bslh.txt

 bslh.txt

@PURPOSE: Provide a framework for hashing types using swappable algorithms.

@MNEMONIC: Basic Standard Library Hashing (bslh)

@DESCRIPTION: The 'bslh' package provides components for a more modular hashing
 implementation than is found in the standard.  This implementation is based on
 ISO C++ Proposal N3980.  An internal proposal for this is available at {TEAM
 BDE:MODULAR HASHING<GO>}.  This package provides hashing algorithms as well as
 'Hash' and 'SeededHash' structs which allow different algorithms to be applied
 to any type that has a 'hashAppend' function.  This document will explain the
 overall benefits of the system, what type implementers need to do to make
 their types hashable in this system, what type users need to do to hash
 different types, how to extend different pieces of this system, and will
 provide a summary of all the components in this package.  All sections are
 independent and can be read and used without having read the other sections.

/Table of Contents
/-----------------
: o Terminology
:   o Avalanche
:   o Denial of Service (DoS)
:   o Funneling
:   o Salient to Hashing
:
: o Why Use This System?
:   o Better and More Predictable Performance
:   o Less Code Duplication
:   o Easier to Make Types Hashable
:   o Hash Combining
:   o Swappable Algorithms
:
: o Type Implementers
:   o 'hashAppend'
:   o Determining what to Hash
:   o Hashing Other User Defined Types
:   o Hashing Pointers (especially 'const char *')
:   o Converting Existing Types
:
: o Type Users
:   o bsl::hash
:   o bslh::Hash
:   o bslh::SeededHash, bslh::SeedGenerator, and Secure Hashing
:     o Seeded Algorithms
:     o bslh::SeedGenerator
:     o bslh::SeededHash
:   o Hashing Performance and Fundamental Integer Types
:   o Choosing a Hashing Algorithm
:
: o Extending the System
:   o Hashing Algorithm Functors
:   o Hashing Algorithm Wrappers (bslh::Hash)
:   o Seed Generator
:
: o Hierarchical Synopsis
:
: o Component Synopsis
:
: o Component Overview
:   o 'bslh_defaulthashalgorithm'
:   o 'bslh_defaultseededhashalgorithm'
:   o 'bslh_hash'
:   o 'bslh_seededhash'
:   o 'bslh_seedgenerator'
:   o 'bslh_siphashalgorithm'
:   o 'bslh_spookyhashalgorithm'
:   o 'bslh_spookyhashalgorithmimp'

/Terminology
/-----------

/Avalanche
/- - - - -
 Changing one bit in the input to the hashing algorithm results in an
 "avalanche", which causes each output bit to have a 50% probability of
 changing.  The avalanche property means that two very similar values will
 produce completely dissimilar hashes.

/Denial of Service (DoS)
/- - - - - - - - - - - -
 Within the context of hash tables, Denial of Service (DoS) attacks refer to an
 attacker causing many hash table keys to collide to the same bucket.  These
 collisions cause look-ups in the hash table to become very time consuming
 linear searches.

/Funneling
/- - - - -
 An undesirable property of hashing algorithms that results in a large number
 of collisions when the inputs differ by only a few bits.  Funneling is related
 to the avalanche property of a hashing algorithm and is essentially the
 opposite of avalanche.  More information can be found at
 'http://burtleburtle.net/bob/hash/evahash.html'.

/Salient to Hashing
/- - - - - - - - -
 A property of attributes (or fields) of a type.  An attribute is considered
 salient to hashing if it should be incorporated into the bytes that are used
 to produce a hash of a given object.  As a general rule is that every
 attribute used in 'operator==' is usually salient to hashing.  This term is
 explored more in depth later.

/Why Use This System?
/--------------------
 There are numerous benefits to both type creators and users in this new
 system:

/Better and More Predictable Performance
/- - - - - - - - - - - - - - - - - - - -
 This modular hashing system allows users to use known good hashing algorithms
 to avoid performance issues.  The graph below depicts 'unordered_map'
 (expected case O(1)) taking longer to find elements that map.  This is a
 real-world benchmark.  Internal users can read more about these tests at {TEAM
 BDE:FLAT MAP<GO>}.  The anomalous behavior arose from the use of a poorly
 written, non-standard, hashing algorithm, which caused similar strings to hash
 to the same value resulting in many collisions:
..
           Time to Find a pair<string, string> in map vs unordered_map

     18 |
        |                      O = map   X = unordered_map
     16 |
  T     |                                                              X
  i  14 |                                                              |
  m     |                                                             /
  e  12 |                                                            /-O
        |                                                           /
  i  10 |                                                        - /
  n     |                                                      -  /
      8 |                                                    O   /
  S     |                                                 --    /
  e   6 |                                               -      /
  c     |                                             --      /
  o   4 |                                       ----O        X
  n     |                               ----O---           --
  d   2 |                      ---O-----              ----
  s     |       XO-------XO-------X--------X--------X
        |____________________________________________________________________
                |        |        |        |        |        |        |
                1        10       100      1000     10000    100000   1000000
                               Elements in Collection
..

/Less Code Duplication
/- - - - - - - - - - -
 In the standard C++03 hashing system, hashing algorithms were often copied
 into the 'bsl::hash' template specialization on each type.  This resulted in
 lots of code duplication.  In the new system, algorithms are not implemented
 directly on the type, so no algorithm duplication occurs.  The little bit of
 code that is written on the type is specific to that type and wouldn't be
 copied elsewhere.

/Easier to Make Types Hashable
/- - - - - - - - - - - - - - -
 In the standard C++03 hashing system, type implementers had to worry about
 writing a hashing algorithm for their type.  This required figuring out what
 constituted a good hash value and how to generate one with a given set of data
 members.  Thinking about what makes a good hash is no longer the job of the
 type implementer.  They simply have to declare what data members they want to
 contribute the hash and then they are done.

/Hash Combining
/ - - - - - - -
 In the standard C++03 hashing system, if a type needed to use more than one
 data member in its hash calculation, there was no proper way for them to
 combine the hashes from multiple data members into one final hash.  Poor
 methods of hash combining such as XORing could result in huge numbers of
 collisions.  The new, modular hashing system offers 'hashAppend' to combine an
 unlimited number of data members into one, good, hash.

/Swappable Algorithms
/- - - - - - - - - -
 In the standard C++03 hashing system, anyone who wanted to hash a type had to
 use the algorithm written by the type implementer, or they had to write their
 own algorithm in a functor (without access to private data members of course).
 Unfortunately, hashing algorithms are not one size fits all.  In some cases, a
 fast identity hash is fine.  In other cases a slower hash that is secure
 against Denial of Service (DoS) attacks is required.  The new, modular hashing
 system offers a suite of hashing algorithms that have been vetted and are know
 to have good characteristics for different situations.  Swapping out one
 algorithm for another only requires a type's users to change one line of code,
 as shown here:
..
  // Uses implicitly 'DefaultHashAlgorithm'.
  bsl::unordered_map<MyType, int, bslh::Hash<>> unorderedMap;

  // Only a single line change is required to use 'SpookyHashAlgorithm'
  bsl::unordered_map<MyType,
                     int,
                     bslh::Hash<bslh::SpookyHashAlgorithm>> unorderedMap;
..

/Type Implementers
/-----------------
 It is the type implementer's job to implement 'hashAppend' (explained below)
 on their type, much like they have to implement 'swap' on their type.  It is
 important to note that implementing 'hashAppend' on 'MyType' is fully backward
 compatible with 'bsl::hash<MyType>'.  That means that when you implement
 'hashAppend' on 'MyNewType', 'bsl::hash<MyNewType>' will automatically pick it
 up without you ever having to write your own template specialization.  For
 existing types that already have a 'bsl::hash<MyExistingType>' specialization,
 it is recommended that type implementers delete 'bsl::hash<MyExistingType>'
 once they have implemented 'hashAppend'.  Deleting the 'bsl::hash' template
 specialization for 'MyExistingType' will not break any code because
 'bsl::hash<TYPE>' automatically redirects to 'bslh::Hash<>'.  Note that
 deleting the 'bsl::hash' template specialization for 'MyExistingType' will
 cause the hash value returned by calls to 'bsl::hash<MyExistingType>' to
 change, but this is explicitly allowed in the 'bsl::hash' contract.

/'hashAppend'
/ - - - - - -
 The fundamental piece of this system, at least for type implementers, is the
 'hashAppend' free function.  What this free function does is pass the data
 members of a class which need to be hashed, into a hashing algorithm.  It
 removes the need for the type implementer to actually write the hashing
 algorithm.  An example implementation can be seen below:
..
  namespace BloombergLP {
  namespace NamespaceForBoxes {

  class Box {
      // A value semantic type that represents a box drawn on to a Cartesian
      // plane.
    private:
      Point d_position;
      int   d_length;
      int   d_width;
    public:
      Box(Point position, int length, int width);
          // Create a box with the specified 'length' and 'width', with its
          // upper left corner at the specified 'position'

      friend bool operator==(const Box &left, const Box &right);

      template <class HASH_ALGORITHM>
      friend
      void hashAppend(HASH_ALGORITHM &hashAlg, const Box &box);
          // Apply the specified 'hashAlg' to the specified 'box'
  };

  Box::Box(Point position, int length, int width)
  : d_position(position)
  , d_length(length)
  , d_width(width)
  {
  }

  bool operator==(const Box &left, const Box &right)
  {
      return (left.d_position == right.d_position)
          && (left.d_length   == right.d_length)
          && (left.d_width    == right.d_width);
  }

  template <class HASH_ALGORITHM>
  void hashAppend(HASH_ALGORITHM &hashAlg, const Box &box)
  {
      using bslh::hashAppend;
      hashAppend(hashAlg, box.d_position);
      hashAppend(hashAlg, box.d_length);
      hashAppend(hashAlg, box.d_width);
  }

  } // close package namespace
  } // close enterprise namespace
..
 A few key features of the 'hashAppend' fuction:

: 1 'hashAppend' is a free function that can be picked up through argument
:   dependent look-up (ADL).
:
:   1 In order to ensure 'hashAppend' can be found through ADL, the function
:     must begin with 'using bslh::hashAppend'.  Note that most of the time
:     this isn't required because most 'HASH_ALGORITHM's will be implemented in
:     'bslh' and thus ADL will already be looking in 'bslh'.  The using
:     statement is still required, however, to support algorithms that are
:     implemented outside of 'blsh'.
:
: 2 'hashAppend' is defined the header file of the type for which it is
:   implemented.
:
: 3 If 'hashAppend' requires access to private data, it is declared as a friend
:   to the type which it is hashing.
:
: 4 'hashAppend' accepts a reference to a templated hashing algorithm functor,
:   and a 'const' reference to the type which it is hashing.
:
: 5 'hashAppend' recursively calls 'hashAppend' on each of the
:   salient-attributes of the type that it is hashing, propagating the hash
:   algorithm functor with each invocation.

 This is the extent of the work for a type implementer.  Once a type
 implementer knows what members need to contribute to a hash value, the type
 implementer simply calls 'hashAppend' on members as shown above.  Those
 members will then be passed into and used by whatever algorithm a consumer of
 the type wants to apply.

/Determining what to Hash
/ - - - - - - - - - - - -
 Type implementers should be familiar with the rules for hash functions.  There
 are two main rules:

: 1 If 'x == y', then both 'x' and 'y' shall produce the same hash.
:
: 2 If 'x != y', then 'x' and 'y' should not produce the same hash.

 For example, if the first rule is violated, 'unordered_map' will encounter
 runtime errors.  If the second rule is violated, collisions will occur and the
 performance of 'unordered_map' will suffer.

 Since the rule about hashing are predicated on 'operator==', the easiest way
 to determine what to hash is to look at 'operator=='.  For example, lets
 reexamine 'operator==' from our earlier code sample:
..
  bool operator==(const Box &left, const Box &right)
  {
      return (left.d_position == right.d_position)
          && (left.d_length   == right.d_length)
          && (left.d_width    == right.d_width);
  }
..
 In order to ensure that the first rule of hashing if followed, we must only
 include data members in our hash if one of the following is true:

: o The data member is used in the 'operator==' comparison.
:
: o The data member is a entirely dependent on data that is used in the
:   'operator==' comparison.
:
: o The data member is constant.

 If we do not meet any of the above criteria, we open ourselves to the
 possibility that two objects that compare equal will hash to different values.
 If two equal objects hash to different values, hash-based data structures
 ('unordered_map' being the primary concern) will break.  And example of
 something not to include in 'hashAppend' would be the 'capacity()' of a
 vector.

 In order to make the second rule of hashing remain true, we should include
 everything that appears in 'operator==' in our hash.  The more data we can put
 into the hashing algorithm, the higher the chances of us getting unique
 outputs.  By following these suggestions, we produced the 'hashAppend'
 function from our earlier code example:
..
  template <class HASH_ALGORITHM>
  void hashAppend(HASH_ALGORITHM &hashAlg, const Box &box)
  {
      using bslh::hashAppend;
      hashAppend(hashAlg, box.d_position);
      hashAppend(hashAlg, box.d_length);
      hashAppend(hashAlg, box.d_width);
  }
..
 It is worth noting that sometimes even data members that don't actually
 contribute entropy can still help us produce a more unique unique outputs.
 For example consider 'vector<vector<int> >'.  If we just hashed the elements
 in the vector, then an empty 'vector<vector<int> >' would generate the same
 hash value as a 'vector<vector<int> >' containing one (or more) empty
 'vector<int>' objects.  This violates the second rule of hashing (above).  By
 also passing 'vector.length()' (which is used in equality) into the algorithm,
 the two vectors will hash to different values.

/Hashing Other User Defined Types
/ - - - - - - - - - - - - - - - -
 As we can see in code sample above showing 'hashAppend', 'hashAppend' is
 called on a data member of the user-defined type, 'Point'.  This code will not
 compile if 'Point' is a type for which 'hashAppend' has not been implemented.
 The best route to take is to have the creator of 'Point' go back and add a
 'hashAppend' function.  If the creator, or somebody knowledgeable about the
 type, is not available to add the 'hashAppend' function, there is a work
 around.  Assuming 'Point' supports the old system and has a 'bsl::hash<Point>'
 specialization, you can just hash the 'Point' and then pass the resulting
 'size_t' into 'hashAppend' just like you would with any other integer data
 member.  The following code sample shows how we can modify 'Box's 'hashAppend'
 function to handle 'Point' without a 'hashAppend' free function for 'Point':
..
  template <class HASH_ALGORITHM>
  void hashAppend(HASH_ALGORITHM &hashAlg, const Box &box)
  {
      using bslh::hashAppend;
      hashAppend(hashAlg, bsl::hash<Point>()(box.d_position));
      hashAppend(hashAlg, box.d_length);
      hashAppend(hashAlg, box.d_width);
  }
..
 In the above code sample, the first call to 'hashAppend' is now effectively
 calling 'hashAppend' on an integer type (the resulting hash from 'bsl::hash'),
 rather than on 'Point'.  This trick allows new development to use this modular
 hashing system without having to wait for existing code to be upgraded.

/Hashing Pointers (especially 'const char *')
/ - - - - - - - - - - - - - - - - - - - - - -
 Pointers, particularly C-Strings (in the 'const char *' form), are an
 unfortunate exception in hashing.  Because the standard mandates that we must
 be able to hash pointers, there is no way to distinguish a null terminated
 'const char *' (C-String) from a regular pointer to a 'char'.  Because of
 this, hashing C-Strings has slightly different semantics.  Instead of calling
 'hashAppend' on the pointer, we must pass our C-String directly into the
 hashing algorithm functor.  All of the hashing algorithm functors in 'bslh'
 have the function signature shown below:
..
  void operator()(const char *data, size_t length);
      // Incorporates the specified 'data' of 'length' bytes into the internal
      // state of the hashing algorithm.
..
 As we can see, the hashing algorithm functor takes a pointer to the start of
 the data and a length in bytes.  To hash a C-String, we call the hashing
 algorithm functor with our pointer and the length of the C-String which we
 have stored or pre-calculated.  An example of this is the 'hashAppend'
 function for string, shown here:
..
  template <class HASHALG,
            class CHAR_TYPE,
            class CHAR_TRAITS,
            class ALLOCATOR>
  void hashAppend(HASHALG&                        hashAlg,
                  const basic_string<CHAR_TYPE,
                                     CHAR_TRAITS,
                                     ALLOCATOR>&  input)
  {
      using bslh::hashAppend;
      hashAlg(input.data(), sizeof(CHAR_TYPE)*input.size());
  }
..
 This technique can be applied to any case where you want to hash contiguous
 data.  It is especially important that the your data is completely contiguous,
 because padding bits and the like could result in the same data hashing to
 different values, which violates the first rule of hashing.

/Converting Existing Types
/- - - - - - - - - - - - -
 The same process as above should be followed when implementing 'hashAppend' on
 user defined types which already specialize 'bsl::hash'.  One extra step that
 is required is the 'bsl::hash' template specialization must be removed once
 'hashAppend' has been implemented.  'unordered_map's will still function after
 the 'bsl::hash' specialization has been removed, since 'bsl::hash<TYPE>'
 automatically calls 'bslh::Hash<>' when no specialization exists.  Note that
 deleting the 'bsl::hash' template specialization will cause the hash value
 returned by 'bsl::hash<YourType>' to change, however, this is explicitly
 allowed by the contract of 'bsl::hash'.

/Type Users
/----------
 The primary use case for 'bslh::Hash' is producing hash values for hash tables
 ('unordered_map's), so we will be focusing on that for the next example, but
 this section does apply generally to any use of the modular hashing system.
 The basic background knowledge required for this section is:

: o 'hashAppend' is a free function, implemented by type implementers, which
:   makes a type hashable.
:
: o 'bslh::Hash<>' is a hashing functor that has the same interface as
:   'bsl::hash<SomeType>', but allows you to supply a hashing algorithm as a
:   template parameter.
:
: o Standard hashing algorithm functors are available in the 'bslh' package.

 Beyond this basic summary, some knowledge of how 'bslh::Hash' and 'bsl::hash'
 interact is required for type users to get the most out of this system.

/'bsl::hash'
/- - - - - -
 Unfortunately, 'bsl::hash' is impossible to completely escape, even with the
 modular hashing system.  The standard mandates that 'unordered_map's with a
 key of 'SomeType' will call 'bsl::hash<SomeType>' by default.  We have done
 our best to make 'bsl::hash' and 'bslh::Hash' interact nicely in most
 scenarios, and the section below shows what behavior can be expected from
 calling 'bsl::hash<SomeType>'.

: o 'bsl::hash' is specialized for 'SomeType' *and* 'hashAppend' is implemented
:   for 'SomeType'.
:
:   o Calling 'bsl::hash<SomeType>' will go to the 'bsl::hash' template
:     specialization.  Note that 'bsl::hash<SomeType>' should normally be
:     deleted once 'hashAppend' has been implemented on 'SomeType'.  If
:     'bsl::hash<SomeType>' has not been deleted, please contact the type's
:     creator and ask them to do so.
:
:   o 'bslh::Hash<AnyImplementedAlgorithm>' can be called directly
:
: o 'bsl::hash' is not specialized for 'SomeType' *and* 'hashAppend' is
:   implemented for 'SomeType'.
:
:   o Calling 'bsl::hash<SomeType>' will automatically redirect to
:     'bslh::Hash<>'.
:
:   o 'bslh::Hash<AnyImplementedAlgorithm>' can be called directly
:
: o 'bsl::hash' is specialized for 'SomeType' *and* 'hashAppend' is not
:   implemented for 'SomeType'.
:
:   o Calling 'bsl::hash<SomeType>' will go to the 'bsl::hash' template
:     specialization.  Please implement 'hashAppend' on 'SomeType'.
:
: o 'bsl::hash' is not specialized for 'SomeType' *and* 'hashAppend' is not
:   implemented for 'SomeType'.
:
:   o Does not compile, please implement 'hashAppend' on 'SomeType'.

 Note that when 'bsl::hash<SomeType>' redirects to 'bslh::Hash<>',
 'bslh::Hash<>' is always using the defualt hashing algorithm.  This is fine
 for most use cases, but if we need to use a special algorithm, such as a
 secure one to prevent Denial of Service (DoS) attacks in a hash table, we must
 directly use 'bslh::Hash' in order to swap out the algorithm template
 parameter.

/'bslh::Hash'
/ - - - - - -
 There are various algorithms that can be swapped into 'bslh::Hash' as template
 parameters.  Algorithms such as SipHash and SpookyHash
 ('bslh::SipHashAlgorithm' and 'bslh::SpookyHashAlgorithm' respectively) are
 implemented and can be swapped into 'bslh::Hash'.  There are also a number of
 wrapper classes such as 'bslh::DefaultHashAlgorithm' and
 'balh::DefaultSeededHashAlgorithm' which are named to allow you to pick them
 based on what you need, meaning you don't need an in depth knowledge of the
 individual hashing algorithms.  The usage of these algorithms can be seen
 below:
..
  // Implicitly uses 'bsl::hash<MyType>', may or may not redirect to
  // 'bslh::Hash<>'.
  bsl::unordered_map<MyType, int> unorderedMap;

  // Implicitly uses 'bslh::DefaultHashAlgorithm', which redirects to the
  // current best default algorithm for hashing.
  bsl::unordered_map<MyType, int, bslh::Hash<>> unorderedMap;

  // Explicitly uses 'bslh::DefaultHashAlgorithm', which redirects to the
  // current best default algorithm for hashing.
  bsl::unordered_map<MyType, int, bslh::Hash<bslh::DefaultHashAlgorithm>>
                                                                  unorderedMap;

  // Explicitly uses 'bslh::SpookyHashAlgorithm', one of the algorithms
  // available in 'bslh'.
  bsl::unordered_map<MyType, int, bslh::Hash<bslh::SpookyHashAlgorithm>>
                                                                  unorderedMap;
..

/'bslh::SeededHash', 'bslh::SeedGenerator', and Secure Hashing
/- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 Some hashing algorithms, such as 'bslh::SipHashAlgorithm', require seeds to
 function.  'bslh::SipHashAlgorithm' was designed by its creators to provide
 protection against hash table DoS attacks.  In order to get the most
 protection, 'bslh::SipHashAlgorithm' requires a cryptographically secure
 random number in order to produce hashes that will be secure against an
 attacker.  'bslh::SeededHash' and 'bslh::SeedGenerator' exist to facilitate
 passing seeds into hashing algorithms.

/Seeded Algorithms
/ -  -  -  -  -  -
 Different algorithms have different seed requirements.  Some require a seed to
 function and others take one optionally.  The table below shows the seed
 requirements of the algorithms in 'bslh':
..
+-----------------------------------+-----------------------------------------+
|        Algorithm                  | Takes Seed? | Requires Seed? | Crypto?* |
+-----------------------------------+-------------+----------------+----------+
|'bslh::DefaultHashAlgorithm'       |      N      |       N        |     N    |
+-----------------------------------+-----------------------------------------+
|'bslh::DefaultSeededHashAlgorithm' |      Y      |       Y        |     N    |
+-----------------------------------+-----------------------------------------+
|'bslh::SipHashAlgorithm'           |      Y      |       Y        |     Y    |
+-----------------------------------+-----------------------------------------+
|'bslh::SpookyHashAlgorithm'        |      Y      |       N        |     N    |
+-----------------------------------+-----------------------------------------+
 [*] "Crypto" is reverting to the requirement on the seed, not the quality of
 the algorithm.  I.e., 'bslh::SipHashAlgorithm' is not a cryptographically
 secure algorithm, but it *is* a cryptographically strong pseudo-random
 function, *if* it's provided a cryptographically secure seed (see
 'bslh_siphashalgorithm' for more information).
..
 Algorithms can require different sized seeds, and different quality of seeds.
 These variances are handled by 'bslh::SeedGenerator'.

/'bslh::SeedGenerator'
/  -  -  -  -  -  -  -
 bslh::SeedGenerator allows users to choose their Random Number Generator (RNG)
 and then handles the actual seed generation for algorithms.  It presents the
 following interface:
..
  template<class RANDOM_NUM_GEN>
  class SeedGenerator : private RANDOM_NUM_GEN {
    private:
      // PRIVATE TYPES
      typedef typename RANDOM_NUM_GEN::result_type result_type;

      // DATA
      enum { k_RNGOUTPUTSIZE = sizeof(typename RANDOM_NUM_GEN::result_type)};

    public:
      // CREATORS
      SeedGenerator();

      explicit SeedGenerator(const RANDOM_NUM_GEN &randomNumberGenerator);

      // MANIPULATORS
      void generateSeed(char *seedLocation, size_t seedLength);
  };
..
 Note that 'bslh::SeedGenerator' takes advantage of the empty base optimization
 when possible.

 'bslh::SeedGenerator' takes a RNG as a template parameter.  The quality of
 this RNG will determine the quality of the seed produced.  That is, if the RNG
 is cryptographically secure, the seed will be as well.  'bslh::SeedGenerator'
 can be either default constructed or constructed with an instance of the
 (template parameter) type 'RANDOM_NUM_GEN'.  Default construction is preferred
 if possible, but the parameterized constructor exists for cases when
 'RANDOM_NUM_GEN' is not default constructable, or when you need to pass in an
 instance of 'RANDOM_NUM_GEN' with a particular state.

 The 'generateSeed' method will be used by 'bslh::SeededHash' to generate seeds
 for any algorithm that takes a seed.

/'bslh::SeededHash'
/-  -  -  -  -  - -
 'bslh::SeededHash' is very similar to 'bslh::Hash'.  Both are wrappers for the
 hashing algorithm functors in 'bslh and both present an interface that meets
 the requirements of the standard for 'std::hash'.  The interface of
 'bslh::SeededHash' can be seen below.
..
  template <class SEED_GENERATOR, class HASH_ALGORITHM =
                                              bslh::DefaultSeededHashAlgorithm>
  struct SeededHash {
    private:
      // DATA
      char seed[HASH_ALGORITHM::k_SEED_LENGTH];

    public:
      // TYPES
      typedef size_t result_type;

      // CREATORS
      SeededHash();

      explicit SeededHash(SEED_GENERATOR& seedGenerator);

      // ACCESSORS
      template <class TYPE>
      result_type operator()(const TYPE& type) const;
  };
..
 Like 'bslh::SeedGenerator', 'bslh::SeededHash' has both default and
 parameterized constructors.  If the (template parameter) type 'SEED_GENERATOR'
 is default constructible, then 'bslh:SeededHash' will be default
 constructible, and it can be used in the exact same way as 'bslh::Hash', as
 shown here:
..
  // Construct an unordered map with a secure hashing algorithm
  bsl::unordered_map<MyType,
                    int,
                    bslh::SeededHash<bslh::SeedGenerator<CryptoRNG>,
                                     bslh::SecureHashAlgorithm> > unorderedMap;
..
 If 'bslh::SeededHash' is not default constructable, or you want to use a
 specific instance of a RNG or seed generator, then 'unordered_map' can no
 longer be default constructed as previously shown.  Instead,
 'bslh::SeededHash' must be passed through the constructor of the unordered
 map, seen here:
..
  // typedefs to make the code smaller
  typedef bslh::SeedGenerator<CryptoRNG> CryptoSeedGen;
  typedef bslh::SeededHash<CryptoSeedGen, bslh::SecureHashAlgorithm>
                                                                  CryptoHasher;

  // Construct the required seed generator and hashing algorithm wrapper from
  // 'someRNGObject'
  CryptoSeedGen seedGenerator(someRNGObject);
  CryptoHasher hashAlg(seedGenerator);

  // Construct our unordered map
  bsl::unordered_map<MyType, int, CryptoHasher> secureUnorderedMap(
                                                                 startIterator,
                                                                 endIterator,
                                                                 hashAlg);
..
 One important difference between 'bslh::Hash' and 'bslh::SeededHash' is that
 'bslh::SeededHash' needs to hold the seed, so it can not benefit from the
 empty base optimization.

/Hashing Performance and Fundamental Integer Types
/- - - - - - - - - - - - - - - - - - - - - - - - -
 Fundamental integer types are a notable exception to the pattern of
 redirecting 'bsl::hash' specializations to 'bslh::Hash'.  Fundamental integer
 types will retain their 'bsl::hash' template specializations that are identity
 functions (they return the supplied integer value its own hash value).  This
 is done for performance reasons, as identity hashing is the fastest possible
 hash.  Please note that this is not a good hashing algorithm and should only
 be used in cases where performance is critical and the data is predicable
 enough that we know minimal numbers of bucket collisions will occur.

/Choosing a Hashing Algorithm
/ - - - - - - - - - - - - - -
 For most purposes, the default supplied hashing algorithm will be best.  It
 has a good combination of speed and key distribution.  In cases where user
 input is directly included in the 'unordered_map', it is recommended to use a
 secure hashing algorithm instead, to prevent Denial of Service (DoS) attacks
 where an attacker causes all of the keys to collide to the same bucket.  Make
 sure to read the component level documentation when looking for an algorithm,
 to be sure that a hashing algorithm has the right trade offs for your use
 case.

/Extending the System
/--------------------
 Every piece of the modular hashing system can be extended and swapped out in
 favor of user defined pieces.  This section defines how to extend the various
 pieces, and the canonical interfaces that must be adhered to.  Both type
 implementers and users may have need of this section.

/Hashing Algorithm Functors
/ - - - - - - - - - - - - -
 Users are free write their own hashing algorithms and make them available via
 functors.  In order to plug into 'bslh::Hash' the algorithms must implement
 the following interface:
..
  class SomeHashAlgorithm
  {
    public:
      // TYPES
      typedef uint64 result_type;

      // CREATORS
      SomeHashAlgorithm();

      // MANIPULATORS
      void operator()(const char * key, size_t len);

      result_type computeHash();
  };
..
 The 'result_type' 'typedef' must define the return type of this particular
 algorithm.  A default constructor (either implicit or explicit) must be
 supplied that creates an algorithm functor that is in a usable state.  An
 'operator()' must be supplied that takes a 'const char' pointer to the data to
 be hashed and a 'size_t' length of bytes to be hashed.  This operator must
 operate on all data uniformly, meaning that regardless of whether data is
 passed in all at once, or one byte at a time, the result returned by
 'computeHash()' will be the same.  'computeHash()' will return the final
 result of the hashing algorithm, in the form of a 'result_type'.
 'computeHash()' is allowed to modify the internal state of the algorithm,
 meaning calling 'computeHash()' more than once might not return the correct
 value.

 Hashing algorithm functors containing algorithms that require seeds must
 implement the interface shown above, with the exception of the default
 constructor.  Seeded algorithm functors must also implement the following
 interface:
..
  class SomeHashAlgorithm
  {
    public:
      // CONSTANTS
      enum { k_SEED_LENGTH = XXX };

      // CREATORS
      explicit SomeHashAlgorithm(const char *seed);
  };
..
 The 'k_SEED_LENGTH' enum must be in the public interface, and 'XXX' must be
 replaced with an integer literal indicating the number of bytes of seed the
 algorithm requires.  The parameterized constructor must accept a 'const char'
 pointer.  This pointer will point to a seed of 'XXX' bytes in size.

/Hashing Algorithm Wrappers ('bslh::Hash')
/- - - - - - - - - - - - - - - - - - - - -
 Users are free to write their own versions of 'bslh::Hash<>' for whatever use
 case they require (in fact, 'bslh::SeededHash<>' is one such example).
 Because no other parts of the modular hashing system rely on 'bslh::Hash<>',
 you are free to use whatever interface is necessary.  The recommended
 interface for maintaining compatibility with components that previously used
 'bsl::hash<>' can be seen here:
..
  template <class HASH_ALGORITHM>
  struct YourHash
  {
      // TYPES
      typedef size_t result_type;

      // ACCESSORS
      template <class TYPE>
      result_type operator()(TYPE const& type) const;
  };
..
 The hashing algorithm wrapper that you create to replace 'blsh::Hash<>' should
 be templated to operate using various 'HASH_ALGORITHM's.  Whether or not there
 is a default option for the template is optional.  The 'result_type' should
 define the type of the hash value that you will return.  The 'operator()'
 should be templated to operate on any 'TYPE'.  'operator()' should take a
 single 'const' reference to 'TYPE' and should return a 'result_type'.  Given
 two 'TYPEs' that compare equal with 'operator==', 'operator()' *must* return
 the same hash.

/Seed Generator
/ - - - - - - -
 Users are free to write their own seed generator, a class of component
 required by 'bslh::SeededHash'.  The seed generator must conform to the
 interface shown here:
..
  class YourSeedGenerator
  {
      // ACCESSORS
      void generateSeed(char *seedLocation, size_t seedLength);
  };
..
 The only mandatory piece of the seed generator interface is the 'generateSeed'
 method which accepts a 'char' pointer to memory to be written and a 'size_t'
 length in bytes.  The generateSeed method must fill the size_t bytes of the
 memory pointed to by the 'char' pointer with a seed.  If possible, it is
 better for the seed generator to be default constructible, however, any sort
 of constructor is acceptable because the seed generator can be constructed and
 passed directly into 'bslh::SeededHash' if required.  Be aware that having a
 non-default constructor makes it more difficult to use the seed generator (see
 the 'bslh::SeededHash' section above to see the difference).

/Hierarchical Synopsis
/---------------------
 The 'bslh' package currently has 14 components having 4 levels of physical
 dependency.  The list below shows the hierarchical ordering of the components.
 The order of components within each level is not architecturally significant,
 just alphabetical.
..
  4. bslh_hashoptional
     bslh_hashpair
     bslh_hashtuple
     bslh_hashvariant
     bslh_seededhash

  3. bslh_hash

  2. bslh_defaulthashalgorithm
     bslh_defaultseededhashalgorithm
     bslh_spookyhashalgorithm

  1. bslh_fibonaccibadhashwrapper
     bslh_seedgenerator
     bslh_siphashalgorithm
     bslh_spookyhashalgorithmimp
     bslh_wyhashalgorithm
..

/Component Synopsis
/------------------
: 'bslh_defaulthashalgorithm':
:      Provide a reasonable hashing algorithm for default use.
:
: 'bslh_defaultseededhashalgorithm':
:      Provide a reasonable seeded hashing algorithm for default use.
:
: 'bslh_fibonaccibadhashwrapper':
:      Provide a wrapper to improve "bad" hash algorithms.
:
: 'bslh_hash':
:      Provide a struct to run 'bslh' hash algorithms on supported types.
:
: 'bslh_hashoptional':
:      Provide 'hashAppend' for 'std::optional'.
:
: 'bslh_hashpair':
:      Provide 'hashAppend' for 'std::pair'.
:
: 'bslh_hashtuple':
:      Provide 'hashAppend' for 'std::tuple'.
:
: 'bslh_hashvariant':
:      Provide 'hashAppend' for 'std::variant'.
:
: 'bslh_seededhash':
:      Provide a struct to run seeded 'bslh' hash algorithms on types.
:
: 'bslh_seedgenerator':
:      Provide a class to generate arbitrary length seeds for algorithms.
:
: 'bslh_siphashalgorithm':
:      Provide an implementation of the SipHash algorithm.
:
: 'bslh_spookyhashalgorithm':
:      Provide an implementation of the SpookyHash algorithm.
:
: 'bslh_spookyhashalgorithmimp':
:      Provide BDE style encapsulation of 3rd party SpookyHash code.
:
: 'bslh_wyhashalgorithm':
:      Provide an implementation of the WyHash algorithm final v3.

/Component Overview
/------------------
 This section provides a brief introduction to each of the components in the
 'bslh' package.  Full details are available in the documentation of each
 component.

/'bslh_defaulthashalgorithm'
/- - - - - - - - - - - - - -
 The 'bslh_defaulthashalgorithm' component provides an unspecified default
 hashing algorithm.  The supplied algorithm is suitable for general purpose use
 in a hash table.  The underlying algorithm is subject to change in future
 releases.

 This class satisfies the requirements for regular 'bslh' hashing algorithms,
 as defined in 'bslh_hash'.

/'bslh_defaultseededhashalgorithm'
/- - - - - - - - - - - - - - - - -
 The 'bslh_defaultseededhashalgorithm' component provides an unspecified
 default seeded hashing algorithm.  The supplied algorithm is suitable for
 general purpose use in a hash table.  The underlying algorithm is subject to
 change in future releases.

 This class satisfies the requirements for seeded 'bslh' hashing algorithms, as
 defined in 'bslh_seededhash'.

/'bslh_hash'
/- - - - - -
 The {'bslh_hash'} component provides a templated 'struct', 'bslh::Hash', which
 provides hashing functionality.  This struct is a drop in replacement for
 'bsl::hash'.  'bslh::Hash' is a wrapper that adapts hashing algorithms from
 'bslh' and 'hashAppend' free functions to match the interface of 'bsl::hash'.
 This component also contains 'hashAppend' definitions for fundamental types,
 which are required to make the hashing algorithms in 'bslh' work.

/'bslh_seededhash'
/- - - - - - - - -
 The {'bslh_seededhash'} component provides a templated struct,
 'bslh::SeededHash', which provides hashing functionality.  This 'struct' is a
 drop in replacement for 'bsl::hash'.  It is similar to 'bslh::Hash', however,
 it is meant for hashes that require a seed.  It takes a seed generator and
 uses that to create seeds to give the hashing algorithm.  'bslh::SeededHash'
 is a wrapper which adapts hashing algorithms from 'bslh' to match the
 interface of 'bsl::hash'.  'bslh::SeededHash' is a universal hashing functor
 that will hash any type that implements 'hashAppend' using the hashing
 algorithm provided as a template parameter.

/'bslh_seedgenerator'
/ - - - - - - - - - -
 The {'bslh_seedgenerator'} component provides a class, 'bslh::SeedGenerator',
 which utilizes a user-supplied random number generator (RNG) to generate
 arbitrary length seeds.  The quality of the seeds will only be as good as the
 quality of the supplied RNG.  A cryptographically secure RNG must be supplied
 in order for 'SeedGenerator' to produce seeds suitable for a cryptographically
 secure algorithm.

 This class satisfies the requirements for a seed generator, as defined in
 'bslh_seededhash'.

/'bslh_siphashalgorithm'
/- - - - - - - - - - - -
 The 'bslh_siphashalgorithm' component provides an implementation of the
 SipHash algorithm.  SipHash is an algorithm designed for speed and security.
 A primary use case for this algorithm is to provide an extra line of defense
 in hash tables (such as the underlying implementation of 'unordered_map')
 against malicious input that could cause Denial of Service (DoS) attacks.  It
 is based on one of the finalists for the SHA-3 cryptographic hash standard.
 Full details of the hash function can be found here:
 '{https://131002.net/siphash/siphash.pdf}'.  This particular implementation
 has been derived from 'siphash.h' in Howard Hinnant's work here:
 '{https://github.com/HowardHinnant/hash_append}' and as much of the original
 code as possible, including comment headers, has been preserved.

 This class satisfies the requirements for seeded 'bslh' hashing algorithms, as
 defined in 'bslh_seededhash'.

/'bslh_spookyhashalgorithm'
/ - - - - - - - - - - - - -
 The 'bslh_spookyhashalgorithm' component provides an implementation of the
 SpookyHash algorithm by Bob Jenkins.  This algorithm is a general purpose
 algorithm that is known to quickly reach good avalanche performance and
 execute in time that is comparable to or faster than other industry standard
 algorithms such as CityHash.  It is a good default choice for hashing values
 in unordered associative containers.  For more information, see
 'http://burtleburtle.net/bob/hash/spooky.html'.

 This class satisfies the requirements for regular 'bslh' hashing algorithms
 and seeded 'bslh' hashing algorithms, as defined in 'bslh_hash' and
 'bslh_seededhash' respectively.

/'bslh_spookyhashalgorithmimp'
/- - - - - - - - - - - - - - -
 The 'bslh_spookyhashalgorithmimp' component provides BDE-style encapsulation
 of Bob Jenkins canonical SpookyHash implementation.  SpookyHash provides a way
 to hash contiguous data all at once, or non-contiguous data in pieces.  More
 information is available at 'http://burtleburtle.net/bob/hash/spooky.html'.