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>
    /// MC-2 — resolves hostile Spitter projectiles against PLAYERS + STRUCTURES (never other enemies — only
    /// PlayerTag / PlacedStructure are snapshotted, so a spit can't friendly-fire the Husks), server-only in the
    /// plain <see cref="SimulationSystemGroup"/> after <see cref="EnemyProjectileMoveSystem"/> (post-move position).
    /// SWEPT planar hit-test (the DR-018 anti-tunnelling discipline): the travel segment is rebuilt from the STORED
    /// <see cref="EnemyProjectile.LastStep"/> (cur - Direction*LastStep), NEVER a fresh delta. REGION-FILTERED: a
    /// target whose <see cref="RegionTag"/>.Region != the spit's Region is skipped — relevancy hides cross-region
    /// ghosts from CLIENTS, but the server world holds base + expedition players 1000u apart, so server damage needs
    /// its own guard (the missing-filter blocker the design review caught). On a hit it appends
    /// DamageEvent{SourceNetworkId=-1, SourceTick=now} (drained the FOLLOWING tick by the predicted
    /// <c>HealthApplyDamageSystem</c> — appending from the predicted loop would double-apply on rollback; SourceTick
    /// makes the dash i-frame negation correct across the 1-tick gap, so dash-through-spit works for free) and
    /// destroys the spit at-most-once; a spit past its Range expires.
    /// </summary>
    [BurstCompile]
    [WorldSystemFilter(WorldSystemFilterFlags.ServerSimulation)]
    [UpdateInGroup(typeof(SimulationSystemGroup))]
    [UpdateAfter(typeof(EnemyProjectileMoveSystem))]
    public partial struct EnemyProjectileDamageSystem : ISystem
    {
        /// <summary>Extra forgiveness for the spit's own size, added to a target's hit radius.</summary>
        const float k_ProjectileRadius = 0.2f;

        /// <summary>Hit radius used for structures, which (by design) bake no HitRadius (so player shots never hit them).</summary>
        const float k_StructureRadius = 1.0f;

        [BurstCompile]
        public void OnCreate(ref SystemState state)
        {
            state.RequireForUpdate<NetworkTime>();
            state.RequireForUpdate<EnemyProjectile>();
        }

        [BurstCompile]
        public void OnUpdate(ref SystemState state)
        {
            uint now = SystemAPI.GetSingleton<NetworkTime>().ServerTick.TickIndexForValidTick;
            var ecb = new EntityCommandBuffer(Allocator.Temp);

            // Snapshot valid targets once (stable query order). PLAYERS carry HitRadius (PlayerAuthoring);
            // STRUCTURES deliberately do NOT (so player projectiles never friendly-fire the base) -> a constant.
            var targetEntities = new NativeList<Entity>(Allocator.Temp);
            var targetPositions = new NativeList<float3>(Allocator.Temp);
            var targetRadii = new NativeList<float>(Allocator.Temp);
            var targetRegions = new NativeList<byte>(Allocator.Temp);

            foreach (var (xform, hitRadius, health, region, e) in
                     SystemAPI.Query<RefRO<LocalTransform>, RefRO<HitRadius>, RefRO<Health>, RefRO<RegionTag>>()
                         .WithAll<PlayerTag>().WithEntityAccess())
            {
                if (health.ValueRO.Current <= 0f) continue; // don't hit a corpse
                targetEntities.Add(e);
                targetPositions.Add(xform.ValueRO.Position);
                targetRadii.Add(hitRadius.ValueRO.Value);
                targetRegions.Add(region.ValueRO.Region);
            }
            foreach (var (xform, health, region, e) in
                     SystemAPI.Query<RefRO<LocalTransform>, RefRO<Health>, RefRO<RegionTag>>()
                         .WithAll<PlacedStructure>().WithEntityAccess())
            {
                if (health.ValueRO.Current <= 0f) continue; // skip a structure pending destroy this tick
                targetEntities.Add(e);
                targetPositions.Add(xform.ValueRO.Position);
                targetRadii.Add(k_StructureRadius);
                targetRegions.Add(region.ValueRO.Region);
            }

            var destroyed = new NativeHashSet<Entity>(16, Allocator.Temp);
            foreach (var (xform, proj, projEntity) in
                     SystemAPI.Query<RefRO<LocalTransform>, RefRO<EnemyProjectile>>().WithEntityAccess())
            {
                float3 cur = xform.ValueRO.Position;
                float2 segEnd = new float2(cur.x, cur.z);
                float2 dir = proj.ValueRO.Direction;
                float2 segStart = segEnd - dir * proj.ValueRO.LastStep; // stored move-step, never a fresh dt
                float2 seg = segEnd - segStart;
                float segLenSq = math.lengthsq(seg);
                byte projRegion = proj.ValueRO.Region;

                int bestIdx = -1;
                float bestT = float.MaxValue;
                for (int i = 0; i < targetEntities.Length; i++)
                {
                    if (targetRegions[i] != projRegion) continue; // server-side damage region guard
                    float2 tp = new float2(targetPositions[i].x, targetPositions[i].z);
                    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)
                {
                    ecb.AppendToBuffer(targetEntities[bestIdx], new DamageEvent
                    {
                        Amount = proj.ValueRO.Damage,
                        SourceNetworkId = -1, // hostile environment, not a player
                        SourceTick = TickUtil.NonZero(now),
                    });
                    if (destroyed.Add(projEntity))
                        ecb.DestroyEntity(projEntity);
                    continue;
                }

                if (proj.ValueRO.DistanceTravelled >= proj.ValueRO.Range && destroyed.Add(projEntity))
                    ecb.DestroyEntity(projEntity);
            }

            ecb.Playback(state.EntityManager);

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