using System;

using System.Linq;

using System.Numerics;

using System.Diagnostics;

​

public static class Program

{

    static int[] preallocated = Enumerable.Range(1, 200_000).ToArray();

    public static void Main()

    {

        int[] array = new int[150_000];

        Array.Clear(array); FillIncremental(array, 1); Validate(array, 1);

        Array.Clear(array); FillIncremental_Simple(array, 1); Validate(array, 1);

        // preallocated

        Validate(preallocated, 1);  

        Array.Clear(array); Array.Copy(preallocated, array, array.Length); Validate(array, 1);

        const int loops = 2000;

        Console.WriteLine($"Array.Length: {array.Length:#,0}, Loops: {loops:#,0}");

        Measure("Vector", () => { for (int i = 0; i < loops; i++) FillIncremental(array, i); });

        Measure("Simple", () => { for (int i = 0; i < loops; i++) FillIncremental_Simple(array, i); });

        // preallocated

        Measure("ArrayCopy", () => { for (int i = 0; i < loops; i++) Array.Copy(preallocated, array, array.Length); });

        static void Validate(int[] array, int start) { if (array.Select((n, i) => n - i - start).Any(n => n != 0)) throw new Exception("Validation failed!"); }

​

        static void Measure(string title, Action action)

        {

            Console.WriteLine();

            Console.WriteLine($"{title}");

            Stopwatch stopwatch = new Stopwatch();

            long mem0 = GC.GetTotalAllocatedBytes(true);

            stopwatch.Start();

            action();

            stopwatch.Stop();

            long mem1 = GC.GetTotalAllocatedBytes(true);

            Console.WriteLine($"- Duration: {stopwatch.Elapsed.TotalMilliseconds:#,0} msec");

            Console.WriteLine($"- Allocated: {mem1 - mem0:#,0} bytes");

        }

    }

​

    /// <summary>

    /// Fills an array of integers with incremented values, starting from the 'startValue'.

    /// </summary>

    public static void FillIncremental(int[] array, int startValue = 0)

    {

        ArgumentNullException.ThrowIfNull(array);

        if (array.Length > 0 && startValue > (Int32.MaxValue - array.Length) + 1)

            throw new ArgumentOutOfRangeException(nameof(startValue));

​

        static void FillSimple(int[] array, int index, int length, int valueOffset)

        {

            int endIndex =  index + length;

            for (int i = index, j = index + valueOffset; i < endIndex; i++, j++)

                array[i] = j;

        }

​

        if (!Vector.IsHardwareAccelerated || array.Length < Vector<int>.Count)

        {

            FillSimple(array, 0, array.Length, startValue);

            return;

        }

        FillSimple(array, 0, Vector<int>.Count, startValue);

        Vector<int> vector = new(array);

        Vector<int> step = new(Vector<int>.Count);

        int endIndex = array.Length - Vector<int>.Count + 1;

        int i;

        for (i = Vector<int>.Count; i < endIndex; i += Vector<int>.Count)

        {

            vector += step;

            vector.CopyTo(array, i);

        }

        FillSimple(array, i, array.Length - i, startValue);

    }

​

    public static void FillIncremental_Simple(int[] array, int startValue)

    {

        ArgumentNullException.ThrowIfNull(array);

        if (array.Length > 0 && startValue > (Int32.MaxValue - array.Length) + 1)

            throw new ArgumentOutOfRangeException(nameof(startValue));

        for (int i = 0, j = 0 + startValue; i < array.Length; i++, j++) array[i] = j;

    }

}

​