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

namespace AcDream.Runtime.Tests.Physics;

/// <summary>
/// #331 — the coverage whose absence made the issue invisible: <b>nothing in
/// the suite asserted that a body-bearing mover makes UPHILL progress on a
/// walkable slope</b>. Every slope assertion we had ran downhill
/// (<see cref="RuntimeRemoteSlopeProjectionTests"/>), and the one uphill test
/// that was written passed <i>vacuously</i> because the body never moved.
///
/// <para>Both tests here drive the production
/// <see cref="AcDream.Runtime.Physics.RuntimeRemotePhysicsUpdater"/> tick over
/// <see cref="RemoteRampHarness"/> and take their expected Z from the
/// fixture's own terrain, never from a re-implementation of the projection.
/// </para>
///
/// <para><b>What #331 turned out to be (2026-08-06).</b> The resolver does not
/// refuse uphill motion. It refuses a sub-step offset that is
/// <i>exactly anti-parallel</i> to a persisted sliding normal — the
/// #137-family absorb, already recorded as retail-faithful in
/// <c>claude-memory/project_physics_collision_digest.md</c>. The chain is:
/// a landing (or spawn settle) on a slope reaches
/// <c>OBJECTINFO::validate_walkable</c> with <c>state &amp; 1</c> (CONTACT)
/// clear, so it calls <c>COLLISIONINFO::set_collision_normal</c> with the
/// <i>terrain</i> plane normal (verified in the PDB-paired binary at
/// <c>0x0050d251-0x0050d26c</c>); <c>CTransition::validate_transition</c> then
/// unconditionally converts that to a sliding normal
/// (<c>0x0050ac19-0x0050ac30</c>), and <c>COLLISIONINFO::set_sliding_normal</c>
/// flattens Z and <b>re-normalizes</b> (<c>0x0050a060</c>) — so even a 1°
/// slope yields a full-length horizontal normal pointing downhill.
/// <c>SetPositionInternal</c> persists it as <c>SLIDING_TS</c>
/// (<c>0x005154c2/0x005154e1</c>), <c>get_object_info</c> re-seeds it next
/// frame (<c>0x00511d44</c>), and <c>CTransition::adjust_offset</c> projects
/// the step onto the crease <c>cross(sliding, contact)</c> — a purely
/// horizontal, purely cross-slope axis. An exactly-up-slope offset has zero
/// component on that axis, so it is annihilated, the sweep aborts at step 0
/// (<c>0x0050c0ed</c>: <c>test ebx,ebx / jne</c> — retail returns
/// <c>i != 0 &amp;&amp; state == OK</c>, exactly as acdream does), and because a
/// failed transition never reaches the writeback the sliding state is never
/// cleared. Latched.</para>
///
/// <para><b>Why it read as "ALL uphill motion".</b> <see cref="RemoteRampHarness"/>
/// builds a ramp whose gradient is exactly along Y, and the probe that found
/// #331 pushed exactly along −Y. Axis-aligned fixture × axis-aligned motion
/// hits the measure-zero anti-parallel case with probability 1. The escape
/// window is the retail <c>F_EPSILON</c> abort: the step needs
/// ≥ 0.0002 m of cross-slope component, i.e. a heading more than about 0.11°
/// off the exact gradient at a 0.1 m step. Measured on this fixture: 0.0001 m
/// of cross-slope stays latched, 0.001 m climbs 0.176 m in five ticks.</para>
/// </summary>
public sealed class RuntimeRemoteUphillProgressTests
{
    /// <summary>
    /// Same ramp the downhill tests use: normal Z = 1/sqrt(1.36) ≈ 0.8575
    /// (30.96°) against retail's 0.6642 <c>floor_z</c> limit (48.4°), so the
    /// slope is comfortably walkable and a failure to climb is unmistakable.
    /// The ramp descends along +Y, so −Y is uphill.
    /// </summary>
    private const float WalkableSlopeGradient = 0.6f;

    private const int TrackedTicks = 30;

    /// <summary>
    /// Body-local root displacement per tick for a running remote: 0.1 m at
    /// 30 Hz is a 3 m/s run, heading about 15° off the exact up-slope
    /// direction — an ordinary heading, well outside the 0.11° absorb window
    /// documented on the class.
    /// </summary>
    private static readonly Vector3 UphillRootMotionPerTick =
        new(0.02588f, -0.09659f, 0f);

    /// <summary>Exactly up-slope: the #331 probe's offset.</summary>
    private static readonly Vector3 ExactlyUpSlopeRootMotionPerTick =
        new(0f, -0.1f, 0f);

    /// <summary>
    /// Same band the downhill tracking test uses. Measured drift on this
    /// fixture is under 1e-4 m.
    /// </summary>
    private const float SurfaceTrackingToleranceMeters = 0.005f;

    /// <summary>
    /// THE MISSING COVERAGE. A remote with a live <see cref="PhysicsBody"/>
    /// running up a walkable slope must gain height every tick and keep its
    /// feet on the ground while doing it.
    ///
    /// <para>Asserted per tick, not start-to-end, so a body that stalls for
    /// part of the run and catches up later still fails.</para>
    ///
    /// <para><b>Sabotage-verified 2026-08-06</b>, both directions.
    /// (SAB-A1) <c>Transition.AdjustOffset</c> → <c>return Vector3.Zero;</c>
    /// reddens it at tick 1 with zero climb. (SAB-A2) flattening the fixture
    /// ramp to gradient 0 reddens it at tick 1 (z 0.00000 → 0.00000), proving
    /// the climb is not an artifact of the settle.</para>
    /// </summary>
    [Fact]
    public void ARemoteWithABodyClimbsAWalkableSlopeAndKeepsItsFeetOnIt()
    {
        using RemoteRampHarness harness =
            RemoteRampHarness.OnRamp(WalkableSlopeGradient);
        PhysicsBody body = harness.Remote.Body;

        Assert.True(body.OnWalkable);

        // A body that has just landed — or been settled, which is the same
        // thing compressed — carries SLIDING with the flattened contact
        // normal (see the sibling test). Retail deletes the up-slope
        // component of exactly ONE step against it, and that step clears the
        // latch. Consume it explicitly, and assert it really was only one, so
        // the climb assertions below measure steady-state running rather than
        // silently tolerating a stall.
        harness.Tick(1, UphillRootMotionPerTick);
        Assert.True(
            (body.TransientState & TransientStateFlags.Sliding) == 0,
            "the landing sliding latch survived its first off-gradient step");

        float startZ = body.Position.Z;
        float previousZ = startZ;

        // The settled resting offset between the body's root and the terrain
        // directly beneath it. Measured, not assumed.
        float restingOffset = body.Position.Z - harness.SurfaceZUnderBody();

        for (int tick = 1; tick <= TrackedTicks; tick++)
        {
            harness.Tick(1, UphillRootMotionPerTick);

            Assert.True(
                body.Position.Z > previousZ,
                $"tick {tick}: body gained no height running uphill "
                + $"(z {previousZ:F5} -> {body.Position.Z:F5}, pos {body.Position})");

            float offset = body.Position.Z - harness.SurfaceZUnderBody();
            Assert.True(
                MathF.Abs(offset - restingOffset) < SurfaceTrackingToleranceMeters,
                $"tick {tick}: body root sits {offset:F5} m above the terrain "
                + $"under it, expected {restingOffset:F5} m (pos {body.Position})");

            previousZ = body.Position.Z;
        }

        float ascent = body.Position.Z - startZ;
        Assert.True(
            ascent > 1.0f,
            $"fixture is not exercising slope ascent: dz = {ascent:F4} m");
    }

    /// <summary>
    /// Characterization pin for the #331 absorb itself, so the next person to
    /// hit it finds the answer instead of re-deriving it. This asserts
    /// RETAIL-FAITHFUL behaviour (every link verified in the PDB-paired binary
    /// — see the class doc comment); it is NOT an approved-defect marker and
    /// must not be "fixed" by loosening the small-offset abort or clearing the
    /// sliding state per frame. Both of those are explicitly on the #137
    /// DO-NOT-RETRY list; the lever, if one is ever wanted, is the PROVENANCE
    /// of the sliding normal.
    ///
    /// <para><b>Sabotage-verified 2026-08-06</b>, both directions.
    /// (SAB-B1) deleting the <c>get_object_info</c> sliding-normal seed in
    /// <c>PhysicsEngine.ResolveWithTransition</c> reddens the absorb assertion
    /// — the body climbs to z 57.7544 instead of standing still — while
    /// leaving the sibling test green. (SAB-A1) <c>AdjustOffset</c> →
    /// <c>Vector3.Zero</c> reddens the escape/climb assertions. (SAB-A2) a
    /// flat ramp reddens the sliding-normal expectation.</para>
    ///
    /// <para><b>What does NOT discriminate here, measured, so nobody infers
    /// it later:</b> making the FINAL tick exactly up-slope leaves this green.
    /// By then the preceding off-gradient tick has already succeeded and its
    /// writeback cleared <c>SLIDING</c>, so there is no persisted normal left
    /// to absorb against. The absorb needs a live latch, not a particular
    /// heading.</para>
    /// </summary>
    [Fact]
    public void AnExactlyUpSlopeOffsetIsAbsorbedByThePersistedSlidingNormal()
    {
        using RemoteRampHarness harness =
            RemoteRampHarness.OnRamp(WalkableSlopeGradient);
        PhysicsBody body = harness.Remote.Body;

        // The state a landing (or the spawn settle that compresses it) leaves
        // behind on any slope: SLIDING carrying the contact plane's normal
        // flattened to XY and re-normalized — here the ramp's exact downhill
        // direction, at full length despite the slope being only 31°.
        Assert.True((body.TransientState & TransientStateFlags.Sliding) != 0);
        Assert.True(
            Vector3.Distance(body.SlidingNormal, new Vector3(0f, 1f, 0f)) < 0.001f,
            $"expected the flattened ramp normal, got {body.SlidingNormal}");

        Vector3 latched = body.Position;

        harness.Tick(5, ExactlyUpSlopeRootMotionPerTick);

        Assert.Equal(latched, body.Position);
        Assert.True((body.TransientState & TransientStateFlags.Sliding) != 0);

        // One ordinary off-gradient tick is itself absorbed — the crease is the
        // pure cross-slope axis, so only the X component survives — but it
        // succeeds, so the writeback clears the latch.
        harness.Tick(1, UphillRootMotionPerTick);

        Assert.True((body.TransientState & TransientStateFlags.Sliding) == 0);
        Assert.True(
            body.Position.X > latched.X,
            $"the cross-slope component was absorbed too (pos {body.Position})");
        Assert.Equal(latched.Z, body.Position.Z, 4);

        // From the next tick on the body climbs normally.
        harness.Tick(1, UphillRootMotionPerTick);

        Assert.True(
            body.Position.Z > latched.Z,
            $"body did not climb once the latch cleared (pos {body.Position})");
    }
}