using UnityEngine;

public partial class MeshPainter
{
    // --- Backend selection / switching ---

    private void SetupBackend(bool gpu, Color32[][] canonical)
    {
        // SetupGpu returns false if the shaders are missing and it fell back to CPU; honor
        // the real outcome so _gpuActive never claims GPU while _gpu is null (would NRE on
        // the next SwitchBackend / paint).
        if (gpu)
        {
            _gpuActive = SetupGpu(canonical);
        }
        else
        {
            SetupCpu(canonical);
            _gpuActive = false;
        }
    }

    /// <summary>Switches the backend at runtime, carrying over all painted maps.</summary>
    private void SwitchBackend(bool toGpu)
    {
        var canonical = new Color32[MAP_COUNT][];
        for (int mi = 0; mi < MAP_COUNT; mi++) canonical[mi] = ReadCanonical(mi);

        RebuildBackend(toGpu, canonical);

        Debug.Log($"[MeshPainter] Backend switched -> {(toGpu ? "GPU" : "CPU")} painting.", this);
    }

    /// <summary>
    /// Tears down the active backend and rebuilds it from externally supplied canonical maps (load /
    /// late-join snapshot / undo). Runs the island/dilation seeding path so edge padding is rebuilt for
    /// the new content. The input arrays are never mutated (SetupBackend clones its seed).
    /// </summary>
    private void ReseedCanonical(Color32[][] canonical) => RebuildBackend(useGpu, canonical);

    /// <summary>Tears down the active backend and sets up the requested one on the given maps.</summary>
    private void RebuildBackend(bool gpu, Color32[][] canonical)
    {
        if (_gpuActive) TeardownGpu();
        else TeardownCpu();
        SetupBackend(gpu, canonical);
        _hasLastScreen = false;
    }

    /// <summary>Seed source for a map: the supplied canonical data when valid, else the material seed.</summary>
    private Color32[] ResolveSeed(int mi, Color32[][] canonical)
    {
        return (canonical != null && canonical[mi] != null && canonical[mi].Length == textureSize * textureSize)
            ? canonical[mi] : _seedCanonical[mi];
    }

    // Backend-agnostic stroke dispatch (GPU vs CPU), shared by local painting and replayed net stamps.

    private void BeginBackendStroke()
    {
        if (_gpuActive) BeginGpuStroke();
        else ResetCpuDirty();
    }

    private void DrawBackendStamp(Vector3 pLocal, float radiusLocal, Vector3 u, Vector3 v, Vector3 n)
    {
        if (_gpuActive) DrawBrushGpu(pLocal, radiusLocal, u, v, n);
        else PaintStamp3D(pLocal, radiusLocal, u, v, n);
    }

    private void EndBackendStroke()
    {
        if (_gpuActive) EndGpuStroke();
        else EndCpuStroke();
    }

    /// <summary>Reads a map from the active backend into canonical (material) layout.</summary>
    private Color32[] ReadCanonical(int mi)
    {
        if (_gpuActive)
        {
            var gm = _gpu[mi];
            // MSO: read the packed final mask RT (R=metallic, G=occlusion, A=smoothness).
            // Others: the display RT. The mask map is linear data; color maps read sRGB.
            if (mi == MAP_MSO) return ReadRtPixels(gm.final, linear: true);
            return ReadRtPixels(gm.display, linear: MapIsLinear(mi));
        }
        return (Color32[])_cpu[mi].pixels.Clone();
    }

    private static bool MapIsLinear(int mi) => mi == MAP_MSO;

    // Material slots per map, as pre-hashed IDs (this runs per stroke end - no per-call allocations).
    // MSO: one mask map drives three URP slots: _MetallicGlossMap (R/A) + _OcclusionMap (G) +
    // _ParallaxMap (G = occlusion reused as a 0.5-centred height for parallax/relief).
    private static readonly int[][] MapPropIds =
    {
        new[] { BaseMapId, MainTexId },                              // MAP_ALBEDO
        new[] { MetallicGlossMapId, OcclusionMapId, ParallaxMapId }, // MAP_MSO
        new[] { EmissionMapId },                                     // MAP_EM
    };

    private void AssignMapTexture(int mi, Texture tex)
    {
        if (!_materialInstance) return;
        foreach (int id in MapPropIds[mi])
            if (_materialInstance.HasProperty(id)) _materialInstance.SetTexture(id, tex);
    }
}