using System;
using System.Linq;
using System.Threading.Tasks;
using Native = System.UInt32;

namespace Cosmos.Core.Memory
{
    /// <summary>
    /// Heap class.
    /// </summary>
    public static unsafe class Heap
    {
        /// <summary>
        /// Init heap.
        /// </summary>
        /// <exception cref="Exception">Thrown on fatal error, contact support.</exception>
        public static void Init()
        {
            HeapSmall.Init();
            HeapMedium.Init();
            HeapLarge.Init();
        }

        /// <summary>
        /// Alloc memory block, of a given size.
        /// </summary>
        /// <param name="aSize">A size of block to alloc, in bytes.</param>
        /// <returns>Byte pointer to the start of the block.</returns>
        public static byte* Alloc(Native aSize)
        {
            if (aSize <= HeapSmall.mMaxItemSize)
            {
                return HeapSmall.Alloc(aSize);
            }
            else if (aSize <= HeapMedium.MaxItemSize)
            {
                return HeapMedium.Alloc(aSize);
            }
            else
            {
                return HeapLarge.Alloc(aSize);
            }
        }

        // Keep as void* and not byte* or other. Reduces typecasting from callers
        // who may have typed the pointer to their own needs.
        /// <summary>
        /// Free a heap item.
        /// </summary>
        /// <param name="aPtr">A pointer to the heap item to be freed.</param>
        /// <exception cref="Exception">Thrown if: 
        /// <list type="bullet">
        /// <item>Page type is not found.</item>
        /// <item>Heap item not found in RAT.</item>
        /// </list>
        /// </exception>
        public static void Free(void* aPtr)
        {
            //TODO find a better way to remove the double look up here for GetPageType and then again in the
            // .Free methods which actually free the entries in the RAT.
            var xType = RAT.GetPageType(aPtr);
            switch (xType)
            {
                case RAT.PageType.HeapLarge:
                    HeapLarge.Free(aPtr);
                    break;

                default:
                    throw new Exception("Heap item not found in RAT.");
            }
        }

    }
}