
+ {
+ /*
+ * This (simplified) hash algorithm decides that if there are
+ * fewer than 100 strings in the container it will hash
+ * a string by summing its characters; otherwise, it will
+ * perform a more complicated operation in order to produce
+ * hash values with fewer collisions.
+ */
+ string::const_iterator it = r_string.begin();
+ size_t hash = 0;
+ if (m_container_size < 100)
+ {
+ // For this size, perform an accumulate type of operation.
+ while (it != r_string.end())
+ hash += static_cast<size_t>(*it++);
+ }
+ else
+ {
+ // For this size, perform a different operation.
+ while (it != r_string.end())
+ {
+ hash += static_cast<size_t>(*it++);
+ hash *= 5;
+ }
+ }
+
+ // The function must, by whatever means, return a size in the
+ // range 0 to m_container_size.
+ return hash % m_container_size;
+ }
+
+ // Swaps content.
+ void
+ swap(simple_string_ranged_hash_fn& other)
+ {
+ std::swap(m_container_size, other.m_container_size);
+ }
+
+private:
+ // Records the size of the container object.
+ size_t m_container_size;
+};
+
+int
+main()
+{
+ // A collision-chaining hash table storing strings.
+ typedef
+ cc_hash_table<
+ string,
+ null_mapped_type,
+ null_hash_fn,
+ equal_to<string>,
+ simple_string_ranged_hash_fn>
+ set_t;
+
+ // Note that in the above, the library determines a resize policy
+ // appropriate for modulo-based range hashing.
+ set_t h;
+
+ // Use the table normally.
+ h.insert("Hello, ");
+ h.insert("world");
+
+ assert(h.size() == 2);
+
+ assert(h.find("Hello, ") != h.end());
+ assert(h.find("world") != h.end());
+
+ assert(h.find("Goodbye, oh cruel world!") == h.end());
+
+ return 0;
+}
+
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