using System;
using System.Collections.Generic;
using System.Linq;
using static System.Buffers.Binary.BinaryPrimitives;

namespace NHSE.Core;

/// <summary>
/// Handles operations for parsing the player inventory.
/// </summary>
/// <remarks>
/// Player Inventory is comprised of multiple values, which we won't bother reimplementing as a convertible data structure.
/// <br>Refer to GSavePlayerItemBaggage's ItemBag &amp; ItemPocket in the dumped c-structure schema.</br>
/// </remarks>
public static class PlayerItemSet
{
    private const int ItemSet_Quantity = 2; // Pouch (Bag) & Pocket.
    private const int ItemSet_ItemCount = 20; // 20 items per item set.
    private const int ItemSet_ItemSize = Item.SIZE * ItemSet_ItemCount;
    private const int ItemSet_MetaSize = 4 + ItemSet_ItemCount;
    private const int ItemSet_TotalSize = (ItemSet_ItemSize + ItemSet_MetaSize) * ItemSet_Quantity;
    private const int ShiftToTopOfStructure = -ItemSet_MetaSize - (Item.SIZE * ItemSet_ItemCount); // shifts slot1 offset => top of data structure

    /// <summary>
    /// Gets the Offset and Size to read from based on the Item 1 RAM offset.
    /// </summary>
    /// <param name="slot1">Item Slot 1 offset in RAM</param>
    /// <param name="offset">Offset to read from</param>
    /// <param name="length">Length of data to read from <see cref="offset"/></param>
    public static void GetOffsetLength(uint slot1, out uint offset, out int length)
    {
        offset = (uint)((int)slot1 + ShiftToTopOfStructure);
        length = ItemSet_TotalSize;
    }

    /// <summary>
    /// Compares the raw data to the expected data layout.
    /// </summary>
    /// <param name="data">Raw RAM from the game from the offset read (as per <see cref="GetOffsetLength"/>).</param>
    /// <returns>True if valid, false if not valid or corrupt.</returns>
    public static bool ValidateItemBinary(ReadOnlySpan<byte> data)
    {
        // Check the unlocked slot count -- expect 0,10,20
        var bagCount = ReadUInt32LittleEndian(data[ItemSet_ItemSize..]);
        if (bagCount > ItemSet_ItemCount || bagCount % 10 != 0) // pouch21-39 count
            return false;

        var pocketCount = ReadUInt32LittleEndian(data[(ItemSet_ItemSize + ItemSet_MetaSize + ItemSet_ItemSize)..]);
        if (pocketCount != ItemSet_ItemCount) // pouch0-19 count should be 20.
            return false;

        // Check the item wheel binding -- expect -1 or [0,7]
        // Disallow duplicate binds!
        // Don't bother checking that bind[i] (when ! -1) is not NONE at items[i]. We don't need to check everything!
        var bound = new List<byte>();
        if (!ValidateBindList(data, ItemSet_ItemSize + 4, bound))
            return false;
        if (!ValidateBindList(data, ItemSet_ItemSize + 4 + (ItemSet_ItemSize + ItemSet_MetaSize), bound))
            return false;

        return true;
    }

    private static bool ValidateBindList(ReadOnlySpan<byte> data, int bindStart, ICollection<byte> bound)
    {
        for (int i = 0; i < ItemSet_ItemCount; i++)
        {
            var bind = data[bindStart + i];
            if (bind == 0xFF) // Not bound
                continue;
            if (bind > 7) // Only [0,7] permitted as the wheel has 8 spots
                return false;
            if (bound.Contains(bind)) // Wheel index is already bound to another item slot
                return false;

            bound.Add(bind);
        }

        return true;
    }

    /// <summary>
    /// Reads the items present in the player inventory packet and returns the list of items.
    /// </summary>
    /// <param name="data">Player Inventory packet</param>
    public static Item[] ReadPlayerInventory(ReadOnlySpan<byte> data)
    {
        var items = GetEmptyItemArray(40);
        ReadPlayerInventory(data, items);
        return items;
    }

    private static Item[] GetEmptyItemArray(int count)
    {
        var items = new Item[count];
        for (int i = 0; i < items.Length; i++)
            items[i] = new Item();
        return items;
    }

    /// <summary>
    /// Reads the items present in the player inventory packet into the <see cref="destination"/> list of items.
    /// </summary>
    /// <param name="data">Player Inventory packet</param>
    /// <param name="destination">40 Item array</param>
    public static void ReadPlayerInventory(ReadOnlySpan<byte> data, IReadOnlyList<Item> destination)
    {
        var pocket2 = destination.Take(20).ToArray();
        var pocket1 = destination.Skip(20).ToArray();
        var p1 = Item.GetArray(data[..ItemSet_ItemSize]);
        var p2 = Item.GetArray(data.Slice(ItemSet_ItemSize + 0x18, ItemSet_ItemSize));

        for (int i = 0; i < pocket1.Length; i++)
            pocket1[i].CopyFrom(p1[i]);

        for (int i = 0; i < pocket2.Length; i++)
            pocket2[i].CopyFrom(p2[i]);
    }

    /// <summary>
    /// Writes the items in the <see cref="source"/> list of items to the player inventory packet.
    /// </summary>
    /// <param name="data">Player Inventory packet</param>
    /// <param name="source">40 Item array</param>
    public static void WritePlayerInventory(Span<byte> data, IReadOnlyList<Item> source)
    {
        var pocket2 = source.Take(20).ToArray();
        var pocket1 = source.Skip(20).ToArray();
        var p1 = Item.SetArray(pocket1);
        var p2 = Item.SetArray(pocket2);

        p1.CopyTo(data);
        p2.CopyTo(data[(ItemSet_ItemSize + 0x18)..]);
    }
}