using System.Collections.Generic;

namespace AcDream.Core.Rendering.Wb;

/// <summary>
/// Computes WorldBuilder's content identity for an EnvCell shell.
/// Equal environment, cell-structure, and ordered surface tuples share one
/// prepared geometry payload even when they occur under different cell IDs.
/// </summary>
public static class EnvCellGeometryIdentity
{
    /// <summary>
    /// Source: WorldBuilder <c>EnvCellRenderManager.GetEnvCellGeomId</c>,
    /// lines 94-103. Bit 33 separates content identities from per-cell IDs,
    /// whose synthetic geometry requests use bit 32.
    /// </summary>
    public static ulong Compute(
        uint environmentId,
        ushort cellStructure,
        IReadOnlyList<ushort> surfaces)
    {
        long hash = 17;
        hash = unchecked(hash * 31 + environmentId);
        hash = unchecked(hash * 31 + cellStructure);
        for (int i = 0; i < surfaces.Count; i++)
            hash = unchecked(hash * 31 + surfaces[i]);
        return (ulong)hash | 0x2_0000_0000UL;
    }
}