acdream/tests/AcDream.Core.Tests/Physics/PhysicsEngineTests.cs

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

namespace AcDream.Core.Tests.Physics;

public class PhysicsEngineTests
{
    private static float[] LinearHeightTable()
    {
        var table = new float[256];
        for (int i = 0; i < 256; i++) table[i] = i * 1.0f;
        return table;
    }

    private static byte[] FlatHeightmap(byte value = 50)
    {
        var heights = new byte[81];
        Array.Fill(heights, value);
        return heights;
    }

    private PhysicsEngine MakeFlatEngine(float terrainZ = 50f)
    {
        var engine = new PhysicsEngine();
        var terrain = new TerrainSurface(FlatHeightmap((byte)terrainZ), LinearHeightTable());
        engine.AddLandblock(0xA9B4FFFFu, terrain, Array.Empty<CellSurface>(),
            worldOffsetX: 0f, worldOffsetY: 0f);
        return engine;
    }

    [Fact]
    public void Resolve_FlatTerrain_ZMatchesTerrain()
    {
        var engine = MakeFlatEngine(terrainZ: 50f);

        var result = engine.Resolve(
            new Vector3(96f, 96f, 50f), cellId: 0x0001, delta: new Vector3(1f, 0f, 0f),
            stepUpHeight: 2f);

        Assert.Equal(50f, result.Position.Z, precision: 1);
        Assert.True(result.IsOnGround);
    }

    [Fact]
    public void Resolve_WalkUpSmallSlope_Accepted()
    {
        // Heights slope from 50 to 52 across X — small enough for step height.
        var heights = new byte[81];
        for (int x = 0; x < 9; x++)
            for (int y = 0; y < 9; y++)
                heights[x * 9 + y] = (byte)(50 + x / 4);  // gentle slope

        var engine = new PhysicsEngine();
        var terrain = new TerrainSurface(heights, LinearHeightTable());
        engine.AddLandblock(0xA9B4FFFFu, terrain, Array.Empty<CellSurface>(),
            worldOffsetX: 0f, worldOffsetY: 0f);

        var result = engine.Resolve(
            new Vector3(48f, 96f, 50f), cellId: 0x0001, delta: new Vector3(48f, 0f, 0f),
            stepUpHeight: 5f);

        Assert.True(result.IsOnGround);
        Assert.True(result.Position.Z >= 50f);  // moved uphill
    }

    [Fact]
    public void Resolve_StepUpExceedsHeight_MovementBlocked()
    {
        // Heights jump sharply: left half = 50, right half = 100.
        var heights = new byte[81];
        for (int x = 0; x < 9; x++)
            for (int y = 0; y < 9; y++)
                heights[x * 9 + y] = (byte)(x < 5 ? 50 : 100);

        var engine = new PhysicsEngine();
        var terrain = new TerrainSurface(heights, LinearHeightTable());
        engine.AddLandblock(0xA9B4FFFFu, terrain, Array.Empty<CellSurface>(),
            worldOffsetX: 0f, worldOffsetY: 0f);

        // Try to walk from the low side to the high side.
        var result = engine.Resolve(
            new Vector3(96f, 96f, 50f), cellId: 0x0001, delta: new Vector3(48f, 0f, 0f),
            stepUpHeight: 2f);

        // Movement should be blocked — Z delta (50→100) exceeds step height (2).
        Assert.Equal(96f, result.Position.X, precision: 1);  // didn't move
        Assert.True(result.IsOnGround);
    }

    [Fact]
    public void Resolve_EnterIndoorCell_TransitionsToCell()
    {
        var engine = new PhysicsEngine();
        var terrain = new TerrainSurface(FlatHeightmap(50), LinearHeightTable());

        // Indoor cell with a floor at Z=55 covering (40..60, 40..60).
        var cellVerts = new Dictionary<ushort, Vector3>
        {
            [0] = new(40f, 40f, 55f),
            [1] = new(60f, 40f, 55f),
            [2] = new(60f, 60f, 55f),
            [3] = new(40f, 60f, 55f),
        };
        var cellPolys = new List<List<short>> { new() { 0, 1, 2, 3 } };
        var cell = new CellSurface(0x0100, cellVerts, cellPolys);

        engine.AddLandblock(0xA9B4FFFFu, terrain, new[] { cell },
            worldOffsetX: 0f, worldOffsetY: 0f);

        // Walk from outdoor (30, 50) into the cell's floor area (50, 50).
        var result = engine.Resolve(
            new Vector3(30f, 50f, 50f), cellId: 0x0001, delta: new Vector3(20f, 0f, 0f),
            stepUpHeight: 10f);

        // Should transition to the indoor cell and snap to its floor Z.
        Assert.Equal(0x0100u, result.CellId);
        Assert.Equal(55f, result.Position.Z, precision: 1);
        Assert.True(result.IsOnGround);
    }

    [Fact]
    public void Resolve_LeaveIndoorCell_TransitionsToOutdoor()
    {
        var engine = new PhysicsEngine();
        var terrain = new TerrainSurface(FlatHeightmap(50), LinearHeightTable());

        var cellVerts = new Dictionary<ushort, Vector3>
        {
            [0] = new(40f, 40f, 55f),
            [1] = new(60f, 40f, 55f),
            [2] = new(60f, 60f, 55f),
            [3] = new(40f, 60f, 55f),
        };
        var cellPolys = new List<List<short>> { new() { 0, 1, 2, 3 } };
        var cell = new CellSurface(0x0100, cellVerts, cellPolys);

        engine.AddLandblock(0xA9B4FFFFu, terrain, new[] { cell },
            worldOffsetX: 0f, worldOffsetY: 0f);

        // Start inside the cell, walk out.
        var result = engine.Resolve(
            new Vector3(50f, 50f, 55f), cellId: 0x0100, delta: new Vector3(-20f, 0f, 0f),
            stepUpHeight: 10f);

        // Should transition back to outdoor.
        Assert.True(result.CellId < 0x0100u);
        Assert.Equal(50f, result.Position.Z, precision: 1);
        Assert.True(result.IsOnGround);
    }

    [Fact]
    public void Resolve_NoSurfaceUnderEntity_NotOnGround()
    {
        var engine = new PhysicsEngine();
        // No landblocks loaded — entity is floating in void.

        var result = engine.Resolve(
            new Vector3(0f, 0f, 100f), cellId: 0x0001, delta: Vector3.Zero,
            stepUpHeight: 2f);

        Assert.False(result.IsOnGround);
    }
}