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CSharp · Console

/Program.cs

using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

public class Program
{
    public static void Main()
    {
        string[] documents = new[] { 'n', 'a', 's', 'j', 'a', 'l', 'v', 'g', 'a', 'f', 'f' }
           .Select((item, index) => $"{item}-{index}")
           .ToArray();
        Console.WriteLine($"Documents: [{String.Join(", ", documents)}]");

        var grouped = documents.GroupBy(item => item[0]); // Group by the first char

        ParallelOptions options = new()
        {
            MaxDegreeOfParallelism = Environment.ProcessorCount
        };

        Parallel.ForEach(grouped, options, grouping =>
        {
            foreach (var document in grouping)
            {
                Console.WriteLine($"Processing: {document}");
                Thread.Sleep(500);
            }
        });
    }
}