using System;
using System.Collections.Generic;
using System.Diagnostics;

namespace Amphora.PerformanceTests
{
    public struct Timing
    {
        public double MedianNanoseconds;
        public double MinimumNanoseconds;
        public double BytesPerCall;
    }

    public static class Measurement
    {
        public const double WarmUpMilliseconds = 100;
        public const double BatchMilliseconds = 5;

        /// <summary>
        /// Odd, so the median is a sample rather than an average of two.
        /// </summary>
        public const int Rounds = 31;

        public const int QuickRounds = 7;

        public static double NanosecondsFor(long ticks)
        {
            return ticks * 1e9 / Stopwatch.Frequency;
        }

        public static double Median(List<double> samples)
        {
            var sorted = new List<double>(samples);
            sorted.Sort();
            return sorted[sorted.Count / 2];
        }

        public static double Minimum(List<double> samples)
        {
            double returnMe = double.MaxValue;
            foreach (double sample in samples)
            {
                returnMe = Math.Min(returnMe, sample);
            }

            return returnMe;
        }

        /// <summary>
        /// Runs the call long enough to JIT it and settle the caches, and returns how many calls fill one batch.
        /// </summary>
        public static int WarmUp(Action call)
        {
            long budget = (long)(WarmUpMilliseconds * Stopwatch.Frequency / 1000);
            long start = Stopwatch.GetTimestamp();
            long calls = 0;
            long elapsed;
            do
            {
                call();
                ++calls;
                elapsed = Stopwatch.GetTimestamp() - start;
            }
            while (elapsed < budget);

            double millisecondsPerCall = elapsed * 1000.0 / Stopwatch.Frequency / calls;
            return Math.Max(1, (int)(BatchMilliseconds / millisecondsPerCall));
        }

        public static double NanosecondsPerCall(Action call, int calls)
        {
            long start = Stopwatch.GetTimestamp();
            for (int i = 0; i < calls; ++i)
            {
                call();
            }

            return NanosecondsFor(Stopwatch.GetTimestamp() - start) / calls;
        }

        public static double BytesPerCall(Action call, int calls)
        {
            long before = GC.GetAllocatedBytesForCurrentThread();
            for (int i = 0; i < calls; ++i)
            {
                call();
            }

            return (double)(GC.GetAllocatedBytesForCurrentThread() - before) / calls;
        }

        /// <summary>
        /// Times each call once warm, one batch at a time, taking turns.
        /// </summary>
        public static Timing[] TimeTakingTurns(IReadOnlyList<Action> calls, int rounds)
        {
            var callsPerBatch = new int[calls.Count];
            var samples = new List<double>[calls.Count];
            for (int call = 0; call < calls.Count; ++call)
            {
                callsPerBatch[call] = WarmUp(calls[call]);
                samples[call] = new List<double>(rounds);
            }

            // Each round starts from a different call, so drift in clock speed as the CPU heats up lands on every call
            // alike rather than on whichever ran last.
            for (int round = 0; round < rounds; ++round)
            {
                for (int turn = 0; turn < calls.Count; ++turn)
                {
                    int call = (round + turn) % calls.Count;
                    samples[call].Add(NanosecondsPerCall(calls[call], callsPerBatch[call]));
                }
            }

            var returnMe = new Timing[calls.Count];
            for (int call = 0; call < calls.Count; ++call)
            {
                returnMe[call] = new Timing
                {
                    MedianNanoseconds = Median(samples[call]),
                    MinimumNanoseconds = Minimum(samples[call]),
                    BytesPerCall = BytesPerCall(calls[call], callsPerBatch[call]),
                };
            }

            return returnMe;
        }
    }
}