using System.Security.Cryptography;
using System.Text;

namespace MatrixImageEditor;

public static class ImageOperations
{
    private static readonly UTF8Encoding Utf8NoBomStrict = new(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);

    public static ImageConfig ReadConfig(string imagePath)
    {
        imagePath = ImagePathRules.ValidateExistingImage(imagePath);
        VerifySidecarIfPresent(imagePath);
        using var disk = new Fat16Disk(imagePath, writable: false);
        return ImageConfigCodec.Read(disk.ReadFile("MSCCFG.BIN"));
    }

    public static void WriteConfig(string imagePath, ImageConfig config)
    {
        imagePath = ImagePathRules.ValidateExistingImage(imagePath);
        VerifySidecarIfPresent(imagePath);
        using var disk = new Fat16Disk(imagePath, writable: true);
        var current = disk.ReadFile("MSCCFG.BIN");
        if (ImageConfigCodec.Read(current).SoftwareVersion != config.SoftwareVersion) throw new InvalidDataException("禁止通过编辑器改变软件版本");
        var update = ImageConfigCodec.EncodeUpdate(current, config);
        disk.WriteFileRange("MSCCFG.BIN", ImageConfigCodec.SlotOffset(update.TargetSlot), update.SlotData);
        var verified = ImageConfigCodec.Read(disk.ReadFile("MSCCFG.BIN"));
        if (!ImageConfigCodec.Serialize(verified).Equals(ImageConfigCodec.Serialize(config), StringComparison.Ordinal)) throw new IOException("配置写后校验失败");
    }

    public static void WriteSha256(string imagePath, IProgress<int>? progress)
    {
        imagePath = ImagePathRules.ValidateExistingImage(imagePath);
        using var stream = new FileStream(imagePath, FileMode.Open, FileAccess.Read, FileShare.Read, 4 * 1024 * 1024, FileOptions.SequentialScan);
        using var hash = IncrementalHash.CreateHash(HashAlgorithmName.SHA256);
        var buffer = new byte[4 * 1024 * 1024];
        long completed = 0;
        int read;
        while ((read = stream.Read(buffer, 0, buffer.Length)) > 0)
        {
            hash.AppendData(buffer, 0, read);
            completed += read;
            progress?.Report((int)(completed * 100 / stream.Length));
        }
        var digest = Convert.ToHexString(hash.GetHashAndReset()).ToLowerInvariant();
        progress?.Report(100);

        var sidecar = imagePath + ".sha256";
        var temporary = sidecar + "." + Guid.NewGuid().ToString("N") + ".tmp";
        try
        {
            var bytes = Utf8NoBomStrict.GetBytes($"{digest}  {Path.GetFileName(imagePath)}\n");
            using (var output = new FileStream(temporary, FileMode.CreateNew, FileAccess.Write, FileShare.None, 4096, FileOptions.WriteThrough))
            {
                output.Write(bytes);
                output.Flush(flushToDisk: true);
            }
            File.Move(temporary, sidecar, overwrite: true);
            var stored = ReadSidecarDigest(sidecar, Path.GetFileName(imagePath));
            if (!stored.Equals(digest, StringComparison.Ordinal)) throw new IOException("镜像 SHA-256 摘要写后校验失败");
        }
        finally
        {
            if (File.Exists(temporary)) File.Delete(temporary);
        }
    }

    private static void VerifySidecarIfPresent(string imagePath)
    {
        var sidecar = imagePath + ".sha256";
        if (!File.Exists(sidecar)) return;
        var expected = ReadSidecarDigest(sidecar, Path.GetFileName(imagePath));
        using var stream = new FileStream(imagePath, FileMode.Open, FileAccess.Read, FileShare.Read, 4 * 1024 * 1024, FileOptions.SequentialScan);
        var actual = Convert.ToHexString(SHA256.HashData(stream)).ToLowerInvariant();
        if (!actual.Equals(expected, StringComparison.Ordinal)) throw new InvalidDataException("镜像内容与 SHA-256 摘要不符，禁止编辑");
    }

    private static string ReadSidecarDigest(string sidecarPath, string expectedFileName)
    {
        if (new FileInfo(sidecarPath).Length > 128 * 1024) throw new InvalidDataException("镜像 SHA-256 摘要文件过大");
        var bytes = File.ReadAllBytes(sidecarPath);
        if (bytes.Length >= 3 && bytes[0] == 0xEF && bytes[1] == 0xBB && bytes[2] == 0xBF) throw new InvalidDataException("镜像 SHA-256 摘要必须使用无 BOM UTF-8 格式");

        string content;
        try
        {
            content = Utf8NoBomStrict.GetString(bytes);
        }
        catch (DecoderFallbackException exception)
        {
            throw new InvalidDataException("镜像 SHA-256 摘要不是有效的 UTF-8 文本", exception);
        }

        if (content.Length < 68 || content[^1] != '\n' || content.AsSpan(0, content.Length - 1).ContainsAny('\r', '\n'))
        {
            throw new InvalidDataException("镜像 SHA-256 摘要文件必须是以 LF 结尾的单行规范格式");
        }

        var line = content.AsSpan(0, content.Length - 1);
        var digest = line[..64];
        if (digest.ContainsAnyExcept("0123456789abcdef") || line[64] != ' ' || line[65] != ' ')
        {
            throw new InvalidDataException("镜像 SHA-256 摘要文件格式无效");
        }

        var storedFileName = line[66..].ToString();
        if (!storedFileName.Equals(expectedFileName, StringComparison.Ordinal))
        {
            throw new InvalidDataException("镜像 SHA-256 摘要中的文件名与当前镜像不一致");
        }
        return digest.ToString();
    }
}