/**
 * Vela Payroll Engine — United States Tax Jurisdiction
 *
 * Based on IRS 2025 tax year.
 * Source: https://www.irs.gov/newsroom/irs-provides-tax-inflation-adjustments-for-tax-year-2025
 *
 * Federal Income Tax — Single Filer
 * ┌──────────────────────────────────┬────────┐
 * │ Taxable Income                   │ Rate   │
 * ├──────────────────────────────────┼────────┤
 * │ $0 – $11,925                     │ 10%    │
 * │ $11,926 – $48,475                │ 12%    │
 * │ $48,476 – $103,350               │ 22%    │
 * │ $103,351 – $197,300              │ 24%    │
 * │ $197,301 – $250,525              │ 32%    │
 * │ $250,526 – $626,350              │ 35%    │
 * │ $626,351 and over                │ 37%    │
 * └──────────────────────────────────┴────────┘
 *
 * Standard Deduction (Single): $15,000 (2025)
 *
 * FICA Taxes:
 * - Social Security: 6.2% employee + 6.2% employer on wages up to $176,100
 * - Medicare: 1.45% employee + 1.45% employer (no cap)
 * - Additional Medicare Tax: 0.9% employee on wages over $200,000 (single)
 *
 * State Tax: Handled via optional `stateTaxRate` parameter (flat rate override).
 * Set to 0 for no-state-tax states (TX, FL, WA, NV, etc.)
 */

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

const FEDERAL_BRACKETS: TaxBracket[] = [
  { min: 0, max: 11_925, rate: 0.10 },
  { min: 11_925, max: 48_475, rate: 0.12 },
  { min: 48_475, max: 103_350, rate: 0.22 },
  { min: 103_350, max: 197_300, rate: 0.24 },
  { min: 197_300, max: 250_525, rate: 0.32 },
  { min: 250_525, max: 626_350, rate: 0.35 },
  { min: 626_350, max: null, rate: 0.37 },
];

const STANDARD_DEDUCTION = 15_000; // 2025 single filer
const SS_RATE = 0.062;             // 6.2% employee + 6.2% employer
const SS_WAGE_BASE = 176_100;      // 2025
const MEDICARE_RATE = 0.0145;      // 1.45% employee + 1.45% employer
const ADDITIONAL_MEDICARE_RATE = 0.009; // 0.9% employee only
const ADDITIONAL_MEDICARE_THRESHOLD = 200_000; // single filer

export interface USTaxOptions {
  /** Flat state income tax rate (e.g. 0.05 for 5%). Set to 0 for no state tax. */
  stateTaxRate?: number;
  /** State name for display purposes */
  stateName?: string;
}

export class USTax implements TaxJurisdiction {
  readonly name = "United States";
  readonly countryCode = "US";
  readonly currency = "USD";

  private stateTaxRate: number;
  private stateName: string;

  constructor(options?: USTaxOptions) {
    this.stateTaxRate = options?.stateTaxRate ?? 0.05; // default 5%
    this.stateName = options?.stateName ?? "State";
  }

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

    // 2. Social Security (capped at wage base)
    const ssWages = Math.min(grossAnnual, SS_WAGE_BASE);
    const ssEmployee = round2(ssWages * SS_RATE);
    const ssEmployer = ssEmployee;

    // 3. Medicare (no cap)
    const medicareEmployee = round2(grossAnnual * MEDICARE_RATE);
    const medicareEmployer = medicareEmployee;

    // 4. Additional Medicare Tax (> $200k for single filer, employee only)
    let additionalMedicare = 0;
    if (grossAnnual > ADDITIONAL_MEDICARE_THRESHOLD) {
      additionalMedicare = round2(
        (grossAnnual - ADDITIONAL_MEDICARE_THRESHOLD) * ADDITIONAL_MEDICARE_RATE
      );
    }

    // 5. State Income Tax (flat rate override)
    const stateTax = round2(grossAnnual * this.stateTaxRate);

    // Build deductions
    const deductions: DeductionLineItem[] = [
      {
        name: "Federal Income Tax",
        amount: round2(federalTax),
        type: "tax",
        description: "IRS federal withholding (2025, single filer)",
      },
      {
        name: "Social Security Tax",
        amount: ssEmployee,
        type: "tax",
        description: `6.2% on wages up to $${SS_WAGE_BASE.toLocaleString()}`,
      },
      {
        name: "Medicare Tax",
        amount: medicareEmployee,
        type: "tax",
        description: "1.45% of gross wages",
      },
    ];

    if (additionalMedicare > 0) {
      deductions.push({
        name: "Additional Medicare Tax",
        amount: additionalMedicare,
        type: "tax",
        description: "0.9% on wages over $200,000 (single)",
      });
    }

    if (this.stateTaxRate > 0) {
      deductions.push({
        name: `${this.stateName} State Income Tax`,
        amount: stateTax,
        type: "tax",
        description: `${(this.stateTaxRate * 100).toFixed(1)}% flat state rate`,
      });
    }

    // Employer contributions
    const employerContributions: EmployerContribution[] = [
      {
        name: "Social Security (Employer)",
        amount: ssEmployer,
        description: "6.2% matching contribution",
      },
      {
        name: "Medicare (Employer)",
        amount: medicareEmployer,
        description: "1.45% matching contribution",
      },
    ];

    return { deductions, employerContributions };
  }
}