using System.Numerics;
using AcDream.Core.Physics;
using Xunit;

namespace AcDream.Core.Tests.Physics;

/// <summary>
/// Conformance tests for <see cref="PhysicsObjUpdate.HandleAllCollisions"/> — the port of
/// retail <c>CPhysicsObj::handle_all_collisions</c> (0x00514780, pc:282647). This is the
/// velocity "bleed on block" decision: reflect (fsf≤1) vs zero (fsf&gt;1).
/// </summary>
public class HandleAllCollisionsTests
{
    private static PhysicsBody Airborne(Vector3 v, int fsf = 0) => new PhysicsBody
    {
        State = PhysicsStateFlags.Gravity | PhysicsStateFlags.ReportCollisions,
        TransientState = TransientStateFlags.None,
        Velocity = v,
        FramesStationaryFall = fsf,
    };

    [Fact]
    public void Fsf0_AirborneWallHit_ReflectsIntoWallComponent()
    {
        var b = Airborne(new Vector3(3f, 0f, 0f));   // moving +X into a wall whose outward normal is -X
        var n = new Vector3(-1f, 0f, 0f);
        PhysicsObjUpdate.HandleAllCollisions(b,
            collisionNormalValid: true, collisionNormal: n,
            prevContact: false, prevOnWalkable: false, nowOnWalkable: false);
        // dot = 3*-1 = -3 < 0 → k = -(-3*(0.05+1)) = 3.15 → v += (-1,0,0)*3.15 → x = 3 - 3.15 = -0.15
        Assert.Equal(-0.15f, b.Velocity.X, precision: 3);
    }

    [Fact]
    public void Fsf0_MovingAwayFromSurface_DoesNotReflect()
    {
        var b = Airborne(new Vector3(0f, 0f, 2f));   // moving up, normal also up (already separating)
        var n = new Vector3(0f, 0f, 1f);
        PhysicsObjUpdate.HandleAllCollisions(b,
            collisionNormalValid: true, collisionNormal: n,
            prevContact: false, prevOnWalkable: false, nowOnWalkable: false);
        Assert.Equal(2f, b.Velocity.Z);   // dot = +2 >= 0 → no reflection
    }

    [Fact]
    public void Fsf2_ZeroesVelocity_TheAirborneStuckBleed()
    {
        // The #182 case: a straight-up jump blocked by a near-horizontal creature normal.
        // At fsf>1 the whole velocity is zeroed so gravity resumes → the player falls/glides off.
        var b = Airborne(new Vector3(0f, 0f, 18f), fsf: 2);
        var n = new Vector3(-0.96f, -0.25f, -0.15f);
        PhysicsObjUpdate.HandleAllCollisions(b,
            collisionNormalValid: true, collisionNormal: n,
            prevContact: false, prevOnWalkable: false, nowOnWalkable: false);
        Assert.Equal(Vector3.Zero, b.Velocity);
    }

    [Fact]
    public void Fsf3_ZeroesVelocity_EvenWithoutCollisionNormal()
    {
        var b = Airborne(new Vector3(1f, 2f, 18f), fsf: 3);
        PhysicsObjUpdate.HandleAllCollisions(b,
            collisionNormalValid: false, collisionNormal: default,
            prevContact: false, prevOnWalkable: false, nowOnWalkable: false);
        Assert.Equal(Vector3.Zero, b.Velocity);
    }

    [Fact]
    public void StayingOnWalkable_DoesNotReflect_CorridorWallSlidePreserved()
    {
        // Grounded before AND after → should_reflect is false → the tangential wall-slide
        // velocity is preserved (retail's rule; the corridor shuffle, not a sticky bounce).
        var b = new PhysicsBody
        {
            Velocity = new Vector3(3f, 0f, 0f),
            TransientState = TransientStateFlags.Contact | TransientStateFlags.OnWalkable,
            FramesStationaryFall = 0,
        };
        var n = new Vector3(-1f, 0f, 0f);
        PhysicsObjUpdate.HandleAllCollisions(b,
            collisionNormalValid: true, collisionNormal: n,
            prevContact: true, prevOnWalkable: true, nowOnWalkable: true);
        Assert.Equal(3f, b.Velocity.X);
    }

    [Fact]
    public void Inelastic_ZeroesInsteadOfReflecting()
    {
        var b = Airborne(new Vector3(3f, 0f, 0f));
        b.State |= PhysicsStateFlags.Inelastic;   // spell projectile / missile
        var n = new Vector3(-1f, 0f, 0f);
        PhysicsObjUpdate.HandleAllCollisions(b,
            collisionNormalValid: true, collisionNormal: n,
            prevContact: false, prevOnWalkable: false, nowOnWalkable: false);
        Assert.Equal(Vector3.Zero, b.Velocity);
    }

    [Fact]
    public void LandingReflects_RetailRuleRestored_NotSuppressed()
    {
        // prev airborne → now grounded (a landing). Retail reflects here too (AD-25 retired);
        // at elasticity 0.05 the effect is a tiny, imperceptible deflection.
        var b = Airborne(new Vector3(0f, 0f, -5f));   // falling
        var n = new Vector3(0f, 0f, 1f);              // floor normal up
        PhysicsObjUpdate.HandleAllCollisions(b,
            collisionNormalValid: true, collisionNormal: n,
            prevContact: false, prevOnWalkable: false, nowOnWalkable: true);
        // dot = -5 < 0 → k = -(-5*1.05) = 5.25 → v.z = -5 + 5.25 = 0.25 (tiny bounce)
        Assert.Equal(0.25f, b.Velocity.Z, precision: 3);
    }
}