ParallelExtensions.cs

namespace System.Linq;

public static class ParallelExtensions
{
    public static ParallelQuery<TResult> BatchSelect<T, TResult>(
        this T[] source,
        Func<Memory<T>, TResult> selector,
        int degreeOfParallelism = 0)
    {
        return source.AsMemory().BatchSelect(selector, degreeOfParallelism);
    }

    public static ParallelQuery<TResult> BatchSelect<T, TResult>(
        this Memory<T> source,
        Func<Memory<T>, TResult> selector,
        int degreeOfParallelism = 0)
    {
        if (degreeOfParallelism <= 0)
        {
            degreeOfParallelism = Environment.ProcessorCount;
        }

        var batchLength = source.Length / degreeOfParallelism;

        return ParallelEnumerable
            .Range(0, degreeOfParallelism + 1)
            .Select(i =>
            {
                var start = i * batchLength;
                var length = i == degreeOfParallelism
                    ? source.Length - start
                    : batchLength;

                return selector(source.Slice(start, length));
            });
    }
}