namespace AcDream.Core.World;

/// <summary>
/// Allocates stable, collision-free local ids for dat-authored
/// <c>LandBlockInfo.Objects</c> and building shells.
/// </summary>
/// <remarks>
/// The top nibble is fixed at <c>0xC</c>. The remaining 28 bits encode the
/// complete landblock X byte, Y byte, and a 12-bit per-landblock counter:
/// <c>0xCXXYYIII</c>. The former <c>0xC0XXYYII</c> packing had only eight
/// counter bits, so a legitimate dense retail landblock aborted after 255
/// entries and was retried every time it streamed into view.
/// </remarks>
public static class LandblockStaticEntityIdAllocator
{
    public const uint MaxCounter = 0xFFFu;

    public static uint Base(uint landblockX, uint landblockY) =>
        0xC0000000u
        | ((landblockX & 0xFFu) << 20)
        | ((landblockY & 0xFFu) << 12);

    public static uint Allocate(uint landblockX, uint landblockY, ref uint counter)
    {
        if (counter > MaxCounter)
        {
            throw new InvalidDataException(
                $"Landblock ({landblockX & 0xFFu:X2},{landblockY & 0xFFu:X2}) exceeds the 4096-entry static entity id namespace.");
        }

        return Base(landblockX, landblockY) + counter++;
    }

    public static bool IsInNamespace(uint entityId) =>
        (entityId & 0xF0000000u) == 0xC0000000u;
}