/**
 * Vela Payroll Engine — United Kingdom Tax Jurisdiction
 *
 * Based on HMRC 2025-26 tax year (6 April 2025 – 5 April 2026).
 * Source: https://www.gov.uk/income-tax-rates
 *
 * Income Tax (PAYE) — England, Wales, Northern Ireland
 * ┌──────────────────────────────────┬─────────────────────────────┐
 * │ Band           │ Taxable Income  │ Tax Rate                    │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ Personal       │ Up to £12,570   │ 0%                          │
 * │ Allowance      │                 │                             │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ Basic Rate     │ £12,571 –       │ 20%                         │
 * │                │ £50,270         │                             │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ Higher Rate    │ £50,271 –       │ 40%                         │
 * │                │ £125,140        │                             │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ Additional     │ Over £125,140   │ 45%                         │
 * └──────────────────────────────────┴─────────────────────────────┘
 *
 * Note: Personal allowance is reduced by £1 for every £2 of income
 * over £100,000 (tapered to zero at £125,140).
 *
 * National Insurance Class 1 — Employee (Primary)
 * ┌──────────────────────────────────┬─────────────────────────────┐
 * │ Earnings Band                    │ Rate                        │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ Up to £12,570/yr (Primary        │ 0%                          │
 * │ Threshold)                       │                             │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ £12,570 – £50,270                │ 8%                          │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ Over £50,270                     │ 2%                          │
 * └──────────────────────────────────┴─────────────────────────────┘
 *
 * National Insurance Class 1 — Employer (Secondary)
 * ┌──────────────────────────────────┬─────────────────────────────┐
 * │ Earnings Band                    │ Rate                        │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ Up to £9,100/yr (Secondary       │ 0%                          │
 * │ Threshold)                       │                             │
 * ├──────────────────────────────────┼─────────────────────────────┤
 * │ Over £9,100                      │ 13.8%                       │
 * └──────────────────────────────────┴─────────────────────────────┘
 *
 * Employment Allowance: Not included in MVP (employer-side relief).
 */

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

// ── 2025-26 Income Tax Brackets ──
const INCOME_TAX_BRACKETS: TaxBracket[] = [
  { min: 0, max: 12_570, rate: 0.0 },
  { min: 12_570, max: 50_270, rate: 0.20 },
  { min: 50_270, max: 125_140, rate: 0.40 },
  { min: 125_140, max: null, rate: 0.45 },
];

// ── Employee NI (Primary) ──
const NI_PRIMARY_PT = 12_570;   // Primary Threshold
const NI_PRIMARY_UPPER = 50_270; // Upper Earnings Limit
const NI_PRIMARY_RATE_LOWER = 0.08; // 8% between PT and UEL
const NI_PRIMARY_RATE_UPPER = 0.02; // 2% above UEL

// ── Employer NI (Secondary) ──
const NI_SECONDARY_PT = 9_100;   // Secondary Threshold
const NI_SECONDARY_RATE = 0.138;  // 13.8% above ST

export class UKTax implements TaxJurisdiction {
  readonly name = "United Kingdom";
  readonly countryCode = "GB";
  readonly currency = "GBP";

  calculate(grossAnnual: number): {
    deductions: DeductionLineItem[];
    employerContributions: EmployerContribution[];
  } {
    // 1. Calculate Personal Allowance (tapered above £100k)
    let personalAllowance = 12_570;
    if (grossAnnual > 100_000) {
      const reduction = Math.floor((grossAnnual - 100_000) / 2);
      personalAllowance = Math.max(0, 12_570 - reduction);
    }

    // 2. Income Tax (PAYE)
    // Adjust brackets with actual personal allowance
    const taxBrackets: TaxBracket[] = [
      { min: 0, max: personalAllowance, rate: 0.0 },
      { min: personalAllowance, max: 50_270, rate: 0.20 },
      { min: 50_270, max: 125_140, rate: 0.40 },
      { min: 125_140, max: null, rate: 0.45 },
    ];
    const incomeTax = progressiveTaxCalc(grossAnnual, taxBrackets);

    // 3. Employee National Insurance
    let employeeNI = 0;
    if (grossAnnual > NI_PRIMARY_PT) {
      const lowerBand = Math.min(grossAnnual, NI_PRIMARY_UPPER) - NI_PRIMARY_PT;
      employeeNI += lowerBand * NI_PRIMARY_RATE_LOWER;
    }
    if (grossAnnual > NI_PRIMARY_UPPER) {
      employeeNI += (grossAnnual - NI_PRIMARY_UPPER) * NI_PRIMARY_RATE_UPPER;
    }
    employeeNI = round2(employeeNI);

    // 4. Employer National Insurance
    let employerNI = 0;
    if (grossAnnual > NI_SECONDARY_PT) {
      employerNI = (grossAnnual - NI_SECONDARY_PT) * NI_SECONDARY_RATE;
    }
    employerNI = round2(employerNI);

    // Build results
    const deductions: DeductionLineItem[] = [
      {
        name: "Income Tax (PAYE)",
        amount: round2(incomeTax),
        type: "tax",
        description: `HMRC PAYE (2025-26, PA: £${personalAllowance.toLocaleString()})`,
      },
      {
        name: "Employee National Insurance",
        amount: employeeNI,
        type: "insurance",
        description: `NI Class 1 — 8% / 2%`,
      },
    ];

    const employerContributions: EmployerContribution[] = [
      {
        name: "Employer National Insurance",
        amount: employerNI,
        description: "NI Class 1 — 13.8% above £9,100",
      },
    ];

    return { deductions, employerContributions };
  }
}