dyn_hash_tbl.h

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/* Copyright (c) 2011 Wildfire Games
 *
 * 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 THE SOFTWARE.
 */

/*
 * dynamic (grow-able) hash table
 */

#ifndef INCLUDED_ADTS_DYN_HASH_TBL
#define INCLUDED_ADTS_DYN_HASH_TBL

#include <string.h> // strcmp

#include "lib/fnv_hash.h"

template<typename Key, typename T> class DHT_Traits
{
public:
    static const size_t initial_entries = 16;
    size_t hash(Key key) const;
    bool equal(Key k1, Key k2) const;
    Key get_key(T t) const;
};

template<> class DHT_Traits<const char*, const char*>
{
public:
    static const size_t initial_entries = 512;
    size_t hash(const char* key) const
    {
        return (size_t)fnv_lc_hash(key);
    }
    bool equal(const char* k1, const char* k2) const
    {
        return !strcmp(k1, k2);
    }
    const char* get_key(const char* t) const
    {
        return t;
    }
};


// intended for pointer types
template<typename Key, typename T, typename Traits=DHT_Traits<Key,T> >
class DynHashTbl
{
    T* tbl;
    u16 num_entries;
    u16 max_entries;    // when initialized, = 2**n for faster modulo
    Traits tr;

    T& get_slot(Key key) const
    {
        size_t hash = tr.hash(key);
        ENSURE(max_entries != 0);   // otherwise, mask will be incorrect
        const size_t mask = max_entries-1;
        for(;;)
        {
            T& t = tbl[hash & mask];
            // empty slot encountered => not found
            if(!t)
                return t;
            // keys are actually equal => found it
            if(tr.equal(key, tr.get_key(t)))
                return t;
            // keep going (linear probing)
            hash++;
        }
    }

    void expand_tbl()
    {
        // alloc a new table (but don't assign it to <tbl> unless successful)
        T* old_tbl = tbl;
        tbl = (T*)calloc(max_entries*2, sizeof(T));
        if(!tbl)
        {
            tbl = old_tbl;
            throw std::bad_alloc();
        }

        max_entries *= 2;
        // must be set before get_slot

        // newly initialized, nothing to copy - done
        if(!old_tbl)
            return;

        // re-hash from old table into the new one
        for(size_t i = 0; i < max_entries/2u; i++)
        {
            T t = old_tbl[i];
            if(t)
                get_slot(tr.get_key(t)) = t;
        }
        free(old_tbl);
    }


public:

    DynHashTbl()
    {
        tbl = 0;
        clear();
    }

    ~DynHashTbl()
    {
        free(tbl);
    }

    void clear()
    {
        // must remain usable after calling clear, so shrink the table to
        // its initial size but don't deallocate it completely
        SAFE_FREE(tbl);
        num_entries = 0;
        // rationale: must not set to 0 because expand_tbl only doubles the size.
        // don't keep the previous size when clearing because it may have become
        // huge and there is no provision for shrinking.
        max_entries = tr.initial_entries/2; // will be doubled in expand_tbl
        expand_tbl();
    }

    void insert(const Key key, const T t)
    {
        // more than 75% full - increase table size.
        // do so before determining slot; this will invalidate previous pnodes.
        if(num_entries*4 >= max_entries*3)
            expand_tbl();

        T& slot = get_slot(key);
        ENSURE(slot == 0);  // not already present
        slot = t;
        num_entries++;
    }

    T find(Key key) const
    {
        return get_slot(key);
    }

    size_t size() const
    {
        return num_entries;
    }


    class iterator
    {
    public:
        typedef std::forward_iterator_tag iterator_category;
        typedef T value_type;
        typedef ptrdiff_t difference_type;
        typedef const T* pointer;
        typedef const T& reference;

        iterator()
        {
        }
        iterator(T* pos_, T* end_) : pos(pos_), end(end_)
        {
        }
        T& operator*() const
        {
            return *pos;
        }
        iterator& operator++()  // pre
        {
            do
            pos++;
            while(pos != end && *pos == 0);
            return (*this);
        }
        bool operator==(const iterator& rhs) const
        {
            return pos == rhs.pos;
        }
        bool operator<(const iterator& rhs) const
        {
            return (pos < rhs.pos);
        }

        // derived
        const T* operator->() const
        {
            return &**this;
        }
        bool operator!=(const iterator& rhs) const
        {
            return !(*this == rhs);
        }
        iterator operator++(int)    // post
        {
            iterator tmp =  *this; ++*this; return tmp;
        }

    protected:
        T* pos;
        T* end;
        // only used when incrementing (avoid going beyond end of table)
    };

    iterator begin() const
    {
        T* pos = tbl;
        while(pos != tbl+max_entries && *pos == 0)
            pos++;
        return iterator(pos, tbl+max_entries);
    }
    iterator end() const
    {
        return iterator(tbl+max_entries, 0);
    }
};


#endif  // #ifndef INCLUDED_ADTS_DYN_HASH_TBL