naive merge sort.cs

using System;
using System.Collections.Generic;
using System.Linq;

var values = GenerateRandomNumber(200);

foreach(var x in MergeSort(values))
    Console.WriteLine(x);

static int[] GenerateRandomNumber(int size)
{
    var array = new int[size];
    var rand = new Random();
    var maxNum = 10000;
    for (int i = 0; i < size; i++)
        array[i] = rand.Next(maxNum + 1);
    return array;
}

IEnumerable<int> MergeSort(IEnumerable<int> source)
{
    var a = new List<int>();
    var b = new List<int>();
    
    var x = false;
    foreach(var item in source)
    {
        x = !x;
        
        if (x)
            a.Add(item);
        else
            b.Add(item);
    }
    
    var totalSize = a.Count + b.Count;
    
    var result = totalSize switch 
    {
        0 => Enumerable.Empty<int>(),
        1 => a.Count == 1 ? a : b,
        _ => Merge(MergeSort(a), MergeSort(b))
    };
    
    return result;
}

IEnumerable<int> Merge(IEnumerable<int> a, IEnumerable<int> b)
{
    a.TryGetNonEnumeratedCount(out var sizeA);
    b.TryGetNonEnumeratedCount(out var sizeB);
    
    var result = new List<int>(sizeA + sizeB);
    
    using var ae = a.GetEnumerator();
    using var be = b.GetEnumerator();
    var af = ae.MoveNext();
    var bf = be.MoveNext();
    
    while(af && bf)
    {
        if (ae.Current <= be.Current)
        {
            result.Add(ae.Current);
            af = ae.MoveNext();
        }
        else
        {
            result.Add(be.Current);
            bf = be.MoveNext();
        }
    }
    
    if (af)
        do
            result.Add(ae.Current);
        while(ae.MoveNext());
    
    if (bf)
        do
            result.Add(be.Current);
        while(be.MoveNext());
    
    return result;
}