bslh_filesystem.h

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/// @file bslh_filesystem.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslh_filesystem.h                                                  -*-C++-*-
#ifndef INCLUDED_BSLH_FILESYSTEM
#define INCLUDED_BSLH_FILESYSTEM
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslh_filesystem bslh_filesystem
/// @brief  Provide `hash` for `std::filesystem::path`.
/// @addtogroup bsl
/// @{
/// @addtogroup bslh
/// @{
/// @addtogroup bslh_filesystem
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslh_filesystem-purpose"> Purpose</a>
/// * <a href="#bslh_filesystem-classes"> Classes </a>
/// * <a href="#bslh_filesystem-description"> Description </a>
///   * <a href="#bslh_filesystem-usage"> Usage </a>
///     * <a href="#bslh_filesystem-example-1-creating-and-using-a-hash-cross-reference"> Example 1: Creating and Using a Hash Cross Reference </a>
///
/// # Purpose {#bslh_filesystem-purpose}
/// Provide `hash` for `std::filesystem::path`.
///
/// # Classes {#bslh_filesystem-classes}
///
///
/// @see  bslh_hash
///
/// # Description {#bslh_filesystem-description}
/// Provides a `hash` specialization for `std::filesystem::path`,
/// delegating to `std::filesystem::path::hash_value` for the implementation
/// since it is already available and conforming.
///
/// ## Usage {#bslh_filesystem-usage}
///
///
/// This section illustrates intended usage of this component.
///
/// ### Example 1: Creating and Using a Hash Cross Reference {#bslh_filesystem-example-1-creating-and-using-a-hash-cross-reference}
///
///
/// Suppose we already have an array of unique values of type `TYPE`, for which
/// `operator==` is defined, and we want to be able to quickly look up whether
/// an element is in the array, without exhaustively applying `operator==` to
/// all the elements in sequence.  The array itself is guaranteed not to change
/// for the duration of our interest in it.
///
/// The problem is much simpler than building a general-purpose hash table,
/// because we know how many elements our cross reference will contain in
/// advance, so we will never have to dynamically grow the number of `buckets`.
/// We do not need to copy the values into our own area, so we don't have to
/// create storage for them, or require that a copy constructor or destructor be
/// available.  We only require that they have a transitive, symmetric
/// equivalence operation `bool operator==` and that a hash function be
/// provided.
///
/// We will need a hash function -- the hash function is a function that will
/// take as input an object of the type stored in our array, and yield a
/// `size_t` value that will be very randomized.  Ideally, the slightest change
/// in the value of the `TYPE` object will result in a large change in the value
/// returned by the hash function.  In a good hash function, typically half the
/// bits of the return value will change for a 1-bit change in the hashed value.
/// We then 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.  We will resolve hash collisions in our array
/// through `linear probing`, where we will search consecutive buckets following
/// the bucket where the collision occurred, testing occupied buckets for
/// equality with the value we are searching on, and concluding that the value
/// is not in the table if we encounter an empty bucket before we encounter one
/// referring to an equal element.
///
/// An important quality of the hash function is that if two values are
/// equivalent, they must yield the same hash value.
///
/// First, we define our `HashCrossReference` template class, with the two type
/// parameters `TYPE` (the type being referenced') and `HASHER`, which defaults
/// to `bslh::Hash<TYPE>`.  This component provides the specialization of
/// `bslh::Hash` for `std::filesystem::path`:
/// @code
/// /// This table leverages a hash table to provide a fast lookup of an
/// /// external, non-owned, array of values of configurable type.
/// ///
/// /// The only requirement for `TYPE` is that it 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
/// /// function of the following signature:
/// /// ```
/// /// size_t operator()(const TYPE)  const; or
/// /// size_t operator()(const TYPE&) const; or
/// /// ```
/// /// and `HASHER` must have a publicly available default constructor and
/// /// destructor.
/// template <class TYPE, class HASHER = bslh::Hash<TYPE> >
/// class HashCrossReference {
///     // DATA
///     const TYPE       *d_values;             // Array of values table is to
///                                             // cross-reference.  Held, not
///                                             // owned.
///     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;
///     bool              d_valid;              // Object was properly
///                                             // initialized.
///     bslma::Allocator *d_allocator_p;        // held, not owned
///
///   private:
///     // PRIVATE ACCESSORS
///
///     /// Look up the specified `value`, having hash value `hashValue`,
///     /// and return its index in `d_bucketArray`.  If not found, return
///     /// the vacant entry in `d_bucketArray` where it should be inserted.
///     /// Return `true` if `value is found and `false` otherwise.
///     bool lookup(size_t      *idx,
///                 const TYPE&  value,
///                 size_t       hashValue) const
///     {
///         const TYPE *ptr;
///         for (*idx = hashValue & d_bucketArrayMask;
///                               (ptr = d_bucketArray[*idx]);
///                                    *idx = (*idx + 1) & d_bucketArrayMask) {
///             if (value == *ptr) {
///                 return true;                                      // RETURN
///             }
///         }
///         // value was not found in table
///
///         return false;
///     }
///
///   public:
///     // CREATORS
///
///     /// Create a hash cross reference referring to the array of value.
///     HashCrossReference(const TYPE       *valuesArray,
///                        size_t            numValues,
///                        bslma::Allocator *allocator = 0)
///     : d_values(valuesArray)
///     , d_numValues(numValues)
///     , d_hasher()
///     , d_valid(true)
///     , d_allocator_p(bslma::Default::allocator(allocator))
///     {
///         size_t bucketArrayLength = 4;
///         while (bucketArrayLength < numValues * 4) {
///             bucketArrayLength *= 2;
///             BSLS_ASSERT_OPT(bucketArrayLength);
///         }
///         d_bucketArrayMask = bucketArrayLength - 1;
///         d_bucketArray = (const TYPE **) d_allocator_p->allocate(
///                                       bucketArrayLength * sizeof(TYPE **));
///         memset(d_bucketArray,  0, bucketArrayLength * sizeof(TYPE *));
///
///         for (unsigned i = 0; i < numValues; ++i) {
///             const TYPE& value = d_values[i];
///             size_t idx;
///             if (lookup(&idx, value, d_hasher(value))) {
///                 // Duplicate value.  Fail.
///
///                 printf("Error: entries %u and %u have the same value\n",
///                             i, (unsigned) (d_bucketArray[idx] - d_values));
///                 d_valid = false;
///
///                 // don't return, continue reporting other redundant
///                 // entries.
///             }
///             else {
///                 d_bucketArray[idx] = &d_values[i];
///             }
///         }
///     }
///
///     /// Free up memory used by this cross-reference.
///     ~HashCrossReference()
///     {
///         d_allocator_p->deallocate(d_bucketArray);
///     }
///
///     // ACCESSORS
///
///     /// Return 1 if the specified `value` is found in the cross
///     /// reference and 0 otherwise.
///     int count(const TYPE& value) const
///     {
///         BSLS_ASSERT_OPT(d_valid);
///
///         size_t idx;
///         return lookup(&idx, value, d_hasher(value));
///     }
///
///     /// Return `true` if this cross reference was successfully
///     /// constructed and `false` otherwise.
///     bool isValid() const
///     {
///         return d_valid;
///     }
/// };
/// @endcode
/// Then, In `main`, we will first use our cross-reference to cross-reference a
/// collection of `std::filesystem::path` values.  Note that the `/` separator
/// is equally valid on Windows and Unix-derived systems when used
/// programmatically.  We define our array and take its length:
/// @code
/// const std::filesystem::path paths[] = { "/2/3",
///                                         "/4/2",
///                                         "/4/7",
///                                         "/5/6",
///                                         "/5/7",
///                                         "/6/1",
///                                         "/6/2",
///                                         "/6/3",
///                                         "/7/0",
///                                         "/7/2",
///                                         "/7/9"
///                                      };
/// enum { NUM_PATHS = sizeof paths / sizeof *paths };
/// @endcode
/// Now, we create our cross-reference `hcri` and verify it constructed
/// properly.  Note that we don't specify the second template parameter `HASHER`
/// and let it default to `bslh::Hash<std::filesystem::path>`, which is already
/// defined by this component:
/// @code
/// HashCrossReference<std::filesystem::path> hcri(paths, NUM_PATHS);
/// assert(hcri.isValid());
/// @endcode
/// Finally, we use `hcri` to verify numbers that were and were not in the
/// collection:
/// @code
/// assert(1 == hcri.count("/2/3"));
/// assert(1 == hcri.count("/4/2"));
/// assert(1 == hcri.count("/4/7"));
/// assert(1 == hcri.count("/5/6"));
/// assert(0 == hcri.count("/a/3"));
/// assert(0 == hcri.count("/3/1"));
/// assert(0 == hcri.count("/3/7"));
/// assert(0 == hcri.count("/5/8"));
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslh
 * @{
 */
/** @addtogroup bslh_filesystem
 * @{
 */
 
#include <bslscm_version.h>
 
#include <bslh_hash.h>
 
#include <bsls_deprecatefeature.h>
#include <bsls_libraryfeatures.h>
#include <bsls_platform.h>
 
#ifdef BSLS_LIBRARYFEATURES_HAS_CPP17_FILESYSTEM
#include <filesystem>
 
#define BSLH_FILESYSTEM_DEPRECATED_CPP17                                      \
    BSLS_DEPRECATE_FEATURE("bsl",                                             \
                           "deprecated_cpp17_standard_library_features",      \
                           "do not use")
 
namespace bslh {
 
 
                          // =========================
                          // class hash specialization
                          // =========================
 
/// Specialization of `hash` for `std::filesystem::path` values.
template <>
struct Hash<std::filesystem::path> {
 
    // STANDARD TYPEDEFS
 
    /// @deprecated  This typedef is depreacted in C++17, for details see
    /// https://isocpp.org/files/papers/p0005r4.html.
    BSLH_FILESYSTEM_DEPRECATED_CPP17
    typedef std::filesystem::path argument_type;
 
    /// @deprecated  This typedef is depreacted in C++17, for details see
    /// https://isocpp.org/files/papers/p0005r4.html.
    BSLH_FILESYSTEM_DEPRECATED_CPP17
    typedef std::size_t result_type;
 
    /// Create a `hash` object.
     hash() = default;
 
    /// Create a `hash` object.  Note that as `hash` is an empty (stateless)
    /// type, this operation has no observable effect.
     hash(const hash& original) = default;
 
    /// Destroy this object.
     ~hash() = default;
 
    // MANIPULATORS
 
    /// Assign to this object the value of the specified 'rhs' object, and
    /// return a reference providing modifiable access to this object.  Note
    /// that as 'hash' is an empty (stateless) type, this operation has no
    /// observable effect.
     hash& operator=(const hash& rhs) = default;
 
    // ACCESSORS
 
    /// Return a hash value computed using the specified `x`.
    std::size_t operator()(const std::filesystem::path &x) const;
};
 
// ============================================================================
//                  TEMPLATE AND INLINE FUNCTION DEFINITIONS
// ============================================================================
 
inline
std::size_t Hash<std::filesystem::path>::operator()(
                                          const std::filesystem::path& x) const
{
    return std::filesystem::hash_value(x);
}
 
/// Pass the specified `input` path to the specified `hashAlg` hashing
/// algorithm of the (template parameter) type `HASHALG`.  Note that this
/// function violates the BDE coding standard, adding a function for a
/// namespace for a different package, and none of the function parameters
/// are from this package either.  This is necessary in order to provide an
/// implementation of `bslh::hashAppend` for the (native) standard library
/// `std::filesystem::path` type as we are not allowed to add overloads
/// directly into namespace `std`.  Also note that this will NOT be found by
/// the compiler if HASHALG is not in `BloombergLP::bslh`.
template <class HASHALG>
BSLS_PLATFORM_AGGRESSIVE_INLINE
void hashAppend(HASHALG& hashAlg, const std::filesystem::path& path)
{
    using BloombergLP::bslh::hashAppend;
    BloombergLP::bslh::Hash<std::filesystem::path> hashFunctor;
 
    hashAppend(hashAlg, hashFunctor(path));
}
 
}  // close package namespace
 
 
#undef BSLH_FILESYSTEM_DEPRECATED_CPP17
 
#endif // BSLS_LIBRARYFEATURES_HAS_CPP17_FILESYSTEM
 
#endif // INCLUDED_BSLH_FILESYSTEM
 
// ----------------------------------------------------------------------------
// Copyright 2022 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */