Project-M/Assets/_Project/Scripts/Simulation/Items/InventoryMath.cs

using Unity.Entities;

namespace ProjectM.Simulation
{
    /// <summary>
    /// Pure, deterministic stacking logic for a player's <see cref="InventorySlot"/> buffer (no RNG /
    /// wall-clock / singleton access, so server and any future prediction agree). Parallels
    /// <see cref="StorageMath"/> in spirit but does NOT collapse into it: StorageMath is an unbounded
    /// single-row merge, whereas this enforces a per-item stack cap and a max slot count and supports multiple
    /// stacks of the same item once a stack fills. DynamicBuffer is a handle, so mutations apply to the
    /// underlying entity buffer; growing it (buffer.Add) is a resize, NOT a structural change, so it is safe to
    /// call while iterating a different query. Unit-tested in EditMode via a plain Entities world.
    /// </summary>
    public static class InventoryMath
    {
        /// <summary>
        /// Add <paramref name="count"/> of <paramref name="itemId"/>: first tops up existing non-full stacks
        /// of that item, then appends new stacks (each capped at <paramref name="stackMax"/>) while a free slot
        /// remains (buffer length &lt; <paramref name="maxSlots"/>). Returns the REMAINDER that did not fit
        /// (0 if everything was deposited). No-op (returns 0) for count &lt;= 0; a positive count of itemId 0
        /// returns the full count (nothing deposited — never writes a 0-id row). stackMax &lt; 1 = unbounded.
        /// </summary>
        public static int Deposit(DynamicBuffer<InventorySlot> buffer, ushort itemId, int count, int stackMax, int maxSlots)
        {
            if (count <= 0) return 0;
            if (itemId == 0) return count;
            if (stackMax < 1) stackMax = int.MaxValue;

            // Top up existing stacks of this item.
            for (int i = 0; i < buffer.Length && count > 0; i++)
            {
                if (buffer[i].ItemId != itemId) continue;
                var e = buffer[i];
                int space = stackMax - e.Count;
                if (space <= 0) continue;
                int add = space < count ? space : count;
                e.Count += add;
                buffer[i] = e;
                count -= add;
            }

            // Append new stacks while a slot is free.
            while (count > 0 && buffer.Length < maxSlots)
            {
                int add = stackMax < count ? stackMax : count;
                buffer.Add(new InventorySlot { ItemId = itemId, Count = add });
                count -= add;
            }

            return count;
        }

        /// <summary>
        /// Remove up to <paramref name="count"/> of <paramref name="itemId"/> across all its stacks, clamped to
        /// what is available; drops a stack that reaches zero. Returns the amount actually withdrawn (0 if none).
        /// Iterates back-to-front so RemoveAt does not skip a stack. No-op for count &lt;= 0 or itemId 0.
        /// </summary>
        public static int Withdraw(DynamicBuffer<InventorySlot> buffer, ushort itemId, int count)
        {
            if (count <= 0 || itemId == 0) return 0;

            int taken = 0;
            for (int i = buffer.Length - 1; i >= 0 && count > 0; i--)
            {
                if (buffer[i].ItemId != itemId) continue;
                var e = buffer[i];
                int t = e.Count < count ? e.Count : count;
                e.Count -= t;
                taken += t;
                count -= t;
                if (e.Count <= 0)
                    buffer.RemoveAt(i);
                else
                    buffer[i] = e;
            }
            return taken;
        }

        /// <summary>
        /// Non-mutating check: would depositing <paramref name="count"/> of <paramref name="itemId"/> FULLY fit
        /// (top-up existing stacks + new stacks within <paramref name="maxSlots"/>)? Used by the equip swap to
        /// guarantee the swapped-out item has room BEFORE any withdrawal (no item loss). Mirrors Deposit's space math.
        /// </summary>
        public static bool CanDeposit(DynamicBuffer<InventorySlot> buffer, ushort itemId, int count, int stackMax, int maxSlots)
        {
            if (count <= 0) return true;
            if (itemId == 0) return false;
            if (stackMax < 1) stackMax = int.MaxValue;

            long space = 0;
            for (int i = 0; i < buffer.Length; i++)
                if (buffer[i].ItemId == itemId)
                    space += stackMax - buffer[i].Count;
            int freeSlots = maxSlots - buffer.Length;
            if (freeSlots > 0)
                space += (long)freeSlots * stackMax;
            return space >= count;
        }

        /// <summary>Total quantity of <paramref name="itemId"/> across all stacks (0 if absent).</summary>
        public static int CountOf(DynamicBuffer<InventorySlot> buffer, ushort itemId)
        {
            int total = 0;
            for (int i = 0; i < buffer.Length; i++)
                if (buffer[i].ItemId == itemId)
                    total += buffer[i].Count;
            return total;
        }
    }
}