using System;
using System.Buffers;
using System.Collections.Generic;
using System.Security.Cryptography;
using System.Text;
using System.Text.Encodings.Web;
using System.Text.Json;

namespace StellaOps.Attestor.Envelope;

public static class DsseEnvelopeSerializer
{
    public static DsseEnvelopeSerializationResult Serialize(DsseEnvelope envelope, DsseEnvelopeSerializationOptions? options = null)
    {
        ArgumentNullException.ThrowIfNull(envelope);

        options ??= new DsseEnvelopeSerializationOptions();

        if (!options.EmitCompactJson && !options.EmitExpandedJson)
        {
            throw new InvalidOperationException("At least one JSON format must be emitted.");
        }

        if (options.CompressionAlgorithm != DsseCompressionAlgorithm.None)
        {
            throw new NotSupportedException("Payload compression is not supported during serialization. Compress the payload before envelope creation and ensure payloadType/metadata reflect the compressed bytes.");
        }

        var originalPayload = envelope.Payload.ToArray();
        var payloadSha256 = Convert.ToHexString(SHA256.HashData(originalPayload)).ToLowerInvariant();
        var payloadBase64 = Convert.ToBase64String(originalPayload);

        if (envelope.DetachedPayload is not null
            && !string.Equals(payloadSha256, envelope.DetachedPayload.Sha256, StringComparison.OrdinalIgnoreCase))
        {
            throw new InvalidOperationException("Detached payload digest does not match the envelope payload.");
        }

        byte[]? compactJson = null;
        if (options.EmitCompactJson)
        {
            compactJson = BuildCompactJson(envelope.PayloadType, payloadBase64, envelope.Signatures);
        }

        byte[]? expandedJson = null;
        if (options.EmitExpandedJson)
        {
            expandedJson = BuildExpandedJson(
                envelope,
                payloadBase64,
                payloadSha256,
                originalPayload.Length,
                originalPayload.Length,
                options,
                originalPayload);
        }

        return new DsseEnvelopeSerializationResult(
            compactJson,
            expandedJson,
            payloadSha256,
            originalPayload.Length,
            originalPayload.Length, // No compression, so processed == original
            options.CompressionAlgorithm,
            envelope.DetachedPayload);
    }

    private static byte[] BuildCompactJson(string payloadType, string payloadBase64, IReadOnlyList<DsseSignature> signatures)
    {
        var buffer = new ArrayBufferWriter<byte>();
        using var writer = new Utf8JsonWriter(buffer, new JsonWriterOptions
        {
            Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
            Indented = false
        });

        writer.WriteStartObject();
        writer.WriteString("payloadType", payloadType);
        writer.WriteString("payload", payloadBase64);
        writer.WritePropertyName("signatures");
        writer.WriteStartArray();

        foreach (var signature in EnumerateCanonicalSignatures(signatures))
        {
            writer.WriteStartObject();
            if (!string.IsNullOrWhiteSpace(signature.KeyId))
            {
                writer.WriteString("keyid", signature.KeyId);
            }

            writer.WriteString("sig", signature.Signature);
            writer.WriteEndObject();
        }

        writer.WriteEndArray();
        writer.WriteEndObject();
        writer.Flush();
        return buffer.WrittenSpan.ToArray();
    }

    private static byte[]? BuildExpandedJson(
        DsseEnvelope envelope,
        string payloadBase64,
        string payloadSha256,
        int originalPayloadLength,
        int embeddedPayloadLength,
        DsseEnvelopeSerializationOptions options,
        byte[] originalPayload)
    {
        var buffer = new ArrayBufferWriter<byte>();
        using var writer = new Utf8JsonWriter(buffer, new JsonWriterOptions
        {
            Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
            Indented = options.IndentExpandedJson
        });

        writer.WriteStartObject();
        writer.WriteString("payloadType", envelope.PayloadType);
        writer.WriteString("payload", payloadBase64);
        writer.WritePropertyName("signatures");
        writer.WriteStartArray();

        foreach (var signature in EnumerateCanonicalSignatures(envelope.Signatures))
        {
            writer.WriteStartObject();
            if (!string.IsNullOrWhiteSpace(signature.KeyId))
            {
                writer.WriteString("keyid", signature.KeyId);
            }

            writer.WriteString("sig", signature.Signature);
            writer.WriteEndObject();
        }

        writer.WriteEndArray();

        writer.WritePropertyName("payloadInfo");
        writer.WriteStartObject();
        writer.WriteString("sha256", payloadSha256);
        writer.WriteNumber("length", originalPayloadLength);

        if (options.CompressionAlgorithm != DsseCompressionAlgorithm.None)
        {
            writer.WritePropertyName("compression");
            writer.WriteStartObject();
            writer.WriteString("algorithm", GetCompressionName(options.CompressionAlgorithm));
            writer.WriteNumber("compressedLength", embeddedPayloadLength);
            writer.WriteEndObject();
        }

        writer.WriteEndObject(); // payloadInfo

        if (options.IncludePayloadPreview && TryWritePayloadPreview(envelope.PayloadContentType, originalPayload, writer))
        {
            // preview already written inside helper
        }

        if (envelope.DetachedPayload is not null)
        {
            writer.WritePropertyName("detachedPayload");
            writer.WriteStartObject();
            writer.WriteString("uri", envelope.DetachedPayload.Uri);
            writer.WriteString("sha256", envelope.DetachedPayload.Sha256);
            if (envelope.DetachedPayload.Length.HasValue)
            {
                writer.WriteNumber("length", envelope.DetachedPayload.Length.Value);
            }

            if (!string.IsNullOrWhiteSpace(envelope.DetachedPayload.MediaType))
            {
                writer.WriteString("mediaType", envelope.DetachedPayload.MediaType);
            }

            writer.WriteEndObject();
        }

        writer.WriteEndObject();
        writer.Flush();
        return buffer.WrittenSpan.ToArray();
    }

    private static bool TryWritePayloadPreview(string? contentType, byte[] originalPayload, Utf8JsonWriter writer)
    {
        if (string.IsNullOrWhiteSpace(contentType))
        {
            return false;
        }

        var lower = contentType.ToLowerInvariant();
        if (!lower.Contains("json") && !lower.StartsWith("text/", StringComparison.Ordinal))
        {
            return false;
        }

        writer.WritePropertyName("payloadPreview");
        writer.WriteStartObject();
        writer.WriteString("mediaType", contentType);

        if (lower.Contains("json") && TryParseJson(originalPayload, out var jsonDocument))
        {
            writer.WritePropertyName("json");
            jsonDocument.WriteTo(writer);
            jsonDocument.Dispose();
        }
        else if (TryDecodeUtf8(originalPayload, out var text))
        {
            writer.WriteString("text", text);
        }

        writer.WriteEndObject();
        return true;
    }

    private static bool TryParseJson(byte[] payload, out JsonDocument document)
    {
        try
        {
            document = JsonDocument.Parse(payload);
            return true;
        }
        catch (JsonException)
        {
            document = null!;
            return false;
        }
    }

    private static bool TryDecodeUtf8(byte[] payload, out string text)
    {
        var utf8 = new UTF8Encoding(false, true);
        try
        {
            text = utf8.GetString(payload);
            return true;
        }
        catch (DecoderFallbackException)
        {
            text = string.Empty;
            return false;
        }
    }

    private static string GetCompressionName(DsseCompressionAlgorithm algorithm)
    {
        return algorithm switch
        {
            DsseCompressionAlgorithm.Gzip => "gzip",
            DsseCompressionAlgorithm.Brotli => "brotli",
            DsseCompressionAlgorithm.None => "none",
            _ => algorithm.ToString().ToLowerInvariant()
        };
    }

    private static IEnumerable<DsseSignature> EnumerateCanonicalSignatures(IReadOnlyList<DsseSignature> signatures)
    {
        if (signatures.Count <= 1)
        {
            return signatures;
        }

        var comparer = CanonicalSignatureComparer.Instance;
        var previous = signatures[0];
        for (var i = 1; i < signatures.Count; i++)
        {
            var current = signatures[i];
            if (comparer.Compare(previous, current) > 0)
            {
                var buffer = new List<DsseSignature>(signatures.Count);
                for (var j = 0; j < signatures.Count; j++)
                {
                    buffer.Add(signatures[j]);
                }

                buffer.Sort(comparer);
                return buffer;
            }

            previous = current;
        }

        return signatures;
    }

    private sealed class CanonicalSignatureComparer : IComparer<DsseSignature>
    {
        public static CanonicalSignatureComparer Instance { get; } = new();

        public int Compare(DsseSignature? x, DsseSignature? y)
        {
            if (ReferenceEquals(x, y))
            {
                return 0;
            }

            ArgumentNullException.ThrowIfNull(x);
            ArgumentNullException.ThrowIfNull(y);

            var keyComparison = string.Compare(x.KeyId, y.KeyId, StringComparison.Ordinal);
            if (keyComparison != 0)
            {
                if (x.KeyId is null)
                {
                    return -1;
                }

                if (y.KeyId is null)
                {
                    return 1;
                }

                return keyComparison;
            }

            return string.Compare(x.Signature, y.Signature, StringComparison.Ordinal);
        }
    }
}