etl/bloom_filter.h

///\file

/******************************************************************************
The MIT License(MIT)

Embedded Template Library.
https://github.com/ETLCPP/etl

Copyright(c) 2014 jwellbelove

Permission is hereby granted, free of charge, to any person obtaining a copy
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******************************************************************************/

#ifndef __ETL_BLOOM_FILTER__
#define __ETL_BLOOM_FILTER__

#include "parameter_type.h"
#include "bitset.h"
#include "type_traits.h"
#include "log.h"
#include "power.h"

///\defgroup bloom_filter bloom_filter
/// A Bloom filter
///\ingroup containers

namespace etl
{
  namespace __private_bloom_filter__
  {
    // Placeholder null hash for defaulted template parameters.
    struct null_hash
    {
      template <typename T>
      size_t operator ()(T)
      {
        return 0;
      }
    };
  }

  //***************************************************************************
  /// An implementation of a bloom filter.
  /// Allows up to three hashes to be defined.
  /// Hashes must support the () operator and define 'argument_type'.
  ///\tparam DESIRED_WIDTH The desired number of hash results that can be stored. Rounded up to best fit the underlying bitset.
  ///\tparam THash1        The first hash generator class.
  ///\tparam THash2        The second hash generator class. If omitted, uses the null hash.
  ///\tparam THash3        The third hash generator class.  If omitted, uses the null hash.
  /// The hash classes must define <b>argument_type</b>.
  ///\ingroup bloom_filter
  //***************************************************************************
  template <const size_t DESIRED_WIDTH,
            typename     THash1,
            typename     THash2 = __private_bloom_filter__::null_hash,
            typename     THash3 = __private_bloom_filter__::null_hash>
  class bloom_filter
  {
  private:

    typedef typename etl::parameter_type<typename THash1::argument_type>::type parameter_t;
    typedef __private_bloom_filter__::null_hash null_hash;

  public:

    enum
    {
      // Make the most efficient use of the bitset.
      WIDTH = etl::bitset<DESIRED_WIDTH>::TOTAL_BITS
    };

    //***************************************************************************
    /// Clears the bloom filter of all entries.
	  //***************************************************************************
	  void clear()
	  {
	    flags.reset();
	  }

    //***************************************************************************
    /// Adds a key to the filter.
    ///\param key The key to add.
    //***************************************************************************
    void add(parameter_t key)
    {
      flags.set(get_hash<THash1>(key));

      if (!etl::is_same<THash2, null_hash>::value)
      {
        flags.set(get_hash<THash2>(key));
      }

      if (!etl::is_same<THash3, null_hash>::value)
      {
        flags.set(get_hash<THash3>(key));
      }
    }

    //***************************************************************************
    /// Tests a key to see if it exists in the filter.
    ///\param  key The key to test.
    ///\return <b>true</b> if the key exists in the filter.
    //***************************************************************************
    bool exists(parameter_t key) const
    {
      bool exists1 = flags[get_hash<THash1>(key)];
      bool exists2 = true;
      bool exists3 = true;

      // Do we have a second hash?
      if (!etl::is_same<THash2, null_hash>::value)
      {
        exists2 = flags[get_hash<THash2>(key)];
      }

      // Do we have a third hash?
      if (!etl::is_same<THash3, null_hash>::value)
      {
        exists2 = flags[get_hash<THash3>(key)];
      }

      return exists1 && exists2 && exists3;
    }

    //***************************************************************************
    /// Returns the width of the Bloom filter.
    //***************************************************************************
    size_t width() const
    {
      return WIDTH;
    }

    //***************************************************************************
    /// Returns the percentage of usage. Range 0 to 100.
    //***************************************************************************
    size_t usage() const
    {
      return (100 * count()) / WIDTH;
    }

    //***************************************************************************
    /// Returns the number of filter flags set.
    //***************************************************************************
    size_t count() const
    {
      return flags.count();
    }

  private:

    //***************************************************************************
    /// Gets the hash for the key.
    ///\param  key The key.
    ///\return The hash value.
    //***************************************************************************
    template <typename THash>
    size_t get_hash(parameter_t key) const
    {
      const size_t mask = etl::power_of_2_round_up<WIDTH>::value - 1;

      size_t hash = THash()(key);

      // Fold the hash down to fit the width.
      size_t folded_hash = 0;

      const size_t shift = etl::log2<etl::power_of_2_round_up<WIDTH>::value>::value;

      // Keep shifting down and XORing the lower bits.
      while (hash >= WIDTH)
      {
        folded_hash ^= hash & mask;
        hash >>= shift;
      }

      // Fold the remaining bits.
      folded_hash ^= hash;

      return folded_hash;
    }

    /// The Bloom filter flags.
    etl::bitset<WIDTH> flags;
  };
}

#endif