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

namespace ProjectM.Server
{
    /// <summary>
    /// Server-only ONE-SHOT arena dressing seeder — the gym-scoped successor to <c>RoomFieldSystem</c>, which was
    /// deleted with the run/room FSM in the 2026-08-07 audit purge (DR-054). That deletion was correct for the FSM
    /// but silently took the arena's furniture with it: the baked <see cref="ClutterFieldSpawner"/>,
    /// <see cref="CoverFieldSpawner"/> and <see cref="GeyserFieldSpawner"/> singletons kept baking and nothing read
    /// them, so the shipping arena spawned ZERO props. Found at the A0 gate — "the world is very static".
    ///
    /// Phase 1 is "no world — a gym", so this seeds ONCE at world start and never tears down. Everything
    /// room-shaped (scatter-in-shape, budget spend-down, per-room reroll) is deliberately dropped; the Phase-2
    /// pocket generator owns that.
    ///
    /// PLACEMENT POLICY (2026-08-07, top-down-arena practice — the props must be usable, not decorative):
    /// - <see cref="CoreClear"/>: a clear core around the landing spot. Props there cramp the spawn and cause
    ///   dash collisions the player never asked for.
    /// - <see cref="BandInner"/>..<see cref="BandOuter"/>: solids live in the FOUGHT-IN band, which is also
    ///   roughly where the diver's travelling light reaches. A prop outside the lit radius reads as an invisible
    ///   wall, which is the worst failure mode in a dark game.
    /// - <see cref="MinSpacing"/> is enforced across clutter AND cover on ONE shared occupancy list. This is a
    ///   navigation guarantee, not an aesthetic: enemy movement has NO pathfinding (a CollisionWorld sphere-cast
    ///   slide plus a depenetrate/nudge backstop), so gaps must stay comfortably wider than a body or movers wedge.
    ///   CLAUDE.md's standing rule is to re-validate movers whenever Environment cover is added.
    /// - Explosive clutter (variant 3) is kept beyond <see cref="ExplosiveInner"/> so a barrel never chains into
    ///   the player at spawn.
    ///
    /// Contracts carried from the original, each load-bearing:
    /// - <c>baked.WithPosition</c>, never <c>FromPosition</c> — the latter resets Scale, which is a [GhostField].
    /// - Geyser <c>NextEruptTick</c> stamped BORN-CORRECT off the live ServerTick via <see cref="TickUtil.NonZero"/>,
    ///   staggered per instance so eruptions desync. 0 stays the "unstamped / not ready" sentinel, never "fire now".
    /// - A fixed seed: a gym wants the SAME arena every session, not a fresh scatter to relearn.
    /// </summary>
    [WorldSystemFilter(WorldSystemFilterFlags.ServerSimulation)]
    [UpdateInGroup(typeof(SimulationSystemGroup))]
    public partial struct ArenaFieldSystem : ISystem
    {
        /// <summary>Clear core around the arena origin — the landing spot and dash room.</summary>
        const float CoreClear = 5f;
        /// <summary>Solids start here: inside kiting range and inside the travelling light.</summary>
        const float BandInner = 6f;
        /// <summary>Solids stop here — beyond this they ring the player in rather than furnish the fight.</summary>
        const float BandOuter = 13f;
        /// <summary>Minimum gap between any two solid props. A NAV guarantee: enemies have no pathfinding.</summary>
        const float MinSpacing = 2.6f;
        /// <summary>Explosive clutter stays beyond this so a barrel never chains into the player at spawn.</summary>
        const float ExplosiveInner = 8f;
        /// <summary>Rejection-sampling attempts before a piece is dropped rather than placed too close.</summary>
        const int PlaceAttempts = 24;

        const uint ArenaSeed = 0x5EABEDu;

        public void OnCreate(ref SystemState state)
        {
            state.RequireForUpdate<ClutterFieldSpawner>();
        }

        public void OnUpdate(ref SystemState state)
        {
            // One-shot for the world's lifetime. Disabling beats a bookkeeping component: no structural change,
            // no re-read tick, and it cannot double-seed if the subscene streams late.
            state.Enabled = false;

            var ecb = new EntityCommandBuffer(Allocator.Temp);
            float3 origin = float3.zero;
            if (SystemAPI.TryGetSingleton<BaseAnchor>(out var anchor))
                origin = BaseGridMath.PlotCenter(anchor);

            // ONE shared occupancy list so clutter and cover cannot overlap each other either.
            var taken = new NativeList<float3>(Allocator.Temp);

            int clutter = SeedClutter(ref state, ecb, origin, ref taken);
            int cover = SeedCover(ref state, ecb, origin, ref taken);
            int geysers = SeedGeysers(ref state, ecb, origin);

            taken.Dispose();
            ecb.Playback(state.EntityManager);
            ecb.Dispose();

            UnityEngine.Debug.Log(
                $"[ArenaFieldSystem] arena seeded: {clutter} clutter, {cover} cover, {geysers} geysers " +
                $"(band {BandInner}-{BandOuter}u, min spacing {MinSpacing}u).");
        }

        /// <summary>Rejection-sampled placement in the fought-in annulus, respecting the shared min-spacing.
        /// Returns false when the band is too crowded to place this piece — the caller DROPS it rather than
        /// jamming a prop into a gap a mover cannot pass.</summary>
        static bool TryPlace(float3 origin, float inner, float outer, ref Random rng,
                             ref NativeList<float3> taken, out float3 pos)
        {
            for (int attempt = 0; attempt < PlaceAttempts; attempt++)
            {
                // sqrt-distributed radius = uniform area coverage, so pieces don't bunch toward the inner edge
                float u = rng.NextFloat();
                float r = math.sqrt(math.lerp(inner * inner, outer * outer, u));
                float a = rng.NextFloat(0f, 2f * math.PI);
                var p = origin + new float3(math.cos(a) * r, 0f, math.sin(a) * r);

                bool clear = true;
                for (int i = 0; i < taken.Length; i++)
                {
                    if (math.distancesq(p.xz, taken[i].xz) < MinSpacing * MinSpacing) { clear = false; break; }
                }
                if (!clear) continue;

                taken.Add(p);
                pos = p;
                return true;
            }
            pos = default;
            return false;
        }

        int SeedClutter(ref SystemState state, EntityCommandBuffer ecb, float3 origin, ref NativeList<float3> taken)
        {
            if (!SystemAPI.TryGetSingleton<ClutterFieldSpawner>(out var spawner) || spawner.Prefab == Entity.Null)
                return 0;
            int want = math.clamp(spawner.Count, 0, 24);
            if (want == 0) return 0;

            var baked = SystemAPI.GetComponent<LocalTransform>(spawner.Prefab);
            var proto = SystemAPI.GetComponent<BlightClutter>(spawner.Prefab);
            var rng = new Random(ArenaSeed ^ 0xC17u);
            float outer = math.max(BandInner + 1f, math.min(BandOuter, spawner.Radius > 0.01f ? spawner.Radius : BandOuter));

            int placed = 0;
            for (int i = 0; i < want; i++)
            {
                if (!TryPlace(origin, BandInner, outer, ref rng, ref taken, out var pos)) continue;
                var e = ecb.Instantiate(spawner.Prefab);
                ecb.SetComponent(e, baked.WithPosition(pos));

                var bc = proto;
                bool farEnoughForBlast = math.distance(pos.xz, origin.xz) >= ExplosiveInner;
                // ~25% EXPLOSIVE (variant 3, the hazard) but never inside the blast keep-out; 0-2 are inert dressing.
                bc.Variant = (farEnoughForBlast && rng.NextFloat() < 0.25f) ? (byte)3 : (byte)(i % 3);
                ecb.SetComponent(e, bc);
                placed++;
            }
            return placed;
        }

        int SeedCover(ref SystemState state, EntityCommandBuffer ecb, float3 origin, ref NativeList<float3> taken)
        {
            if (!SystemAPI.TryGetSingleton<CoverFieldSpawner>(out var spawner) || spawner.Prefab == Entity.Null)
                return 0;
            int want = math.clamp(spawner.Count, 0, 6);
            if (want == 0) return 0;

            var baked = SystemAPI.GetComponent<LocalTransform>(spawner.Prefab);
            var rng = new Random(ArenaSeed ^ 0xC0Eu);

            int placed = 0;
            for (int i = 0; i < want; i++)
            {
                if (!TryPlace(origin, BandInner, BandOuter, ref rng, ref taken, out var pos)) continue;
                var e = ecb.Instantiate(spawner.Prefab);
                ecb.SetComponent(e, baked.WithPosition(pos));
                placed++;
            }
            return placed;
        }

        int SeedGeysers(ref SystemState state, EntityCommandBuffer ecb, float3 origin)
        {
            if (!SystemAPI.TryGetSingleton<GeyserFieldSpawner>(out var spawner) || spawner.Prefab == Entity.Null)
                return 0;
            int count = math.clamp(spawner.Count, 0, 4);
            if (count == 0) return 0;

            // Born-correct scheduling: read the LIVE tick. 0 stays the "unstamped" sentinel and means NOT ready.
            uint stamp = 0u;
            if (SystemAPI.TryGetSingleton<NetworkTime>(out var nt) && nt.ServerTick.IsValid)
                stamp = nt.ServerTick.TickIndexForValidTick;

            var baked = SystemAPI.GetComponent<LocalTransform>(spawner.Prefab);
            var rng = new Random(ArenaSeed ^ 0x6E7u);

            for (int i = 0; i < count; i++)
            {
                // Geysers carry no collider, so they are exempt from the spacing list — but they still respect the
                // clear core: an eruption on the landing spot is an unfair hit the player never saw coming.
                float r = math.lerp(CoreClear + 2f, BandOuter, (i + 0.5f) / count);
                float a = i * 2.399963f + rng.NextFloat(-0.4f, 0.4f);   // golden angle: spread, not clumped
                var pos = origin + new float3(math.cos(a) * r, 0f, math.sin(a) * r);

                var e = ecb.Instantiate(spawner.Prefab);
                ecb.SetComponent(e, baked.WithPosition(pos));
                uint next = stamp != 0u
                    ? TickUtil.NonZero(stamp + Tuning.GeyserPeriodTicks + (uint)i * 60u)   // stagger so they desync
                    : 0u;
                ecb.SetComponent(e, new Geyser { NextEruptTick = next });
            }
            return count;
        }
    }
}