Project-M/Assets/_Project/Scripts/Simulation/Building/BuildPreviewMath.cs

using Unity.Mathematics;

namespace ProjectM.Simulation
{
    /// <summary>
    /// Pure validity check for the client build-placement PREVIEW (the ground-ghost colour) — the same legality
    /// the server re-validates authoritatively in BuildPlaceSystem, computed client-side so the ghost can read
    /// green (valid) vs red (why-not). No managed types / RNG / wall-clock → unit-testable. The caller supplies
    /// the live occupancy result + the affordability inputs (it owns the structure scan + the ledger read).
    /// </summary>
    public static class BuildPreviewMath
    {
        public const byte Valid = 0;
        public const byte OutOfPlot = 1;
        public const byte Occupied = 2;
        public const byte Unaffordable = 3;

        /// <summary>
        /// Evaluate placement at <paramref name="cell"/>: must be in-plot, unoccupied, and affordable.
        /// <paramref name="occupied"/> = the caller's live-structure cell check; <paramref name="have"/>/<paramref name="cost"/>
        /// the resource on hand vs the catalog cost. Returns the first failing reason, else <see cref="Valid"/>.
        /// </summary>
        public static byte Evaluate(in BaseAnchor anchor, int2 cell, bool occupied, int have, int cost)
        {
            if (!BaseGridMath.IsCellInPlot(anchor, cell)) return OutOfPlot;
            if (occupied) return Occupied;
            if (have < cost) return Unaffordable;
            return Valid;
        }
    }
}