bslh_spookyhashalgorithm.h

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/// @file bslh_spookyhashalgorithm.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslh_spookyhashalgorithm.h                                         -*-C++-*-
#ifndef INCLUDED_BSLH_SPOOKYHASHALGORITHM
#define INCLUDED_BSLH_SPOOKYHASHALGORITHM
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslh_spookyhashalgorithm bslh_spookyhashalgorithm
/// @brief  Provide an implementation of the SpookyHash algorithm.
/// @addtogroup bsl
/// @{
/// @addtogroup bslh
/// @{
/// @addtogroup bslh_spookyhashalgorithm
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslh_spookyhashalgorithm-purpose"> Purpose</a>
/// * <a href="#bslh_spookyhashalgorithm-classes"> Classes </a>
/// * <a href="#bslh_spookyhashalgorithm-description"> Description </a>
///   * <a href="#bslh_spookyhashalgorithm-security"> Security </a>
///   * <a href="#bslh_spookyhashalgorithm-speed"> Speed </a>
///   * <a href="#bslh_spookyhashalgorithm-hash-distribution"> Hash Distribution </a>
///   * <a href="#bslh_spookyhashalgorithm-hash-consistency"> Hash Consistency </a>
///   * <a href="#bslh_spookyhashalgorithm-subdivision-invariance"> Subdivision-Invariance </a>
///   * <a href="#bslh_spookyhashalgorithm-usage"> Usage </a>
///     * <a href="#bslh_spookyhashalgorithm-example-creating-and-using-a-hash-table"> Example: Creating and Using a Hash Table </a>
///
/// # Purpose {#bslh_spookyhashalgorithm-purpose}
/// Provide an implementation of the SpookyHash algorithm.
///
/// # Classes {#bslh_spookyhashalgorithm-classes}
///
/// -  bslh::SpookyHashAlgorithm: functor implementing the SpookyHash algorithm
///
/// @see  bslh_hash, bslh_spookyhashalgorithmimp
///
/// # Description {#bslh_spookyhashalgorithm-description}
/// `bslh::SpookyHashAlgorithm` implements 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, defined in `bslh_hash.h` and
/// `bslh_seededhash.h` respectively.  More information can be found in the
/// package level documentation for `bslh` (internal users can also find
/// information here {TEAM BDE:USING MODULAR HASHING<GO>})
///
/// ## Security {#bslh_spookyhashalgorithm-security}
///
///
/// In this context "security" refers to the ability of the algorithm to produce
/// hashes that are not predictable by an attacker.  Security is a concern when
/// an attacker may be able to provide malicious input into a hash table,
/// thereby causing hashes to collide to buckets, which degrades performance.
/// There are *no* security guarantees made by `bslh::SpookyHashAlgorithm`,
/// meaning attackers may be able to engineer keys that will cause a Denial of
/// Service (DoS) attack in hash tables using this algorithm.  Note that even if
/// an attacker does not know the seed used to initialize this algorithm, they
/// may still be able to produce keys that will cause a DoS attack in hash
/// tables using this algorithm.  If security is required, an algorithm that
/// documents better secure properties should be used, such as
/// `bslh::SipHashAlgorithm`.
///
/// ## Speed {#bslh_spookyhashalgorithm-speed}
///
///
/// This algorithm will compute a hash on the order of O(n) where `n` is the
/// length of the input data.  Note that this algorithm will produce hashes fast
/// enough to be used to hash keys in a hash table.  It is quicker than
/// specialized algorithms such as SipHash, but not as fast as hashing using the
/// identity function.
///
/// ## Hash Distribution {#bslh_spookyhashalgorithm-hash-distribution}
///
///
/// Output hashes will be well distributed and will avalanche, which means
/// changing one bit of the input will change approximately 50% of the output
/// bits.  This will prevent similar values from funneling to the same hash or
/// bucket.
///
/// ## Hash Consistency {#bslh_spookyhashalgorithm-hash-consistency}
///
///
/// This hash algorithm is endian-specific.  It is designed for little-endian
/// machines, however, it will run on big-endian machines.  On big-endian
/// machines, the Performance and Security Guarantees still apply, however the
/// hashes produced will be different from those produced by the canonical
/// implementation.  The creator of this algorithm acknowledges this and says
/// that the big-endian hashes are just as good as the little-endian ones.  It
/// is not recommended to send hashes from `bslh::SpookyHashAlgorihtm` over a
/// network because of the differences in hashes across architectures.  It is
/// also not recommended to write hashes from `bslh::SpookyHashAlgorihtm` to any
/// memory accessible by multiple machines.
///
/// ## Subdivision-Invariance {#bslh_spookyhashalgorithm-subdivision-invariance}
///
///
/// Note that this algorithm is *subdivision-invariant* (see
/// {@ref bslh_hash |Subdivision-Invariance}).
///
/// ## Usage {#bslh_spookyhashalgorithm-usage}
///
///
/// This section illustrates intended usage of this component.
///
/// ### Example: Creating and Using a Hash Table {#bslh_spookyhashalgorithm-example-creating-and-using-a-hash-table}
///
///
/// Suppose we have any array of types that define `operator==`, and we want a
/// fast way to find out if values are contained in the array.  We can create a
/// `HashTable` data structure that is capable of looking up values in O(1)
/// time.
///
/// Further suppose that we will be storing futures (the financial instruments)
/// in this table.  Since futures have standardized names, we don't have to
/// worry about any malicious values causing collisions.  We will want to use a
/// general purpose hashing algorithm with a good hash distribution and good
/// speed.  This algorithm will need to be in the form of a hash functor -- an
/// object that will take objects stored in our array as input, and yield a
/// 64-bit int value.  The functor can pass the attributes of the `TYPE` that
/// are salient to hashing into the hashing algorithm, and then return the hash
/// that is produced.
///
/// We can use the result of the hash function to index into our array of
/// `buckets`.  Each `bucket` is simply a pointer to a value in our original
/// array of `TYPE` objects.
///
/// First, we define our `HashTable` template class, with the two type
/// parameters: `TYPE` (the type being referenced) and `HASHER` (a functor that
/// produces the hash).
/// @code
/// template <class TYPE, class HASHER>
/// class HashTable {
///     // This class template implements a hash table providing fast lookup of
///     // an external, non-owned, array of values of (template parameter)
///     // 'TYPE'.
///     //
///     // The (template parameter) 'TYPE' shall have a transitive, symmetric
///     // 'operator==' function.  There is no requirement that it have any
///     // kind of creator defined.
///     //
///     // The 'HASHER' template parameter type must be a functor with a method
///     // having the following signature:
///     //..
///     //  size_t operator()(TYPE)  const;
///     //                   -OR-
///     //  size_t operator()(const TYPE&) const;
///     //..
///     // and 'HASHER' shall have a publicly accessible default constructor
///     // and destructor.
///     //
///     // Note that this hash table has numerous simplifications because we
///     // know the size of the array and never have to resize the table.
///
///     // DATA
///     const TYPE       *d_values;             // Array of values table is to
///                                             // hold
///     size_t            d_numValues;          // Length of 'd_values'.
///     const TYPE      **d_bucketArray;        // Contains ptrs into
///                                             // 'd_values'
///     size_t            d_bucketArrayMask;    // Will always be '2^N - 1'.
///     HASHER            d_hasher;
///
///   private:
///     // PRIVATE ACCESSORS
///     bool lookup(size_t      *idx,
///                 const TYPE&  value,
///                 size_t       hashValue) const;
///         // Look up the specified 'value', having the specified 'hashValue',
///         // and load its index in 'd_bucketArray' into the specified 'idx'.
///         // If not found, return the vacant entry in 'd_bucketArray' where
///         // it should be inserted.  Return 'true' if 'value' is found and
///         // 'false' otherwise.
///
///   public:
///     // CREATORS
///     HashTable(const TYPE *valuesArray,
///               size_t      numValues);
///         // Create a hash table referring to the specified 'valuesArray'
///         // having length of the specified 'numValues'.  No value in
///         // 'valuesArray' shall have the same value as any of the other
///         // values in 'valuesArray'
///
///     ~HashTable();
///         // Free up memory used by this hash table.
///
///     // ACCESSORS
///     bool contains(const TYPE& value) const;
///         // Return true if the specified 'value' is found in the table and
///         // false otherwise.
/// };
/// @endcode
/// Then, we define a `Future` class, which holds a c-string `name`, char
/// `callMonth`, and short `callYear`.
/// @code
/// class Future {
///     // This class identifies a future contract.  It tracks the name, call
///     // month and year of the contract it represents, and allows equality
///     // comparison.
///
///     // DATA
///     const char *d_name;    // held, not owned
///     const char  d_callMonth;
///     const short d_callYear;
///
///   public:
///     // CREATORS
///     Future(const char *name, const char callMonth, const short callYear)
///     : d_name(name), d_callMonth(callMonth), d_callYear(callYear)
///         // Create a 'Future' object out of the specified 'name',
///         // 'callMonth', and 'callYear'.
///     {}
///
///     Future() : d_name(""), d_callMonth('\0'), d_callYear(0)
///         // Create a 'Future' with default values.
///     {}
///
///     // ACCESSORS
///     const char * getMonth() const
///         // Return the month that this future expires.
///     {
///         return &d_callMonth;
///     }
///
///     const char * getName() const
///         // Return the name of this future
///     {
///         return d_name;
///     }
///
///     const short * getYear() const
///         // Return the year that this future expires
///     {
///         return &d_callYear;
///     }
///
///     bool operator==(const Future& other) const
///         // Compare this to the specified 'other' object and return true if
///         // they are equal
///     {
///         return (!strcmp(d_name, other.d_name))  &&
///            d_callMonth == other.d_callMonth &&
///            d_callYear  == other.d_callYear;
///     }
/// };
///
/// bool operator!=(const Future& lhs, const Future& rhs)
///     // Compare compare the specified 'lhs' and 'rhs' objects and return
///     // true if they are not equal
/// {
///     return !(lhs == rhs);
/// }
/// @endcode
/// Next, we need a hash functor for `Future`.  We are going to use the
/// `SpookyHashAlgorithm` because it is a fast, general purpose hashing
/// algorithm that will provide an easy way to combine the attributes of
/// `Future` objects that are salient to hashing into one reasonable hash that
/// will distribute the items evenly throughout the hash table.
/// @code
/// struct HashFuture {
///     // This struct is a  functor that will apply the SpookyHashAlgorithm to
///     // objects of type 'Future'.
///
///     size_t operator()(const Future& future) const
///         // Return the hash of the of the specified 'future'.  Note that
///         // this uses the 'SpookyHashAlgorithm' to quickly combine the
///         // attributes of 'Future' objects that are salient to hashing into
///         // a hash suitable for a hash table.
///     {
///         SpookyHashAlgorithm hash;
///
///         hash(future.getName(),  strlen(future.getName()));
///         hash(future.getMonth(), sizeof(char));
///         hash(future.getYear(),  sizeof(short));
///
///         return static_cast<size_t>(hash.computeHash());
///     }
/// };
/// @endcode
/// Then, we want to actually use our hash table on `Future` objects.  We create
/// an array of `Future`s based on data that was originally from some external
/// source:
/// @code
///     Future futures[] = { Future("Swiss Franc", 'F', 2014),
///                          Future("US Dollar", 'G', 2015),
///                          Future("Canadian Dollar", 'Z', 2014),
///                          Future("British Pound", 'M', 2015),
///                          Future("Deutsche Mark", 'X', 2016),
///                          Future("Eurodollar", 'Q', 2017)};
///     enum { NUM_FUTURES = sizeof futures / sizeof *futures };
/// @endcode
/// Next, we create our HashTable `hashTable`.  We pass the functor that we
/// defined above as the second argument:
/// @code
///     HashTable<Future, HashFuture> hashTable(futures, NUM_FUTURES);
/// @endcode
/// Now, we verify that each element in our array registers with count:
/// @code
///     for ( int i = 0; i < 6; ++i) {
///         assert(hashTable.contains(futures[i]));
///     }
/// @endcode
/// Finally, we verify that futures not in our original array are correctly
/// identified as not being in the set:
/// @code
///     assert(!hashTable.contains(Future("French Franc", 'N', 2019)));
///     assert(!hashTable.contains(Future("Swiss Franc", 'X', 2014)));
///     assert(!hashTable.contains(Future("US Dollar", 'F', 2014)));
///
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslh
 * @{
 */
/** @addtogroup bslh_spookyhashalgorithm
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslh_spookyhashalgorithmimp.h>
 
#include <bslmf_isbitwisemoveable.h>
 
#include <bsls_assert.h>
#include <bsls_byteorder.h>
#include <bsls_platform.h>
#include <bsls_types.h>
 
#include <stddef.h>  // for 'size_t'
#include <string.h>  // for 'memcpy'
 
 
namespace bslh {
 
 
                        // ===============================
                        // class bslh::SpookyHashAlgorithm
                        // ===============================
 
/// This class wraps an implementation of the "SpookyHash" hash algorithm in
/// an interface that is usable in the modular hashing system in `bslh`.
///
/// See @ref bslh_spookyhashalgorithm 
class SpookyHashAlgorithm {
 
  private:
    // PRIVATE TYPES
 
    /// Typedef for a 64-bit integer type used in the hashing algorithm.
    typedef bsls::Types::Uint64 Uint64;
 
    // DATA
 
    // Object that contains the actual implementation of the SpookHash
    // algorithm.
    SpookyHashAlgorithmImp d_state;
 
    // NOT IMPLEMENTED
 
    /// Do not allow copy construction.
    SpookyHashAlgorithm(const SpookyHashAlgorithm& original); // = delete;
 
    /// Do not allow assignment.
    SpookyHashAlgorithm& operator=(const SpookyHashAlgorithm& rhs);// = delete;
 
  public:
    // TYPES
 
    /// Typedef indicating the value type returned by this algorithm.
    typedef bsls::Types::Uint64 result_type;
 
 
    // CONSTANTS
    enum { k_SEED_LENGTH = 16 }; // Seed length in bytes.
 
  private:
    // PRIVATE CLASS METHODS
 
    /// The specified `seed` points to a possibly non-aligned `Uint64`,
    /// return the value of that `Uint64` without violating the host
    /// machine's alignment requirements.
    static
    Uint64 getSeed(const char *seed);
 
  public:
    // CREATORS
 
    /// Create a `SpookyHashAlgorithm` using a default initial seed.
    SpookyHashAlgorithm();
 
    /// Create a `bslh::SpookyHashAlgorithm`, seeded with a 128-bit
    /// (`k_SEED_LENGTH` bytes) seed pointed to by the specified `seed`.
    /// Each bit of the supplied seed will contribute to the final hash
    /// produced by `computeHash()`.  The behaviour is undefined unless
    /// `seed` points to at least 16 bytes of initialized memory.
    explicit SpookyHashAlgorithm(const char *seed);
 
     ~SpookyHashAlgorithm() = default;
        // Destroy this object.
 
    // MANIPULATORS
 
    /// Incorporate the specified `data`, of at least the specified
    /// `numBytes`, into the internal state of the hashing algorithm.  Every
    /// bit of data incorporated into the internal state of the algorithm
    /// will contribute to the final hash produced by `computeHash()`.  The
    /// same hash value will be produced regardless of whether a sequence of
    /// bytes is passed in all at once or through multiple calls to this
    /// member function.  Input where `numBytes` is 0 will have no effect on
    /// the internal state of the algorithm.  The behaviour is undefined
    /// unless `data` points to a valid memory location with at least
    /// `numBytes` bytes of initialized memory or `numBytes` is zero.
    void operator()(const void *data, size_t numBytes);
 
    /// Return the finalized version of the hash that has been accumulated.
    /// Note that this changes the internal state of the object, so calling
    /// `computeHash()` multiple times in a row will return different
    /// results, and only the first result returned will match the expected
    /// result of the algorithm.  Also note that a value will be returned,
    /// even if data has not been passed into `operator()`
    result_type computeHash();
};
 
// ============================================================================
//                            INLINE DEFINITIONS
// ============================================================================
 
// PRIVATE CLASS METHODS
inline
SpookyHashAlgorithm::Uint64 SpookyHashAlgorithm::getSeed(const char *seed)
{
    Uint64 ret;
    ::memcpy(&ret, seed, sizeof(ret));
    return ret;
}
 
// CREATORS
inline
SpookyHashAlgorithm::SpookyHashAlgorithm()
: d_state(1, 2)
{}
 
inline
SpookyHashAlgorithm::SpookyHashAlgorithm(const char *seed)
: d_state(getSeed(seed), getSeed(seed + sizeof(Uint64)))
{}
 
// MANIPULATORS
inline
void SpookyHashAlgorithm::operator()(const void *data, size_t numBytes)
{
    BSLS_ASSERT(0 != data || 0 == numBytes);
 
    d_state.update(data, numBytes);
}
 
inline
SpookyHashAlgorithm::result_type SpookyHashAlgorithm::computeHash()
{
    bsls::Types::Uint64 h1, h2;
    d_state.finalize(&h1, &h2);
    return h1;
}
 
}  // close package namespace
 
// ============================================================================
//                                TYPE TRAITS
// ============================================================================
 
namespace bslmf {
template <>
struct IsBitwiseMoveable<bslh::SpookyHashAlgorithm>
    : bsl::true_type {};
}  // close namespace bslmf
 
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2014 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */