using System.Numerics;
using AcDream.Core.Net;
using AcDream.Core.Net.Messages;
using AcDream.Core.Physics;
using AcDream.Core.Physics.Motion;
using AcDream.Runtime.Entities;
using AcDream.Runtime.Physics;

namespace AcDream.Runtime.Tests.Physics;

/// <summary>
/// Bug B (2026-08-04) — remote characters froze on steep surfaces instead of
/// sliding. The per-tick remote owner forged retail's two contact transients
/// (<c>Contact | OnWalkable</c>) before every sweep, discarded the
/// authoritative velocity, decided its landing edge from the contact-derived
/// <c>ResolveResult.IsOnGround</c> rather than the plane-derived
/// <c>OnWalkable</c>, and cleared the persistent Gravity state bit.
///
/// <para>
/// Retail derives all of it: <c>CPhysicsObj::SetPositionInternal</c>
/// (<c>0x00515330</c>) writes CONTACT_TS from
/// <c>collision_info.contact_plane_valid</c> (0x00515430) and then routes
/// ON_WALKABLE_TS through <c>set_on_walkable</c> (<c>0x00511310</c>) purely on
/// <c>contact_plane.N.z &lt; PhysicsGlobals::floor_z</c>
/// (0x00515465-0x0051548E). <c>set_on_walkable</c> is the SOLE source of
/// <c>MovementManager::HitGround</c>/<c>::LeaveGround</c>. Gravity survives a
/// steep contact because <c>calc_acceleration</c> (<c>0x00510950</c>) only
/// zeroes acceleration when CONTACT and ON_WALKABLE are BOTH set, and
/// <c>calc_friction</c> (<c>0x0050EE70</c>) returns at its first line when
/// ON_WALKABLE is clear.
/// </para>
///
/// <para>
/// Every test here runs the production <see cref="RuntimeRemotePhysicsUpdater"/>
/// tick over a synthetic landblock whose terrain is a single constant-gradient
/// ramp, so the contact plane the sweep finds is a real geometric result, not a
/// stubbed value.
/// </para>
/// </summary>
public sealed class RuntimeRemoteSteepContactSlideTests
{
    /// <summary>
    /// Ramp gradient chosen so the terrain-plane normal's Z lands just under
    /// retail's walkable limit — 52.4 degrees against a 48.4-degree limit, the
    /// same relationship as the live house roof that produced the freeze
    /// (measured contact-plane Normal.Z 0.6097 versus FloorZ 0.6642).
    /// </summary>
    private const float SteepGradient = 1.30f;

    /// <summary>A gentle ramp that is comfortably walkable.</summary>
    private const float WalkableGradient = 0.10f;

    [Fact]
    public void SteepTerrainProducesANonWalkableContactPlane()
    {
        using RemoteRampHarness harness = RemoteRampHarness.OnRamp(SteepGradient);

        Assert.True(harness.Remote.Body.ContactPlaneValid);
        Assert.InRange(
            harness.Remote.Body.ContactPlane.Normal.Z,
            0.55f,
            PhysicsGlobals.FloorZ - 0.001f);
    }

    /// <summary>
    /// The landing edge must be the sweep's plane-derived
    /// <c>OnWalkable</c>, never <c>IsOnGround</c> (which is
    /// <c>inContact || …</c> and is therefore TRUE on a steep contact).
    /// </summary>
    [Fact]
    public void SteepContactDoesNotLatchALanding()
    {
        using RemoteRampHarness harness = RemoteRampHarness.OnRamp(SteepGradient);
        harness.Remote.Airborne = true;
        int groundEdges = 0;
        harness.Remote.Motion.RemoveLinkAnimations = () => groundEdges++;

        harness.Tick(40);

        Assert.True(harness.Remote.Body.InContact);
        Assert.False(harness.Remote.Body.OnWalkable);
        Assert.Equal(0, groundEdges);
    }

    /// <summary>
    /// Gravity is a persistent object property in retail; nothing on a ground
    /// edge may clear it. Before the fix both landing blocks did, which is why
    /// <c>calc_acceleration</c> returned zero forever afterwards.
    /// </summary>
    [Fact]
    public void GravityPersistsAcrossTicksOnASteepContact()
    {
        using RemoteRampHarness harness = RemoteRampHarness.OnRamp(SteepGradient);
        harness.Remote.Airborne = true;

        harness.Tick(40);

        Assert.True(harness.Remote.Body.HasGravity);
        Assert.True(harness.Remote.Body.Acceleration.Z < -1f);
    }

    /// <summary>
    /// The visible consequence: a remote resting on a non-walkable face keeps
    /// moving. Before the fix the body reported <c>moved=0.0000</c> on every
    /// tick, forever.
    /// </summary>
    [Fact]
    public void SteepContactKeepsTheBodySlidingDownhill()
    {
        using RemoteRampHarness harness = RemoteRampHarness.OnRamp(SteepGradient);
        Vector3 start = harness.Remote.Body.Position;

        harness.Tick(40);

        Vector3 travelled = harness.Remote.Body.Position - start;
        Assert.True(
            travelled.Length() > 0.25f,
            $"expected a slide, body moved {travelled.Length():F4} m");
        Assert.True(
            travelled.Z < -0.1f,
            $"expected downhill travel, dz = {travelled.Z:F4} m");
    }

    /// <summary>
    /// The direct statement of "stop forging inputs": with no sweep to derive
    /// from — no starting cell, so <c>ResolveWithTransition</c> is skipped
    /// entirely — the tick must leave both retail transients exactly as it
    /// found them. Retail's only writer is <c>SetPositionInternal</c>
    /// (<c>0x00515330</c>), which a skipped transition never reaches.
    /// </summary>
    [Fact]
    public void TheTickNeverAssertsContactOrWalkableWithoutASweep()
    {
        using RemoteRampHarness harness = RemoteRampHarness.OnRamp(WalkableGradient);
        harness.Remote.CellId = 0u;
        harness.Remote.Body.TransientState &= ~(TransientStateFlags.Contact
            | TransientStateFlags.OnWalkable);
        harness.Remote.Airborne = false;

        harness.Tick(1);

        Assert.False(harness.Remote.Body.InContact);
        Assert.False(harness.Remote.Body.OnWalkable);
    }

    /// <summary>
    /// The tick immediately after a body crossed from walkable ground onto a
    /// steep face: it enters carrying last tick's grounded transients and its
    /// downhill speed. The tick must NOT re-assert those transients — with
    /// <c>Contact | OnWalkable</c> forced, <c>calc_acceleration</c>
    /// (<c>0x00510950</c>) returns zero and <c>calc_friction</c>
    /// (<c>0x0050EE70</c>) engages, so the body decelerates to a stop on a face
    /// retail would keep accelerating it down.
    /// </summary>
    [Fact]
    public void AGroundedTickOnASteepFaceReleasesTheBodyInsteadOfPinningIt()
    {
        using RemoteRampHarness harness = RemoteRampHarness.OnRamp(SteepGradient);
        harness.Remote.Body.TransientState |=
            TransientStateFlags.Contact | TransientStateFlags.OnWalkable;
        harness.Remote.Airborne = false;
        harness.Remote.Body.Velocity =
            Vector3.Normalize(new Vector3(0f, 1f, -SteepGradient)) * 3f;
        Vector3 start = harness.Remote.Body.Position;

        harness.Tick(40);

        Assert.False(harness.Remote.Body.OnWalkable);
        float travelled = (harness.Remote.Body.Position - start).Length();
        Assert.True(
            travelled > 2f,
            $"expected the steep face to release the body, travelled {travelled:F3} m");
    }

    /// <summary>
    /// Retail's <c>MoveOrTeleport</c> (<c>0x00516330</c>) never reads or writes
    /// the wire velocity for a remote; the deleted per-tick
    /// <c>Body.Velocity = Zero</c> threw away whatever ACE delivered through
    /// <c>0xF74E</c> as well as everything gravity had accumulated.
    /// </summary>
    [Fact]
    public void AuthoritativeVelocityIsNotDiscardedOnAGroundedTick()
    {
        using RemoteRampHarness harness = RemoteRampHarness.OnRamp(WalkableGradient);
        Assert.False(harness.Remote.Airborne);
        harness.Remote.Body.Velocity = new Vector3(2.146f, 2.264f, -3.549f);

        harness.Tick(1);

        Assert.NotEqual(Vector3.Zero, harness.Remote.Body.Velocity);
        Assert.True(
            harness.Remote.Body.Velocity.X > 0.5f,
            $"velocity X was {harness.Remote.Body.Velocity.X:F4}");
    }

    /// <summary>
    /// The committed transients must be the ones the sweep's contact plane
    /// implies — Contact from plane validity, OnWalkable from
    /// <c>Normal.Z &gt;= floor_z</c> — and never an independently asserted pair.
    ///
    /// <para>
    /// Deliberately NOT stated as the two equalities
    /// <c>ContactPlaneValid == InContact</c> and
    /// <c>IsWalkableContact(committed plane) == OnWalkable</c>. Neither is an
    /// invariant of the production code, and this test asserted both until the
    /// 2026-08-04 review: <c>PhysicsEngine.ResolveWithTransition</c> publishes
    /// the contact plane whenever the transition returned <c>ok</c>, while the
    /// transient commit additionally requires <c>candidateMoved</c>
    /// (<c>RuntimeRemotePhysicsUpdater</c>'s SetPositionInternal commit,
    /// matching retail <c>UpdateObjectInternal</c> pc:283657), so a zero-move
    /// frame can legitimately leave the two one tick apart. The same writeback
    /// also falls back to <c>LastKnownContactPlane</c>, which keeps
    /// <c>ContactPlaneValid</c> true across a contact-FREE frame by design.
    /// The old assertions passed only because every body in this fixture moves
    /// on every tick. What is asserted instead is what the commit path DOES
    /// guarantee — the two implications — plus each fixture's known ramp
    /// geometry checked on BOTH sides of retail's walkability comparison, so
    /// re-forging <c>Contact | OnWalkable</c> still fails the steep case.
    /// </para>
    /// </summary>
    [Fact]
    public void CommittedTransientsAgreeWithTheCommittedContactPlane()
    {
        using RemoteRampHarness steep = RemoteRampHarness.OnRamp(SteepGradient);
        steep.Tick(20);
        AssertTransientsAreContactPlaneDerived(
            steep.Remote.Body, expectWalkable: false);

        using RemoteRampHarness gentle = RemoteRampHarness.OnRamp(WalkableGradient);
        gentle.Tick(20);
        AssertTransientsAreContactPlaneDerived(
            gentle.Remote.Body, expectWalkable: true);
    }

    private static void AssertTransientsAreContactPlaneDerived(
        PhysicsBody body,
        bool expectWalkable)
    {
        // Unconditional: OnWalkable is only ever written as
        // `inContact && onWalkable`
        // (PhysicsObjUpdate.CommitSetPositionContactPrefix), so it cannot
        // outlive Contact on any frame, committed or not.
        Assert.True(
            !body.OnWalkable || body.InContact,
            "OnWalkable without Contact — the two transients were asserted "
            + "independently of the contact plane");

        // Contact is written from the sweep's contact-plane validity by the
        // same resolve that publishes the plane, so a body in contact carries
        // a valid plane. The CONVERSE is not guaranteed — see the summary.
        Assert.True(
            !body.InContact || body.ContactPlaneValid,
            "Contact without a valid contact plane — Contact was not "
            + "plane-derived");

        // Both sides of retail's walkability comparison
        // (SetPositionInternal 0x00515465-0x0051548E) against this fixture's
        // known constant-gradient ramp: the plane the sweep found, and the
        // transient that plane drove.
        Assert.Equal(
            expectWalkable,
            body.ContactPlane.Normal.Z >= PhysicsGlobals.FloorZ);
        Assert.Equal(expectWalkable, body.OnWalkable);
    }

    /// <summary>
    /// The other half of the edge: a genuine walkable landing must still fire
    /// retail's <c>set_on_walkable(1)</c> -> <c>MovementManager::HitGround</c>
    /// exactly once and leave the body grounded.
    /// </summary>
    [Fact]
    public void WalkableLandingStillLandsAndFiresTheGroundEdgeOnce()
    {
        using RemoteRampHarness harness = RemoteRampHarness.Airborne(WalkableGradient, height: 3f);
        int groundEdges = 0;
        harness.Remote.Motion.RemoveLinkAnimations = () => groundEdges++;

        harness.Tick(60);

        Assert.True(harness.Remote.Body.OnWalkable);
        Assert.False(harness.Remote.Airborne);
        Assert.Equal(1, groundEdges);
    }

    /// <summary>
    /// GRAVITY_PS is set by the retail <c>CPhysicsObj</c> constructor
    /// (state 0x400C08 @0x00512508) and thereafter assigned wholesale from the
    /// wire by <c>set_description</c>'s <c>set_state</c> (<c>0x00514DD0</c>),
    /// which post-processes only lighting/nodraw/hidden. No ground edge
    /// anywhere in retail toggles it — acdream's two landing blocks did, which
    /// is what left a landed remote permanently unable to fall again.
    /// </summary>
    [Fact]
    public void WalkableLandingDoesNotClearTheGravityStateBit()
    {
        using RemoteRampHarness harness = RemoteRampHarness.Airborne(WalkableGradient, height: 3f);

        harness.Tick(60);

        Assert.True(harness.Remote.Body.OnWalkable);
        Assert.True(harness.Remote.Body.HasGravity);
    }
}