namespace QWERTYkez.Mensura.Extensions;

public static partial class CollectionsMultiplyExtensions
{
    // === MultiplyCore === SIMD
    internal static void MultiplyCore<T, R>(this ReadOnlySpan<T> units, double multiplicator, int len, Span<R> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
        where R : struct, IMensuraUnit, IEquatable<R>
    {
        ReadOnlySpan<double> srcDouble = MemoryMarshal.Cast<T, double>(units);
        Span<double> dstDouble = MemoryMarshal.Cast<R, double>(destination);

        var vectorizedMultiplicator = new Vector<double>(multiplicator);
        int vectorSize = Vector<double>.Count;
        int i = 0;

        ref double srcRef = ref MemoryMarshal.GetReference(srcDouble);
        ref double dstRef = ref MemoryMarshal.GetReference(dstDouble);

        int simdEnd = len & ~(vectorSize - 1);

        for (; i < simdEnd; i += vectorSize)
        {
            ref double currentSrc = ref Unsafe.Add(ref srcRef, i);
            ref double currentDst = ref Unsafe.Add(ref dstRef, i);

            var vector = Unsafe.As<double, Vector<double>>(ref currentSrc);
            var multiplied = vector * vectorizedMultiplicator;

            Unsafe.As<double, Vector<double>>(ref currentDst) = multiplied;
        }

        for (; i < len; i++)
        {
            Unsafe.Add(ref dstRef, i) = Unsafe.Add(ref srcRef, i) * multiplicator;
        }
    }
    internal static void MultiplyCore<T, R>(this ReadOnlySpan<T?> units, double multiplicator, int len, Span<R?> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
        where R : struct, IMensuraUnit, IEquatable<R>
    {
        // Получаем прямые неуправляемые ref-ссылки на начало буферов за 0 тактов
        ref var srcRef = ref MemoryMarshal.GetReference(units);
        ref var dstRef = ref MemoryMarshal.GetReference(destination);

        int i = 0;
        int unrollEnd = len & ~3; // Граница развернутого цикла (кратная 4)

        // 1. ОСНОВНОЙ ЦИКЛ: Обрабатываем конвейером по 4 элемента за итерацию
        for (; i < unrollEnd; i += 4)
        {
            T? u0 = Unsafe.Add(ref srcRef, i);
            T? u1 = Unsafe.Add(ref srcRef, i + 1);
            T? u2 = Unsafe.Add(ref srcRef, i + 2);
            T? u3 = Unsafe.Add(ref srcRef, i + 3);

            // Получаем ref-ссылки на ячейки назначения (zero-cost адресация)
            ref var d0 = ref Unsafe.Add(ref dstRef, i);
            ref var d1 = ref Unsafe.Add(ref dstRef, i + 1);
            ref var d2 = ref Unsafe.Add(ref dstRef, i + 2);
            ref var d3 = ref Unsafe.Add(ref dstRef, i + 3);

            // Пишем строго по месту. Если HasValue == false, в ячейку dX запишется null (сбросятся байты флага)
            d0 = u0.HasValue ? (u0.Value.ToDouble() * multiplicator).ToUnit<R>() : null;
            d1 = u1.HasValue ? (u1.Value.ToDouble() * multiplicator).ToUnit<R>() : null;
            d2 = u2.HasValue ? (u2.Value.ToDouble() * multiplicator).ToUnit<R>() : null;
            d3 = u3.HasValue ? (u3.Value.ToDouble() * multiplicator).ToUnit<R>() : null;
        }

        // 2. ХВОСТ ЦИКЛА: Довычисляем остаток элементов (от 1 до 3 штук)
        for (; i < len; i++)
        {
            T? unit = Unsafe.Add(ref srcRef, i);
            ref var dst = ref Unsafe.Add(ref dstRef, i);

            dst = unit.HasValue ? (unit.Value.ToDouble() * multiplicator).ToUnit<R>() : null;
        }
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    internal static void MultiplyCore<T, R>(this double multiplicator, ReadOnlySpan<T> units, int len, Span<R> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
        where R : struct, IMensuraUnit, IEquatable<R> => units.MultiplyCore(multiplicator, len, destination);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    internal static void MultiplyCore<T, R>(this double multiplicator, ReadOnlySpan<T?> units, int len, Span<R?> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
        where R : struct, IMensuraUnit, IEquatable<R> => units.MultiplyCore(multiplicator, len, destination);




    // === ReadOnlySpan
    public static void Multiply<T>(this ReadOnlySpan<T> units, double multiplicator, Span<T> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units.IsEmpty) return;
        int len = units.Length;
        if (len > destination.Length)
            throw new ArgumentException("Целевой буфер destination меньше исходного source.");

        units.MultiplyCore(multiplicator, len, destination);
    }
    public static void Multiply<T>(this ReadOnlySpan<T?> units, double multiplicator, Span<T?> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units.IsEmpty) return;
        int len = units.Length;
        if (len > destination.Length)
            throw new ArgumentException("Целевой буфер destination меньше исходного source.");

        units.MultiplyCore(multiplicator, len, destination);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void Multiply<T>(this double multiplicator, ReadOnlySpan<T> units, Span<T> destination)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator, destination);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void Multiply<T>(this double multiplicator, ReadOnlySpan<T?> units, Span<T?> destination)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator, destination);

    // === Array ===
    public static T[] Multiply<T>(this T[] units, double multiplicator)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return null!;
        int len = units.Length;
        if (len == 0) return [];

        var result = new T[len];
        Multiply(units, multiplicator, result);
        return result;
    }
    public static T?[] Multiply<T>(this T?[] units, double multiplicator)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return null!;
        int len = units.Length;
        if (len == 0) return [];

        var result = new T?[len];
        Multiply(units, multiplicator, result);
        return result;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T[] Multiply<T>(this double multiplicator, T[] units)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T?[] Multiply<T>(this double multiplicator, T?[] units)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator);

    // === List<T> ===
    public static List<T> Multiply<T>(this List<T> units, double multiplicator)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return null!;
        int len = units.Count;
        if (len == 0) return [];

        var resultArray = new T[len];
        Multiply(CollectionsMarshal.AsSpan(units), multiplicator, resultArray);
        return resultArray.WrapAsList();
    }
    public static List<T?> Multiply<T>(this List<T?> units, double multiplicator)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return null!;
        int count = units.Count;
        if (count == 0) return [];

        var resultArray = new T?[count];
        Multiply(CollectionsMarshal.AsSpan(units), multiplicator, resultArray);
        return resultArray.WrapAsList();
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static List<T> Multiply<T>(this double multiplicator, List<T> units)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static List<T?> Multiply<T>(this double multiplicator, List<T?> units)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator);

    // === ICollection<T> ===
    public static void Multiply<T>(this ICollection<T> units, double multiplicator, Span<T> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return;
        int count = units.Count;
        if (count == 0) return;
        if (destination.Length < count)
            throw new ArgumentException("Destination too short");

        if (units is T[] array) { array.Multiply(multiplicator, destination); return; }
        if (units is List<T> list) { CollectionsMarshal.AsSpan(list).Multiply(multiplicator, destination); return; }

        int i = 0;
        foreach (var item in units)
            destination[i++] = (item.ToDouble() * multiplicator).ToUnit<T>();
    }
    public static void Multiply<T>(this ICollection<T?> units, double multiplicator, Span<T?> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return;
        int count = units.Count;
        if (count == 0) return;
        if (destination.Length < count)
            throw new ArgumentException("Destination too short");

        if (units is T?[] array) { array.Multiply(multiplicator, destination); return; }
        if (units is List<T?> list) { CollectionsMarshal.AsSpan(list).Multiply(multiplicator, destination); return; }

        int i = 0;
        foreach (var item in units)
            destination[i++] = item.HasValue
                ? (item.Value.ToDouble() * multiplicator).ToUnit<T>() : null;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void Multiply<T>(this double multiplicator, ICollection<T> units, Span<T> destination)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator, destination);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void Multiply<T>(this double multiplicator, ICollection<T?> units, Span<T?> destination)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator, destination);

    // === IReadOnlyCollection<T> ===
    public static void Multiply<T>(this IReadOnlyCollection<T> units, double multiplicator, Span<T> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return;
        int count = units.Count;
        if (count == 0) return;
        if (destination.Length < count)
            throw new ArgumentException("Destination too short");

        if (units is T[] array) { array.Multiply(multiplicator, destination); return; }
        if (units is List<T> list) { CollectionsMarshal.AsSpan(list).Multiply(multiplicator, destination); return; }

        int i = 0;
        foreach (var item in units)
            destination[i++] = (item.ToDouble() * multiplicator).ToUnit<T>();
    }
    public static void Multiply<T>(this IReadOnlyCollection<T?> units, double multiplicator, Span<T?> destination)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return;
        int count = units.Count;
        if (count == 0) return;
        if (destination.Length < count)
            throw new ArgumentException("Destination too short");

        if (units is T?[] array) { array.Multiply(multiplicator, destination); return; }
        if (units is List<T?> list) { CollectionsMarshal.AsSpan(list).Multiply(multiplicator, destination); return; }

        int i = 0;
        foreach (var item in units)
            destination[i++] = item.HasValue
                ? (item.Value.ToDouble() * multiplicator).ToUnit<T>() : null;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void Multiply<T>(this double multiplicator, IReadOnlyCollection<T> units, Span<T> destination)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator, destination);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void Multiply<T>(this double multiplicator, IReadOnlyCollection<T?> units, Span<T?> destination)
        where T : struct, IMensuraUnit, IEquatable<T> => units.Multiply(multiplicator, destination);

    // === IEnumerable<T> + yeild ===
    static IEnumerable<T> MultiplyIterator<T>(IEnumerable<T> units, double multiplicator)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        foreach (var item in units)
            yield return (item.ToDouble() * multiplicator).ToUnit<T>();
    }
    static IEnumerable<T?> MultiplyNullableIterator<T>(IEnumerable<T?> units, double multiplicator)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        foreach (T? item in units)
            yield return item.HasValue
                ? (item.Value.ToDouble() * multiplicator).ToUnit<T>() : null;
    }

    // === IEnumerable<T> ===
    public static IEnumerable<T> Multiply<T>(this IEnumerable<T> units, double multiplicator)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return null!;
        if (units is T[] array) return array.Multiply(multiplicator);
        if (units is List<T> list) return list.Multiply(multiplicator);
        if (units is ICollection<T> col)
        {
            var arr = new T[col.Count];
            col.Multiply(multiplicator, arr);
            return arr;
        }
        if (units is IReadOnlyCollection<T> roc)
        {
            var arr = new T[roc.Count];
            roc.Multiply(multiplicator, arr);
            return arr;
        }
        return MultiplyIterator(units, multiplicator);
    }
    public static IEnumerable<T?> Multiply<T>(this IEnumerable<T?> units, double multiplicator)
        where T : struct, IMensuraUnit, IEquatable<T>
    {
        if (units is null) return null!;
        if (units is T?[] array) return array.Multiply(multiplicator);
        if (units is List<T?> list) return list.Multiply(multiplicator);
        if (units is ICollection<T?> col)
        {
            var arr = new T?[col.Count];
            col.Multiply(multiplicator, arr);
            return arr;
        }
        if (units is IReadOnlyCollection<T?> roc)
        {
            var arr = new T?[roc.Count];
            roc.Multiply(multiplicator, arr);
            return arr;
        }
        return MultiplyNullableIterator(units, multiplicator);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static IEnumerable<T> Multiply<T>(this double multiplicator, IEnumerable<T> units)
        where T : struct, IMensuraUnit, IEquatable<T> => Multiply(units, multiplicator);

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static IEnumerable<T?> Multiply<T>(this double multiplicator, IEnumerable<T?> units)
        where T : struct, IMensuraUnit, IEquatable<T> => Multiply(units, multiplicator);
}