/**
 * 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/.
 */

/**
 * Aggregation for the two TDS reports.
 *
 * Every rupee figure here comes out of `tds-15gh.compute` — the reports do no
 * tax arithmetic of their own. Writing it twice is how the register and the
 * return end up disagreeing, so the engine is the single authority and this
 * module only groups and totals what it returns.
 */

import {
  DeclarationForm,
  FY_TAX_CONFIG,
  Quarter,
  TdsMode,
  TdsSchedule,
  ageInFy,
  buildSchedule,
  collectTdsCredits,
  declarationFormFor,
  formKindFor,
  fyOf,
  isSeniorInFy,
  isTdsApplicable,
  isWellFormedPan,
  lateDeductionInterest,
  monthsOrPartBetween,
  tdsDeducteeOf,
  tdsModeFor
} from '../letter-mgmt/certificates/form-15gh/tds-15gh.compute';
import { TdsChallan, TdsDeductee, tdsChallans, tdsDeductees } from './tds-reports-data';

// ─────────────────────────────── Filtering ───────────────────────────────

export interface TdsReportFilter {
  fy: string;
  /** '' means every branch. */
  branchCode: string;
  /** 0 means the whole year. */
  quarter: 0 | Quarter;
  /** '' means both declaration types. */
  band: '' | 'GENERAL' | 'SENIOR';
  /** Free text over name, CIF and PAN. */
  search: string;
  /** Only customers whose aggregate crossed the deduction trigger. */
  crossedOnly: boolean;
  /** Only customers sitting just below the trigger — the follow-up list. */
  nearThresholdOnly: boolean;
}

/** How close to the trigger counts as "worth chasing a declaration for". */
export const NEAR_THRESHOLD_BAND = 6_000;

// ───────────────────────── Report 1: interest ─────────────────────────

export interface TdsInterestRow {
  deductee: TdsDeductee;
  branchCode: string;
  /** The form in force for the year — '121' from FY 2026-27. */
  form: DeclarationForm;
  /** Which deduction trigger applies. Age-based, even after the forms merged. */
  band: 'GENERAL' | 'SENIOR';
  /**
   * The 15G/15H label. A form distinction only up to FY 2025-26; from FY 2026-27
   * the two merged, so it duplicates `band` and is kept for pre-2026 years.
   */
  kind: '15G' | '15H';
  age: number;
  /** Settlement timing, from the customer's deposit products. */
  mode: TdsMode;
  /** Interest on time deposits where this customer is the deductee, by quarter. */
  quarters: Record<Quarter, number>;
  /** Total of the above — the figure the deduction trigger is tested against. */
  tdsApplicableInterest: number;
  /** Savings interest, shown alongside but outside the charge on time deposits. */
  exemptInterest: number;
  /** Interest on deposits where somebody else is the first-named holder. */
  jointHeldInterest: number;
  threshold: number;
  crossed: boolean;
  /** The quarter the aggregate crossed the trigger, if it did. */
  crossedInQuarter: Quarter | null;
  /** How much more interest before the trigger bites. */
  headroom: number;
  nearThreshold: boolean;
  schedule: TdsSchedule;
  hasDeclaration: boolean;
  declarationUin: string | null;
  declarationAcceptedOn: string | null;
  /** Tax the declaration kept out of the customer's pocket. */
  tdsSaved: number;
  /** Interest paid gross on the strength of the declaration — reportable in the return. */
  paidWithoutDeduction: number;
  panOnRecord: boolean;
  rateApplied: number;
}

function emptyQuarters(): Record<Quarter, number> {
  return { 1: 0, 2: 0, 3: 0, 4: 0 };
}

/**
 * One row per deductee for the selected year.
 *
 * Memoised on the year alone: building a row runs two schedules, and the getters
 * that feed the template are read many times per change-detection pass.
 */
let rowCacheKey = '';
let rowCache: TdsInterestRow[] = [];

export function tdsInterestRowsForFy(fy: string): TdsInterestRow[] {
  if (rowCacheKey === fy) {
    return rowCache;
  }

  const rows: TdsInterestRow[] = [];

  for (const deductee of tdsDeductees()) {
    const ctx = {
      fy,
      dateOfBirth: deductee.dob,
      pan: deductee.pan,
      resident: true
    };

    // Rules 8 to 12 live in the engine: savings interest dropped, joint deposits
    // attributed to the first-named holder, every branch aggregated.
    const credits = collectTdsCredits(deductee.clientId, deductee.accounts, fy);
    if (!credits.length && !deductee.accounts.length) {
      continue;
    }

    const declaration = deductee.declarations[fy] ?? null;
    const mode = overallModeFor(deductee);
    const schedule = buildSchedule(mode, credits, { ...ctx, declaration });
    const withoutDeclaration = declaration ? buildSchedule(mode, credits, { ...ctx, declaration: null }) : schedule;

    const quarters = emptyQuarters();
    for (const line of schedule.lines) {
      quarters[line.quarter] = line.interest;
    }

    let exemptInterest = 0;
    let jointHeldInterest = 0;
    for (const account of deductee.accounts) {
      const inYear = account.credits.filter((c) => fyOf(c.date) === fy).reduce((s, c) => s + c.amount, 0);
      if (!isTdsApplicable(account)) {
        exemptInterest += inYear;
      } else if (tdsDeducteeOf(account) !== deductee.clientId) {
        jointHeldInterest += inYear;
      }
    }

    const crossedLine = schedule.lines.find((l) => l.thresholdCrossedHere);
    const headroom = Math.max(0, schedule.threshold - schedule.totalInterest);

    rows.push({
      deductee,
      branchCode: deductee.branchCode,
      form: declarationFormFor(fy, deductee.dob),
      band: isSeniorInFy(deductee.dob, fy) ? 'SENIOR' : 'GENERAL',
      kind: formKindFor(deductee.dob, fy),
      age: ageInFy(deductee.dob, fy),
      mode,
      quarters,
      tdsApplicableInterest: schedule.totalInterest,
      exemptInterest,
      jointHeldInterest,
      threshold: schedule.threshold,
      crossed: schedule.totalInterest > schedule.threshold,
      crossedInQuarter: crossedLine?.quarter ?? null,
      headroom,
      nearThreshold: headroom > 0 && headroom <= NEAR_THRESHOLD_BAND,
      schedule,
      hasDeclaration: !!declaration,
      declarationUin: declaration?.uin ?? null,
      declarationAcceptedOn: declaration?.acceptedOn ?? null,
      tdsSaved: declaration ? Math.max(0, withoutDeclaration.totalTds - schedule.totalTds) : 0,
      paidWithoutDeduction: schedule.shieldedInterest,
      panOnRecord: isWellFormedPan(deductee.pan),
      rateApplied: schedule.rateApplied
    });
  }

  rows.sort((a, b) => b.tdsApplicableInterest - a.tdsApplicableInterest);
  rowCacheKey = fy;
  rowCache = rows;
  return rows;
}

/**
 * When tax settles for a customer whose deposits post interest on different
 * cycles. A quarterly deposit anywhere means a deduction falls due at the first
 * quarter end, so that governs.
 */
function overallModeFor(deductee: TdsDeductee): TdsMode {
  const applicable = deductee.accounts.filter((a) => isTdsApplicable(a) && tdsDeducteeOf(a) === deductee.clientId);
  return applicable.some((a) => tdsModeFor(a) === 'QUARTERLY') ? 'QUARTERLY' : 'ANNUAL';
}

export function tdsInterestRows(filter: TdsReportFilter): TdsInterestRow[] {
  const needle = filter.search.trim().toLowerCase();
  return tdsInterestRowsForFy(filter.fy).filter((row) => {
    if (filter.branchCode && row.branchCode !== filter.branchCode) {
      return false;
    }
    if (filter.band && row.band !== filter.band) {
      return false;
    }
    if (filter.crossedOnly && !row.crossed) {
      return false;
    }
    if (filter.nearThresholdOnly && !row.nearThreshold) {
      return false;
    }
    // A quarter filter narrows to customers who actually had interest in it.
    if (filter.quarter && !row.quarters[filter.quarter]) {
      return false;
    }
    if (!needle) {
      return true;
    }
    const d = row.deductee;
    return (
      d.name.toLowerCase().includes(needle) ||
      d.customerId.toLowerCase().includes(needle) ||
      d.pan.toLowerCase().includes(needle)
    );
  });
}

export interface TdsInterestSummary {
  deductees: number;
  /** Interest on time deposits — the base the charge applies to. */
  applicableInterest: number;
  /** Savings interest, outside the section. */
  exemptInterest: number;
  totalTds: number;
  crossed: number;
  nearThreshold: number;
  declarations: number;
  tdsSaved: number;
  paidWithoutDeduction: number;
  noPan: number;
  byQuarter: { quarter: Quarter; interest: number; tds: number }[];
}

export function tdsInterestSummary(filter: TdsReportFilter): TdsInterestSummary {
  const rows = tdsInterestRows(filter);
  const byQuarter = ([
      1,
      2,
      3,
      4
    ] as Quarter[]).map((quarter) => ({
    quarter,
    interest: rows.reduce((s, r) => s + r.quarters[quarter], 0),
    tds: rows.reduce((s, r) => s + (r.schedule.lines.find((l) => l.quarter === quarter)?.tds ?? 0), 0)
  }));

  return {
    deductees: rows.length,
    applicableInterest: rows.reduce((s, r) => s + r.tdsApplicableInterest, 0),
    exemptInterest: rows.reduce((s, r) => s + r.exemptInterest, 0),
    totalTds: rows.reduce((s, r) => s + r.schedule.totalTds, 0),
    crossed: rows.filter((r) => r.crossed).length,
    nearThreshold: rows.filter((r) => r.nearThreshold).length,
    declarations: rows.filter((r) => r.hasDeclaration).length,
    tdsSaved: rows.reduce((s, r) => s + r.tdsSaved, 0),
    paidWithoutDeduction: rows.reduce((s, r) => s + r.paidWithoutDeduction, 0),
    noPan: rows.filter((r) => !r.panOnRecord).length,
    byQuarter
  };
}

// ─────────────────────── Report 2: remittance ───────────────────────

export interface TdsRemittanceRow extends TdsChallan {
  /** Statutory due date has passed with nothing deposited. */
  overdue: boolean;
  /** Deposited, but after the due date. */
  late: boolean;
  /** Months or part months of delay. */
  delayMonths: number;
  /** Interest charged for depositing late. */
  defaultInterest: number;
  /** Return the quarter's deduction is reported in. */
  returnDueDate: string;
}

const RETURN_DUE: Record<Quarter, string> = {
  1: '31 Jul',
  2: '31 Oct',
  3: '31 Jan',
  4: '31 May'
};

/** Tax deducted per quarter, from the engine, for one year. */
export function deductedByQuarter(fy: string): Record<number, number> {
  const out: Record<number, number> = { 1: 0, 2: 0, 3: 0, 4: 0 };
  for (const row of tdsInterestRowsForFy(fy)) {
    for (const line of row.schedule.lines) {
      out[line.quarter] += line.tds;
    }
  }
  return out;
}

export function tdsRemittanceRows(fy: string, asAt: string): TdsRemittanceRow[] {
  const deducted: Record<string, Record<number, number>> = {};
  for (const year of Object.keys(FY_TAX_CONFIG)) {
    deducted[year] = deductedByQuarter(year);
  }
  const challans = tdsChallans(deducted)[fy] ?? [];

  return challans.map((c) => {
    const overdue = !c.depositedOn && c.dueDate < asAt;
    const late = !!c.depositedOn && c.depositedOn > c.dueDate;
    // While a payment is overdue the clock runs to today; once paid it stops on
    // the payment date.
    const settledOn = c.depositedOn ?? asAt;
    const delayMonths = late || overdue ? monthsOrPartBetween(c.dueDate, settledOn) : 0;
    const defaultInterest =
      late || overdue ? lateDeductionInterest(c.amount, c.dueDate, settledOn, 'DEPOSIT', fy).interest : 0;

    return {
      ...c,
      overdue,
      late,
      delayMonths,
      defaultInterest,
      returnDueDate: RETURN_DUE[c.quarter]
    };
  });
}

/**
 * The year-end position — what the quarterly TDS return has to reconcile to.
 *
 * `difference` is the control figure: tax the engine says was deducted, less what
 * actually reached the Government. Anything other than zero, once the pending
 * challans are settled, means something is wrong and must be found before filing.
 */
export interface Tds26qSummary {
  fy: string;
  deductees: number;
  totalInterestPaid: number;
  interestWithDeduction: number;
  /** Paid gross against a declaration — a mandatory disclosure in the return. */
  interestWithoutDeduction: number;
  tdsAtStandardRate: number;
  tdsAtHigherRate: number;
  totalDeducted: number;
  totalDeposited: number;
  pendingDeposit: number;
  /** Deducted less deposited less still-pending. Must be zero. */
  difference: number;
  defaultInterest: number;
  lateChallans: number;
  overdueChallans: number;
}

export function tds26qSummary(fy: string, asAt: string): Tds26qSummary {
  const rows = tdsInterestRowsForFy(fy);
  const challans = tdsRemittanceRows(fy, asAt);
  const standard = rows.filter((r) => r.panOnRecord);
  const higher = rows.filter((r) => !r.panOnRecord);
  const totalDeducted = rows.reduce((s, r) => s + r.schedule.totalTds, 0);
  const totalDeposited = challans.filter((c) => c.depositedOn).reduce((s, c) => s + c.amount, 0);
  const pendingDeposit = challans.filter((c) => !c.depositedOn).reduce((s, c) => s + c.amount, 0);

  return {
    fy,
    deductees: rows.length,
    totalInterestPaid: rows.reduce((s, r) => s + r.tdsApplicableInterest, 0),
    interestWithDeduction: rows.reduce((s, r) => s + r.schedule.liableInterest, 0),
    interestWithoutDeduction: rows.reduce((s, r) => s + r.paidWithoutDeduction, 0),
    tdsAtStandardRate: standard.reduce((s, r) => s + r.schedule.totalTds, 0),
    tdsAtHigherRate: higher.reduce((s, r) => s + r.schedule.totalTds, 0),
    totalDeducted,
    totalDeposited,
    pendingDeposit,
    difference: Math.round(totalDeducted - totalDeposited - pendingDeposit),
    defaultInterest: challans.reduce((s, c) => s + c.defaultInterest, 0),
    lateChallans: challans.filter((c) => c.late).length,
    overdueChallans: challans.filter((c) => c.overdue).length
  };
}