// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System;
 
namespace Microsoft.Extensions.Diagnostics.ResourceMonitoring;
 
/// <summary>
/// A Utilization Calculator.
/// </summary>
internal static class Calculator
{
    private const double Hundred = 100.0;
 
    /// <summary>
    /// Calculate utilization based on two successive samples.
    /// </summary>
    /// <param name="first">Sample one.</param>
    /// <param name="second">Sample two.</param>
    /// <param name="systemResources">CPU and memory limits of the system.</param>
    /// <returns>A utilization score.</returns>
    public static ResourceUtilization CalculateUtilization(in Snapshot first, in Snapshot second, in SystemResources systemResources)
    {
        // Compute the length of the interval between samples.
        long runtimeTickDelta = second.TotalTimeSinceStart.Ticks - first.TotalTimeSinceStart.Ticks;
 
        // Compute the total number of ticks available on the machine during that interval
        double totalSystemTicks = runtimeTickDelta;
 
        // fudge to avoid divide by zero
        if (totalSystemTicks <= 0)
        {
            totalSystemTicks = 1;
        }
 
        // Now, compute the amount of usage between the intervals
        long oldUsageTicks = first.KernelTimeSinceStart.Ticks + first.UserTimeSinceStart.Ticks;
        long newUsageTicks = second.KernelTimeSinceStart.Ticks + second.UserTimeSinceStart.Ticks;
        long totalUsageTickDelta = newUsageTicks - oldUsageTicks;
 
        double cpuUtilization = Math.Max(0.0, totalUsageTickDelta / totalSystemTicks * Hundred);
 
        return new ResourceUtilization(cpuUtilization, second.MemoryUsageInBytes, systemResources, second);
    }
}