C++ implementation of a PerfectHash algorithm as described in: http://www.drdobbs.com/architecture-and-design/generating-perfect-hash-functions/184404506

perfecthash.h

/*
 * Perfect Hash implementation as described in:
 * http://www.drdobbs.com/architecture-and-design/generating-perfect-hash-functions/184404506
 * 
 * Copyright 2015-2020 Dinesh Manajipet <saidinesh5@gmail.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * 
 * * Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * * Redistributions in binary form must reproduce the above
 *   copyright notice, this list of conditions and the following disclaimer
 *   in the documentation and/or other materials provided with the
 *   distribution.
 * * Neither the name of the  nor the names of its
 *   contributors may be used to endorse or promote products derived from
 *   this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 * 
 */

#include <iostream>
#include <initializer_list>
#include <utility>
#include <cmath>
#include <algorithm>

using namespace std;

//Your Key must support std::div() and operator==
template <typename K, typename V,
           const K invalid_key, const V invalid_value,
           const int n> // maximum allowed number of buckets
class PerfectHash {
  public:
    PerfectHash( initializer_list< pair<K,V> > data )
    {
      if(!initializeData(data))
      {
        m_count = 0;
        m_rows = 0;
      }
    }
    
    PerfectHash( const vector< pair <K,V> >& data )
    {
	  if(!initializeData(data))
      {
        m_count = 0;
        m_rows = 0;
      }
    }

    // How many (key,value) pairs could this list have taken
    inline int capacity() const { return m_data.size(); } 

    // How many (key, value) pairs are present in this list
    inline int count() const { return m_count; }

    // Is there a (key,value) pair with this key?
    inline bool contains(K key) const
    {
      const auto d = std::div(key , m_rows);
      if(d.quot >= (int)m_rows)
        return false;

      const int offset = m_rowOffsets[d.quot];
      if(offset < 0)
        return false;

      const unsigned int c = offset + d.rem;
      return c < m_data.size() && m_data[ c ].first == key;
    }

    // Return the value associated with this key
    // If nothing found, this returns a default value
    inline V value(K key) const
    {
      const auto d = std::div(key, m_rows);
      if(d.quot >= (int)m_rows)
        return invalid_value;

      const int offset = m_rowOffsets[d.quot];
      if(offset < 0)
        return invalid_value;

      const unsigned int c = offset + d.rem;

      if(c < m_data.size() && m_data[c].first == key)
        return m_data[c].second;
	  return invalid_value;
    }
    
    //Overloading value(K key)
    inline V operator[](K key) const { return value(key); }

  private:  
    typedef vector< vector<K> > ScratchPad;
    struct RowInfo { int index, count; };

    bool initializeData(const vector< pair<K,V> >& rawData)
    {
      K kMax = max_element(rawData.begin(), rawData.end())->first;
      m_rows = sqrt(kMax) + 1;
	  
      //The 2D array for inserting the keys into
      ScratchPad scratchpad(m_rows, vector<K>(m_rows, invalid_key) );
      vector<RowInfo> rowInfo;

      //No row is shifted at all
      for(unsigned int r = 0; r < m_rows; r++)
      {
        m_rowOffsets.push_back(-1);
        rowInfo.push_back({r,0}); // Assume that each row has 0 keys
      }

      //Insert the keys into the Key Array
      for(const auto element : rawData)
      {
        K k = element.first;
        int r = k/m_rows;
        int c = k%m_rows;
        if(scratchpad[r][c] == invalid_key)
        {
          scratchpad[r][c] = k;
          rowInfo[r].count++;
        }
      }

      // Process the keys and populate the hashmap
      return firstFitDescending(rawData, scratchpad, rowInfo);
    }

    bool firstFitDescending( const vector< pair<K,V> >& rawData,
                              ScratchPad& scratchPad,
                              vector<RowInfo> &rowInfo )
    {
      // A temporary large array to hold all the keys being hashed
      vector<K> keys = vector<K>(n, invalid_key);

      // 0. Sort the Rows based on the number of elements they contain
      sort(rowInfo.begin(), rowInfo.end(),
          [](const RowInfo& A, const RowInfo& B){ return A.count > B.count; });

      // For each non-empty row:
      for(int r = 0; rowInfo[r].count > 0; r++)
      {
        unsigned int row = rowInfo[r].index;
        // 1. Shift the row right until none of its items collide with
        // any of the items in previous rows.
        for(unsigned int offset = 0; offset < n - m_rows - 1; offset++)
        {
          unsigned int y = 0;

          for(y = 0; y < m_rows; y++)
          {
            if( (keys[offset + y] != invalid_key) && (scratchPad[row][y] != invalid_key) )
              break;
          }

          if( y == m_rows )
          {
            // 2. Record the shift amount in m_rowOffsets
            //cout << row << "--->" << offset << endl;
            m_rowOffsets[row] = offset;

            // 3. Copy the keys from this row into the keys already hashed
            for(unsigned int x = 0; x < m_rows; x++)
            {
              if(scratchPad[row][x] != invalid_key)
              {
                keys[offset + x] = scratchPad[row][x];
                //cout << "Keys[" << offset + x << "] = " << keys[offset+x] <<endl;
              }
			}

            //Move on to the next row
            break;
          }

          if(offset == n - m_rows - 1)
          {
            cout << "Error::Failed to fit the Row " << row << " in hash table." << endl;
            return false;
          }
        }
      }
	  
	  // Now copy the elements of rawData into their appropriate buckets
      int capacity = 0;
      for(int i = 0; i < n; i++)
        if(keys[i] != invalid_key) capacity = i + 1;


      m_data = vector< pair<K,V> >(capacity, { invalid_key, invalid_value} );
      m_count = 0;

      for(unsigned int i = 0; i < m_data.size(); i++)
      {
        const auto currentKey = keys[i];
        //Bother only if currentKey is a valid key
        if( currentKey == invalid_key) continue;
        
        for(const auto element : rawData)
        {
          if(element.first == currentKey)
          {
            //cout << "Bucket " << offset + x << " :: " << element.first << "," << element.second << endl;
            m_data[i] = element;
            break;
          }
        }
        //cout << i << "::" << m_data[i].first << "," << m_data[i].second << endl;
        m_count++;
      }

      return true;
    }

    unsigned int m_rows; // Number of rows in the scratchPad
    vector<int> m_rowOffsets; // Offset of each row in m_data

    unsigned int m_count; // Total number of keys hashed into m_data
    vector< pair<K,V> > m_data; // Actual data
};

int main(int argc, char* argv[])
{
  vector < pair<int, char> > items;
  for(int i = 0; i<26; i++)
  {
    items.push_back( {i+'A',i+'a'} );
    items.push_back( {i+'a',i+'A'} );
  }
  
  PerfectHash<int, char, -1, -1, 1000> hash{ items };
  
  cout << "Hash capacity:" << hash.capacity() << endl;
  cout << "Hash count:" << hash.count() << endl;
  for(int i = 0 ; i < 125 ; i++ )
  {
    if(hash.contains(i)) cout << "Hash[" << (char)i << "] = " << (char)(hash[i])  << endl;
    else cout << "Hash[" << i << "] doesn't exist" << endl;
  }
  return 0;
}