// -----------------------------------------------------------------------------
// file="UnmanagedArray"
// Project: StgSharp
// Copyright (c) Nitload.
// SPDX-License-Identifier: MIT
// -----------------------------------------------------------------------------

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace StgSharp.Collections
{
    /// <summary>
    ///   Zero-overhead view over a contiguous region of unmanaged memory. Does not own the memory —
    ///   caller is responsible for lifetime management.
    /// </summary>
    /// <typeparam name="T">
    ///   Element type. Must be unmanaged.
    /// </typeparam>
    public readonly unsafe struct UnmanagedArray<T> where T : unmanaged
    {

        private static readonly nint ElementSize = sizeof(T);

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public UnmanagedArray(
               T* pointer,
               int length
        )
        {
            Address = pointer;
            Length = length;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public UnmanagedArray(
               nuint address,
               int length
        )
        {
            Address = (T*)address;
            Length = length;
        }

        /// <summary>
        ///   Indexed access. No bounds check in Release builds.
        /// </summary>
        public ref T this[
                     int index
        ]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get
            {
                Debug.Assert((uint)index < (uint)Length,
                    "UnmanagedArray index out of range");
                return ref Address[index];
            }
        }

        /// <summary>
        ///   Raw pointer to the first element.
        /// </summary>
        public T* Address
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get;
            init;
        }

        /// <summary>
        ///   Element count.
        /// </summary>
        public int Length
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get;
            init;
        }

        /// <summary>
        ///   Total byte size of the array (for allocation calculations).
        /// </summary>
        public int ByteSize => Length * sizeof(T);

        /// <summary>
        ///   Wraps the array as a <see cref="Span{T}" />.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Span<T> AsSpan()
        {
            return new Span<T>(Address, Length);
        }

        /// <summary>
        ///   Wraps a sub-range as a <see cref="Span{T}" />.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public Span<T> AsSpan(
                       int start,
                       int length
        )
        {
            Debug.Assert(((uint)start) + ((uint)length) <= (uint)Length,
                "UnmanagedArray.AsSpan range out of bounds");
            return new Span<T>(Address + start, length);
        }

        /// <summary>
        ///   Zero-fill all elements.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Clear()
        {
            Unsafe.InitBlock(Address, 0, (uint)(Length * sizeof(T)));
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public int IndexOf(
                   T* ptr
        )
        {
            int offset = (int)((((nint)ptr) - ((nint)Address)) / ElementSize);
            ArgumentOutOfRangeException.ThrowIfNegative(offset, nameof(ptr));
            ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual(offset, Length, nameof(ptr));
            return offset;
        }

        /// <summary>
        ///   Returns a pointer to element at <paramref name="index" />. Useful for feeding SIMD
        ///   load instructions (e.g. Sse2.LoadVector128).
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T* PtrAt(
                  int index
        )
        {
            Debug.Assert((uint)index < (uint)Length,
                "UnmanagedArray.PtrAt index out of range");
            return Address + index;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public int UncheckedIndexOf(
                   T* ptr
        )
        {
            return (int)((((nint)ptr) - ((nint)Address)) / ElementSize);
        }

    }
}