using System.Security.Cryptography;

namespace StellaOps.Cryptography.Plugin.OfflineVerification;

/// <summary>
/// Cryptographic provider for offline/air-gapped environments using .NET BCL cryptography.
/// This provider wraps System.Security.Cryptography in the ICryptoProvider abstraction
/// to enable configuration-driven crypto while maintaining offline verification capabilities.
/// </summary>
public sealed class OfflineVerificationCryptoProvider : ICryptoProvider
{
    /// <summary>
    /// Provider name for registry resolution.
    /// </summary>
    public string Name => "offline-verification";

    /// <summary>
    /// Checks if this provider supports the specified capability and algorithm.
    /// </summary>
    public bool Supports(CryptoCapability capability, string algorithmId)
    {
        return capability switch
        {
            CryptoCapability.Signing => algorithmId is "ES256" or "ES384" or "ES512"
                                                    or "RS256" or "RS384" or "RS512"
                                                    or "PS256" or "PS384" or "PS512",
            CryptoCapability.Verification => algorithmId is "ES256" or "ES384" or "ES512"
                                                          or "RS256" or "RS384" or "RS512"
                                                          or "PS256" or "PS384" or "PS512",
            CryptoCapability.ContentHashing => algorithmId is "SHA-256" or "SHA-384" or "SHA-512"
                                                            or "SHA256" or "SHA384" or "SHA512",
            CryptoCapability.PasswordHashing => algorithmId is "PBKDF2" or "Argon2id",
            _ => false
        };
    }

    /// <summary>
    /// Not supported for offline verification - no password hashing.
    /// </summary>
    public IPasswordHasher GetPasswordHasher(string algorithmId)
    {
        throw new NotSupportedException(
            $"Password hashing is not supported by the offline verification provider.");
    }

    /// <summary>
    /// Gets a content hasher for the specified algorithm.
    /// </summary>
    public ICryptoHasher GetHasher(string algorithmId)
    {
        var normalized = NormalizeAlgorithmId(algorithmId);
        return normalized switch
        {
            "SHA-256" => new BclHasher("SHA-256", HashAlgorithmName.SHA256),
            "SHA-384" => new BclHasher("SHA-384", HashAlgorithmName.SHA384),
            "SHA-512" => new BclHasher("SHA-512", HashAlgorithmName.SHA512),
            _ => throw new NotSupportedException($"Hash algorithm '{algorithmId}' is not supported.")
        };
    }

    /// <summary>
    /// Gets a signer for the specified algorithm and key reference.
    /// </summary>
    public ICryptoSigner GetSigner(string algorithmId, CryptoKeyReference keyReference)
    {
        return algorithmId switch
        {
            "ES256" => new EcdsaSigner(algorithmId, ECCurve.NamedCurves.nistP256, HashAlgorithmName.SHA256, keyReference),
            "ES384" => new EcdsaSigner(algorithmId, ECCurve.NamedCurves.nistP384, HashAlgorithmName.SHA384, keyReference),
            "ES512" => new EcdsaSigner(algorithmId, ECCurve.NamedCurves.nistP521, HashAlgorithmName.SHA512, keyReference),
            "RS256" => new RsaSigner(algorithmId, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1, keyReference),
            "RS384" => new RsaSigner(algorithmId, HashAlgorithmName.SHA384, RSASignaturePadding.Pkcs1, keyReference),
            "RS512" => new RsaSigner(algorithmId, HashAlgorithmName.SHA512, RSASignaturePadding.Pkcs1, keyReference),
            "PS256" => new RsaSigner(algorithmId, HashAlgorithmName.SHA256, RSASignaturePadding.Pss, keyReference),
            "PS384" => new RsaSigner(algorithmId, HashAlgorithmName.SHA384, RSASignaturePadding.Pss, keyReference),
            "PS512" => new RsaSigner(algorithmId, HashAlgorithmName.SHA512, RSASignaturePadding.Pss, keyReference),
            _ => throw new NotSupportedException($"Signing algorithm '{algorithmId}' is not supported.")
        };
    }

    /// <summary>
    /// Creates an ephemeral verifier from raw public key bytes (verification-only).
    /// </summary>
    public ICryptoSigner CreateEphemeralVerifier(string algorithmId, ReadOnlySpan<byte> publicKeyBytes)
    {
        return algorithmId switch
        {
            "ES256" => new EcdsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA256, publicKeyBytes.ToArray()),
            "ES384" => new EcdsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA384, publicKeyBytes.ToArray()),
            "ES512" => new EcdsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA512, publicKeyBytes.ToArray()),
            "RS256" => new RsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1, publicKeyBytes.ToArray()),
            "RS384" => new RsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA384, RSASignaturePadding.Pkcs1, publicKeyBytes.ToArray()),
            "RS512" => new RsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA512, RSASignaturePadding.Pkcs1, publicKeyBytes.ToArray()),
            "PS256" => new RsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA256, RSASignaturePadding.Pss, publicKeyBytes.ToArray()),
            "PS384" => new RsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA384, RSASignaturePadding.Pss, publicKeyBytes.ToArray()),
            "PS512" => new RsaEphemeralVerifier(algorithmId, HashAlgorithmName.SHA512, RSASignaturePadding.Pss, publicKeyBytes.ToArray()),
            _ => throw new NotSupportedException($"Verification algorithm '{algorithmId}' is not supported.")
        };
    }

    /// <summary>
    /// Not supported - offline verification provider does not manage keys.
    /// </summary>
    public void UpsertSigningKey(CryptoSigningKey signingKey)
    {
        throw new NotSupportedException(
            "The offline verification provider does not support key management.");
    }

    /// <summary>
    /// Not supported - offline verification provider does not manage keys.
    /// </summary>
    public bool RemoveSigningKey(string keyId)
    {
        throw new NotSupportedException(
            "The offline verification provider does not support key management.");
    }

    /// <summary>
    /// Returns empty collection - offline verification provider does not manage keys.
    /// </summary>
    public IReadOnlyCollection<CryptoSigningKey> GetSigningKeys()
    {
        return Array.Empty<CryptoSigningKey>();
    }

    private static string NormalizeAlgorithmId(string algorithmId)
    {
        if (string.IsNullOrEmpty(algorithmId))
            return algorithmId ?? string.Empty;

        return algorithmId.ToUpperInvariant() switch
        {
            "SHA256" or "SHA-256" => "SHA-256",
            "SHA384" or "SHA-384" => "SHA-384",
            "SHA512" or "SHA-512" => "SHA-512",
            _ => algorithmId
        };
    }

    /// <summary>
    /// Internal hasher implementation using .NET BCL.
    /// </summary>
    private sealed class BclHasher : ICryptoHasher
    {
        private readonly HashAlgorithmName _hashAlgorithmName;

        public string AlgorithmId { get; }

        public BclHasher(string algorithmId, HashAlgorithmName hashAlgorithmName)
        {
            AlgorithmId = algorithmId;
            _hashAlgorithmName = hashAlgorithmName;
        }

        public byte[] ComputeHash(ReadOnlySpan<byte> data)
        {
            // Use System.Security.Cryptography internally - this is allowed within plugin
            return _hashAlgorithmName.Name switch
            {
                "SHA256" => SHA256.HashData(data),
                "SHA384" => SHA384.HashData(data),
                "SHA512" => SHA512.HashData(data),
                _ => throw new NotSupportedException($"Hash algorithm '{_hashAlgorithmName}' is not supported.")
            };
        }

        public string ComputeHashHex(ReadOnlySpan<byte> data)
        {
            var hash = ComputeHash(data);
            return Convert.ToHexString(hash).ToLowerInvariant();
        }
    }

    /// <summary>
    /// Internal ECDSA signer using .NET BCL.
    /// </summary>
    private sealed class EcdsaSigner : ICryptoSigner
    {
        private readonly ECCurve _curve;
        private readonly HashAlgorithmName _hashAlgorithm;
        private readonly CryptoKeyReference _keyReference;

        public string KeyId { get; }
        public string AlgorithmId { get; }

        public EcdsaSigner(string algorithmId, ECCurve curve, HashAlgorithmName hashAlgorithm, CryptoKeyReference keyReference)
        {
            AlgorithmId = algorithmId;
            _curve = curve;
            _hashAlgorithm = hashAlgorithm;
            _keyReference = keyReference;
            KeyId = keyReference.KeyId;
        }

        public ValueTask<byte[]> SignAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken = default)
        {
            // Use System.Security.Cryptography internally - this is allowed within plugin
            using var ecdsa = ECDsa.Create(_curve);

            // In a real implementation, would load key material from _keyReference
            // For now, generate ephemeral key (caller should provide key material)

            var signature = ecdsa.SignData(data.Span, _hashAlgorithm);
            return new ValueTask<byte[]>(signature);
        }

        public ValueTask<bool> VerifyAsync(ReadOnlyMemory<byte> data, ReadOnlyMemory<byte> signature, CancellationToken cancellationToken = default)
        {
            // Use System.Security.Cryptography internally - this is allowed within plugin
            using var ecdsa = ECDsa.Create(_curve);

            // In a real implementation, would load public key from _keyReference
            var isValid = ecdsa.VerifyData(data.Span, signature.Span, _hashAlgorithm);
            return new ValueTask<bool>(isValid);
        }

        public Microsoft.IdentityModel.Tokens.JsonWebKey ExportPublicJsonWebKey()
        {
            // In a real implementation, would export actual public key
            // For offline verification, this is typically not needed
            throw new NotSupportedException("JWK export not supported in offline verification mode.");
        }
    }

    /// <summary>
    /// Internal RSA signer using .NET BCL.
    /// </summary>
    private sealed class RsaSigner : ICryptoSigner
    {
        private readonly HashAlgorithmName _hashAlgorithm;
        private readonly RSASignaturePadding _padding;
        private readonly CryptoKeyReference _keyReference;

        public string KeyId { get; }
        public string AlgorithmId { get; }

        public RsaSigner(string algorithmId, HashAlgorithmName hashAlgorithm, RSASignaturePadding padding, CryptoKeyReference keyReference)
        {
            AlgorithmId = algorithmId;
            _hashAlgorithm = hashAlgorithm;
            _padding = padding;
            _keyReference = keyReference;
            KeyId = keyReference.KeyId;
        }

        public ValueTask<byte[]> SignAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken = default)
        {
            // Use System.Security.Cryptography internally - this is allowed within plugin
            using var rsa = RSA.Create();

            // In a real implementation, would load key material from _keyReference
            // For now, generate ephemeral key (caller should provide key material)

            var signature = rsa.SignData(data.Span, _hashAlgorithm, _padding);
            return new ValueTask<byte[]>(signature);
        }

        public ValueTask<bool> VerifyAsync(ReadOnlyMemory<byte> data, ReadOnlyMemory<byte> signature, CancellationToken cancellationToken = default)
        {
            // Use System.Security.Cryptography internally - this is allowed within plugin
            using var rsa = RSA.Create();

            // In a real implementation, would load public key from _keyReference
            var isValid = rsa.VerifyData(data.Span, signature.Span, _hashAlgorithm, _padding);
            return new ValueTask<bool>(isValid);
        }

        public Microsoft.IdentityModel.Tokens.JsonWebKey ExportPublicJsonWebKey()
        {
            // In a real implementation, would export actual public key
            // For offline verification, this is typically not needed
            throw new NotSupportedException("JWK export not supported in offline verification mode.");
        }
    }

    /// <summary>
    /// Ephemeral RSA verifier using raw public key bytes (verification-only).
    /// </summary>
    private sealed class RsaEphemeralVerifier : ICryptoSigner
    {
        private readonly HashAlgorithmName _hashAlgorithm;
        private readonly RSASignaturePadding _padding;
        private readonly byte[] _publicKeyBytes;

        public string KeyId { get; } = "ephemeral";
        public string AlgorithmId { get; }

        public RsaEphemeralVerifier(string algorithmId, HashAlgorithmName hashAlgorithm, RSASignaturePadding padding, byte[] publicKeyBytes)
        {
            AlgorithmId = algorithmId;
            _hashAlgorithm = hashAlgorithm;
            _padding = padding;
            _publicKeyBytes = publicKeyBytes;
        }

        public ValueTask<byte[]> SignAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken = default)
        {
            throw new NotSupportedException("Ephemeral verifier does not support signing operations.");
        }

        public ValueTask<bool> VerifyAsync(ReadOnlyMemory<byte> data, ReadOnlyMemory<byte> signature, CancellationToken cancellationToken = default)
        {
            // Use System.Security.Cryptography internally - this is allowed within plugin
            using var rsa = RSA.Create();
            rsa.ImportSubjectPublicKeyInfo(_publicKeyBytes, out _);
            var isValid = rsa.VerifyData(data.Span, signature.Span, _hashAlgorithm, _padding);
            return new ValueTask<bool>(isValid);
        }

        public Microsoft.IdentityModel.Tokens.JsonWebKey ExportPublicJsonWebKey()
        {
            throw new NotSupportedException("JWK export not supported for ephemeral verifiers.");
        }
    }

    /// <summary>
    /// Ephemeral ECDSA verifier using raw public key bytes (verification-only).
    /// </summary>
    private sealed class EcdsaEphemeralVerifier : ICryptoSigner
    {
        private readonly HashAlgorithmName _hashAlgorithm;
        private readonly byte[] _publicKeyBytes;

        public string KeyId { get; } = "ephemeral";
        public string AlgorithmId { get; }

        public EcdsaEphemeralVerifier(string algorithmId, HashAlgorithmName hashAlgorithm, byte[] publicKeyBytes)
        {
            AlgorithmId = algorithmId;
            _hashAlgorithm = hashAlgorithm;
            _publicKeyBytes = publicKeyBytes;
        }

        public ValueTask<byte[]> SignAsync(ReadOnlyMemory<byte> data, CancellationToken cancellationToken = default)
        {
            throw new NotSupportedException("Ephemeral verifier does not support signing operations.");
        }

        public ValueTask<bool> VerifyAsync(ReadOnlyMemory<byte> data, ReadOnlyMemory<byte> signature, CancellationToken cancellationToken = default)
        {
            // Use System.Security.Cryptography internally - this is allowed within plugin
            using var ecdsa = ECDsa.Create();
            ecdsa.ImportSubjectPublicKeyInfo(_publicKeyBytes, out _);
            var isValid = ecdsa.VerifyData(data.Span, signature.Span, _hashAlgorithm);
            return new ValueTask<bool>(isValid);
        }

        public Microsoft.IdentityModel.Tokens.JsonWebKey ExportPublicJsonWebKey()
        {
            throw new NotSupportedException("JWK export not supported for ephemeral verifiers.");
        }
    }
}