using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace StellaOps.Auth.Client;

/// <summary>
/// Delegating handler that attaches bearer credentials and tenant headers to outbound requests.
/// </summary>
internal sealed class StellaOpsBearerTokenHandler : DelegatingHandler
{
    private readonly string clientName;
    private readonly IOptionsMonitor<StellaOpsApiAuthenticationOptions> apiAuthOptions;
    private readonly IOptionsMonitor<StellaOpsAuthClientOptions> authClientOptions;
    private readonly IStellaOpsTokenClient tokenClient;
    private readonly TimeProvider timeProvider;
    private readonly ILogger<StellaOpsBearerTokenHandler>? logger;
    private readonly SemaphoreSlim refreshLock = new(1, 1);

    private StellaOpsTokenResult? cachedToken;
    private string? cachedTokenKey;

    public StellaOpsBearerTokenHandler(
        string clientName,
        IOptionsMonitor<StellaOpsApiAuthenticationOptions> apiAuthOptions,
        IOptionsMonitor<StellaOpsAuthClientOptions> authClientOptions,
        IStellaOpsTokenClient tokenClient,
        TimeProvider? timeProvider,
        ILogger<StellaOpsBearerTokenHandler>? logger)
    {
        this.clientName = clientName ?? throw new ArgumentNullException(nameof(clientName));
        this.apiAuthOptions = apiAuthOptions ?? throw new ArgumentNullException(nameof(apiAuthOptions));
        this.authClientOptions = authClientOptions ?? throw new ArgumentNullException(nameof(authClientOptions));
        this.tokenClient = tokenClient ?? throw new ArgumentNullException(nameof(tokenClient));
        this.timeProvider = timeProvider ?? TimeProvider.System;
        this.logger = logger;
    }

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var options = apiAuthOptions.Get(clientName);

        if (!string.IsNullOrWhiteSpace(options.Tenant))
        {
            request.Headers.Remove(options.TenantHeader);
            request.Headers.TryAddWithoutValidation(options.TenantHeader, options.Tenant);
        }

        var token = await ResolveTokenAsync(options, cancellationToken).ConfigureAwait(false);
        if (!string.IsNullOrEmpty(token))
        {
            request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
        }

        return await base.SendAsync(request, cancellationToken).ConfigureAwait(false);
    }

    private async Task<string?> ResolveTokenAsync(StellaOpsApiAuthenticationOptions options, CancellationToken cancellationToken)
    {
        if (options.Mode == StellaOpsApiAuthMode.PersonalAccessToken)
        {
            return options.PersonalAccessToken;
        }

        var buffer = GetRefreshBuffer(options);
        var now = timeProvider.GetUtcNow();
        var clientOptions = authClientOptions.CurrentValue;
        var cacheKey = BuildCacheKey(options, clientOptions);
        var token = cachedToken;

        if (token is not null && cachedTokenKey == cacheKey && token.ExpiresAt - buffer > now)
        {
            return token.AccessToken;
        }

        await refreshLock.WaitAsync(cancellationToken).ConfigureAwait(false);
        try
        {
            token = cachedToken;
            now = timeProvider.GetUtcNow();
            if (token is not null && cachedTokenKey == cacheKey && token.ExpiresAt - buffer > now)
            {
                return token.AccessToken;
            }

            var cachedEntry = await tokenClient.GetCachedTokenAsync(cacheKey, cancellationToken).ConfigureAwait(false);
            if (cachedEntry is not null && !cachedEntry.IsExpired(timeProvider, buffer))
            {
                cachedToken = new StellaOpsTokenResult(
                    cachedEntry.AccessToken,
                    cachedEntry.TokenType,
                    cachedEntry.ExpiresAtUtc,
                    cachedEntry.Scopes,
                    cachedEntry.RefreshToken,
                    cachedEntry.IdToken,
                    null);
                cachedTokenKey = cacheKey;
                return cachedEntry.AccessToken;
            }
            else if (cachedEntry is not null)
            {
                await tokenClient.ClearCachedTokenAsync(cacheKey, cancellationToken).ConfigureAwait(false);
            }

            StellaOpsTokenResult result = options.Mode switch
            {
                StellaOpsApiAuthMode.ClientCredentials => await tokenClient.RequestClientCredentialsTokenAsync(
                    options.Scope,
                    null,
                    cancellationToken).ConfigureAwait(false),
                StellaOpsApiAuthMode.Password => await tokenClient.RequestPasswordTokenAsync(
                    options.Username!,
                    options.Password!,
                    options.Scope,
                    null,
                    cancellationToken).ConfigureAwait(false),
                _ => throw new InvalidOperationException($"Unsupported authentication mode '{options.Mode}'.")
            };

            cachedToken = result;
            cachedTokenKey = cacheKey;
            await tokenClient.CacheTokenAsync(cacheKey, result.ToCacheEntry(), cancellationToken).ConfigureAwait(false);
            logger?.LogDebug("Issued access token for client {ClientName}; expires at {ExpiresAt}.", clientName, result.ExpiresAt);
            return result.AccessToken;
        }
        finally
        {
            refreshLock.Release();
        }
    }

    private TimeSpan GetRefreshBuffer(StellaOpsApiAuthenticationOptions options)
    {
        var authOptions = authClientOptions.CurrentValue;
        var buffer = options.RefreshBuffer;
        if (buffer <= TimeSpan.Zero)
        {
            return authOptions.ExpirationSkew;
        }

        return buffer > authOptions.ExpirationSkew ? buffer : authOptions.ExpirationSkew;
    }

    private string BuildCacheKey(StellaOpsApiAuthenticationOptions apiOptions, StellaOpsAuthClientOptions clientOptions)
    {
        var resolvedScope = ResolveScope(apiOptions.Scope, clientOptions);
        var authority = clientOptions.AuthorityUri?.ToString() ?? clientOptions.Authority;

        var builder = new StringBuilder();
        builder.Append("stellaops|");
        builder.Append(clientName).Append('|');
        builder.Append(authority).Append('|');
        builder.Append(clientOptions.ClientId ?? string.Empty).Append('|');
        builder.Append(apiOptions.Mode).Append('|');
        builder.Append(resolvedScope ?? string.Empty).Append('|');
        builder.Append(apiOptions.Username ?? string.Empty).Append('|');
        builder.Append(apiOptions.Tenant ?? string.Empty);

        return builder.ToString();
    }

    private static string? ResolveScope(string? scope, StellaOpsAuthClientOptions clientOptions)
    {
        var resolved = scope;
        if (string.IsNullOrWhiteSpace(resolved) && clientOptions.NormalizedScopes.Count > 0)
        {
            resolved = string.Join(' ', clientOptions.NormalizedScopes);
        }

        return string.IsNullOrWhiteSpace(resolved) ? null : resolved.Trim();
    }
}