Project-M/Assets/_Project/Scripts/Server/Combat/ProjectileDamageSystem.cs

using ProjectM.Simulation;
using Unity.Burst;
using Unity.Collections;
using Unity.Entities;
using Unity.Mathematics;
using Unity.NetCode;
using Unity.Transforms;

namespace ProjectM.Server
{
    /// <summary>
    /// Server-authoritative projectile resolution: applies hits to damageable entities and expires
    /// projectiles past their range. Runs in the server world only
    /// (<see cref="WorldSystemFilterFlags.ServerSimulation"/>) inside the
    /// <see cref="PredictedSimulationSystemGroup"/>, ordered <see cref="ProjectileMoveSystem"/> so the
    /// projectile's <see cref="LocalTransform"/> is the post-move position for this tick.
    ///
    /// Hit detection is a <b>swept</b> planar (XZ) test: rather than checking the projectile's point
    /// position (which tunnels straight through a target when the per-tick step exceeds the target's
    /// radius — e.g. a fast projectile, or any projectile while the server is tick-batching under load),
    /// it reconstructs the segment the projectile traversed this tick
    /// (<c>[curPos - dir*speed*dt, curPos]</c>) and tests each target's hit radius against the closest
    /// point on that segment. The target hit earliest along the path (smallest segment parameter) wins.
    /// A target whose <see cref="GhostOwner"/> matches the projectile's owner is skipped (no self-hits);
    /// dummies carry no <see cref="GhostOwner"/> and are therefore always valid targets.
    ///
    /// On a hit the system appends a <see cref="DamageEvent"/> to the target (consumed by
    /// <c>HealthApplyDamageSystem</c>) and destroys the projectile. Deferring damage to a buffer lets a
    /// single tick stack hits from multiple projectiles. All structural changes go through an
    /// <see cref="EntityCommandBuffer"/> that plays back immediately to the
    /// <see cref="EntityManager"/> (Temp allocator) — keeping this server-only, once-per-tick system
    /// self-contained and plain-world testable without a separate ECB system.
    /// </summary>
    [BurstCompile]
    [WorldSystemFilter(WorldSystemFilterFlags.ServerSimulation)]
    [UpdateInGroup(typeof(PredictedSimulationSystemGroup))]
    [UpdateAfter(typeof(ProjectileMoveSystem))]
    public partial struct ProjectileDamageSystem : ISystem
    {
        /// <summary>Lookup used to read a target's owner so a projectile never hits its own caster.</summary>
        ComponentLookup<GhostOwner> m_GhostOwnerLookup;

        /// <summary>RW lookup to stamp server-only knockback on a hit Husk (Husks bake KnockbackState; players/dummies don't).</summary>
        ComponentLookup<KnockbackState> m_KnockbackLookup;

        /// <summary>Extra forgiveness added to a target's hit radius to approximate the projectile's own size.</summary>
        const float k_ProjectileRadius = 0.2f;

        [BurstCompile]
        public void OnCreate(ref SystemState state)
        {
            m_GhostOwnerLookup = state.GetComponentLookup<GhostOwner>(isReadOnly: true);
            m_KnockbackLookup = state.GetComponentLookup<KnockbackState>(isReadOnly: false);

            // No projectiles → nothing to expire or hit-test; skip the tick (and its allocations) entirely.
            state.RequireForUpdate<Projectile>();
        }

        [BurstCompile]
        public void OnUpdate(ref SystemState state)
        {
            m_GhostOwnerLookup.Update(ref state);
            m_KnockbackLookup.Update(ref state);
            bool haveTick = SystemAPI.TryGetSingleton<NetworkTime>(out var nt);

            float dt = SystemAPI.Time.DeltaTime;
            var ecb = new EntityCommandBuffer(Allocator.Temp);

            // Snapshot all damageable targets once for this tick. Stable iteration order (query order).
            var targetEntities = new NativeList<Entity>(Allocator.Temp);
            var targetPositions = new NativeList<float3>(Allocator.Temp);
            var targetRadii = new NativeList<float>(Allocator.Temp);

            foreach (var (xform, hitRadius, targetEntity) in
                     SystemAPI.Query<RefRO<LocalTransform>, RefRO<HitRadius>>()
                         .WithAll<Health>()
                         .WithEntityAccess())
            {
                targetEntities.Add(targetEntity);
                targetPositions.Add(xform.ValueRO.Position);
                targetRadii.Add(hitRadius.ValueRO.Value);
            }

            foreach (var (xform, proj, owner, projectileEntity) in
                     SystemAPI.Query<RefRO<LocalTransform>, RefRO<Projectile>, RefRO<GhostOwner>>()
                         .WithEntityAccess())
            {
                int projOwnerId = owner.ValueRO.NetworkId;

                // This tick's planar travel segment: [segStart -> segEnd]. Sweeping the segment (rather
                // than testing only segEnd) is what prevents fast projectiles from tunnelling targets.
                float3 cur = xform.ValueRO.Position;
                float2 segEnd = new float2(cur.x, cur.z);
                float2 segStart = segEnd - proj.ValueRO.Direction * (proj.ValueRO.Speed * dt);
                float2 seg = segEnd - segStart;
                float segLenSq = math.lengthsq(seg);

                int bestIdx = -1;
                float bestT = float.MaxValue;
                for (int i = 0; i < targetEntities.Length; i++)
                {
                    var target = targetEntities[i];

                    // Skip the caster: a target whose GhostOwner matches the projectile owner is the
                    // shooter (or another ghost they own). Dummies have no GhostOwner, so never skipped.
                    if (m_GhostOwnerLookup.HasComponent(target) &&
                        m_GhostOwnerLookup[target].NetworkId == projOwnerId)
                        continue;

                    float2 tp = new float2(targetPositions[i].x, targetPositions[i].z);

                    // Closest point on the travel segment to the target centre.
                    float t = segLenSq > 1e-8f
                        ? math.saturate(math.dot(tp - segStart, seg) / segLenSq)
                        : 0f;
                    float2 closest = segStart + t * seg;

                    float hitDist = targetRadii[i] + k_ProjectileRadius;
                    if (math.distancesq(tp, closest) <= hitDist * hitDist && t < bestT)
                    {
                        bestT = t;
                        bestIdx = i;
                    }
                }

                if (bestIdx >= 0)
                {
                    // Earliest target along the travel path: deal damage and consume the projectile.
                    ecb.AppendToBuffer(targetEntities[bestIdx], new DamageEvent
                    {
                        Amount = proj.ValueRO.Damage,
                        SourceNetworkId = projOwnerId,
                    });
                    var hitTarget = targetEntities[bestIdx];
                    if (haveTick && Tuning.KnockbackSpeed > 0f && m_KnockbackLookup.HasComponent(hitTarget))
                    {
                        m_KnockbackLookup[hitTarget] = new KnockbackState
                        {
                            Dir = proj.ValueRO.Direction,
                            Speed = Tuning.KnockbackSpeed,
                            UntilTick = TickUtil.NonZero(nt.ServerTick.TickIndexForValidTick + (uint)math.max(1, Tuning.KnockbackDurationTicks)),
                        };
                    }
                    ecb.DestroyEntity(projectileEntity);
                    continue;
                }

                // Nothing hit this tick: expire the projectile once it has travelled its full range.
                if (proj.ValueRO.DistanceTravelled >= proj.ValueRO.Range)
                    ecb.DestroyEntity(projectileEntity);
            }

            ecb.Playback(state.EntityManager);

            ecb.Dispose();
            targetEntities.Dispose();
            targetPositions.Dispose();
            targetRadii.Dispose();
        }
    }
}