/**
 * Vela Payroll Engine — Singapore Tax Jurisdiction
 *
 * Based on IRAS Year of Assessment 2025 (income earned in 2024).
 * Source: https://www.iras.gov.sg/taxes/individual-income-tax/basic-individual-income-tax/individual-income-tax-rates
 *
 * IMPORTANT: Singapore does NOT use payroll withholding (PAYE).
 * Employees file and pay their own taxes directly to IRAS.
 * There is NO income tax deduction from the payslip.
 *
 * CPF (Central Provident Fund) — Mandatory savings for
 * Singapore Citizens and Permanent Residents.
 * Foreign employees (EP, S Pass) are NOT subject to CPF.
 *
 * Employee CPF Contributions (2025, age ≤ 55)
 * ┌─────────────────────────────┬──────────────┬────────────────┐
 * │ Monthly Wages               │ Employee     │ Employer       │
 * │                             │ Rate         │ Rate           │
 * ├─────────────────────────────┼──────────────┼────────────────┤
 * │ ≤ $750                      │ 0%           │ 0%             │
 * │ > $750                      │ 20%          │ 17%            │
 * └─────────────────────────────┴──────────────┴────────────────┘
 *
 * Ordinary Wage Ceiling: $6,800/month ($81,600/year)
 * Additional Wage Ceiling: $102,000/year - OW for the year
 * (For MVP, we only handle ordinary wages / annual salary)
 *
 * Source: https://www.cpf.gov.sg/service/faq/employer/employer-contributions
 */

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

const CPF_EMPLOYEE_RATE = 0.20;   // 20% employee contribution
const CPF_EMPLOYER_RATE = 0.17;   // 17% employer contribution
const CPF_OW_MONTHLY_CEILING = 6_800;
const MIN_WAGE_THRESHOLD = 750;   // No CPF below $750/month

export interface SingaporeTaxOptions {
  /** Is the employee a Singapore Citizen or PR? CPF only applies if true. */
  isCPRorSC?: boolean;
  /** Employee age group for CPF rate tiers (simplified). */
  ageGroup?: "55-and-below" | "55-to-60" | "60-to-65" | "above-65";
}

export class SingaporeTax implements TaxJurisdiction {
  readonly name = "Singapore";
  readonly countryCode = "SG";
  readonly currency = "SGD";

  private isCPRorSC: boolean;

  constructor(options?: SingaporeTaxOptions) {
    this.isCPRorSC = options?.isCPRorSC ?? true;
  }

  calculate(grossAnnual: number): {
    deductions: DeductionLineItem[];
    employerContributions: EmployerContribution[];
  } {
    const monthlyWage = grossAnnual / 12;

    // Singapore has NO payroll withholding — income tax is self-filed
    // So there are NO tax deductions from the payslip

    let employeeCPF = 0;
    let employerCPF = 0;

    if (this.isCPRorSC && monthlyWage > MIN_WAGE_THRESHOLD) {
      // CPF is calculated on wages up to the OW ceiling
      const cpfWage = Math.min(monthlyWage, CPF_OW_MONTHLY_CEILING);
      employeeCPF = cpfWage * CPF_EMPLOYEE_RATE;
      employerCPF = cpfWage * CPF_EMPLOYER_RATE;

      // Annualise
      employeeCPF = round2(employeeCPF * 12);
      employerCPF = round2(employerCPF * 12);
    }

    const deductions: DeductionLineItem[] = [];
    const employerContributions: EmployerContribution[] = [];

    if (employeeCPF > 0) {
      deductions.push({
        name: "CPF (Employee)",
        amount: employeeCPF,
        type: "pension",
        description: `Central Provident Fund — 20% employee contribution`,
      });
    }

    if (employerCPF > 0) {
      employerContributions.push({
        name: "CPF (Employer)",
        amount: employerCPF,
        description: "Central Provident Fund — 17% employer contribution",
      });
    }

    // Note: Income tax is self-filed, not deducted at source
    if (this.isCPRorSC) {
      deductions.push({
        name: "Income Tax",
        amount: 0,
        type: "tax",
        description: "Self-filed directly with IRAS — not deducted from payslip",
      });
    }

    return { deductions, employerContributions };
  }
}