using GB5Shared.DTO.Framework.Login;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using PayRollBLL.SAPPaySlip.Client;
using PayRollBLL.SAPPaySlip.Config;
using PayRollBLL.SAPPaySlip.DTO;
using PayRollDAL.CustomeCode.SAPPaySlip;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;

namespace PayRollBLL.SAPPaySlip
{
    public class SAPPaySlipBLL : ISAPPaySlipBLL
    {
        private readonly ISapPaySlipApiClient _sapClient;
        private readonly ISAPPaySlipDAL       _dal;
        private readonly SapPaySlipOptions    _options;
        private readonly ILogger<SAPPaySlipBLL> _logger;

        public SAPPaySlipBLL(
            ISapPaySlipApiClient      sapClient,
            ISAPPaySlipDAL            dal,
            IOptions<SapPaySlipOptions> options,
            ILogger<SAPPaySlipBLL>    logger)
        {
            _sapClient = sapClient;
            _dal       = dal;
            _options   = options.Value;
            _logger    = logger;
        }

        public async Task<List<SapPaySlipResultDTO>> GetPaySlipAsync(
            string employeeId,
            string fiscalYear,
            string month,
            LoginDTO login,
            CancellationToken ct)
        {
            // ── Step 1: Validate query parameters ────────────────────────────────────────
            ValidateQueryParameters(employeeId, fiscalYear, month);

            // ── Step 2: Resolve SAP credentials from configuration ────────────────────────
            string username = ResolveConfiguredUsername();
            string password = ResolveConfiguredPassword();

            // ── Step 4: Resolve SAP client / mandt from morganizationunit ────────────────
            string sapClient = await ResolveSapClientCodeAsync(login, ct).ConfigureAwait(false);

            // Normalise month to 2-digit ("1" → "01") after validation confirms 1–12.
            month = month.Trim().PadLeft(2, '0');

            _logger.LogInformation(
                "Fetching SAP PaySlip — EmployeeID={EmployeeID}, FiscalYear={FiscalYear}, " +
                "Month={Month}, SapClient={SapClient}",
                employeeId, fiscalYear, month, sapClient);

            // ── Step 5: Call SAP OData API ────────────────────────────────────────────────
            var apiResult = await _sapClient
                .GetAsync(employeeId, fiscalYear.Trim(), month,
                          username, password, sapClient, ct)
                .ConfigureAwait(false);

            // ── Step 6: Validate HTTP response ────────────────────────────────────────────
            ValidateApiResponse(apiResult, employeeId, fiscalYear, month);

            // ── Step 7: Deserialize OData envelope ────────────────────────────────────────
            SapPaySlipODataResponse? odataResponse =
                DeserializeODataResponse(apiResult.ResponseBody, employeeId, fiscalYear, month);

            var items = odataResponse?.D?.Results ?? new List<SapPaySlipODataItem>();

            if (items.Count == 0)
            {
                _logger.LogInformation(
                    "No payslip found in SAP for EmployeeID={EmployeeID}, " +
                    "FiscalYear={FiscalYear}, Month={Month}",
                    employeeId, fiscalYear, month);

                return new List<SapPaySlipResultDTO>();
            }

            // ── Step 8: Validate each item — including Base64 integrity ───────────────────
            var results = new List<SapPaySlipResultDTO>(items.Count);

            for (int i = 0; i < items.Count; i++)
            {
                var item = items[i];
                ValidateODataItem(item, index: i + 1, employeeId, fiscalYear, month);
                results.Add(new SapPaySlipResultDTO(
                    item.EmployeeID,
                    item.FiscalYear,
                    item.Month,
                    item.PayslipForm));
            }

            _logger.LogInformation(
                "SAP returned {Count} valid payslip record(s) for EmployeeID={EmployeeID}, " +
                "FiscalYear={FiscalYear}, Month={Month}",
                results.Count, employeeId, fiscalYear, month);

            return results;
        }

        // ─────────────────────────────────────────────────────────────────────────────────
        // Resolution helpers
        // ─────────────────────────────────────────────────────────────────────────────────

        private string ResolveConfiguredUsername()
        {
            if (string.IsNullOrWhiteSpace(_options.Username))
                throw new InvalidOperationException(
                    "SAP Username is not configured. " +
                    "Set SapPaySlip:Username in appsettings.json or the corresponding " +
                    "environment variable, then restart the service.");

            return _options.Username;
        }

        private string ResolveConfiguredPassword()
        {
            if (string.IsNullOrWhiteSpace(_options.Password))
                throw new InvalidOperationException(
                    "SAP Password is not configured. " +
                    "Set SapPaySlip:Password in appsettings.json or the corresponding " +
                    "environment variable, then restart the service.");

            return _options.Password;
        }

        private async Task<string> ResolveSapClientCodeAsync(LoginDTO login, CancellationToken ct)
        {
 

            string? sapClient = await _dal
                .GetSapClientCodeAsync(login.ClientId, login, ct)
                .ConfigureAwait(false);

            if (string.IsNullOrWhiteSpace(sapClient))
                throw new InvalidOperationException(
                    $"No SAP client code found in morganizationunit for ClientId={login.ClientId}. " +
                    "Ensure the organization unit has an OrganizationUnitCode configured " +
                    "that matches the SAP client / mandt number (e.g. '633').");

            return sapClient.Trim();
        }

        // ─────────────────────────────────────────────────────────────────────────────────
        // Validation helpers
        // ─────────────────────────────────────────────────────────────────────────────────

        private static void ValidateQueryParameters(string employeeId, string fiscalYear, string month)
        {
            var errors = new List<string>();

            // ── EmployeeID ────────────────────────────────────────────────────────────────
            if (string.IsNullOrWhiteSpace(employeeId))
                errors.Add("EmployeeID is required.");

            // ── FiscalYear ────────────────────────────────────────────────────────────────
            if (string.IsNullOrWhiteSpace(fiscalYear))
            {
                errors.Add("FiscalYear is required.");
            }
            else
            {
                string fy = fiscalYear.Trim();
                if (fy.Length != 4 || !int.TryParse(fy, out int year))
                    errors.Add(
                        $"FiscalYear must be a 4-digit numeric year (e.g. '2025'). " +
                        $"Received: '{fy}'.");
                else if (year < 2000 || year > 2100)
                    errors.Add(
                        $"FiscalYear must be between 2000 and 2100. Received: {year}.");
            }

            // ── Month ─────────────────────────────────────────────────────────────────────
            if (string.IsNullOrWhiteSpace(month))
            {
                errors.Add("Month is required.");
            }
            else if (!int.TryParse(month.Trim(), out int m))
            {
                errors.Add(
                    $"Month must be a numeric value between 1 and 12. Received: '{month}'.");
            }
            else if (m < 1 || m > 12)
            {
                errors.Add(
                    $"Month must be between 1 and 12. Received: {m}.");
            }

            if (errors.Count > 0)
                throw new ArgumentException(
                    $"Request validation failed ({errors.Count} error(s)):\n" +
                    string.Join("\n", errors.Select((e, i) => $"  {i + 1}. {e}")));
        }

        private void ValidateApiResponse(
            SapPaySlipApiResultDTO apiResult,
            string employeeId, string fiscalYear, string month)
        {
            if (apiResult.IsSuccess) return;

            var sb = new StringBuilder();

            if (apiResult.StatusCode == 0)
            {
                // Network-level failure — no HTTP response received from SAP.
                sb.AppendLine("Cannot reach the SAP PaySlip server.");
                sb.AppendLine(apiResult.NetworkFailureReason ?? "Unknown network error.");
            }
            else
            {
                // HTTP-level failure — SAP responded but indicated an error.
                string hint = apiResult.StatusCode switch
                {
                    400 => "The SAP request was malformed. " +
                           "Check the EmployeeID format or OData filter syntax.",
                    401 => "SAP authentication failed. " +
                           "Verify SapPaySlip:Username and SapPaySlip:Password in appsettings.json.",
                    403 => "SAP denied access. " +
                           "The configured SAP user may not have authorisation to access ZPY_PAYSLIP_SRV.",
                    404 => "SAP OData service endpoint not found. " +
                           "Verify that ZPY_PAYSLIP_SRV is activated in SAP " +
                           "(transaction /IWFND/MAINT_SERVICE) and that SapPaySlip:BaseUrl " +
                           "points to the correct SAP system.",
                    408 or 504 =>
                           "The SAP server timed out processing the request. " +
                           "Retry later or contact the SAP BASIS team.",
                    500 => "SAP internal server error. Contact the SAP BASIS team.",
                    503 => "SAP service is temporarily unavailable. " +
                           "Retry later or contact the SAP BASIS team.",
                    _   => $"SAP responded with HTTP {apiResult.StatusCode}."
                };

                sb.AppendLine($"SAP HTTP {apiResult.StatusCode}: {hint}");

                if (!string.IsNullOrWhiteSpace(apiResult.SapErrorCode))
                    sb.AppendLine($"SAP Error Code    : {apiResult.SapErrorCode}");

                if (!string.IsNullOrWhiteSpace(apiResult.SapErrorMessage))
                    sb.AppendLine($"SAP Error Message : {apiResult.SapErrorMessage}");

                if (!string.IsNullOrWhiteSpace(apiResult.SapTransactionId))
                    sb.AppendLine($"SAP Transaction ID: {apiResult.SapTransactionId} " +
                                  "(provide this to the SAP BASIS team for server-side log tracing)");
            }

            if (!string.IsNullOrWhiteSpace(apiResult.AttemptedUri))
                sb.AppendLine($"Endpoint          : {apiResult.AttemptedUri}");

            sb.Append($"Context           : EmployeeID={employeeId}, FiscalYear={fiscalYear}, Month={month}");

            string errorMessage = sb.ToString().Trim();

            _logger.LogError(
                "SAP PaySlip API failure — EmployeeID={EmployeeID}, FiscalYear={FiscalYear}, " +
                "Month={Month}, StatusCode={StatusCode}, SapErrorCode={SapErrorCode}, " +
                "SapErrorMessage={SapErrorMessage}, SapTransactionId={SapTransactionId}, " +
                "Uri={AttemptedUri}",
                employeeId, fiscalYear, month,
                apiResult.StatusCode, apiResult.SapErrorCode, apiResult.SapErrorMessage,
                apiResult.SapTransactionId, apiResult.AttemptedUri);

            throw new InvalidOperationException(errorMessage);
        }

        private SapPaySlipODataResponse? DeserializeODataResponse(
            string responseBody,
            string employeeId, string fiscalYear, string month)
        {
            if (string.IsNullOrWhiteSpace(responseBody))
            {
                _logger.LogWarning(
                    "SAP PaySlip API returned an empty response body for " +
                    "EmployeeID={EmployeeID}, FiscalYear={FiscalYear}, Month={Month}.",
                    employeeId, fiscalYear, month);
                return null;
            }

            try
            {
                return JsonSerializer.Deserialize<SapPaySlipODataResponse>(
                    responseBody,
                    new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
            }
            catch (JsonException ex)
            {
                _logger.LogError(ex,
                    "Failed to deserialize SAP OData response for " +
                    "EmployeeID={EmployeeID}, FiscalYear={FiscalYear}, Month={Month}. " +
                    "Response preview: {Preview}",
                    employeeId, fiscalYear, month, TruncateForLog(responseBody, 300));

                throw new InvalidOperationException(
                    "SAP PaySlip API returned an unrecognised response format. " +
                    $"JSON deserialization error: {ex.Message}");
            }
        }

        private void ValidateODataItem(
            SapPaySlipODataItem item, int index,
            string employeeId, string fiscalYear, string month)
        {
            if (string.IsNullOrWhiteSpace(item.EmployeeID))
                _logger.LogWarning(
                    "SAP PaySlip result #{Index} — EmployeeID is empty in the SAP response " +
                    "(filter was EmployeeID={EmployeeID}, FiscalYear={FiscalYear}, Month={Month}).",
                    index, employeeId, fiscalYear, month);

            if (string.IsNullOrWhiteSpace(item.FiscalYear))
                _logger.LogWarning(
                    "SAP PaySlip result #{Index} (EmployeeID={EmployeeID}) — FiscalYear is empty.",
                    index, item.EmployeeID);

            if (string.IsNullOrWhiteSpace(item.Month))
                _logger.LogWarning(
                    "SAP PaySlip result #{Index} (EmployeeID={EmployeeID}) — Month is empty.",
                    index, item.EmployeeID);

            // ── PayslipForm: presence check ───────────────────────────────────────────────
            if (string.IsNullOrWhiteSpace(item.PayslipForm))
            {
                _logger.LogError(
                    "SAP PaySlip result #{Index} (EmployeeID={EmployeeID}, " +
                    "FiscalYear={FiscalYear}, Month={Month}) — PayslipForm is empty. " +
                    "The payslip PDF has not been generated in SAP for this period.",
                    index, item.EmployeeID, item.FiscalYear, item.Month);

                throw new InvalidOperationException(
                    $"SAP returned a payslip record for EmployeeID '{item.EmployeeID}' " +
                    $"(FiscalYear={item.FiscalYear}, Month={item.Month}) " +
                    "but the PayslipForm (PDF data) is empty. " +
                    "The payslip may not have been generated yet in SAP for this period.");
            }

            // ── PayslipForm: Base64 integrity check ───────────────────────────────────────
            try
            {
                Convert.FromBase64String(item.PayslipForm);
            }
            catch (FormatException ex)
            {
                _logger.LogError(ex,
                    "SAP PaySlip result #{Index} (EmployeeID={EmployeeID}, " +
                    "FiscalYear={FiscalYear}, Month={Month}) — PayslipForm is not valid Base64. " +
                    "Data length={Length} chars. First 100 chars: {Sample}",
                    index, item.EmployeeID, item.FiscalYear, item.Month,
                    item.PayslipForm.Length,
                    item.PayslipForm.Length > 100 ? item.PayslipForm[..100] : item.PayslipForm);

                throw new InvalidOperationException(
                    $"SAP returned corrupt payslip data for EmployeeID '{item.EmployeeID}' " +
                    $"(FiscalYear={item.FiscalYear}, Month={item.Month}). " +
                    "The PayslipForm field is not valid Base64-encoded PDF data. " +
                    $"Error: {ex.Message}");
            }

            _logger.LogDebug(
                "SAP PaySlip result #{Index} — EmployeeID={EmployeeID}, FiscalYear={FiscalYear}, " +
                "Month={Month}, PayslipForm Base64 length={Length} chars — VALID.",
                index, item.EmployeeID, item.FiscalYear, item.Month, item.PayslipForm.Length);
        }

        private static string TruncateForLog(string message, int maxLength = 500)
        {
            if (string.IsNullOrEmpty(message) || message.Length <= maxLength)
                return message;

            return message[..maxLength] +
                   $"... [truncated — full length: {message.Length} chars]";
        }
    }
}