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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
//
//      https://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.
//
// -----------------------------------------------------------------------------
// File: hashtablez_sampler.h
// -----------------------------------------------------------------------------
//
// This header file defines the API for a low level library to sample hashtables
// and collect runtime statistics about them.
//
// `HashtablezSampler` controls the lifecycle of `HashtablezInfo` objects which
// store information about a single sample.
//
// `Record*` methods store information into samples.
// `Sample()` and `Unsample()` make use of a single global sampler with
// properties controlled by the flags hashtablez_enabled,
// hashtablez_sample_rate, and hashtablez_max_samples.
//
// WARNING
//
// Using this sampling API may cause sampled Swiss tables to use the global
// allocator (operator `new`) in addition to any custom allocator.  If you
// are using a table in an unusual circumstance where allocation or calling a
// linux syscall is unacceptable, this could interfere.
//
// This utility is internal-only. Use at your own risk.

#ifndef ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
#define ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_

#include <atomic>
#include <functional>
#include <memory>
#include <vector>

#include "absl/base/config.h"
#include "absl/base/internal/per_thread_tls.h"
#include "absl/base/optimization.h"
#include "absl/profiling/internal/sample_recorder.h"
#include "absl/synchronization/mutex.h"
#include "absl/utility/utility.h"

namespace absl
{
    ABSL_NAMESPACE_BEGIN
    namespace container_internal
    {

        // Stores information about a sampled hashtable.  All mutations to this *must*
        // be made through `Record*` functions below.  All reads from this *must* only
        // occur in the callback to `HashtablezSampler::Iterate`.
        struct HashtablezInfo : public profiling_internal::Sample<HashtablezInfo>
        {
            // Constructs the object but does not fill in any fields.
            HashtablezInfo();
            ~HashtablezInfo();
            HashtablezInfo(const HashtablezInfo&) = delete;
            HashtablezInfo& operator=(const HashtablezInfo&) = delete;

            // Puts the object into a clean state, fills in the logically `const` members,
            // blocking for any readers that are currently sampling the object.
            void PrepareForSampling(int64_t stride, size_t inline_element_size_value)
                ABSL_EXCLUSIVE_LOCKS_REQUIRED(init_mu);

            // These fields are mutated by the various Record* APIs and need to be
            // thread-safe.
            std::atomic<size_t> capacity;
            std::atomic<size_t> size;
            std::atomic<size_t> num_erases;
            std::atomic<size_t> num_rehashes;
            std::atomic<size_t> max_probe_length;
            std::atomic<size_t> total_probe_length;
            std::atomic<size_t> hashes_bitwise_or;
            std::atomic<size_t> hashes_bitwise_and;
            std::atomic<size_t> hashes_bitwise_xor;
            std::atomic<size_t> max_reserve;

            // All of the fields below are set by `PrepareForSampling`, they must not be
            // mutated in `Record*` functions.  They are logically `const` in that sense.
            // These are guarded by init_mu, but that is not externalized to clients,
            // which can read them only during `SampleRecorder::Iterate` which will hold
            // the lock.
            static constexpr int kMaxStackDepth = 64;
            absl::Time create_time;
            int32_t depth;
            void* stack[kMaxStackDepth];
            size_t inline_element_size;  // How big is the slot?
        };

        inline void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length)
        {
#ifdef ABSL_INTERNAL_HAVE_SSE2
            total_probe_length /= 16;
#else
            total_probe_length /= 8;
#endif
            info->total_probe_length.store(total_probe_length, std::memory_order_relaxed);
            info->num_erases.store(0, std::memory_order_relaxed);
            // There is only one concurrent writer, so `load` then `store` is sufficient
            // instead of using `fetch_add`.
            info->num_rehashes.store(
                1 + info->num_rehashes.load(std::memory_order_relaxed),
                std::memory_order_relaxed
            );
        }

        inline void RecordReservationSlow(HashtablezInfo* info, size_t target_capacity)
        {
            info->max_reserve.store(
                (std::max)(info->max_reserve.load(std::memory_order_relaxed), target_capacity),
                std::memory_order_relaxed
            );
        }

        inline void RecordClearedReservationSlow(HashtablezInfo* info)
        {
            info->max_reserve.store(0, std::memory_order_relaxed);
        }

        inline void RecordStorageChangedSlow(HashtablezInfo* info, size_t size, size_t capacity)
        {
            info->size.store(size, std::memory_order_relaxed);
            info->capacity.store(capacity, std::memory_order_relaxed);
            if (size == 0)
            {
                // This is a clear, reset the total/num_erases too.
                info->total_probe_length.store(0, std::memory_order_relaxed);
                info->num_erases.store(0, std::memory_order_relaxed);
            }
        }

        void RecordInsertSlow(HashtablezInfo* info, size_t hash, size_t distance_from_desired);

        inline void RecordEraseSlow(HashtablezInfo* info)
        {
            info->size.fetch_sub(1, std::memory_order_relaxed);
            // There is only one concurrent writer, so `load` then `store` is sufficient
            // instead of using `fetch_add`.
            info->num_erases.store(
                1 + info->num_erases.load(std::memory_order_relaxed),
                std::memory_order_relaxed
            );
        }

        struct SamplingState
        {
            int64_t next_sample;
            // When we make a sampling decision, we record that distance so we can weight
            // each sample.
            int64_t sample_stride;
        };

        HashtablezInfo* SampleSlow(SamplingState& next_sample, size_t inline_element_size);
        void UnsampleSlow(HashtablezInfo* info);

#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
#error ABSL_INTERNAL_HASHTABLEZ_SAMPLE cannot be directly set
#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)

#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
        class HashtablezInfoHandle
        {
        public:
            explicit HashtablezInfoHandle() :
                info_(nullptr)
            {
            }
            explicit HashtablezInfoHandle(HashtablezInfo* info) :
                info_(info)
            {
            }
            ~HashtablezInfoHandle()
            {
                if (ABSL_PREDICT_TRUE(info_ == nullptr))
                    return;
                UnsampleSlow(info_);
            }

            HashtablezInfoHandle(const HashtablezInfoHandle&) = delete;
            HashtablezInfoHandle& operator=(const HashtablezInfoHandle&) = delete;

            HashtablezInfoHandle(HashtablezInfoHandle&& o) noexcept
                :
                info_(absl::exchange(o.info_, nullptr))
            {
            }
            HashtablezInfoHandle& operator=(HashtablezInfoHandle&& o) noexcept
            {
                if (ABSL_PREDICT_FALSE(info_ != nullptr))
                {
                    UnsampleSlow(info_);
                }
                info_ = absl::exchange(o.info_, nullptr);
                return *this;
            }

            inline void RecordStorageChanged(size_t size, size_t capacity)
            {
                if (ABSL_PREDICT_TRUE(info_ == nullptr))
                    return;
                RecordStorageChangedSlow(info_, size, capacity);
            }

            inline void RecordRehash(size_t total_probe_length)
            {
                if (ABSL_PREDICT_TRUE(info_ == nullptr))
                    return;
                RecordRehashSlow(info_, total_probe_length);
            }

            inline void RecordReservation(size_t target_capacity)
            {
                if (ABSL_PREDICT_TRUE(info_ == nullptr))
                    return;
                RecordReservationSlow(info_, target_capacity);
            }

            inline void RecordClearedReservation()
            {
                if (ABSL_PREDICT_TRUE(info_ == nullptr))
                    return;
                RecordClearedReservationSlow(info_);
            }

            inline void RecordInsert(size_t hash, size_t distance_from_desired)
            {
                if (ABSL_PREDICT_TRUE(info_ == nullptr))
                    return;
                RecordInsertSlow(info_, hash, distance_from_desired);
            }

            inline void RecordErase()
            {
                if (ABSL_PREDICT_TRUE(info_ == nullptr))
                    return;
                RecordEraseSlow(info_);
            }

            friend inline void swap(HashtablezInfoHandle& lhs, HashtablezInfoHandle& rhs)
            {
                std::swap(lhs.info_, rhs.info_);
            }

        private:
            friend class HashtablezInfoHandlePeer;
            HashtablezInfo* info_;
        };
#else
        // Ensure that when Hashtablez is turned off at compile time, HashtablezInfo can
        // be removed by the linker, in order to reduce the binary size.
        class HashtablezInfoHandle
        {
        public:
            explicit HashtablezInfoHandle() = default;
            explicit HashtablezInfoHandle(std::nullptr_t)
            {
            }

            inline void RecordStorageChanged(size_t /*size*/, size_t /*capacity*/)
            {
            }
            inline void RecordRehash(size_t /*total_probe_length*/)
            {
            }
            inline void RecordReservation(size_t /*target_capacity*/)
            {
            }
            inline void RecordClearedReservation()
            {
            }
            inline void RecordInsert(size_t /*hash*/, size_t /*distance_from_desired*/)
            {
            }
            inline void RecordErase()
            {
            }

            friend inline void swap(HashtablezInfoHandle& /*lhs*/, HashtablezInfoHandle& /*rhs*/)
            {
            }
        };
#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)

#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
        extern ABSL_PER_THREAD_TLS_KEYWORD SamplingState global_next_sample;
#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)

        // Returns an RAII sampling handle that manages registration and unregistation
        // with the global sampler.
        inline HashtablezInfoHandle Sample(
            size_t inline_element_size ABSL_ATTRIBUTE_UNUSED
        )
        {
#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
            if (ABSL_PREDICT_TRUE(--global_next_sample.next_sample > 0))
            {
                return HashtablezInfoHandle(nullptr);
            }
            return HashtablezInfoHandle(
                SampleSlow(global_next_sample, inline_element_size)
            );
#else
            return HashtablezInfoHandle(nullptr);
#endif  // !ABSL_PER_THREAD_TLS
        }

        using HashtablezSampler =
            ::absl::profiling_internal::SampleRecorder<HashtablezInfo>;

        // Returns a global Sampler.
        HashtablezSampler& GlobalHashtablezSampler();

        using HashtablezConfigListener = void (*)();
        void SetHashtablezConfigListener(HashtablezConfigListener l);

        // Enables or disables sampling for Swiss tables.
        bool IsHashtablezEnabled();
        void SetHashtablezEnabled(bool enabled);
        void SetHashtablezEnabledInternal(bool enabled);

        // Sets the rate at which Swiss tables will be sampled.
        int32_t GetHashtablezSampleParameter();
        void SetHashtablezSampleParameter(int32_t rate);
        void SetHashtablezSampleParameterInternal(int32_t rate);

        // Sets a soft max for the number of samples that will be kept.
        int32_t GetHashtablezMaxSamples();
        void SetHashtablezMaxSamples(int32_t max);
        void SetHashtablezMaxSamplesInternal(int32_t max);

        // Configuration override.
        // This allows process-wide sampling without depending on order of
        // initialization of static storage duration objects.
        // The definition of this constant is weak, which allows us to inject a
        // different value for it at link time.
        extern "C" bool ABSL_INTERNAL_C_SYMBOL(AbslContainerInternalSampleEverything)();

    }  // namespace container_internal
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_