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// Copyright 2018 The Abseil Authors.
// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include <ostream>
#include <string>
#include <vector>
#include "absl/hash/hash.h"
#include "absl/strings/match.h"
#include "absl/strings/str_format.h"
#include "absl/strings/str_join.h"
namespace absl {
namespace hash_internal {
// SpyHashState is an implementation of the HashState API that simply
// accumulates all input bytes in an internal buffer. This makes it useful
// for testing AbslHashValue overloads (so long as they are templated on the
// HashState parameter), since it can report the exact hash representation
// that the AbslHashValue overload produces.
// Sample usage:
// EXPECT_EQ(SpyHashState::combine(SpyHashState(), foo),
// SpyHashState::combine(SpyHashState(), bar));
template <typename T>
class SpyHashStateImpl : public HashStateBase<SpyHashStateImpl<T>> {
SpyHashStateImpl() : error_(std::make_shared<absl::optional<std::string>>()) {
static_assert(std::is_void<T>::value, "");
// Move-only
SpyHashStateImpl(const SpyHashStateImpl&) = delete;
SpyHashStateImpl& operator=(const SpyHashStateImpl&) = delete;
SpyHashStateImpl(SpyHashStateImpl&& other) noexcept {
*this = std::move(other);
SpyHashStateImpl& operator=(SpyHashStateImpl&& other) noexcept {
hash_representation_ = std::move(other.hash_representation_);
error_ = other.error_;
moved_from_ = other.moved_from_;
other.moved_from_ = true;
return *this;
template <typename U>
SpyHashStateImpl(SpyHashStateImpl<U>&& other) { // NOLINT
hash_representation_ = std::move(other.hash_representation_);
error_ = other.error_;
moved_from_ = other.moved_from_;
other.moved_from_ = true;
template <typename A, typename... Args>
static SpyHashStateImpl combine(SpyHashStateImpl s, const A& a,
const Args&... args) {
// Pass an instance of SpyHashStateImpl<A> when trying to combine `A`. This
// allows us to test that the user only uses this instance for combine calls
// and does not call AbslHashValue directly.
// See AbslHashValue implementation at the bottom.
s = SpyHashStateImpl<A>::HashStateBase::combine(std::move(s), a);
return SpyHashStateImpl::combine(std::move(s), args...);
static SpyHashStateImpl combine(SpyHashStateImpl s) {
if (direct_absl_hash_value_error_) {
*s.error_ = "AbslHashValue should not be invoked directly.";
} else if (s.moved_from_) {
*s.error_ = "Used moved-from instance of the hash state object.";
return s;
static void SetDirectAbslHashValueError() {
direct_absl_hash_value_error_ = true;
// Two SpyHashStateImpl objects are equal if they hold equal hash
// representations.
friend bool operator==(const SpyHashStateImpl& lhs,
const SpyHashStateImpl& rhs) {
return lhs.hash_representation_ == rhs.hash_representation_;
friend bool operator!=(const SpyHashStateImpl& lhs,
const SpyHashStateImpl& rhs) {
return !(lhs == rhs);
enum class CompareResult {
static CompareResult Compare(const SpyHashStateImpl& a,
const SpyHashStateImpl& b) {
const std::string a_flat = absl::StrJoin(a.hash_representation_, "");
const std::string b_flat = absl::StrJoin(b.hash_representation_, "");
if (a_flat == b_flat) return CompareResult::kEqual;
if (absl::EndsWith(a_flat, b_flat)) return CompareResult::kBSuffixA;
if (absl::EndsWith(b_flat, a_flat)) return CompareResult::kASuffixB;
return CompareResult::kUnequal;
// operator<< prints the hash representation as a hex and ASCII dump, to
// facilitate debugging.
friend std::ostream& operator<<(std::ostream& out,
const SpyHashStateImpl& hash_state) {
out << "[\n";
for (auto& s : hash_state.hash_representation_) {
size_t offset = 0;
for (char c : s) {
if (offset % 16 == 0) {
out << absl::StreamFormat("\n0x%04x: ", offset);
if (offset % 2 == 0) {
out << " ";
out << absl::StreamFormat("%02x", c);
out << "\n";
return out << "]";
// The base case of the combine recursion, which writes raw bytes into the
// internal buffer.
static SpyHashStateImpl combine_contiguous(SpyHashStateImpl hash_state,
const unsigned char* begin,
size_t size) {
reinterpret_cast<const char*>(begin), size);
return hash_state;
using SpyHashStateImpl::HashStateBase::combine_contiguous;
absl::optional<std::string> error() const {
if (moved_from_) {
return "Returned a moved-from instance of the hash state object.";
return *error_;
template <typename U>
friend class SpyHashStateImpl;
// This is true if SpyHashStateImpl<T> has been passed to a call of
// AbslHashValue with the wrong type. This detects that the user called
// AbslHashValue directly (because the hash state type does not match).
static bool direct_absl_hash_value_error_;
std::vector<std::string> hash_representation_;
// This is a shared_ptr because we want all instances of the particular
// SpyHashState run to share the field. This way we can set the error for
// use-after-move and all the copies will see it.
std::shared_ptr<absl::optional<std::string>> error_;
bool moved_from_ = false;
template <typename T>
bool SpyHashStateImpl<T>::direct_absl_hash_value_error_;
template <bool& B>
struct OdrUse {
constexpr OdrUse() {}
bool& b = B;
template <void (*)()>
struct RunOnStartup {
static bool run;
static constexpr OdrUse<run> kOdrUse{};
template <void (*f)()>
bool RunOnStartup<f>::run = (f(), true);
template <
typename T, typename U,
// Only trigger for when (T != U),
typename = absl::enable_if_t<!std::is_same<T, U>::value>,
// This statement works in two ways:
// - First, it instantiates RunOnStartup and forces the initialization of
// `run`, which set the global variable.
// - Second, it triggers a SFINAE error disabling the overload to prevent
// compile time errors. If we didn't disable the overload we would get
// ambiguous overload errors, which we don't want.
int = RunOnStartup<SpyHashStateImpl<T>::SetDirectAbslHashValueError>::run>
void AbslHashValue(SpyHashStateImpl<T>, const U&);
using SpyHashState = SpyHashStateImpl<void>;
} // namespace hash_internal
} // namespace absl