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.
    ///
    /// Phase 1.7 mechanic-changer boons ride the server-only <see cref="ProjectileEffectState"/> (seeded at
    /// spawn by <c>AbilityFireSystem</c> from the owner's <see cref="BoonEffects"/>): PIERCE lets the projectile
    /// survive a hit (decrement, don't destroy), CHAIN retargets its (replicated) <see cref="Projectile.Direction"/>
    /// toward the next-nearest living enemy, and PULL flips the knockback heading toward the shooter. A per-projectile
    /// hit-set is excluded DURING target selection so a surviving projectile never re-hits a target across ticks; the
    /// set full (or no pierce/chain left) destroys as before. Projectiles WITHOUT the component behave exactly as
    /// before (destroy on hit) — graceful degradation. Exactly ONE destroy per projectile per tick is preserved.
    ///
    /// On a hit the system appends a <see cref="DamageEvent"/> to the target (consumed by
    /// <c>HealthApplyDamageSystem</c>). 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).
    /// </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>RW lookup to stamp the server-only homing ReelState on a Reel-flagged (HookPull) hit — the Harpooner reel.</summary>
        ComponentLookup<ReelState> m_ReelLookup;

        /// <summary>Knockback stamp lookup. The former BossState immunity gate went with the boss purge
        /// knockback-immune (A4) so a solo player can't perma-stunlock it out of its slam wind-ups.</summary>

        /// <summary>RW lookup for the per-projectile Phase-1.7 pierce/chain/pull state + re-hit set.</summary>
        ComponentLookup<ProjectileEffectState> m_FxLookup;

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

        /// <summary>Max planar distance a Ricochet chain will reach for its next target (tunable).</summary>
        const float k_ChainRange = 8f;

        /// <summary>Harpooner reel pull speed (world units/sec) written into the reeled Husk's KnockbackState.</summary>
        const float k_ReelSpeed = 16f;

        /// <summary>Reel leash: max ticks the homing pull lasts before releasing (~60 ticks/sec).</summary>
        const uint k_ReelLeashTicks = 90u;

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

            m_FxLookup = state.GetComponentLookup<ProjectileEffectState>(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);
            m_ReelLookup.Update(ref state);

            m_FxLookup.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>()
                         .WithNone<Dying>() // B3: corpses are not shields - a shot passes through to living targets
                         .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>, RefRW<Projectile>, RefRO<GhostOwner>>()
                         .WithAll<Simulate>() // predicted-group convention: only simulate what this tick simulates
                         .WithEntityAccess())
            {
                int projOwnerId = owner.ValueRO.NetworkId;

                bool hasFx = m_FxLookup.HasComponent(projectileEntity);
                ProjectileEffectState fx = hasFx ? m_FxLookup[projectileEntity] : default;

                // 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;

                    // Phase 1.7: a surviving (pierced/chained) projectile never re-hits a target already struck —
                    // excluded DURING selection, not post-filtered.
                    if (hasFx && HitSetContains(in fx, target))
                        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)
                {
                    var hitTarget = targetEntities[bestIdx];

                    // Earliest target along the travel path: deal damage.
                    ecb.AppendToBuffer(hitTarget, new DamageEvent
                    {
                        Amount = proj.ValueRO.Damage,
                        SourceNetworkId = projOwnerId,
                        SourceTick = haveTick ? TickUtil.NonZero(nt.ServerTick.TickIndexForValidTick) : 0u,
                    });

                    // Knockback / REEL. Reel (HookPull) stamps the HOMING ReelState (ReelSystem re-aims toward the
                    // caster's live position each tick); otherwise the classic frozen knockback (PULL flips toward the shooter).
                    if (haveTick && m_KnockbackLookup.HasComponent(hitTarget))
                    {
                        bool reel = hasFx && (fx.Flags & ProjectileEffectFlag.Reel) != 0;
                        if (reel && m_ReelLookup.HasComponent(hitTarget))
                        {
                            m_ReelLookup[hitTarget] = new ReelState
                            {
                                CasterNetworkId = projOwnerId,
                                Speed = k_ReelSpeed,
                                ExpireTick = TickUtil.NonZero(nt.ServerTick.TickIndexForValidTick + k_ReelLeashTicks),
                            };
                        }
                        else if (Tuning.KnockbackSpeed > 0f)
                        {
                            bool pull = hasFx && (fx.Flags & ProjectileEffectFlag.Pull) != 0;
                            float2 kdir = pull ? -proj.ValueRO.Direction : proj.ValueRO.Direction;
                            m_KnockbackLookup[hitTarget] = new KnockbackState
                            {
                                Dir = kdir,
                                Speed = Tuning.KnockbackSpeed,
                                UntilTick = TickUtil.NonZero(nt.ServerTick.TickIndexForValidTick + (uint)math.max(1, Tuning.KnockbackDurationTicks)),
                            };
                        }
                    }

                    // Phase 1.7: pierce/chain let the projectile SURVIVE; else it is consumed. Record the target so it
                    // can't be re-hit. A full hit-set is a natural cap → destroy.
                    bool survive = false;
                    if (hasFx)
                    {
                        if (fx.Hit.Length < fx.Hit.Capacity)
                        {
                            fx.Hit.Add(hitTarget);
                            if (fx.PierceRemaining > 0)
                            {
                                fx.PierceRemaining = (byte)(fx.PierceRemaining - 1);
                                survive = true;
                            }
                            else if (fx.ChainRemaining > 0)
                            {
                                int nextIdx = FindChainTarget(targetEntities, targetPositions, cur, projOwnerId, in fx);
                                if (nextIdx >= 0)
                                {
                                    float2 to = new float2(targetPositions[nextIdx].x - cur.x, targetPositions[nextIdx].z - cur.z);
                                    if (math.lengthsq(to) > 1e-6f)
                                    {
                                        proj.ValueRW.Direction = math.normalize(to);
                                        fx.ChainRemaining = (byte)(fx.ChainRemaining - 1);
                                        survive = true;
                                    }
                                }
                            }
                        }
                        m_FxLookup[projectileEntity] = fx;
                    }

                    if (survive)
                    {
                        // A surviving projectile still expires once it has travelled its full range.
                        if (proj.ValueRO.DistanceTravelled >= proj.ValueRO.Range)
                            ecb.DestroyEntity(projectileEntity);
                        continue;
                    }

                    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();
        }

        /// <summary>True when <paramref name="target"/> is already in the projectile's re-hit history.</summary>
        static bool HitSetContains(in ProjectileEffectState fx, Entity target)
        {
            for (int i = 0; i < fx.Hit.Length; i++)
                if (fx.Hit[i] == target) return true;
            return false;
        }

        /// <summary>Nearest living target to <paramref name="from"/> within <see cref="k_ChainRange"/> that is neither
        /// the caster's own ghost nor already in the projectile's hit-set. Returns the snapshot index or -1.</summary>
        int FindChainTarget(in NativeList<Entity> targetEntities, in NativeList<float3> targetPositions,
            float3 from, int projOwnerId, in ProjectileEffectState fx)
        {
            int best = -1;
            float bestDistSq = k_ChainRange * k_ChainRange;
            for (int i = 0; i < targetEntities.Length; i++)
            {
                var target = targetEntities[i];
                if (m_GhostOwnerLookup.HasComponent(target) && m_GhostOwnerLookup[target].NetworkId == projOwnerId)
                    continue;
                if (HitSetContains(in fx, target))
                    continue;
                float2 d = new float2(targetPositions[i].x - from.x, targetPositions[i].z - from.z);
                float dsq = math.lengthsq(d);
                if (dsq <= bestDistSq)
                {
                    bestDistSq = dsq;
                    best = i;
                }
            }
            return best;
        }
    }
}