/**
 * Vela Payroll Engine — Canada Tax Jurisdiction
 *
 * Based on CRA 2025 tax year.
 * Source: https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-return/completing-a-tax-return/personal-income-tax-line-numbers.html
 *
 * Federal Income Tax — 2025
 * ┌──────────────────────────────────┬────────┐
 * │ Taxable Income                   │ Rate   │
 * ├──────────────────────────────────┼────────┤
 * │ $0 – $57,375                     │ 15.0%  │
 * │ $57,376 – $114,750               │ 20.5%  │
 * │ $114,751 – $177,882              │ 26.0%  │
 * │ $177,883 – $253,414              │ 29.0%  │
 * │ $253,415 and over                │ 33.0%  │
 * └──────────────────────────────────┴────────┘
 *
 * Basic Personal Amount (Federal): $15,705 (2025)
 *
 * Canada Pension Plan (CPP) — 2025
 * - Employee rate: 5.95% on pensionable earnings between YMIP and YAMPE
 * - Employer rate: 5.95% matching
 * - Year's Maximum Pensionable Earnings (YAMPE): $71,300
 * - Year's Maximum Insurable Pension (YMIP / basic exemption): $3,500
 * - Additional CPP (enhancement): 4.00% on earnings between YAMPE and
 *   the additional CPP ceiling ($81,200) — both employee and employer.
 *   For MVP simplicity, this is NOT included. Only base CPP.
 *
 * Employment Insurance (EI) — 2025
 * - Employee rate: 1.37% (approximate — set annually)
 * - Maximum insurable earnings: $65,700
 * - Employer rate: 1.4 × employee rate (not included in employee deductions)
 *
 * Provincial Tax: Not included in MVP. Use FlatRateTax with
 * a combined federal+provincial rate for non-Canada jurisdictions,
 * or extend with a province-specific plugin.
 */

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

// ── 2025 Federal Tax Brackets ──
const FEDERAL_BRACKETS: TaxBracket[] = [
  { min: 0, max: 57_375, rate: 0.150 },
  { min: 57_375, max: 114_750, rate: 0.205 },
  { min: 114_750, max: 177_882, rate: 0.260 },
  { min: 177_882, max: 253_414, rate: 0.290 },
  { min: 253_414, max: null, rate: 0.330 },
];

const BASIC_PERSONAL_AMOUNT = 15_705; // 2025 federal

// ── CPP 2025 ──
const CPP_RATE = 0.0595;           // 5.95% employee + 5.95% employer
const CPP_YAMPE = 71_300;          // Year's Maximum Pensionable Earnings
const CPP_YMIP = 3_500;            // Basic exemption

// ── EI 2025 ──
const EI_RATE = 0.0137;             // 1.37% employee
const EI_MAX_INSURABLE = 65_700;    // Maximum insurable earnings
const EI_EMPLOYER_MULTIPLIER = 1.4; // Employer pays 1.4× employee

export class CanadaTax implements TaxJurisdiction {
  readonly name = "Canada";
  readonly countryCode = "CA";
  readonly currency = "CAD";

  calculate(grossAnnual: number): {
    deductions: DeductionLineItem[];
    employerContributions: EmployerContribution[];
  } {
    // 1. Federal Income Tax (on taxable income after basic personal amount)
    const taxableIncome = Math.max(0, grossAnnual - BASIC_PERSONAL_AMOUNT);
    const federalTax = progressiveTaxCalc(taxableIncome, FEDERAL_BRACKETS);

    // 2. CPP Contributions
    const cppPensionableEarnings = Math.max(
      0,
      Math.min(grossAnnual, CPP_YAMPE) - CPP_YMIP
    );
    const cppEmployee = round2(cppPensionableEarnings * CPP_RATE);
    const cppEmployer = cppEmployee;

    // 3. EI Premiums
    const eiInsurableEarnings = Math.min(grossAnnual, EI_MAX_INSURABLE);
    const eiEmployee = round2(eiInsurableEarnings * EI_RATE);
    const eiEmployer = round2(eiEmployee * EI_EMPLOYER_MULTIPLIER);

    // Build results
    const deductions: DeductionLineItem[] = [
      {
        name: "Federal Income Tax",
        amount: round2(federalTax),
        type: "tax",
        description: `CRA federal withholding (2025, BPA: $${BASIC_PERSONAL_AMOUNT.toLocaleString()})`,
      },
      {
        name: "CPP Contribution",
        amount: cppEmployee,
        type: "pension",
        description: `5.95% on pensionable earnings ($${CPP_YMIP.toLocaleString()} – $${CPP_YAMPE.toLocaleString()})`,
      },
      {
        name: "EI Premium",
        amount: eiEmployee,
        type: "insurance",
        description: `1.37% on insurable earnings (max $${EI_MAX_INSURABLE.toLocaleString()})`,
      },
    ];

    const employerContributions: EmployerContribution[] = [
      {
        name: "CPP (Employer)",
        amount: cppEmployer,
        description: "5.95% matching contribution",
      },
      {
        name: "EI (Employer)",
        amount: eiEmployer,
        description: "1.4× employee premium",
      },
    ];

    return { deductions, employerContributions };
  }
}