using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using ZB.MOM.WW.OtOpcUa.Core.Authorization;
using ZB.MOM.WW.OtOpcUa.OpcUaServer.Security;

namespace ZB.MOM.WW.OtOpcUa.Host.OpcUa;

/// <summary>
///     Production <see cref="IOpcUaUserAuthenticator"/> for the machine-to-machine data plane (#520):
///     the client presents the static username <c>apikey</c> with an
///     <c>otua_&lt;keyId&gt;_&lt;secret&gt;</c> credential as the password. Replaces the LDAP adapter.
/// </summary>
/// <remarks>
///     <para>
///         <b>This runs inline on session activation.</b> The SDK raises <c>ImpersonateUser</c>
///         synchronously while holding a SessionManager-wide event lock, so a slow authenticator does not
///         stall one request — it serializes every session activation server-wide. That is why
///         verification is a single indexed lookup plus one HMAC, and why there is no directory call,
///         no retry, and no network I/O on this path at all. It is the main practical advantage of keys
///         over the LDAP bind this replaces.
///     </para>
///     <para>
///         <b>Fail closed, and say little.</b> Every rejection returns the same opaque message. The
///         reason is logged server-side but never returned, so a caller cannot distinguish "no such key"
///         from "wrong secret" from "key disabled" and use the server as an oracle.
///     </para>
/// </remarks>
public sealed class ApiKeyOpcUaUserAuthenticator : IOpcUaUserAuthenticator
{
    /// <summary>
    ///     Returned for every failure mode, deliberately uninformative. Public so a test can assert that
    ///     the failure modes really are indistinguishable, rather than restating the literal and passing
    ///     even if the implementation started differentiating them.
    /// </summary>
    public const string DenyMessage = "Invalid API key.";

    private readonly IApiKeyStore _store;
    private readonly IOptions<ApiKeyOptions> _options;
    private readonly ILogger<ApiKeyOpcUaUserAuthenticator> _log;
    private readonly TimeProvider _time;

    /// <summary>Initializes a new instance of the <see cref="ApiKeyOpcUaUserAuthenticator"/> class.</summary>
    /// <param name="store">The API key source.</param>
    /// <param name="options">The <c>Security:ApiKeys</c> options.</param>
    /// <param name="log">The logger.</param>
    /// <param name="time">Time provider, injected so expiry is testable.</param>
    public ApiKeyOpcUaUserAuthenticator(
        IApiKeyStore store,
        IOptions<ApiKeyOptions> options,
        ILogger<ApiKeyOpcUaUserAuthenticator> log,
        TimeProvider? time = null)
    {
        _store = store;
        _options = options;
        _log = log;
        _time = time ?? TimeProvider.System;
    }

    /// <inheritdoc />
    public async Task<OpcUaUserAuthResult> AuthenticateUserNameAsync(
        string username, string password, CancellationToken ct)
    {
        var opts = _options.Value;

        // The username carries no information, but check it rather than ignore it: a client sending the
        // wrong username gets an explicit denial it can diagnose, instead of "any username works".
        if (!string.Equals(username, opts.Username, StringComparison.Ordinal))
        {
            _log.LogInformation(
                "ApiKeyAuth: denied — username {Presented} is not the configured {Expected}",
                username, opts.Username);
            return OpcUaUserAuthResult.Deny(DenyMessage);
        }

        if (!ApiKeySecret.TryParse(password, out var apiKeyId, out var secret))
        {
            _log.LogInformation("ApiKeyAuth: denied — credential is not a well-formed otua_ key");
            return OpcUaUserAuthResult.Deny(DenyMessage);
        }

        var now = _time.GetUtcNow().UtcDateTime;

        ApiKeyLookup? lookup;
        try
        {
            lookup = await _store.FindAsync(apiKeyId, now, ct).ConfigureAwait(false);
        }
        catch (Exception ex)
        {
            // The contract forbids throwing: an exception here escapes into the SDK's session-activation
            // path and surfaces as BadInternalError. A store outage denies, it does not crash.
            _log.LogError(ex, "ApiKeyAuth: key store threw for key id {ApiKeyId}", apiKeyId);
            return OpcUaUserAuthResult.Deny(DenyMessage);
        }

        if (lookup is null)
        {
            _log.LogInformation("ApiKeyAuth: denied — no enabled, unexpired key with id {ApiKeyId}", apiKeyId);
            return OpcUaUserAuthResult.Deny(DenyMessage);
        }

        bool verified;
        try
        {
            verified = ApiKeySecret.Verify(secret, opts.Pepper, lookup.Key.SecretHash);
        }
        catch (ArgumentException ex)
        {
            // Thrown only when the pepper is unset. This is a deployment error, not a bad credential —
            // log it loudly, because otherwise it presents as "every key is rejected" with no clue why.
            _log.LogError(ex,
                "ApiKeyAuth: {Section}:Pepper is not configured — every key will be rejected until it is",
                ApiKeyOptions.SectionName);
            return OpcUaUserAuthResult.Deny(DenyMessage);
        }

        if (!verified)
        {
            _log.LogInformation("ApiKeyAuth: denied — secret mismatch for key id {ApiKeyId}", apiKeyId);
            return OpcUaUserAuthResult.Deny(DenyMessage);
        }

        var roles = ApiKeyInterimRoleMapper.Map(lookup.Grants, lookup.Key.Principal);

        _log.LogInformation(
            "ApiKeyAuth: granted key {Name} ({ApiKeyId}) as principal {Principal} with roles [{Roles}]",
            lookup.Key.Name, apiKeyId, lookup.Key.Principal, string.Join(",", roles));

        return OpcUaUserAuthResult.Allow(lookup.Key.Principal, roles);
    }
}