using System.Buffers.Text;
using System.Globalization;
using System.Security.Cryptography;
using System.Text;

namespace Fleet.Identity;

/// <summary>passkey（WebAuthn）对一段文本签出的断言：<c>challenge = SHA-256(Statement 的 UTF-8)</c>。四个字段都是 base64url。</summary>
public sealed record SignedStatement(string Statement, string CredentialId, string AuthenticatorData, string ClientDataJson, string Signature);

/// <summary>一个操作员凭证：passkey 的凭证 id 和公钥（SPKI）。</summary>
public sealed record OperatorKey(string CredentialId, byte[] PublicKeySpki);

/// <summary>
/// 操作员集合的一个版本。<c>RpId</c> 和 <c>Origins</c> 也在里面：节点验 WebAuthn 断言要用，而它们被操作员的签名覆盖，Backend 改不了。
/// 创世版本（1）由它自己的某把凭证签名，指纹（<see cref="OperatorTrust.Fingerprint"/>）在安装时固定到节点；之后每个版本由上一版里的凭证签名。
/// </summary>
/// <param name="Servers">服务器密钥（SPKI）：操作员授权的 Backend 签名密钥。集合里登记了服务器密钥之后，节点要求服务器在认证挑战里用它签名，
/// 并在这条链路上用应用层加密，所以 DNS 或证书层面的冒充拿不出这把密钥就骗不了节点。空 = 还没有启用。</param>
public sealed record OperatorSet(int Version, string Previous, string RpId, IReadOnlyList<string> Origins, IReadOnlyList<OperatorKey> Keys, IReadOnlyList<byte[]>? Servers = null);

/// <summary>
/// 信任根：操作员（passkey）→ 操作员集合 → 节点本地固定的属主。设计见 docs/security/access.md 第 8 节。
/// 安装命令里带着创世指纹和属主（passkey 的凭证 id）：节点把它们固定在本地，之后只认从创世出发、由操作员 passkey 签过名的集合版本，
/// 属主必须是集合里的凭证。Backend 能转发集合，但签不出新的版本，也改不了节点已经固定的东西。
/// 全是纯函数，只用 BCL，节点和 Backend 共用。语句是 <c>key=value</c> 的文本，第一行是带版本的标题；签名覆盖整段文本。
/// </summary>
public static class OperatorTrust
{
    public const string SetHeader = "moshiro-opset-v1";
    public const string Genesis = "0";

    public static string Encode(OperatorSet set)
    {
        var text = new StringBuilder();
        text.Append(SetHeader).Append('\n')
            .Append("version=").Append(set.Version.ToString(CultureInfo.InvariantCulture)).Append('\n')
            .Append("prev=").Append(set.Previous).Append('\n')
            .Append("rp=").Append(set.RpId).Append('\n')
            .Append("origins=").Append(string.Join(',', set.Origins));
        foreach (OperatorKey key in set.Keys)
        {
            text.Append("\nkey=").Append(key.CredentialId).Append(':').Append(Convert.ToBase64String(key.PublicKeySpki));
        }

        foreach (byte[] server in set.Servers ?? [])
        {
            text.Append("\nserver=").Append(Convert.ToBase64String(server));
        }

        return text.ToString();
    }

    /// <summary>严格解析：多一行、少一行、顺序不对都算坏。</summary>
    public static OperatorSet? ParseSet(string text)
    {
        string[] lines = text.Split('\n');
        if (lines.Length < 6 || lines[0] != SetHeader || !TryValue(lines[1], "version", out string version) || !int.TryParse(version, NumberStyles.None, CultureInfo.InvariantCulture, out int number) || number < 1
            || !TryValue(lines[2], "prev", out string previous) || !TryValue(lines[3], "rp", out string rp) || rp.Length == 0 || !TryValue(lines[4], "origins", out string origins) || origins.Length == 0)
        {
            return null;
        }

        var keys = new List<OperatorKey>();
        var servers = new List<byte[]>();
        foreach (string line in lines.Skip(5))
        {
            if (servers.Count == 0 && TryValue(line, "key", out string value) && value.IndexOf(':', StringComparison.Ordinal) is > 0 and int split && TryBase64(value[(split + 1)..], out byte[] spki))
            {
                keys.Add(new OperatorKey(value[..split], spki));
            }
            else if (TryValue(line, "server", out string serverText) && TryBase64(serverText, out byte[] serverKey))
            {
                servers.Add(serverKey);
            }
            else
            {
                return null;
            }
        }

        return keys.Count == 0 || (number == 1) != (previous == Genesis) ? null : new OperatorSet(number, previous, rp, origins.Split(','), keys, servers);
    }

    /// <summary>一段语句的指纹（SHA-256，小写十六进制）。创世版本的指纹就是安装命令里固定的那个。</summary>
    public static string Fingerprint(string statement) => Convert.ToHexStringLower(SHA256.HashData(Encoding.UTF8.GetBytes(statement)));

    /// <summary>
    /// 验证操作员集合的链：第一项是创世版本（指纹等于 <paramref name="pinnedGenesis"/>，自签），其后每一项由上一版里的凭证签名、版本加一、<c>prev</c> 指向上一版的指纹、域名不变。
    /// 返回最后一版；不对返回 null。
    /// </summary>
    public static OperatorSet? VerifyChain(string pinnedGenesis, IReadOnlyList<SignedStatement> chain)
    {
        if (chain.Count == 0 || !string.Equals(Fingerprint(chain[0].Statement), pinnedGenesis, StringComparison.OrdinalIgnoreCase))
        {
            return null;
        }

        OperatorSet? current = null;
        string? previousFingerprint = null;
        foreach (SignedStatement link in chain)
        {
            OperatorSet? next = ParseSet(link.Statement);
            if (next is null)
            {
                return null;
            }

            if (current is null)
            {
                if (next.Version != 1 || !VerifyOperator(link, next))
                {
                    return null;
                }
            }
            else if (next.Version != current.Version + 1 || next.Previous != previousFingerprint || next.RpId != current.RpId || !VerifyOperator(link, current))
            {
                return null;
            }

            current = next;
            previousFingerprint = Fingerprint(link.Statement);
        }

        return current;
    }

    /// <summary>断言是不是这个集合里某把凭证对 <c>Statement</c> 签的（要求用户验证；挑战是语句的 SHA-256）。</summary>
    public static bool VerifyOperator(SignedStatement signed, OperatorSet set)
    {
        OperatorKey? key = set.Keys.FirstOrDefault(k => k.CredentialId == signed.CredentialId);
        if (key is null || !TryDecode(signed.AuthenticatorData, out byte[] authData) || !TryDecode(signed.ClientDataJson, out byte[] clientData) || !TryDecode(signed.Signature, out byte[] signature))
        {
            return false;
        }

        return WebAuthnAssertion.Verify(key.PublicKeySpki, authData, clientData, signature, set.RpId, SHA256.HashData(Encoding.UTF8.GetBytes(signed.Statement)), set.Origins.Contains, requireUserVerification: true, out _);
    }

    private static bool TryValue(string line, string key, out string value)
    {
        value = "";
        if (!line.StartsWith(key + "=", StringComparison.Ordinal))
        {
            return false;
        }

        value = line[(key.Length + 1)..];
        return true;
    }

    private static bool TryBase64(string text, out byte[] data)
    {
        try
        {
            data = Convert.FromBase64String(text);
            return data.Length > 0;
        }
        catch (FormatException)
        {
            data = [];
            return false;
        }
    }

    private static bool TryDecode(string text, out byte[] data)
    {
        data = [];
        return Base64Url.IsValid(text) && (data = Base64Url.DecodeFromChars(text)).Length > 0;
    }
}