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

namespace AcDream.Core.Tests.Physics;

// Component D — cell-march landblock preservation (#145 residual, 2026-06-22 spec §3 D).
//
// Scenario: a player arrives via teleport at outdoor cell 0xA9B40019 (lbX=0xA9, lbY=0xB4).
// The destination landblock has NOT yet been applied to the physics engine (priority-apply
// is in flight). The body's CarriedBlockOrigin is null (either the body has no seeded outdoor
// cell yet, or the incoming cell differs from the body's last-known cell).
//
// Without the fix: BuildCellSetAndPickContaining's TryGetTerrainOrigin returns false with
// blockOrigin=(0,0,0). AddAllOutsideCells treats worldSphereCenter as block-local and marches
// the cell one block in whatever direction the player is moving — lbX→lbX−1 or lbY→lbY−1
// per tick, cascading until lbX or lbY underflows to 0x00. The outbound wire then encodes
// the marched cell and ACE rejects every move (MOVEMENT SPEED / failed transition).
//
// With the fix: when TryGetTerrainOrigin returns false (block not resident), return the
// seed currentCellId verbatim — "no block-local frame → preserve" (same contract as
// PhysicsEngine.Resolve's NO-LANDBLOCK verbatim branch). The player cell stays correct
// until the landblock applies.
//
// The entry point exercised here is CellTransit.FindCellSet — the same function that
// RunCheckOtherCellsAndAdvance (TransitionTypes.cs) calls on every per-frame physics tick.
public class CellMarchLandblockPreservationTests
{
    private static int LbX(uint cellId) => (int)((cellId >> 24) & 0xFFu);
    private static int LbY(uint cellId) => (int)((cellId >> 16) & 0xFFu);

    // ── West-edge crossing: no anchor, unstreamed terrain → must NOT march lbX ──────────────
    [Fact]
    public void WestEdge_UnstreamedLandblock_NoAnchor_PreservesSeedCell()
    {
        // Empty cache — landblock 0xA9B4 has not been applied (TryGetTerrainOrigin → false).
        var cache = new PhysicsDataCache();

        // Sphere just west of 0xA9B4's origin (world X = −0.088 → block-local X = −0.088
        // relative to (0,0) origin → floor(−0.088/24) = −1 → lbX marches without fix).
        var spheres = new[] { new DatReaderWriter.Types.Sphere { Origin = new Vector3(-0.088f, 14.8f, 12f), Radius = 0.48f } };
        const uint currentCell = 0xA9B40019u;

        // No carried anchor (the teleport-arrival first tick: body still carries an old
        // cell or is null-seeded, so ResolveWithTransition passes null).
        uint result = CellTransit.FindCellSet(cache, spheres, 1, currentCell, out _, null);

        // The cell must be preserved unchanged — lbX must NOT drop below 0xA9.
        Assert.Equal(0xA9, LbX(result));
        // The full cell id must equal the seed (no march at all).
        Assert.Equal(currentCell, result);
    }

    // ── South-edge crossing: no anchor, unstreamed terrain → must NOT march lbY ─────────────
    [Fact]
    public void SouthEdge_UnstreamedLandblock_NoAnchor_PreservesSeedCell()
    {
        var cache = new PhysicsDataCache();
        var spheres = new[] { new DatReaderWriter.Types.Sphere { Origin = new Vector3(14.8f, -0.088f, 12f), Radius = 0.48f } };
        const uint currentCell = 0xA9B40001u;  // south-west landcell of 0xA9B4

        uint result = CellTransit.FindCellSet(cache, spheres, 1, currentCell, out _, null);

        // Without fix: lbY drops from 0xB4 to 0xB3 on the first tick, then cascades.
        Assert.Equal(0xB4, LbY(result));
        Assert.Equal(currentCell, result);
    }

    // ── Carried anchor STILL works: if the anchor is supplied, the correct neighbour ────────
    // landblock cell is returned even when terrain is unstreamed (the original #145 fix).
    [Fact]
    public void WithCarriedAnchor_UnstreamedNeighbour_ReturnsCorrectCell()
    {
        var cache = new PhysicsDataCache();
        // Player crossed the south edge into 0xA9B3 (origin (0, −192)).
        var spheres = new[] { new DatReaderWriter.Types.Sphere { Origin = new Vector3(14.8f, -0.088f, 12f), Radius = 0.48f } };
        const uint currentCell = 0xA9B30001u;
        var anchor = new Vector3(0f, -192f, 0f);  // body.Position − CellPosition.Origin

        uint result = CellTransit.FindCellSet(cache, spheres, 1, currentCell, out _, anchor);

        // The anchor gives the correct block-local frame → stays in 0xA9B3 (no march back to B4).
        Assert.Equal(0xB3, LbY(result));
    }
}