/**
 * Copyright since 2025 Mifos Initiative
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */

/**
 * The three things the bank actually has to file or hand over.
 *
 *   1. The consolidated statement of declarations received — monthly from
 *      FY 2026-27, due on the 7th of the following month, carrying the UIN the
 *      branch allotted to each Form 121.
 *   2. The deductee annexure of the quarterly return (Form 140), which needs the
 *      UIN against every payee paid gross so the department can tie a
 *      nil-deduction back to a declaration.
 *   3. The certificate issued to each deductee (Form 131), which is what the
 *      customer uses to claim credit in their own return.
 *
 * All figures come from the TDS engine. Nothing here recomputes tax — a return
 * that disagrees with the register is worse than no return at all.
 */

import {
  FY_TAX_CONFIG,
  Quarter,
  buildUin,
  declarationStatementDueDate,
  isWellFormedPan,
  normaliseFy,
  quarterEnd,
  quarterOf
} from '../letter-mgmt/certificates/form-15gh/tds-15gh.compute';
import { TdsInterestRow, tdsInterestRowsForFy } from './tds-reports.compute';

/** The bank's own deductor particulars, which head every filing. */
export interface DeductorParticulars {
  tan: string;
  pan: string;
  name: string;
  branchAddress: string;
  responsiblePerson: string;
  designation: string;
}

/**
 * Placeholder particulars for the demo book. In a live install these come from
 * the organisation's tax registration, not from a constant.
 */
export const DEDUCTOR: DeductorParticulars = {
  tan: 'MUMC12345E',
  pan: 'AAACC1234C',
  name: 'CBS Nexora Co-operative Bank Ltd.',
  branchAddress: 'Head Office, Mumbai 400001',
  responsiblePerson: 'Chief Financial Officer',
  designation: 'Principal Officer'
};

// ───────────────── 1. Consolidated statement of declarations ─────────────────

export interface DeclarationStatementLine {
  /** Running number within the statement. */
  serial: number;
  uin: string;
  payeeName: string;
  pan: string;
  customerId: string;
  branchCode: string;
  /** Date the branch accepted the declaration. */
  acceptedOn: string;
  /**
   * Income the declaration covers, as estimated when it was accepted. The
   * statement reports the amount the payer expects to pay gross.
   */
  estimatedIncome: number;
  /** Interest actually paid without deduction so far. */
  paidWithoutDeduction: number;
  /** Whether the declaration would survive scrutiny on the branch's own record. */
  eligible: boolean;
}

export interface DeclarationStatementPeriod {
  /** 'MONTHLY' periods are keyed '2026-04'; quarterly ones 'Q1'. */
  key: string;
  label: string;
  /** Statutory upload deadline for this period. */
  dueDate: string;
  overdue: boolean;
  lines: DeclarationStatementLine[];
  totalPaidWithoutDeduction: number;
}

export interface DeclarationStatement {
  fy: string;
  form: string;
  rule: string;
  frequency: 'MONTHLY' | 'QUARTERLY';
  periods: DeclarationStatementPeriod[];
  totalDeclarations: number;
  /** Declarations on record that the branch could not lawfully have accepted. */
  ineligibleCount: number;
  /** Declarations with no UIN allotted — these cannot be filed at all. */
  missingUinCount: number;
}

function monthLabel(key: string): string {
  const months = [
    'Jan',
    'Feb',
    'Mar',
    'Apr',
    'May',
    'Jun',
    'Jul',
    'Aug',
    'Sep',
    'Oct',
    'Nov',
    'Dec'
  ];
  return `${months[Number(key.slice(5, 7)) - 1]} ${key.slice(0, 4)}`;
}

/**
 * Groups every declaration on record for the year into the periods the payer has
 * to upload, and flags the ones that are already past their due date.
 *
 * A declaration with no UIN is counted separately rather than filed with a blank:
 * the portal rejects the line, and a silently dropped row is how a bank ends up
 * reporting fewer declarations than it holds.
 */
export function declarationStatement(fy: string, asAt: string): DeclarationStatement {
  const year = normaliseFy(fy);
  const cfg = FY_TAX_CONFIG[year];
  if (!cfg) {
    throw new Error(`No tax configuration for FY ${year}`);
  }

  const declared = tdsInterestRowsForFy(year).filter((r) => r.hasDeclaration && r.declarationAcceptedOn);
  const buckets = new Map<string, DeclarationStatementLine[]>();
  let serial = 0;
  let ineligibleCount = 0;
  let missingUinCount = 0;

  for (const row of declared) {
    const acceptedOn = row.declarationAcceptedOn as string;
    const key = cfg.declarationStatementFrequency === 'MONTHLY' ? acceptedOn.slice(0, 7) : `Q${quarterOf(acceptedOn)}`;

    if (!row.declarationUin) {
      missingUinCount++;
    }
    // The branch cannot compute income earned elsewhere, so eligibility here is
    // only what the register can see: a valid PAN on an individual.
    const eligible = isWellFormedPan(row.deductee.pan);
    if (!eligible) {
      ineligibleCount++;
    }

    const line: DeclarationStatementLine = {
      serial: ++serial,
      uin: row.declarationUin ?? '',
      payeeName: row.deductee.name,
      pan: row.deductee.pan,
      customerId: row.deductee.customerId,
      branchCode: row.branchCode,
      acceptedOn,
      estimatedIncome: row.tdsApplicableInterest,
      paidWithoutDeduction: row.paidWithoutDeduction,
      eligible
    };

    const list = buckets.get(key);
    if (list) {
      list.push(line);
    } else {
      buckets.set(key, [line]);
    }
  }

  const periods: DeclarationStatementPeriod[] = [...buckets.entries()]
    .sort((a, b) => a[0].localeCompare(b[0]))
    .map(
      ([
        key,
        lines
      ]) => {
        const dueDate = declarationStatementDueDate(lines[0].acceptedOn, year);
        return {
          key,
          label: cfg.declarationStatementFrequency === 'MONTHLY' ? monthLabel(`${key}-01`) : key,
          dueDate,
          overdue: dueDate < asAt,
          lines,
          totalPaidWithoutDeduction: lines.reduce((s, l) => s + l.paidWithoutDeduction, 0)
        };
      }
    );

  return {
    fy: year,
    form: cfg.declarationForm === '121' ? 'Form 121' : 'Form 15G / 15H',
    rule: cfg.declarationRule,
    frequency: cfg.declarationStatementFrequency,
    periods,
    totalDeclarations: declared.length,
    ineligibleCount,
    missingUinCount
  };
}

/**
 * Allots UINs to declarations that do not have one.
 *
 * Sequence numbers continue from the highest already in use for the year, so a
 * re-run does not renumber declarations that have already been filed.
 */
export function allotMissingUins(rows: TdsInterestRow[], fy: string, tan: string = DEDUCTOR.tan): Map<string, string> {
  const out = new Map<string, string>();
  let next = rows.reduce((max, r) => {
    const seq = r.declarationUin ? Number(r.declarationUin.slice(1, 10)) : 0;
    return Number.isFinite(seq) && seq > max ? seq : max;
  }, 0);

  for (const row of rows) {
    if (row.hasDeclaration && !row.declarationUin) {
      out.set(row.deductee.customerId, buildUin(++next, fy, tan));
    }
  }
  return out;
}

// ──────────────────── 2. Deductee annexure of the return ────────────────────

/** One deductee line in the quarterly return's annexure. */
export interface ReturnAnnexureLine {
  serial: number;
  payeeName: string;
  pan: string;
  /** 'Individual' or 'HUF', from the fourth character of the PAN. */
  payeeType: string;
  /** Amount paid or credited in the quarter. */
  amountPaid: number;
  /** Tax deducted in the quarter. */
  taxDeducted: number;
  rate: number;
  /**
   * Reason the payer deducted nothing or less than the standard rate.
   * Blank where tax was deducted normally.
   */
  remark: string;
  /** UIN of the declaration, required wherever the remark cites one. */
  uin: string;
  branchCode: string;
}

export interface ReturnAnnexure {
  fy: string;
  quarter: Quarter;
  form: string;
  rule: string;
  section: string;
  /** Last day of the quarter the return covers. */
  periodEnd: string;
  dueDate: string;
  lines: ReturnAnnexureLine[];
  totalPaid: number;
  totalDeducted: number;
  /** Lines carrying a nil-deduction remark — these need a UIN. */
  nilDeductionLines: number;
  /** Nil-deduction lines with no UIN. The portal will reject these. */
  missingUinLines: number;
}

const RETURN_DUE_DATE: Record<Quarter, (startYear: number) => string> = {
  1: (y) => `${y}-07-31`,
  2: (y) => `${y}-10-31`,
  3: (y) => `${y + 1}-01-31`,
  4: (y) => `${y + 1}-05-31`
};

/**
 * Remark codes for a nil or lower deduction.
 *
 * The department's file specification assigns a letter per reason. The letters
 * have been revised more than once, so they live here as named constants and
 * must be checked against the current specification before a file is generated —
 * a wrong code fails validation at upload, or worse, passes and misreports.
 */
export const NIL_DEDUCTION_REMARK = {
  /** Payee filed a declaration for receipt without deduction. */
  DECLARATION: 'B',
  /** Aggregate stayed within the threshold, so nothing was deductible. */
  BELOW_THRESHOLD: 'Y',
  /** Higher rate applied because PAN was not furnished. */
  NO_PAN: 'C'
};

function payeeTypeOf(pan: string): string {
  const p = (pan || '').toUpperCase();
  if (!isWellFormedPan(p)) {
    return 'Unknown';
  }
  return p[3] === 'H' ? 'HUF' : p[3] === 'P' ? 'Individual' : 'Entity';
}

/**
 * The deductee annexure for one quarter.
 *
 * Every payee who was paid interest in the quarter appears, whether or not tax
 * came off — a payee paid gross against a declaration is precisely what the
 * department cross-checks, so omitting them is the error to avoid.
 */
export function returnAnnexure(fy: string, quarter: Quarter): ReturnAnnexure {
  const year = normaliseFy(fy);
  const cfg = FY_TAX_CONFIG[year];
  if (!cfg) {
    throw new Error(`No tax configuration for FY ${year}`);
  }
  const startYear = Number(year.slice(0, 4));
  const lines: ReturnAnnexureLine[] = [];
  let serial = 0;

  for (const row of tdsInterestRowsForFy(year)) {
    const line = row.schedule.lines.find((l) => l.quarter === quarter);
    const amountPaid = line?.interest ?? 0;
    const taxDeducted = line?.tds ?? 0;
    if (amountPaid === 0 && taxDeducted === 0) {
      continue;
    }

    // The remark explains why the deduction is not simply rate × amount.
    let remark = '';
    if (taxDeducted <= 0 && amountPaid > 0) {
      if (row.hasDeclaration && line?.declarationActive) {
        remark = NIL_DEDUCTION_REMARK.DECLARATION;
      } else if (!row.crossed) {
        remark = NIL_DEDUCTION_REMARK.BELOW_THRESHOLD;
      }
    } else if (taxDeducted > 0 && !row.panOnRecord) {
      remark = NIL_DEDUCTION_REMARK.NO_PAN;
    }

    lines.push({
      serial: ++serial,
      payeeName: row.deductee.name,
      pan: row.deductee.pan,
      payeeType: payeeTypeOf(row.deductee.pan),
      amountPaid,
      taxDeducted,
      rate: row.rateApplied,
      remark,
      uin: row.declarationUin ?? '',
      branchCode: row.branchCode
    });
  }

  const nilDeduction = lines.filter((l) => l.remark === NIL_DEDUCTION_REMARK.DECLARATION);

  return {
    fy: year,
    quarter,
    form: cfg.tdsStatementForm,
    rule: cfg.tdsStatementRule,
    section: cfg.interestTdsSection,
    periodEnd: quarterEnd(quarter, year),
    dueDate: RETURN_DUE_DATE[quarter](startYear),
    lines,
    totalPaid: lines.reduce((s, l) => s + l.amountPaid, 0),
    totalDeducted: lines.reduce((s, l) => s + l.taxDeducted, 0),
    nilDeductionLines: nilDeduction.length,
    missingUinLines: nilDeduction.filter((l) => !l.uin).length
  };
}

// ───────────────────── 3. Certificate for the deductee ─────────────────────

export interface TdsCertificateQuarterLine {
  quarter: Quarter;
  label: string;
  amountPaid: number;
  taxDeducted: number;
  /** Tax the bank has actually remitted for this quarter. */
  taxDeposited: number;
}

export interface TdsCertificate {
  fy: string;
  form: string;
  section: string;
  /** Certificate number, unique per deductee per year. */
  certificateNo: string;
  deductor: DeductorParticulars;
  payeeName: string;
  pan: string;
  customerId: string;
  branchCode: string;
  lines: TdsCertificateQuarterLine[];
  totalPaid: number;
  totalDeducted: number;
  /** Interest paid gross against a declaration, disclosed on the certificate. */
  paidWithoutDeduction: number;
  declarationUin: string | null;
  rate: number;
  /** A certificate cannot be issued without a valid PAN to issue it against. */
  issuable: boolean;
  blockingReason: string | null;
}

/**
 * The TDS certificate for one deductee.
 *
 * Quarterly deposits are taken as equal to the quarter's deduction: the branch
 * remits what it deducts, and the remittance report is where any gap between the
 * two is surfaced. Wiring the certificate to actual challans is the right next
 * step once challan records come from the backend rather than the seed.
 */
export function tdsCertificate(row: TdsInterestRow, fy: string): TdsCertificate {
  const year = normaliseFy(fy);
  const cfg = FY_TAX_CONFIG[year];
  if (!cfg) {
    throw new Error(`No tax configuration for FY ${year}`);
  }

  const lines: TdsCertificateQuarterLine[] = row.schedule.lines.map((l) => ({
    quarter: l.quarter,
    label: l.label,
    amountPaid: l.interest,
    taxDeducted: l.tds,
    taxDeposited: Math.max(0, l.tds)
  }));

  const issuable = isWellFormedPan(row.deductee.pan);
  return {
    fy: year,
    form: cfg.certificateForm,
    section: cfg.interestTdsSection,
    certificateNo: `${DEDUCTOR.tan}/${uinSafe(year)}/${row.deductee.customerId}`,
    deductor: DEDUCTOR,
    payeeName: row.deductee.name,
    pan: row.deductee.pan,
    customerId: row.deductee.customerId,
    branchCode: row.branchCode,
    lines,
    totalPaid: row.tdsApplicableInterest,
    totalDeducted: row.schedule.totalTds,
    paidWithoutDeduction: row.paidWithoutDeduction,
    declarationUin: row.declarationUin,
    rate: row.rateApplied,
    issuable,
    blockingReason: issuable
      ? null
      : `No valid PAN on record. A certificate cannot be issued without one — the deductee would be unable to claim credit.`
  };
}

function uinSafe(fy: string): string {
  return normaliseFy(fy).replace('-', '');
}

/** Certificates for every deductee who had tax deducted in the year. */
export function tdsCertificates(fy: string): TdsCertificate[] {
  return tdsInterestRowsForFy(normaliseFy(fy))
    .filter((r) => r.schedule.totalTds > 0 || r.paidWithoutDeduction > 0)
    .map((r) => tdsCertificate(r, fy));
}