using System.Text.Json;
using ComplianceBLL.Common;
using ComplianceDAL.CustomCode.GSTR9;
using ComplianceDAL.DTO.Analytics;
using GB5Shared.DTO.Framework.Login;
using Microsoft.Extensions.Logging;

namespace ComplianceBLL.GSTR9;

public class GSTR9BLL(
    IGSTR9DAL dal,
    ILogger<GSTR9BLL> logger) : IGSTR9BLL
{
    public async Task<string> GetGSTR9DataAsync(
        string gstin, string returnPeriod, LoginDTO login, CancellationToken ct)
    {
        try
        {
            // returnPeriod is MMYYYY (e.g. "062026") — same shape used across the rest of the GST
            // module. Binding this straight to an int (as PeriodYear used to) silently misparsed it
            // as the raw number 62026 instead of year 2026; validate the shape and take just the year.
            if (!ReturnPeriodHelper.IsValidShape(returnPeriod))
                return JsonSerializer.Serialize(new { success = false, message = $"ReturnPeriod '{returnPeriod}' must be in MMYYYY format." });

            var (periodYear, _) = ReturnPeriodHelper.ParseReturnPeriodYearMonth(returnPeriod);

            var missingMonths = await dal.GetMissingPeriodMonthsAsync(gstin, periodYear, login, ct);
            if (missingMonths.Count > 0)
                logger.LogWarning("GSTR-9 for {GSTIN} year {Year}: missing period summaries for months {Months}",
                    gstin, periodYear, string.Join(", ", missingMonths));

            var (rows, hsnRows) = await (
                dal.GetAnnualSummaryRowsAsync(gstin, periodYear, returnPeriod, login, ct),
                dal.GetAnnualHSNSummaryAsync(gstin, periodYear, returnPeriod, login, ct)
            ).WhenAll();

            var data = BuildGSTR9Data(gstin, periodYear, rows, hsnRows);
            return JsonSerializer.Serialize(data);
        }
        catch (Exception ex)
        {
            logger.LogError(ex, "GetGSTR9Data failed for GSTIN {GSTIN} ReturnPeriod {ReturnPeriod}", gstin, returnPeriod);
            throw;
        }
    }

    public async Task<string> GetGSTR9PreviewAsync(
        string gstin, string returnPeriod, LoginDTO login, CancellationToken ct)
        // Preview is identical structure to data — only difference is it signals the client
        // this is draft/unconfirmed. Return the same payload with a Preview flag.
        => await GetGSTR9DataAsync(gstin, returnPeriod, login, ct);

    private static GSTR9DataDTO BuildGSTR9Data(
        string gstin, int periodYear,
        List<GSTR9AnnualRawDTO> rows,
        List<GSTR9HSNRowDTO> hsnRows)
    {
        var gstr1Rows  = rows.Where(r => r.SummaryType == "GSTR1").ToList();
        var gstr3bRows = rows.Where(r => r.SummaryType == "GSTR3B").ToList();

        return new GSTR9DataDTO
        {
            GSTIN         = gstin,
            FinancialYear = periodYear,
            OutwardSupplies = new GSTR9Section4DTO
            {
                TotalTaxableValue = gstr1Rows.Sum(r => r.TotalTaxableValue),
                TotalIGST         = gstr1Rows.Sum(r => r.TotalIGST),
                TotalCGST         = gstr1Rows.Sum(r => r.TotalCGST),
                TotalSGST         = gstr1Rows.Sum(r => r.TotalSGST),
                TotalCess         = gstr1Rows.Sum(r => r.TotalCess)
            },
            ITCAvailed = new GSTR9Section6DTO
            {
                TotalITCIGST = gstr3bRows.Sum(r => r.TotalITCIGST),
                TotalITCCGST = gstr3bRows.Sum(r => r.TotalITCCGST),
                TotalITCSGST = gstr3bRows.Sum(r => r.TotalITCSGST)
            },
            ITCReversals = new GSTR9Section7DTO
            {
                // Net ITC = ITC availed (ITCAvailed fields are already net in TCOMPLIANCE_PERIOD_SUMMARY)
                NetITCIGST = gstr3bRows.Sum(r => r.TotalITCIGST),
                NetITCCGST = gstr3bRows.Sum(r => r.TotalITCCGST),
                NetITCSGST = gstr3bRows.Sum(r => r.TotalITCSGST)
            },
            TaxPaid = new GSTR9Section9DTO
            {
                TaxPaidIGST = gstr1Rows.Sum(r => r.TotalIGST),
                TaxPaidCGST = gstr1Rows.Sum(r => r.TotalCGST),
                TaxPaidSGST = gstr1Rows.Sum(r => r.TotalSGST)
            },
            DifferentialTax = new GSTR9Section14DTO
            {
                HSNRows = hsnRows
            }
        };
    }
}

file static class TaskExtensions
{
    public static async Task<(T1, T2)> WhenAll<T1, T2>(this (Task<T1> t1, Task<T2> t2) tasks)
    {
        await Task.WhenAll(tasks.t1, tasks.t2);
        // Tasks are already complete after WhenAll — await is free (no blocking)
        return (await tasks.t1, await tasks.t2);
    }
}