text: //===-- llvm/ADT/Hashing.h - Utilities for hashing --------------*- C++ -*-===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This file implements the newly proposed standard C++ interfaces for hashing
  // arbitrary data and building hash functions for user-defined types. This
  // interface was originally proposed in N3333[1] and is currently under review
  // for inclusion in a future TR and/or standard.
  //
  // The primary interfaces provide are comprised of one type and three functions:
  //
  //  -- 'hash_code' class is an opaque type representing the hash code for some
  //     data. It is the intended product of hashing, and can be used to implement
  //     hash tables, checksumming, and other common uses of hashes. It is not an
  //     integer type (although it can be converted to one) because it is risky
  //     to assume much about the internals of a hash_code. In particular, each
  //     execution of the program has a high probability of producing a different
  //     hash_code for a given input. Thus their values are not stable to save or
  //     persist, and should only be used during the execution for the
  //     construction of hashing datastructures.
  //
  //  -- 'hash_value' is a function designed to be overloaded for each
  //     user-defined type which wishes to be used within a hashing context. It
  //     should be overloaded within the user-defined type's namespace and found
  //     via ADL. Overloads for primitive types are provided by this library.
  //
  //  -- 'hash_combine' and 'hash_combine_range' are functions designed to aid
  //      programmers in easily and intuitively combining a set of data into
  //      a single hash_code for their object. They should only logically be used
  //      within the implementation of a 'hash_value' routine or similar context.
  //
  // Note that 'hash_combine_range' contains very special logic for hashing
  // a contiguous array of integers or pointers. This logic is *extremely* fast,
  // on a modern Intel "Gainestown" Xeon (Nehalem uarch) @2.2 GHz, these were
  // benchmarked at over 6.5 GiB/s for large keys, and <20 cycles/hash for keys
  // under 32-bytes.
  //
  //===----------------------------------------------------------------------===//
  
  #ifndef LLVM_ADT_HASHING_H
  #define LLVM_ADT_HASHING_H
  
  #include "llvm/ADT/STLExtras.h"
  #include "llvm/Support/DataTypes.h"
  #include "llvm/Support/Host.h"
  #include "llvm/Support/SwapByteOrder.h"
  #include "llvm/Support/type_traits.h"
  #include <algorithm>
  #include <cassert>
  #include <cstring>
  #include <iterator>
  #include <utility>
  
  // Allow detecting C++11 feature availability when building with Clang without
  // breaking other compilers.
  #ifndef __has_feature
  # define __has_feature(x) 0
  #endif
  
  namespace llvm {
  
  /// \brief An opaque object representing a hash code.
  ///
  /// This object represents the result of hashing some entity. It is intended to
  /// be used to implement hashtables or other hashing-based data structures.
  /// While it wraps and exposes a numeric value, this value should not be
  /// trusted to be stable or predictable across processes or executions.
  ///
  /// In order to obtain the hash_code for an object 'x':
  /// \code
  ///   using llvm::hash_value;
  ///   llvm::hash_code code = hash_value(x);
  /// \endcode
  class hash_code {
    size_t value;
  
  public:
    /// \brief Default construct a hash_code.
    /// Note that this leaves the value uninitialized.
    hash_code() {}
  
    /// \brief Form a hash code directly from a numerical value.
    hash_code(size_t value) : value(value) {}
  
    /// \brief Convert the hash code to its numerical value for use.
    /*explicit*/ operator size_t() const { return value; }
  
    friend bool operator==(const hash_code &lhs, const hash_code &rhs) {
      return lhs.value == rhs.value;
    }
    friend bool operator!=(const hash_code &lhs, const hash_code &rhs) {
      return lhs.value != rhs.value;
    }
  
    /// \brief Allow a hash_code to be directly run through hash_value.
    friend size_t hash_value(const hash_code &code) { return code.value; }
  };
  
  /// \brief Compute a hash_code for any integer value.
  ///
  /// Note that this function is intended to compute the same hash_code for
  /// a particular value without regard to the pre-promotion type. This is in
  /// contrast to hash_combine which may produce different hash_codes for
  /// differing argument types even if they would implicit promote to a common
  /// type without changing the value.
  template <typename T>
  typename enable_if<is_integral_or_enum<T>, hash_code>::type hash_value(T value);
  
  /// \brief Compute a hash_code for a pointer's address.
  ///
  /// N.B.: This hashes the *address*. Not the value and not the type.
  template <typename T> hash_code hash_value(const T *ptr);
  
  /// \brief Compute a hash_code for a pair of objects.
  template <typename T, typename U>
  hash_code hash_value(const std::pair<T, U> &arg);
  
  /// \brief Compute a hash_code for a standard string.
  template <typename T>
  hash_code hash_value(const std::basic_string<T> &arg);
  
  
  /// \brief Override the execution seed with a fixed value.
  ///
  /// This hashing library uses a per-execution seed designed to change on each
  /// run with high probability in order to ensure that the hash codes are not
  /// attackable and to ensure that output which is intended to be stable does
  /// not rely on the particulars of the hash codes produced.
  ///
  /// That said, there are use cases where it is important to be able to
  /// reproduce *exactly* a specific behavior. To that end, we provide a function
  /// which will forcibly set the seed to a fixed value. This must be done at the
  /// start of the program, before any hashes are computed. Also, it cannot be
  /// undone. This makes it thread-hostile and very hard to use outside of
  /// immediately on start of a simple program designed for reproducible
  /// behavior.
  void set_fixed_execution_hash_seed(size_t fixed_value);
  
  
  // All of the implementation details of actually computing the various hash
  // code values are held within this namespace. These routines are included in
  // the header file mainly to allow inlining and constant propagation.
  namespace hashing {
  namespace detail {
  
  inline uint64_t fetch64(const char *p) {
    uint64_t result;
    memcpy(&result, p, sizeof(result));
    if (sys::isBigEndianHost())
      return sys::SwapByteOrder(result);
    return result;
  }
  
  inline uint32_t fetch32(const char *p) {
    uint32_t result;
    memcpy(&result, p, sizeof(result));
    if (sys::isBigEndianHost())
      return sys::SwapByteOrder(result);
    return result;
  }
  
  /// Some primes between 2^63 and 2^64 for various uses.
  static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
  static const uint64_t k1 = 0xb492b66fbe98f273ULL;
  static const uint64_t k2 = 0x9ae16a3b2f90404fULL;
  static const uint64_t k3 = 0xc949d7c7509e6557ULL;
  
  /// \brief Bitwise right rotate.
  /// Normally this will compile to a single instruction, especially if the
  /// shift is a manifest constant.
  inline uint64_t rotate(uint64_t val, size_t shift) {
    // Avoid shifting by 64: doing so yields an undefined result.
    return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
  }
  
  inline uint64_t shift_mix(uint64_t val) {
    return val ^ (val >> 47);
  }
  
  inline uint64_t hash_16_bytes(uint64_t low, uint64_t high) {
    // Murmur-inspired hashing.
    const uint64_t kMul = 0x9ddfea08eb382d69ULL;
    uint64_t a = (low ^ high) * kMul;
    a ^= (a >> 47);
    uint64_t b = (high ^ a) * kMul;
    b ^= (b >> 47);
    b *= kMul;
    return b;
  }
  
  inline uint64_t hash_1to3_bytes(const char *s, size_t len, uint64_t seed) {
    uint8_t a = s[0];
    uint8_t b = s[len >> 1];
    uint8_t c = s[len - 1];
    uint32_t y = static_cast<uint32_t>(a) + (static_cast<uint32_t>(b) << 8);
    uint32_t z = len + (static_cast<uint32_t>(c) << 2);
    return shift_mix(y * k2 ^ z * k3 ^ seed) * k2;
  }
  
  inline uint64_t hash_4to8_bytes(const char *s, size_t len, uint64_t seed) {
    uint64_t a = fetch32(s);
    return hash_16_bytes(len + (a << 3), seed ^ fetch32(s + len - 4));
  }
  
  inline uint64_t hash_9to16_bytes(const char *s, size_t len, uint64_t seed) {
    uint64_t a = fetch64(s);
    uint64_t b = fetch64(s + len - 8);
    return hash_16_bytes(seed ^ a, rotate(b + len, len)) ^ b;
  }
  
  inline uint64_t hash_17to32_bytes(const char *s, size_t len, uint64_t seed) {
    uint64_t a = fetch64(s) * k1;
    uint64_t b = fetch64(s + 8);
    uint64_t c = fetch64(s + len - 8) * k2;
    uint64_t d = fetch64(s + len - 16) * k0;
    return hash_16_bytes(rotate(a - b, 43) + rotate(c ^ seed, 30) + d,
                         a + rotate(b ^ k3, 20) - c + len + seed);
  }
  
  inline uint64_t hash_33to64_bytes(const char *s, size_t len, uint64_t seed) {
    uint64_t z = fetch64(s + 24);
    uint64_t a = fetch64(s) + (len + fetch64(s + len - 16)) * k0;
    uint64_t b = rotate(a + z, 52);
    uint64_t c = rotate(a, 37);
    a += fetch64(s + 8);
    c += rotate(a, 7);
    a += fetch64(s + 16);
    uint64_t vf = a + z;
    uint64_t vs = b + rotate(a, 31) + c;
    a = fetch64(s + 16) + fetch64(s + len - 32);
    z = fetch64(s + len - 8);
    b = rotate(a + z, 52);
    c = rotate(a, 37);
    a += fetch64(s + len - 24);
    c += rotate(a, 7);
    a += fetch64(s + len - 16);
    uint64_t wf = a + z;
    uint64_t ws = b + rotate(a, 31) + c;
    uint64_t r = shift_mix((vf + ws) * k2 + (wf + vs) * k0);
    return shift_mix((seed ^ (r * k0)) + vs) * k2;
  }
  
  inline uint64_t hash_short(const char *s, size_t length, uint64_t seed) {
    if (length >= 4 && length <= 8)
      return hash_4to8_bytes(s, length, seed);
    if (length > 8 && length <= 16)
      return hash_9to16_bytes(s, length, seed);
    if (length > 16 && length <= 32)
      return hash_17to32_bytes(s, length, seed);
    if (length > 32)
      return hash_33to64_bytes(s, length, seed);
    if (length != 0)
      return hash_1to3_bytes(s, length, seed);
  
    return k2 ^ seed;
  }
  
  /// \brief The intermediate state used during hashing.
  /// Currently, the algorithm for computing hash codes is based on CityHash and
  /// keeps 56 bytes of arbitrary state.
  struct hash_state {
    uint64_t h0, h1, h2, h3, h4, h5, h6;
    uint64_t seed;
  
    /// \brief Create a new hash_state structure and initialize it based on the
    /// seed and the first 64-byte chunk.
    /// This effectively performs the initial mix.
    static hash_state create(const char *s, uint64_t seed) {
      hash_state state = {
        0, seed, hash_16_bytes(seed, k1), rotate(seed ^ k1, 49),
        seed * k1, shift_mix(seed), 0, seed };
      state.h6 = hash_16_bytes(state.h4, state.h5);
      state.mix(s);
      return state;
    }
  
    /// \brief Mix 32-bytes from the input sequence into the 16-bytes of 'a'
    /// and 'b', including whatever is already in 'a' and 'b'.
    static void mix_32_bytes(const char *s, uint64_t &a, uint64_t &b) {
      a += fetch64(s);
      uint64_t c = fetch64(s + 24);
      b = rotate(b + a + c, 21);
      uint64_t d = a;
      a += fetch64(s + 8) + fetch64(s + 16);
      b += rotate(a, 44) + d;
      a += c;
    }
  
    /// \brief Mix in a 64-byte buffer of data.
    /// We mix all 64 bytes even when the chunk length is smaller, but we
    /// record the actual length.
    void mix(const char *s) {
      h0 = rotate(h0 + h1 + h3 + fetch64(s + 8), 37) * k1;
      h1 = rotate(h1 + h4 + fetch64(s + 48), 42) * k1;
      h0 ^= h6;
      h1 += h3 + fetch64(s + 40);
      h2 = rotate(h2 + h5, 33) * k1;
      h3 = h4 * k1;
      h4 = h0 + h5;
      mix_32_bytes(s, h3, h4);
      h5 = h2 + h6;
      h6 = h1 + fetch64(s + 16);
      mix_32_bytes(s + 32, h5, h6);
      std::swap(h2, h0);
    }
  
    /// \brief Compute the final 64-bit hash code value based on the current
    /// state and the length of bytes hashed.
    uint64_t finalize(size_t length) {
      return hash_16_bytes(hash_16_bytes(h3, h5) + shift_mix(h1) * k1 + h2,
                           hash_16_bytes(h4, h6) + shift_mix(length) * k1 + h0);
    }
  };
  
  
  /// \brief A global, fixed seed-override variable.
  ///
  /// This variable can be set using the \see llvm::set_fixed_execution_seed
  /// function. See that function for details. Do not, under any circumstances,
  /// set or read this variable.
  extern size_t fixed_seed_override;
  
  inline size_t get_execution_seed() {
    // FIXME: This needs to be a per-execution seed. This is just a placeholder
    // implementation. Switching to a per-execution seed is likely to flush out
    // instability bugs and so will happen as its own commit.
    //
    // However, if there is a fixed seed override set the first time this is
    // called, return that instead of the per-execution seed.
    const uint64_t seed_prime = 0xff51afd7ed558ccdULL;
    static size_t seed = fixed_seed_override ? fixed_seed_override
                                             : (size_t)seed_prime;
    return seed;
  }
  
  
  /// \brief Trait to indicate whether a type's bits can be hashed directly.
  ///
  /// A type trait which is true if we want to combine values for hashing by
  /// reading the underlying data. It is false if values of this type must
  /// first be passed to hash_value, and the resulting hash_codes combined.
  //
  // FIXME: We want to replace is_integral_or_enum and is_pointer here with
  // a predicate which asserts that comparing the underlying storage of two
  // values of the type for equality is equivalent to comparing the two values
  // for equality. For all the platforms we care about, this holds for integers
  // and pointers, but there are platforms where it doesn't and we would like to
  // support user-defined types which happen to satisfy this property.
  template <typename T> struct is_hashable_data
    : integral_constant<bool, ((is_integral_or_enum<T>::value ||
                                is_pointer<T>::value) &&
                               64 % sizeof(T) == 0)> {};
  
  // Special case std::pair to detect when both types are viable and when there
  // is no alignment-derived padding in the pair. This is a bit of a lie because
  // std::pair isn't truly POD, but it's close enough in all reasonable
  // implementations for our use case of hashing the underlying data.
  template <typename T, typename U> struct is_hashable_data<std::pair<T, U> >
    : integral_constant<bool, (is_hashable_data<T>::value &&
                               is_hashable_data<U>::value &&
                               (sizeof(T) + sizeof(U)) ==
                                sizeof(std::pair<T, U>))> {};
  
  /// \brief Helper to get the hashable data representation for a type.
  /// This variant is enabled when the type itself can be used.
  template <typename T>
  typename enable_if<is_hashable_data<T>, T>::type
  get_hashable_data(const T &value) {
    return value;
  }
  /// \brief Helper to get the hashable data representation for a type.
  /// This variant is enabled when we must first call hash_value and use the
  /// result as our data.
  template <typename T>
  typename enable_if_c<!is_hashable_data<T>::value, size_t>::type
  get_hashable_data(const T &value) {
    using ::llvm::hash_value;
    return hash_value(value);
  }
  
  /// \brief Helper to store data from a value into a buffer and advance the
  /// pointer into that buffer.
  ///
  /// This routine first checks whether there is enough space in the provided
  /// buffer, and if not immediately returns false. If there is space, it
  /// copies the underlying bytes of value into the buffer, advances the
  /// buffer_ptr past the copied bytes, and returns true.
  template <typename T>
  bool store_and_advance(char *&buffer_ptr, char *buffer_end, const T& value,
                         size_t offset = 0) {
    size_t store_size = sizeof(value) - offset;
    if (buffer_ptr + store_size > buffer_end)
      return false;
    const char *value_data = reinterpret_cast<const char *>(&value);
    memcpy(buffer_ptr, value_data + offset, store_size);
    buffer_ptr += store_size;
    return true;
  }
  
  /// \brief Implement the combining of integral values into a hash_code.
  ///
  /// This overload is selected when the value type of the iterator is
  /// integral. Rather than computing a hash_code for each object and then
  /// combining them, this (as an optimization) directly combines the integers.
  template <typename InputIteratorT>
  hash_code hash_combine_range_impl(InputIteratorT first, InputIteratorT last) {
    const size_t seed = get_execution_seed();
    char buffer[64], *buffer_ptr = buffer;
    char *const buffer_end = buffer_ptr + array_lengthof(buffer);
    while (first != last && store_and_advance(buffer_ptr, buffer_end,
                                              get_hashable_data(*first)))
      ++first;
    if (first == last)
      return hash_short(buffer, buffer_ptr - buffer, seed);
    assert(buffer_ptr == buffer_end);
  
    hash_state state = state.create(buffer, seed);
    size_t length = 64;
    while (first != last) {
      // Fill up the buffer. We don't clear it, which re-mixes the last round
      // when only a partial 64-byte chunk is left.
      buffer_ptr = buffer;
      while (first != last && store_and_advance(buffer_ptr, buffer_end,
                                                get_hashable_data(*first)))
        ++first;
  
      // Rotate the buffer if we did a partial fill in order to simulate doing
      // a mix of the last 64-bytes. That is how the algorithm works when we
      // have a contiguous byte sequence, and we want to emulate that here.
      std::rotate(buffer, buffer_ptr, buffer_end);
  
      // Mix this chunk into the current state.
      state.mix(buffer);
      length += buffer_ptr - buffer;
    };
  
    return state.finalize(length);
  }
  
  /// \brief Implement the combining of integral values into a hash_code.
  ///
  /// This overload is selected when the value type of the iterator is integral
  /// and when the input iterator is actually a pointer. Rather than computing
  /// a hash_code for each object and then combining them, this (as an
  /// optimization) directly combines the integers. Also, because the integers
  /// are stored in contiguous memory, this routine avoids copying each value
  /// and directly reads from the underlying memory.
  template <typename ValueT>
  typename enable_if<is_hashable_data<ValueT>, hash_code>::type
  hash_combine_range_impl(ValueT *first, ValueT *last) {
    const size_t seed = get_execution_seed();
    const char *s_begin = reinterpret_cast<const char *>(first);
    const char *s_end = reinterpret_cast<const char *>(last);
    const size_t length = std::distance(s_begin, s_end);
    if (length <= 64)
      return hash_short(s_begin, length, seed);
  
    const char *s_aligned_end = s_begin + (length & ~63);
    hash_state state = state.create(s_begin, seed);
    s_begin += 64;
    while (s_begin != s_aligned_end) {
      state.mix(s_begin);
      s_begin += 64;
    }
    if (length & 63)
      state.mix(s_end - 64);
  
    return state.finalize(length);
  }
  
  } // namespace detail
  } // namespace hashing
  
  
  /// \brief Compute a hash_code for a sequence of values.
  ///
  /// This hashes a sequence of values. It produces the same hash_code as
  /// 'hash_combine(a, b, c, ...)', but can run over arbitrary sized sequences
  /// and is significantly faster given pointers and types which can be hashed as
  /// a sequence of bytes.
  template <typename InputIteratorT>
  hash_code hash_combine_range(InputIteratorT first, InputIteratorT last) {
    return ::llvm::hashing::detail::hash_combine_range_impl(first, last);
  }
  
  
  // Implementation details for hash_combine.
  namespace hashing {
  namespace detail {
  
  /// \brief Helper class to manage the recursive combining of hash_combine
  /// arguments.
  ///
  /// This class exists to manage the state and various calls involved in the
  /// recursive combining of arguments used in hash_combine. It is particularly
  /// useful at minimizing the code in the recursive calls to ease the pain
  /// caused by a lack of variadic functions.
  struct hash_combine_recursive_helper {
    char buffer[64];
    hash_state state;
    const size_t seed;
  
  public:
    /// \brief Construct a recursive hash combining helper.
    ///
    /// This sets up the state for a recursive hash combine, including getting
    /// the seed and buffer setup.
    hash_combine_recursive_helper()
      : seed(get_execution_seed()) {}
  
    /// \brief Combine one chunk of data into the current in-flight hash.
    ///
    /// This merges one chunk of data into the hash. First it tries to buffer
    /// the data. If the buffer is full, it hashes the buffer into its
    /// hash_state, empties it, and then merges the new chunk in. This also
    /// handles cases where the data straddles the end of the buffer.
    template <typename T>
    char *combine_data(size_t &length, char *buffer_ptr, char *buffer_end, T data) {
      if (!store_and_advance(buffer_ptr, buffer_end, data)) {
        // Check for skew which prevents the buffer from being packed, and do
        // a partial store into the buffer to fill it. This is only a concern
        // with the variadic combine because that formation can have varying
        // argument types.
        size_t partial_store_size = buffer_end - buffer_ptr;
        memcpy(buffer_ptr, &data, partial_store_size);
  
        // If the store fails, our buffer is full and ready to hash. We have to
        // either initialize the hash state (on the first full buffer) or mix
        // this buffer into the existing hash state. Length tracks the *hashed*
        // length, not the buffered length.
        if (length == 0) {
          state = state.create(buffer, seed);
          length = 64;
        } else {
          // Mix this chunk into the current state and bump length up by 64.
          state.mix(buffer);
          length += 64;
        }
        // Reset the buffer_ptr to the head of the buffer for the next chunk of
        // data.
        buffer_ptr = buffer;
  
        // Try again to store into the buffer -- this cannot fail as we only
        // store types smaller than the buffer.
        if (!store_and_advance(buffer_ptr, buffer_end, data,
                               partial_store_size))
          abort();
      }
      return buffer_ptr;
    }
  
  #if defined(__has_feature) && __has_feature(__cxx_variadic_templates__)
  
    /// \brief Recursive, variadic combining method.
    ///
    /// This function recurses through each argument, combining that argument
    /// into a single hash.
    template <typename T, typename ...Ts>
    hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
                      const T &arg, const Ts &...args) {
      buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg));
  
      // Recurse to the next argument.
      return combine(length, buffer_ptr, buffer_end, args...);
    }
  
  #else
    // Manually expanded recursive combining methods. See variadic above for
    // documentation.
  
    template <typename T1, typename T2, typename T3, typename T4, typename T5,
              typename T6>
    hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
                      const T1 &arg1, const T2 &arg2, const T3 &arg3,
                      const T4 &arg4, const T5 &arg5, const T6 &arg6) {
      buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg1));
      return combine(length, buffer_ptr, buffer_end, arg2, arg3, arg4, arg5, arg6);
    }
    template <typename T1, typename T2, typename T3, typename T4, typename T5>
    hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
                      const T1 &arg1, const T2 &arg2, const T3 &arg3,
                      const T4 &arg4, const T5 &arg5) {
      buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg1));
      return combine(length, buffer_ptr, buffer_end, arg2, arg3, arg4, arg5);
    }
    template <typename T1, typename T2, typename T3, typename T4>
    hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
                      const T1 &arg1, const T2 &arg2, const T3 &arg3,
                      const T4 &arg4) {
      buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg1));
      return combine(length, buffer_ptr, buffer_end, arg2, arg3, arg4);
    }
    template <typename T1, typename T2, typename T3>
    hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
                      const T1 &arg1, const T2 &arg2, const T3 &arg3) {
      buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg1));
      return combine(length, buffer_ptr, buffer_end, arg2, arg3);
    }
    template <typename T1, typename T2>
    hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
                      const T1 &arg1, const T2 &arg2) {
      buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg1));
      return combine(length, buffer_ptr, buffer_end, arg2);
    }
    template <typename T1>
    hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
                      const T1 &arg1) {
      buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg1));
      return combine(length, buffer_ptr, buffer_end);
    }
  
  #endif
  
    /// \brief Base case for recursive, variadic combining.
    ///
    /// The base case when combining arguments recursively is reached when all
    /// arguments have been handled. It flushes the remaining buffer and
    /// constructs a hash_code.
    hash_code combine(size_t length, char *buffer_ptr, char *buffer_end) {
      // Check whether the entire set of values fit in the buffer. If so, we'll
      // use the optimized short hashing routine and skip state entirely.
      if (length == 0)
        return hash_short(buffer, buffer_ptr - buffer, seed);
  
      // Mix the final buffer, rotating it if we did a partial fill in order to
      // simulate doing a mix of the last 64-bytes. That is how the algorithm
      // works when we have a contiguous byte sequence, and we want to emulate
      // that here.
      std::rotate(buffer, buffer_ptr, buffer_end);
  
      // Mix this chunk into the current state.
      state.mix(buffer);
      length += buffer_ptr - buffer;
  
      return state.finalize(length);
    }
  };
  
  } // namespace detail
  } // namespace hashing
  
  
  #if __has_feature(__cxx_variadic_templates__)
  
  /// \brief Combine values into a single hash_code.
  ///
  /// This routine accepts a varying number of arguments of any type. It will
  /// attempt to combine them into a single hash_code. For user-defined types it
  /// attempts to call a \see hash_value overload (via ADL) for the type. For
  /// integer and pointer types it directly combines their data into the
  /// resulting hash_code.
  ///
  /// The result is suitable for returning from a user's hash_value
  /// *implementation* for their user-defined type. Consumers of a type should
  /// *not* call this routine, they should instead call 'hash_value'.
  template <typename ...Ts> hash_code hash_combine(const Ts &...args) {
    // Recursively hash each argument using a helper class.
    ::llvm::hashing::detail::hash_combine_recursive_helper helper;
    return helper.combine(0, helper.buffer, helper.buffer + 64, args...);
  }
  
  #else
  
  // What follows are manually exploded overloads for each argument width. See
  // the above variadic definition for documentation and specification.
  
  template <typename T1, typename T2, typename T3, typename T4, typename T5,
            typename T6>
  hash_code hash_combine(const T1 &arg1, const T2 &arg2, const T3 &arg3,
                         const T4 &arg4, const T5 &arg5, const T6 &arg6) {
    ::llvm::hashing::detail::hash_combine_recursive_helper helper;
    return helper.combine(0, helper.buffer, helper.buffer + 64,
                          arg1, arg2, arg3, arg4, arg5, arg6);
  }
  template <typename T1, typename T2, typename T3, typename T4, typename T5>
  hash_code hash_combine(const T1 &arg1, const T2 &arg2, const T3 &arg3,
                         const T4 &arg4, const T5 &arg5) {
    ::llvm::hashing::detail::hash_combine_recursive_helper helper;
    return helper.combine(0, helper.buffer, helper.buffer + 64,
                          arg1, arg2, arg3, arg4, arg5);
  }
  template <typename T1, typename T2, typename T3, typename T4>
  hash_code hash_combine(const T1 &arg1, const T2 &arg2, const T3 &arg3,
                         const T4 &arg4) {
    ::llvm::hashing::detail::hash_combine_recursive_helper helper;
    return helper.combine(0, helper.buffer, helper.buffer + 64,
                          arg1, arg2, arg3, arg4);
  }
  template <typename T1, typename T2, typename T3>
  hash_code hash_combine(const T1 &arg1, const T2 &arg2, const T3 &arg3) {
    ::llvm::hashing::detail::hash_combine_recursive_helper helper;
    return helper.combine(0, helper.buffer, helper.buffer + 64, arg1, arg2, arg3);
  }
  template <typename T1, typename T2>
  hash_code hash_combine(const T1 &arg1, const T2 &arg2) {
    ::llvm::hashing::detail::hash_combine_recursive_helper helper;
    return helper.combine(0, helper.buffer, helper.buffer + 64, arg1, arg2);
  }
  template <typename T1>
  hash_code hash_combine(const T1 &arg1) {
    ::llvm::hashing::detail::hash_combine_recursive_helper helper;
    return helper.combine(0, helper.buffer, helper.buffer + 64, arg1);
  }
  
  #endif
  
  
  // Implementation details for implementations of hash_value overloads provided
  // here.
  namespace hashing {
  namespace detail {
  
  /// \brief Helper to hash the value of a single integer.
  ///
  /// Overloads for smaller integer types are not provided to ensure consistent
  /// behavior in the presence of integral promotions. Essentially,
  /// "hash_value('4')" and "hash_value('0' + 4)" should be the same.
  inline hash_code hash_integer_value(uint64_t value) {
    // Similar to hash_4to8_bytes but using a seed instead of length.
    const uint64_t seed = get_execution_seed();
    const char *s = reinterpret_cast<const char *>(&value);
    const uint64_t a = fetch32(s);
    return hash_16_bytes(seed + (a << 3), fetch32(s + 4));
  }
  
  } // namespace detail
  } // namespace hashing
  
  // Declared and documented above, but defined here so that any of the hashing
  // infrastructure is available.
  template <typename T>
  typename enable_if<is_integral_or_enum<T>, hash_code>::type
  hash_value(T value) {
    return ::llvm::hashing::detail::hash_integer_value(value);
  }
  
  // Declared and documented above, but defined here so that any of the hashing
  // infrastructure is available.
  template <typename T> hash_code hash_value(const T *ptr) {
    return ::llvm::hashing::detail::hash_integer_value(
      reinterpret_cast<uintptr_t>(ptr));
  }
  
  // Declared and documented above, but defined here so that any of the hashing
  // infrastructure is available.
  template <typename T, typename U>
  hash_code hash_value(const std::pair<T, U> &arg) {
    return hash_combine(arg.first, arg.second);
  }
  
  // Declared and documented above, but defined here so that any of the hashing
  // infrastructure is available.
  template <typename T>
  hash_code hash_value(const std::basic_string<T> &arg) {
    return hash_combine_range(arg.begin(), arg.end());
  }
  
  } // namespace llvm
  
  #endif
  