using StbImageSharp;
using StbImageWriteSharp;

namespace MrGameEng.Atlases;

/// <summary>Options for one <see cref="AtlasBuilder.Build"/> run.</summary>
public sealed class AtlasBuildOptions
{
    /// <summary>Directory scanned recursively for source images (png/jpg/jpeg/bmp).</summary>
    public required string SourceDirectory { get; init; }

    /// <summary>Directory the <c>.atlas</c> metadata and page images are written to.</summary>
    public required string OutputDirectory { get; init; }

    /// <summary>
    /// How many leading directories of a source-relative path form the atlas group:
    /// 0 packs everything into one atlas, 1 packs per top-level folder, and so on.
    /// </summary>
    public int GroupDepth { get; init; } = 1;

    /// <summary>Maximum page width/height in pixels.</summary>
    public int MaxPageSize { get; init; } = 2048;

    /// <summary>Gap in pixels between packed images and page edges (bleed protection).</summary>
    public int Padding { get; init; } = 2;

    /// <summary>Atlas name for images that have fewer directories than <see cref="GroupDepth"/>.</summary>
    public string RootAtlasName { get; init; } = "Atlas";

    /// <summary>Rebuild every atlas even when sources are unchanged.</summary>
    public bool Force { get; init; }
}

/// <summary>Build outcome for one atlas group.</summary>
/// <param name="Name">Atlas name (group key with '/' replaced by '.').</param>
/// <param name="RegionCount">Number of packed source images.</param>
/// <param name="PageCount">Number of page images written.</param>
/// <param name="Skipped">True when the atlas was up to date and not rebuilt.</param>
public sealed record AtlasGroupResult(string Name, int RegionCount, int PageCount, bool Skipped);

/// <summary>Result of an <see cref="AtlasBuilder.Build"/> run.</summary>
/// <param name="Groups">Per-atlas outcomes, sorted by name.</param>
/// <param name="DeletedOrphans">Output files of atlases whose source group no longer exists.</param>
public sealed record AtlasBuildResult(IReadOnlyList<AtlasGroupResult> Groups, IReadOnlyList<string> DeletedOrphans);

/// <summary>
/// Build-time utility converting a directory tree of loose images into texture atlases:
/// page images plus an <see cref="AtlasMetadata"/> JSON per group. Pure CPU (StbImageSharp),
/// no graphics device — intended for tools and build scripts, not for the render loop.
/// Region keys are source-relative paths without extension, so game code addresses sprites
/// by the same path it would have used for the loose file.
/// </summary>
public static class AtlasBuilder
{
    private static readonly string[] SourceExtensions = [".png", ".jpg", ".jpeg", ".bmp"];

    /// <summary>Snapshot of one source image taken at scan time (size/mtime feed the staleness check).</summary>
    private readonly record struct SourceFile(string FullPath, string Key, long Size, long ModifiedTicks);

    /// <summary>Builds (or incrementally refreshes) all atlases for <paramref name="options"/>.</summary>
    public static AtlasBuildResult Build(AtlasBuildOptions options)
    {
        var sourceRoot = Path.GetFullPath(options.SourceDirectory);
        if (!Directory.Exists(sourceRoot))
        {
            throw new DirectoryNotFoundException($"Atlas source directory not found: '{sourceRoot}'.");
        }

        Directory.CreateDirectory(options.OutputDirectory);

        var groups = ScanGroups(sourceRoot, options);
        var results = new List<AtlasGroupResult>();
        foreach (var (name, files) in groups)
        {
            results.Add(BuildGroup(name, files, options));
        }

        var orphans = DeleteOrphans(options.OutputDirectory, groups.Keys);
        return new AtlasBuildResult(results, orphans);
    }

    /// <summary>Maps a source-relative image path to its atlas name and region key.</summary>
    internal static (string AtlasName, string Key) ClassifyPath(string relativePath, int groupDepth, string rootAtlasName)
    {
        var normalized = relativePath.Replace('\\', '/');
        var key = normalized[..normalized.LastIndexOf('.')];
        var segments = normalized.Split('/');
        var depth = Math.Min(groupDepth, segments.Length - 1);
        var name = depth == 0 ? rootAtlasName : string.Join('.', segments[..depth]);
        return (name, key);
    }

    private static SortedDictionary<string, List<SourceFile>> ScanGroups(
        string sourceRoot, AtlasBuildOptions options)
    {
        var groups = new SortedDictionary<string, List<SourceFile>>(StringComparer.Ordinal);
        var keys = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
        foreach (var fullPath in Directory.EnumerateFiles(sourceRoot, "*", SearchOption.AllDirectories))
        {
            if (!SourceExtensions.Contains(Path.GetExtension(fullPath), StringComparer.OrdinalIgnoreCase))
            {
                continue;
            }

            var relative = Path.GetRelativePath(sourceRoot, fullPath);
            var (atlasName, key) = ClassifyPath(relative, options.GroupDepth, options.RootAtlasName);
            if (keys.TryGetValue(key, out var existing))
            {
                throw new InvalidDataException($"Duplicate region key '{key}': '{existing}' and '{relative}'.");
            }

            keys.Add(key, relative);
            if (!groups.TryGetValue(atlasName, out var list))
            {
                list = [];
                groups.Add(atlasName, list);
            }

            var info = new FileInfo(fullPath);
            list.Add(new SourceFile(fullPath, key, info.Length, info.LastWriteTimeUtc.Ticks));
        }

        return groups;
    }

    private static AtlasGroupResult BuildGroup(
        string name, List<SourceFile> files, AtlasBuildOptions options)
    {
        var metadataPath = Path.Combine(options.OutputDirectory, name + ".atlas");
        if (!options.Force && IsUpToDate(metadataPath, files, options, out var existingPages))
        {
            return new AtlasGroupResult(name, files.Count, existingPages, Skipped: true);
        }

        // Декодирование — самая дорогая фаза, параллелим (билд-тайм, аллокации допустимы).
        var images = new ImageResult[files.Count];
        Parallel.For(0, files.Count, i =>
        {
            using var stream = File.OpenRead(files[i].FullPath);
            images[i] = ImageResult.FromStream(stream, StbImageSharp.ColorComponents.RedGreenBlueAlpha);
        });

        var items = new PackItem[files.Count];
        for (var i = 0; i < files.Count; i++)
        {
            items[i] = new PackItem(files[i].Key, images[i].Width, images[i].Height);
        }

        var packed = ShelfPacker.Pack(items, options.MaxPageSize, options.Padding);

        var pixelsByKey = new Dictionary<string, ImageResult>(files.Count, StringComparer.Ordinal);
        for (var i = 0; i < files.Count; i++)
        {
            pixelsByKey.Add(files[i].Key, images[i]);
        }

        WritePages(name, packed, pixelsByKey, options.OutputDirectory);
        WriteMetadata(name, packed, files, options, metadataPath);
        DeleteExtraPages(name, packed.PageSizes.Count, options.OutputDirectory);

        return new AtlasGroupResult(name, files.Count, packed.PageSizes.Count, Skipped: false);
    }

    private static bool IsUpToDate(
        string metadataPath, List<SourceFile> files, AtlasBuildOptions options, out int pages)
    {
        pages = 0;
        if (!File.Exists(metadataPath))
        {
            return false;
        }

        AtlasMetadata metadata;
        try
        {
            metadata = AtlasMetadata.FromJson(File.ReadAllText(metadataPath));
        }
        catch (Exception)
        {
            return false;
        }

        if (metadata.Version != AtlasMetadata.CurrentVersion ||
            metadata.PageSize != options.MaxPageSize || metadata.Padding != options.Padding)
        {
            return false;
        }

        var outputDirectory = Path.GetDirectoryName(metadataPath)!;
        if (metadata.Pages.Any(page => !File.Exists(Path.Combine(outputDirectory, page.File))))
        {
            return false;
        }

        // Источники сравниваются по точному снапшоту (ключ + размер + mtime), а не по
        // «новее метаданных»: переименования и копии с сохранением времени тоже ловятся.
        if (metadata.Sources.Count != files.Count)
        {
            return false;
        }

        var sourcesByKey = metadata.Sources.ToDictionary(s => s.Key, StringComparer.Ordinal);
        foreach (var file in files)
        {
            if (!sourcesByKey.TryGetValue(file.Key, out var source) ||
                source.Size != file.Size || source.Modified != file.ModifiedTicks)
            {
                return false;
            }
        }

        pages = metadata.Pages.Count;
        return true;
    }

    private static void WritePages(
        string name, PackResult packed, Dictionary<string, ImageResult> pixelsByKey, string outputDirectory)
    {
        Parallel.For(0, packed.PageSizes.Count, page =>
        {
            var (width, height) = packed.PageSizes[page];
            var buffer = new byte[width * height * 4];
            foreach (var placement in packed.Placements)
            {
                if (placement.Page != page)
                {
                    continue;
                }

                var source = pixelsByKey[placement.Key];
                for (var row = 0; row < source.Height; row++)
                {
                    Array.Copy(
                        source.Data, row * source.Width * 4,
                        buffer, ((placement.Y + row) * width + placement.X) * 4,
                        source.Width * 4);
                }
            }

            using var stream = File.Create(Path.Combine(outputDirectory, PageFileName(name, page)));
            new ImageWriter().WritePng(
                buffer, width, height, StbImageWriteSharp.ColorComponents.RedGreenBlueAlpha, stream);
        });
    }

    private static void WriteMetadata(
        string name, PackResult packed, List<SourceFile> files, AtlasBuildOptions options, string metadataPath)
    {
        var metadata = new AtlasMetadata
        {
            Name = name,
            PageSize = options.MaxPageSize,
            Padding = options.Padding,
            Sources = files
                .OrderBy(f => f.Key, StringComparer.Ordinal)
                .Select(f => new AtlasSource { Key = f.Key, Size = f.Size, Modified = f.ModifiedTicks })
                .ToList(),
            Pages = packed.PageSizes
                .Select((size, index) => new AtlasPage
                {
                    File = PageFileName(name, index),
                    Width = size.Width,
                    Height = size.Height,
                })
                .ToList(),
            Regions = packed.Placements
                .OrderBy(p => p.Key, StringComparer.Ordinal)
                .Select(p => new AtlasRegion
                {
                    Key = p.Key,
                    Page = p.Page,
                    X = p.X,
                    Y = p.Y,
                    Width = p.Width,
                    Height = p.Height,
                })
                .ToList(),
        };

        File.WriteAllText(metadataPath, metadata.ToJson());
    }

    private static string PageFileName(string atlasName, int page) => $"{atlasName}.atlas.{page}.png";

    private static void DeleteExtraPages(string name, int pageCount, string outputDirectory)
    {
        for (var page = pageCount; ; page++)
        {
            var path = Path.Combine(outputDirectory, PageFileName(name, page));
            if (!File.Exists(path))
            {
                return;
            }

            File.Delete(path);
        }
    }

    private static List<string> DeleteOrphans(string outputDirectory, IEnumerable<string> liveAtlasNames)
    {
        var live = liveAtlasNames.ToHashSet(StringComparer.Ordinal);
        var deleted = new List<string>();
        foreach (var metadataPath in Directory.EnumerateFiles(outputDirectory, "*.atlas"))
        {
            var name = Path.GetFileNameWithoutExtension(metadataPath);
            if (live.Contains(name))
            {
                continue;
            }

            File.Delete(metadataPath);
            deleted.Add(metadataPath);
            DeleteExtraPages(name, 0, outputDirectory);
        }

        return deleted;
    }
}