/**
 * Vela Payroll Engine — New Zealand Tax Jurisdiction
 *
 * Based on IRD 2025 tax year (1 April 2025 – 31 March 2026).
 * Source: https://www.ird.govt.nz/income-tax-rates
 *
 * PAYE Income Tax — 2025-26
 * ┌──────────────────────────────────┬────────┐
 * │ Taxable Income                   │ Rate   │
 * ├──────────────────────────────────┼────────┤
 * │ $0 – $15,600                     │ 10.5%  │
 * │ $15,601 – $53,500                │ 17.5%  │
 * │ $53,501 – $78,100                │ 30.0%  │
 * │ $78,101 – $180,000               │ 33.0%  │
 * │ $180,001 and over                │ 39.0%  │
 * └──────────────────────────────────┴────────┘
 *
 * ACC Earner Levy: 1.60% of gross earnings (capped at max annual earner levy)
 * ACC Employer Levy: ~1.39% (WorkSafe account, varies by industry)
 * — Not included in MVP as it's industry-specific.
 *
 * KiwiSaver: Voluntary employee savings scheme (3%, 4%, 6%, 8%, 10%).
 * Employer must contribute 3% minimum if employee is a KiwiSaver member.
 * Not auto-included in MVP — would require member status input.
 *
 * No social security / pension contributions are mandatory beyond ACC.
 */

import {
  TaxJurisdiction,
  TaxBracket,
  DeductionLineItem,
  EmployerContribution,
  progressiveTaxCalc,
  round2,
} from "../types";

// ── 2025-26 PAYE Tax Brackets ──
const TAX_BRACKETS: TaxBracket[] = [
  { min: 0, max: 15_600, rate: 0.105 },
  { min: 15_600, max: 53_500, rate: 0.175 },
  { min: 53_500, max: 78_100, rate: 0.30 },
  { min: 78_100, max: 180_000, rate: 0.33 },
  { min: 180_000, max: null, rate: 0.39 },
];

const ACC_EARNER_LEVY_RATE = 0.016; // 1.60% of gross
// Max earner levy per year is around $32.74 — but for simplicity
// we apply the flat rate (the cap is small enough to be negligible
// for most salaries; can be added as an enhancement).

export class NewZealandTax implements TaxJurisdiction {
  readonly name = "New Zealand";
  readonly countryCode = "NZ";
  readonly currency = "NZD";

  calculate(grossAnnual: number): {
    deductions: DeductionLineItem[];
    employerContributions: EmployerContribution[];
  } {
    // 1. PAYE Income Tax
    const incomeTax = progressiveTaxCalc(grossAnnual, TAX_BRACKETS);

    // 2. ACC Earner Levy (1.60% of gross, capped)
    const accEarner = round2(Math.min(grossAnnual * ACC_EARNER_LEVY_RATE, 32.74));

    // No mandatory employer contributions in MVP
    // (ACC Employer Levy is industry-specific and billed separately by WorkSafe)
    const employerContributions: EmployerContribution[] = [];

    const deductions: DeductionLineItem[] = [
      {
        name: "PAYE Income Tax",
        amount: round2(incomeTax),
        type: "tax",
        description: "IRD PAYE withholding (2025-26 brackets)",
      },
      {
        name: "ACC Earner Levy",
        amount: accEarner,
        type: "levy",
        description: "1.60% of gross earnings (accident compensation)",
      },
    ];

    return { deductions, employerContributions };
  }
}