// <copyright file="MetricReader.cs" company="OpenTelemetry Authors">
// Copyright The OpenTelemetry 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
//
//     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.
// </copyright>

using System;
using System.Diagnostics;
using System.Diagnostics.Metrics;
using System.Threading;
using System.Threading.Tasks;
using OpenTelemetry.Internal;

namespace OpenTelemetry.Metrics
{
    /// <summary>
    /// MetricReader base class.
    /// </summary>
    public abstract partial class MetricReader : IDisposable
    {
        private const MetricReaderTemporalityPreference MetricReaderTemporalityPreferenceUnspecified = (MetricReaderTemporalityPreference)0;

        private static Func<Type, AggregationTemporality> cumulativeTemporalityPreferenceFunc =
            (instrumentType) => AggregationTemporality.Cumulative;

        private static Func<Type, AggregationTemporality> monotonicDeltaTemporalityPreferenceFunc = (instrumentType) =>
        {
            return instrumentType.GetGenericTypeDefinition() switch
            {
                var type when type == typeof(Counter<>) => AggregationTemporality.Delta,
                var type when type == typeof(ObservableCounter<>) => AggregationTemporality.Delta,
                var type when type == typeof(Histogram<>) => AggregationTemporality.Delta,

                // Temporality is not defined for gauges, so this does not really affect anything.
                var type when type == typeof(ObservableGauge<>) => AggregationTemporality.Delta,

                var type when type == typeof(UpDownCounter<>) => AggregationTemporality.Cumulative,
                var type when type == typeof(ObservableUpDownCounter<>) => AggregationTemporality.Cumulative,

                // TODO: Consider logging here because we should not fall through to this case.
                _ => AggregationTemporality.Delta,
            };
        };

        private readonly object newTaskLock = new();
        private readonly object onCollectLock = new();
        private readonly TaskCompletionSource<bool> shutdownTcs = new();
        private MetricReaderTemporalityPreference temporalityPreference = MetricReaderTemporalityPreferenceUnspecified;
        private Func<Type, AggregationTemporality> temporalityFunc = cumulativeTemporalityPreferenceFunc;
        private int shutdownCount;
        private TaskCompletionSource<bool> collectionTcs;
        private BaseProvider parentProvider;

        public MetricReaderTemporalityPreference TemporalityPreference
        {
            get
            {
                if (this.temporalityPreference == MetricReaderTemporalityPreferenceUnspecified)
                {
                    this.temporalityPreference = MetricReaderTemporalityPreference.Cumulative;
                }

                return this.temporalityPreference;
            }

            set
            {
                if (this.temporalityPreference != MetricReaderTemporalityPreferenceUnspecified)
                {
                    throw new NotSupportedException($"The temporality preference cannot be modified (the current value is {this.temporalityPreference}).");
                }

                this.temporalityPreference = value;
                switch (value)
                {
                    case MetricReaderTemporalityPreference.Delta:
                        this.temporalityFunc = monotonicDeltaTemporalityPreferenceFunc;
                        break;
                    case MetricReaderTemporalityPreference.Cumulative:
                    default:
                        this.temporalityFunc = cumulativeTemporalityPreferenceFunc;
                        break;
                }
            }
        }

        /// <summary>
        /// Attempts to collect the metrics, blocks the current thread until
        /// metrics collection completed, shutdown signaled or timed out.
        /// If there are asynchronous instruments involved, their callback
        /// functions will be triggered.
        /// </summary>
        /// <param name="timeoutMilliseconds">
        /// The number (non-negative) of milliseconds to wait, or
        /// <c>Timeout.Infinite</c> to wait indefinitely.
        /// </param>
        /// <returns>
        /// Returns <c>true</c> when metrics collection succeeded; otherwise,
        /// <c>false</c>.
        /// </returns>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown when the <c>timeoutMilliseconds</c> is smaller than -1.
        /// </exception>
        /// <remarks>
        /// This function guarantees thread-safety. If multiple calls occurred
        /// simultaneously, they might get folded and result in less calls to
        /// the <c>OnCollect</c> callback for improved performance, as long as
        /// the semantic can be preserved.
        /// </remarks>
        public bool Collect(int timeoutMilliseconds = Timeout.Infinite)
        {
            Guard.ThrowIfInvalidTimeout(timeoutMilliseconds);

            OpenTelemetrySdkEventSource.Log.MetricReaderEvent("MetricReader.Collect method called.");
            var shouldRunCollect = false;
            var tcs = this.collectionTcs;

            if (tcs == null)
            {
                lock (this.newTaskLock)
                {
                    tcs = this.collectionTcs;

                    if (tcs == null)
                    {
                        shouldRunCollect = true;
                        tcs = new TaskCompletionSource<bool>();
                        this.collectionTcs = tcs;
                    }
                }
            }

            if (!shouldRunCollect)
            {
                return Task.WaitAny(tcs.Task, this.shutdownTcs.Task, Task.Delay(timeoutMilliseconds)) == 0 && tcs.Task.Result;
            }

            var result = false;
            try
            {
                lock (this.onCollectLock)
                {
                    this.collectionTcs = null;
                    result = this.OnCollect(timeoutMilliseconds);
                }
            }
            catch (Exception ex)
            {
                OpenTelemetrySdkEventSource.Log.MetricReaderException(nameof(this.Collect), ex);
            }

            tcs.TrySetResult(result);

            if (result)
            {
                OpenTelemetrySdkEventSource.Log.MetricReaderEvent("MetricReader.Collect succeeded.");
            }
            else
            {
                OpenTelemetrySdkEventSource.Log.MetricReaderEvent("MetricReader.Collect failed.");
            }

            return result;
        }

        /// <summary>
        /// Attempts to shutdown the processor, blocks the current thread until
        /// shutdown completed or timed out.
        /// </summary>
        /// <param name="timeoutMilliseconds">
        /// The number (non-negative) of milliseconds to wait, or
        /// <c>Timeout.Infinite</c> to wait indefinitely.
        /// </param>
        /// <returns>
        /// Returns <c>true</c> when shutdown succeeded; otherwise, <c>false</c>.
        /// </returns>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown when the <c>timeoutMilliseconds</c> is smaller than -1.
        /// </exception>
        /// <remarks>
        /// This function guarantees thread-safety. Only the first call will
        /// win, subsequent calls will be no-op.
        /// </remarks>
        public bool Shutdown(int timeoutMilliseconds = Timeout.Infinite)
        {
            Guard.ThrowIfInvalidTimeout(timeoutMilliseconds);

            OpenTelemetrySdkEventSource.Log.MetricReaderEvent("MetricReader.Shutdown called.");

            if (Interlocked.CompareExchange(ref this.shutdownCount, 1, 0) != 0)
            {
                return false; // shutdown already called
            }

            var result = false;
            try
            {
                result = this.OnShutdown(timeoutMilliseconds);
            }
            catch (Exception ex)
            {
                OpenTelemetrySdkEventSource.Log.MetricReaderException(nameof(this.Shutdown), ex);
            }

            this.shutdownTcs.TrySetResult(result);

            if (result)
            {
                OpenTelemetrySdkEventSource.Log.MetricReaderEvent("MetricReader.Shutdown succeeded.");
            }
            else
            {
                OpenTelemetrySdkEventSource.Log.MetricReaderEvent("MetricReader.Shutdown failed.");
            }

            return result;
        }

        /// <inheritdoc/>
        public void Dispose()
        {
            this.Dispose(true);
            GC.SuppressFinalize(this);
        }

        internal virtual void SetParentProvider(BaseProvider parentProvider)
        {
            this.parentProvider = parentProvider;
        }

        /// <summary>
        /// Processes a batch of metrics.
        /// </summary>
        /// <param name="metrics">Batch of metrics to be processed.</param>
        /// <param name="timeoutMilliseconds">
        /// The number (non-negative) of milliseconds to wait, or
        /// <c>Timeout.Infinite</c> to wait indefinitely.
        /// </param>
        /// <returns>
        /// Returns <c>true</c> when metrics processing succeeded; otherwise,
        /// <c>false</c>.
        /// </returns>
        internal virtual bool ProcessMetrics(in Batch<Metric> metrics, int timeoutMilliseconds)
        {
            return true;
        }

        /// <summary>
        /// Called by <c>Collect</c>. This function should block the current
        /// thread until metrics collection completed, shutdown signaled or
        /// timed out.
        /// </summary>
        /// <param name="timeoutMilliseconds">
        /// The number (non-negative) of milliseconds to wait, or
        /// <c>Timeout.Infinite</c> to wait indefinitely.
        /// </param>
        /// <returns>
        /// Returns <c>true</c> when metrics collection succeeded; otherwise,
        /// <c>false</c>.
        /// </returns>
        /// <remarks>
        /// This function is called synchronously on the threads which called
        /// <c>Collect</c>. This function should not throw exceptions.
        /// </remarks>
        protected virtual bool OnCollect(int timeoutMilliseconds)
        {
            OpenTelemetrySdkEventSource.Log.MetricReaderEvent("MetricReader.OnCollect called.");

            var sw = timeoutMilliseconds == Timeout.Infinite
                ? null
                : Stopwatch.StartNew();

            var collectObservableInstruments = this.parentProvider.GetObservableInstrumentCollectCallback();
            collectObservableInstruments?.Invoke();

            OpenTelemetrySdkEventSource.Log.MetricReaderEvent("Observable instruments collected.");

            var metrics = this.GetMetricsBatch();

            bool result;
            if (sw == null)
            {
                OpenTelemetrySdkEventSource.Log.MetricReaderEvent("ProcessMetrics called.");
                result = this.ProcessMetrics(metrics, Timeout.Infinite);
                if (result)
                {
                    OpenTelemetrySdkEventSource.Log.MetricReaderEvent("ProcessMetrics succeeded.");
                }
                else
                {
                    OpenTelemetrySdkEventSource.Log.MetricReaderEvent("ProcessMetrics failed.");
                }

                return result;
            }
            else
            {
                var timeout = timeoutMilliseconds - sw.ElapsedMilliseconds;

                if (timeout <= 0)
                {
                    OpenTelemetrySdkEventSource.Log.MetricReaderEvent("OnCollect failed timeout period has elapsed.");
                    return false;
                }

                OpenTelemetrySdkEventSource.Log.MetricReaderEvent("ProcessMetrics called.");
                result = this.ProcessMetrics(metrics, (int)timeout);
                if (result)
                {
                    OpenTelemetrySdkEventSource.Log.MetricReaderEvent("ProcessMetrics succeeded.");
                }
                else
                {
                    OpenTelemetrySdkEventSource.Log.MetricReaderEvent("ProcessMetrics failed.");
                }

                return result;
            }
        }

        /// <summary>
        /// Called by <c>Shutdown</c>. This function should block the current
        /// thread until shutdown completed or timed out.
        /// </summary>
        /// <param name="timeoutMilliseconds">
        /// The number (non-negative) of milliseconds to wait, or
        /// <c>Timeout.Infinite</c> to wait indefinitely.
        /// </param>
        /// <returns>
        /// Returns <c>true</c> when shutdown succeeded; otherwise, <c>false</c>.
        /// </returns>
        /// <remarks>
        /// This function is called synchronously on the thread which made the
        /// first call to <c>Shutdown</c>. This function should not throw
        /// exceptions.
        /// </remarks>
        protected virtual bool OnShutdown(int timeoutMilliseconds)
        {
            return this.Collect(timeoutMilliseconds);
        }

        /// <summary>
        /// Releases the unmanaged resources used by this class and optionally
        /// releases the managed resources.
        /// </summary>
        /// <param name="disposing">
        /// <see langword="true"/> to release both managed and unmanaged resources;
        /// <see langword="false"/> to release only unmanaged resources.
        /// </param>
        protected virtual void Dispose(bool disposing)
        {
        }
    }
}
