datasynth_generators/hr/

payroll_generator.rs

//! Payroll generator for the Hire-to-Retire (H2R) process.
//!
//! Generates payroll runs with individual employee line items, computing
//! gross pay (base salary + overtime + bonus), deductions (tax, social security,
//! health insurance, retirement), and net pay.

use chrono::NaiveDate;
use datasynth_config::schema::PayrollConfig;
use datasynth_core::country::schema::TaxBracket;
use datasynth_core::models::{PayrollLineItem, PayrollRun, PayrollRunStatus};
use datasynth_core::utils::{sample_decimal_range, seeded_rng};
use datasynth_core::uuid_factory::{DeterministicUuidFactory, GeneratorType};
use datasynth_core::CountryPack;
use rand::prelude::*;
use rand_chacha::ChaCha8Rng;
use rust_decimal::Decimal;
use tracing::debug;

/// Resolved payroll deduction rates used during generation.
#[derive(Debug, Clone)]
struct PayrollRates {
    /// Combined income tax rate (federal + state, or equivalent).
    income_tax_rate: Decimal,
    /// Progressive income tax brackets from country pack (if available).
    income_tax_brackets: Vec<TaxBracket>,
    /// Social security / FICA rate.
    fica_rate: Decimal,
    /// Employee health insurance rate.
    health_rate: Decimal,
    /// Employee retirement / pension rate.
    retirement_rate: Decimal,
    /// Employer-side social security matching rate.
    employer_fica_rate: Decimal,
}

/// Country-pack-derived deduction labels applied to every line item in a run.
#[derive(Debug, Clone, Default)]
struct DeductionLabels {
    tax_withholding: Option<String>,
    social_security: Option<String>,
    health_insurance: Option<String>,
    retirement_contribution: Option<String>,
    employer_contribution: Option<String>,
}

/// Generates [`PayrollRun`] and [`PayrollLineItem`] records from employee data.
pub struct PayrollGenerator {
    rng: ChaCha8Rng,
    uuid_factory: DeterministicUuidFactory,
    line_uuid_factory: DeterministicUuidFactory,
    config: PayrollConfig,
    country_pack: Option<CountryPack>,
    /// Pool of real employee IDs for approved_by / posted_by references.
    employee_ids_pool: Vec<String>,
    /// Pool of real cost center IDs (unused directly here since cost_center
    /// comes from the employee tuple, but kept for consistency).
    cost_center_ids_pool: Vec<String>,
}

impl PayrollGenerator {
    /// Create a new payroll generator with default configuration.
    pub fn new(seed: u64) -> Self {
        Self {
            rng: seeded_rng(seed, 0),
            uuid_factory: DeterministicUuidFactory::new(seed, GeneratorType::PayrollRun),
            line_uuid_factory: DeterministicUuidFactory::with_sub_discriminator(
                seed,
                GeneratorType::PayrollRun,
                1,
            ),
            config: PayrollConfig::default(),
            country_pack: None,
            employee_ids_pool: Vec::new(),
            cost_center_ids_pool: Vec::new(),
        }
    }

    /// Create a payroll generator with custom configuration.
    pub fn with_config(seed: u64, config: PayrollConfig) -> Self {
        Self {
            rng: seeded_rng(seed, 0),
            uuid_factory: DeterministicUuidFactory::new(seed, GeneratorType::PayrollRun),
            line_uuid_factory: DeterministicUuidFactory::with_sub_discriminator(
                seed,
                GeneratorType::PayrollRun,
                1,
            ),
            config,
            country_pack: None,
            employee_ids_pool: Vec::new(),
            cost_center_ids_pool: Vec::new(),
        }
    }

    /// Set ID pools for cross-reference coherence.
    ///
    /// When pools are non-empty, the generator selects `approved_by` and
    /// `posted_by` from `employee_ids` instead of fabricating placeholder IDs.
    pub fn with_pools(mut self, employee_ids: Vec<String>, cost_center_ids: Vec<String>) -> Self {
        self.employee_ids_pool = employee_ids;
        self.cost_center_ids_pool = cost_center_ids;
        self
    }

    /// Set the country pack for localized deduction labels.
    ///
    /// When a country pack is set, generated [`PayrollLineItem`] records will
    /// carry localized deduction labels derived from the pack's
    /// `payroll.statutory_deductions` and `payroll.employer_contributions`.
    /// The stored pack is also used by `generate` to resolve deduction rates,
    /// so callers no longer need to pass the pack explicitly.
    pub fn set_country_pack(&mut self, pack: CountryPack) {
        self.country_pack = Some(pack);
    }

    /// Generate a payroll run and line items for the given employees and period.
    ///
    /// Uses tax rates from the [`PayrollConfig`] (defaults: 22% federal, 5% state,
    /// 7.65% FICA, 3% health, 5% retirement).  If a country pack has been set via
    /// `set_country_pack`, the stored pack is used to resolve both rates and
    /// localized deduction labels automatically.
    ///
    /// # Arguments
    ///
    /// * `company_code` - Company code owning the payroll
    /// * `employees` - Slice of (employee_id, base_salary, cost_center, department)
    /// * `period_start` - Start of the pay period (inclusive)
    /// * `period_end` - End of the pay period (inclusive)
    /// * `currency` - ISO 4217 currency code
    pub fn generate(
        &mut self,
        company_code: &str,
        employees: &[(String, Decimal, Option<String>, Option<String>)],
        period_start: NaiveDate,
        period_end: NaiveDate,
        currency: &str,
    ) -> (PayrollRun, Vec<PayrollLineItem>) {
        debug!(company_code, employee_count = employees.len(), %period_start, %period_end, currency, "Generating payroll run");
        if let Some(pack) = self.country_pack.as_ref() {
            let rates = self.rates_from_country_pack(pack);
            let labels = Self::labels_from_country_pack(pack);
            self.generate_with_rates_and_labels(
                company_code,
                employees,
                period_start,
                period_end,
                currency,
                &rates,
                &labels,
            )
        } else {
            let rates = self.rates_from_config();
            self.generate_with_rates_and_labels(
                company_code,
                employees,
                period_start,
                period_end,
                currency,
                &rates,
                &DeductionLabels::default(),
            )
        }
    }

    /// Generate a payroll run using statutory deduction rates from a country pack.
    ///
    /// Iterates over `pack.payroll.statutory_deductions` to resolve rates by
    /// deduction code / English name.  Any rate not found in the pack falls back
    /// to the corresponding value from the generator's [`PayrollConfig`].
    ///
    /// # Deduction mapping
    ///
    /// | Pack code / `name_en` pattern              | Resolves to         |
    /// |--------------------------------------------|---------------------|
    /// | `FIT`, `LOHNST`, or `*Income Tax*` (not state) | federal income tax  |
    /// | `SIT` or `*State Income Tax*`              | state income tax    |
    /// | `FICA` or `*Social Security*`              | FICA / social security |
    /// | `*Health Insurance*`                       | health insurance    |
    /// | `*Pension*` or `*Retirement*`              | retirement / pension |
    ///
    /// For packs that have many small deductions (e.g. DE with pension, health,
    /// unemployment, long-term care, solidarity surcharge, church tax), the rates
    /// are summed into the closest category. Deductions not matching any category
    /// above are accumulated into the FICA/social-security bucket.
    pub fn generate_with_country_pack(
        &mut self,
        company_code: &str,
        employees: &[(String, Decimal, Option<String>, Option<String>)],
        period_start: NaiveDate,
        period_end: NaiveDate,
        currency: &str,
        pack: &CountryPack,
    ) -> (PayrollRun, Vec<PayrollLineItem>) {
        let rates = self.rates_from_country_pack(pack);
        let labels = Self::labels_from_country_pack(pack);
        self.generate_with_rates_and_labels(
            company_code,
            employees,
            period_start,
            period_end,
            currency,
            &rates,
            &labels,
        )
    }

    // ------------------------------------------------------------------
    // Private helpers
    // ------------------------------------------------------------------

    /// Build [`PayrollRates`] from the generator's config (original behaviour).
    fn rates_from_config(&self) -> PayrollRates {
        let federal_rate = Decimal::from_f64_retain(self.config.tax_rates.federal_effective)
            .unwrap_or(Decimal::ZERO);
        let state_rate = Decimal::from_f64_retain(self.config.tax_rates.state_effective)
            .unwrap_or(Decimal::ZERO);
        let fica_rate =
            Decimal::from_f64_retain(self.config.tax_rates.fica).unwrap_or(Decimal::ZERO);

        PayrollRates {
            income_tax_rate: federal_rate + state_rate,
            income_tax_brackets: Vec::new(),
            fica_rate,
            health_rate: Decimal::from_f64_retain(0.03).unwrap_or(Decimal::ZERO),
            retirement_rate: Decimal::from_f64_retain(0.05).unwrap_or(Decimal::ZERO),
            employer_fica_rate: fica_rate,
        }
    }

    /// Compute progressive tax using marginal brackets.
    ///
    /// Iterates brackets in ascending order. Each bracket taxes only the
    /// portion of income within that bracket at the bracket's rate. The
    /// terminal bracket (no `up_to`) taxes all remaining income.
    fn compute_progressive_tax(annual_income: Decimal, brackets: &[TaxBracket]) -> Decimal {
        let mut total_tax = Decimal::ZERO;
        let mut taxed_up_to = Decimal::ZERO;

        for bracket in brackets {
            let bracket_floor = bracket
                .above
                .and_then(Decimal::from_f64_retain)
                .unwrap_or(taxed_up_to);
            let bracket_rate = Decimal::from_f64_retain(bracket.rate).unwrap_or(Decimal::ZERO);

            if annual_income <= bracket_floor {
                break;
            }

            let taxable_in_bracket = if let Some(ceiling) = bracket.up_to {
                let ceiling = Decimal::from_f64_retain(ceiling).unwrap_or(Decimal::ZERO);
                (annual_income.min(ceiling) - bracket_floor).max(Decimal::ZERO)
            } else {
                // Terminal bracket — tax all remaining income
                (annual_income - bracket_floor).max(Decimal::ZERO)
            };

            total_tax += (taxable_in_bracket * bracket_rate).round_dp(2);
            taxed_up_to = bracket
                .up_to
                .and_then(Decimal::from_f64_retain)
                .unwrap_or(annual_income);
        }

        total_tax.round_dp(2)
    }

    /// Build [`PayrollRates`] from a [`CountryPack`], falling back to config
    /// values for any category not found.
    fn rates_from_country_pack(&self, pack: &CountryPack) -> PayrollRates {
        let fallback = self.rates_from_config();

        // Accumulators – start at zero; we only use the fallback when a
        // category has *no* matching deduction in the pack at all.
        let mut federal_tax = Decimal::ZERO;
        let mut state_tax = Decimal::ZERO;
        let mut fica = Decimal::ZERO;
        let mut health = Decimal::ZERO;
        let mut retirement = Decimal::ZERO;

        // Track which categories were populated from the pack.
        let mut found_federal = false;
        let mut found_state = false;
        let mut found_fica = false;
        let mut found_health = false;
        let mut found_retirement = false;

        for ded in &pack.payroll.statutory_deductions {
            let code_upper = ded.code.to_uppercase();
            let name_en_lower = ded.name_en.to_lowercase();
            let rate = Decimal::from_f64_retain(ded.rate).unwrap_or(Decimal::ZERO);

            // Progressive (bracket-based) income taxes have rate 0.0 as a
            // placeholder. Mark the category as found so the config fallback
            // is skipped — the actual tax will be computed per-employee from
            // the bracket table in generate_with_rates_and_labels().
            if (ded.deduction_type == "progressive" || ded.type_field == "progressive")
                && ded.rate == 0.0
            {
                if code_upper == "FIT"
                    || code_upper == "LOHNST"
                    || (name_en_lower.contains("income tax") && !name_en_lower.contains("state"))
                {
                    found_federal = true;
                }
                continue;
            }

            if code_upper == "FIT"
                || code_upper == "LOHNST"
                || (name_en_lower.contains("income tax") && !name_en_lower.contains("state"))
            {
                federal_tax += rate;
                found_federal = true;
            } else if code_upper == "SIT" || name_en_lower.contains("state income tax") {
                state_tax += rate;
                found_state = true;
            } else if code_upper == "FICA" || name_en_lower.contains("social security") {
                fica += rate;
                found_fica = true;
            } else if name_en_lower.contains("health insurance") {
                health += rate;
                found_health = true;
            } else if name_en_lower.contains("pension") || name_en_lower.contains("retirement") {
                retirement += rate;
                found_retirement = true;
            } else {
                // Unrecognised statutory deductions (solidarity surcharge,
                // church tax, unemployment insurance, long-term care, etc.)
                // are accumulated into the social-security / FICA bucket so
                // that total deductions still reflect the country's burden.
                fica += rate;
                found_fica = true;
            }
        }

        PayrollRates {
            income_tax_rate: if found_federal || found_state {
                let f = if found_federal {
                    federal_tax
                } else {
                    fallback.income_tax_rate
                        - Decimal::from_f64_retain(self.config.tax_rates.state_effective)
                            .unwrap_or(Decimal::ZERO)
                };
                let s = if found_state {
                    state_tax
                } else {
                    Decimal::from_f64_retain(self.config.tax_rates.state_effective)
                        .unwrap_or(Decimal::ZERO)
                };
                f + s
            } else {
                fallback.income_tax_rate
            },
            income_tax_brackets: pack.tax.payroll_tax.income_tax_brackets.clone(),
            fica_rate: if found_fica { fica } else { fallback.fica_rate },
            health_rate: if found_health {
                health
            } else {
                fallback.health_rate
            },
            retirement_rate: if found_retirement {
                retirement
            } else {
                fallback.retirement_rate
            },
            employer_fica_rate: if found_fica {
                fica
            } else {
                fallback.employer_fica_rate
            },
        }
    }

    /// Build [`DeductionLabels`] from a country pack.
    ///
    /// Walks the pack's `statutory_deductions` and `employer_contributions` and
    /// picks the matching deduction's localized `name` (falling back to
    /// `name_en` when `name` is empty) for each category.  The matching logic
    /// mirrors [`rates_from_country_pack`] so labels and rates stay consistent.
    fn labels_from_country_pack(pack: &CountryPack) -> DeductionLabels {
        let mut labels = DeductionLabels::default();

        for ded in &pack.payroll.statutory_deductions {
            let code_upper = ded.code.to_uppercase();
            let name_en_lower = ded.name_en.to_lowercase();

            // Pick the best human-readable label: prefer localized `name`, fall
            // back to `name_en`.
            let label = if ded.name.is_empty() {
                ded.name_en.clone()
            } else {
                ded.name.clone()
            };
            if label.is_empty() {
                continue;
            }

            // For progressive placeholders (rate 0), still capture the label
            // since the config-fallback rate will be used for the amount.
            if (ded.deduction_type == "progressive" || ded.type_field == "progressive")
                && ded.rate == 0.0
            {
                if code_upper == "FIT"
                    || code_upper == "LOHNST"
                    || (name_en_lower.contains("income tax") && !name_en_lower.contains("state"))
                {
                    if labels.tax_withholding.is_none() {
                        labels.tax_withholding = Some(label);
                    }
                } else if code_upper == "SIT" || name_en_lower.contains("state income tax") {
                    labels.tax_withholding = Some(match labels.tax_withholding.take() {
                        Some(existing) => format!("{existing}; {label}"),
                        None => label,
                    });
                }
                continue;
            }

            if code_upper == "FIT"
                || code_upper == "LOHNST"
                || code_upper == "SIT"
                || name_en_lower.contains("income tax")
                || name_en_lower.contains("state income tax")
            {
                // All income-tax-related deductions (federal, state, combined)
                // are grouped under the tax_withholding label.
                labels.tax_withholding = Some(match labels.tax_withholding.take() {
                    Some(existing) => format!("{existing}; {label}"),
                    None => label,
                });
            } else if code_upper == "FICA" || name_en_lower.contains("social security") {
                labels.social_security = Some(match labels.social_security.take() {
                    Some(existing) => format!("{existing}; {label}"),
                    None => label,
                });
            } else if name_en_lower.contains("health insurance") {
                if labels.health_insurance.is_none() {
                    labels.health_insurance = Some(label);
                }
            } else if name_en_lower.contains("pension") || name_en_lower.contains("retirement") {
                if labels.retirement_contribution.is_none() {
                    labels.retirement_contribution = Some(label);
                }
            } else {
                // Misc deductions (unemployment, church tax, etc.) — append to
                // social_security label since those rates go into that bucket.
                labels.social_security = Some(match labels.social_security.take() {
                    Some(existing) => format!("{existing}; {label}"),
                    None => label,
                });
            }
        }

        // Employer contributions
        let emp_labels: Vec<String> = pack
            .payroll
            .employer_contributions
            .iter()
            .filter_map(|c| {
                let l = if c.name.is_empty() {
                    c.name_en.clone()
                } else {
                    c.name.clone()
                };
                if l.is_empty() {
                    None
                } else {
                    Some(l)
                }
            })
            .collect();
        if !emp_labels.is_empty() {
            labels.employer_contribution = Some(emp_labels.join("; "));
        }

        labels
    }

    /// Core generation logic parameterised on resolved rates and labels.
    fn generate_with_rates_and_labels(
        &mut self,
        company_code: &str,
        employees: &[(String, Decimal, Option<String>, Option<String>)],
        period_start: NaiveDate,
        period_end: NaiveDate,
        currency: &str,
        rates: &PayrollRates,
        labels: &DeductionLabels,
    ) -> (PayrollRun, Vec<PayrollLineItem>) {
        let payroll_id = self.uuid_factory.next().to_string();

        let mut line_items = Vec::with_capacity(employees.len());
        let mut total_gross = Decimal::ZERO;
        let mut total_deductions = Decimal::ZERO;
        let mut total_net = Decimal::ZERO;
        let mut total_employer_cost = Decimal::ZERO;

        let benefits_enrolled = self.config.benefits_enrollment_rate;
        let retirement_participating = self.config.retirement_participation_rate;

        for (employee_id, base_salary, cost_center, department) in employees {
            let line_id = self.line_uuid_factory.next().to_string();

            // Monthly base component (annual salary / 12)
            let monthly_base = (*base_salary / Decimal::from(12)).round_dp(2);

            // Overtime: 10% chance, 1-20 hours at 1.5x hourly rate
            let (overtime_pay, overtime_hours) = if self.rng.random_bool(0.10) {
                let ot_hours = self.rng.random_range(1.0..=20.0);
                // Hourly rate = annual salary / (52 weeks * 40 hours)
                let hourly_rate = *base_salary / Decimal::from(2080);
                let ot_rate = hourly_rate * Decimal::from_f64_retain(1.5).unwrap_or(Decimal::ONE);
                let ot_pay = (ot_rate
                    * Decimal::from_f64_retain(ot_hours).unwrap_or(Decimal::ZERO))
                .round_dp(2);
                (ot_pay, ot_hours)
            } else {
                (Decimal::ZERO, 0.0)
            };

            // Bonus: 5% chance for a monthly bonus (1-10% of monthly base)
            let bonus = if self.rng.random_bool(0.05) {
                let pct = self.rng.random_range(0.01..=0.10);
                (monthly_base * Decimal::from_f64_retain(pct).unwrap_or(Decimal::ZERO)).round_dp(2)
            } else {
                Decimal::ZERO
            };

            let gross_pay = monthly_base + overtime_pay + bonus;

            // Deductions — use progressive brackets when available
            let tax_withholding = if !rates.income_tax_brackets.is_empty() {
                let annual = gross_pay * Decimal::from(12);
                Self::compute_progressive_tax(annual, &rates.income_tax_brackets)
                    / Decimal::from(12)
            } else {
                (gross_pay * rates.income_tax_rate).round_dp(2)
            };
            let social_security = (gross_pay * rates.fica_rate).round_dp(2);

            let health_insurance = if self.rng.random_bool(benefits_enrolled) {
                (gross_pay * rates.health_rate).round_dp(2)
            } else {
                Decimal::ZERO
            };

            let retirement_contribution = if self.rng.random_bool(retirement_participating) {
                (gross_pay * rates.retirement_rate).round_dp(2)
            } else {
                Decimal::ZERO
            };

            // Small random other deductions (garnishments, etc.): ~3% chance
            let other_deductions = if self.rng.random_bool(0.03) {
                sample_decimal_range(&mut self.rng, Decimal::from(50), Decimal::from(500))
                    .round_dp(2)
            } else {
                Decimal::ZERO
            };

            let total_ded = tax_withholding
                + social_security
                + health_insurance
                + retirement_contribution
                + other_deductions;
            let net_pay = gross_pay - total_ded;

            // Standard 160 regular hours per month (8h * 20 business days)
            let hours_worked = 160.0;

            // Employer-side cost: gross + employer contribution match
            let employer_contrib = (gross_pay * rates.employer_fica_rate).round_dp(2);
            let employer_cost = gross_pay + employer_contrib;

            total_gross += gross_pay;
            total_deductions += total_ded;
            total_net += net_pay;
            total_employer_cost += employer_cost;

            line_items.push(PayrollLineItem {
                payroll_id: payroll_id.clone(),
                employee_id: employee_id.clone(),
                line_id,
                gross_pay,
                base_salary: monthly_base,
                overtime_pay,
                bonus,
                tax_withholding,
                social_security,
                health_insurance,
                retirement_contribution,
                other_deductions,
                net_pay,
                hours_worked,
                overtime_hours,
                pay_date: period_end,
                cost_center: cost_center.clone(),
                department: department.clone(),
                tax_withholding_label: labels.tax_withholding.clone(),
                social_security_label: labels.social_security.clone(),
                health_insurance_label: labels.health_insurance.clone(),
                retirement_contribution_label: labels.retirement_contribution.clone(),
                employer_contribution_label: labels.employer_contribution.clone(),
            });
        }

        // Determine status
        let status_roll: f64 = self.rng.random();
        let status = if status_roll < 0.60 {
            PayrollRunStatus::Posted
        } else if status_roll < 0.85 {
            PayrollRunStatus::Approved
        } else if status_roll < 0.95 {
            PayrollRunStatus::Calculated
        } else {
            PayrollRunStatus::Draft
        };

        let approved_by = if matches!(
            status,
            PayrollRunStatus::Approved | PayrollRunStatus::Posted
        ) {
            if !self.employee_ids_pool.is_empty() {
                let idx = self.rng.random_range(0..self.employee_ids_pool.len());
                Some(self.employee_ids_pool[idx].clone())
            } else {
                Some(format!("USR-{:04}", self.rng.random_range(201..=400)))
            }
        } else {
            None
        };

        let posted_by = if status == PayrollRunStatus::Posted {
            if !self.employee_ids_pool.is_empty() {
                let idx = self.rng.random_range(0..self.employee_ids_pool.len());
                Some(self.employee_ids_pool[idx].clone())
            } else {
                Some(format!("USR-{:04}", self.rng.random_range(401..=500)))
            }
        } else {
            None
        };

        let run = PayrollRun {
            company_code: company_code.to_string(),
            payroll_id: payroll_id.clone(),
            pay_period_start: period_start,
            pay_period_end: period_end,
            run_date: period_end,
            status,
            total_gross,
            total_deductions,
            total_net,
            total_employer_cost,
            employee_count: employees.len() as u32,
            currency: currency.to_string(),
            posted_by,
            approved_by,
        };

        (run, line_items)
    }

    /// Generate a payroll run, adjusting each employee's salary based on
    /// `EmployeeChangeEvent` records before delegating to `generate`.
    ///
    /// For each employee, all `SalaryAdjustment` events whose `effective_date`
    /// falls on or before `period_end` are examined.  The most recent such
    /// event determines the effective annual salary for the period (prorated
    /// when the effective date falls mid-period).
    ///
    /// # Arguments
    ///
    /// * `company_code`  - Company code owning the payroll
    /// * `employees`     - Slice of (employee_id, base_annual_salary, cost_center, department)
    /// * `period_start`  - Start of the pay period (inclusive)
    /// * `period_end`    - End of the pay period (inclusive)
    /// * `currency`      - ISO 4217 currency code
    /// * `changes`       - Employee change history to apply
    pub fn generate_with_changes(
        &mut self,
        company_code: &str,
        employees: &[(String, Decimal, Option<String>, Option<String>)],
        period_start: NaiveDate,
        period_end: NaiveDate,
        currency: &str,
        changes: &[datasynth_core::models::EmployeeChangeEvent],
    ) -> (PayrollRun, Vec<PayrollLineItem>) {
        let adjusted: Vec<(String, Decimal, Option<String>, Option<String>)> = employees
            .iter()
            .map(|(id, salary, cc, dept)| {
                let adjusted_salary =
                    Self::apply_salary_changes(id, *salary, period_start, period_end, changes);
                (id.clone(), adjusted_salary, cc.clone(), dept.clone())
            })
            .collect();
        self.generate(company_code, &adjusted, period_start, period_end, currency)
    }

    /// Compute the effective annual salary for one employee over a pay period,
    /// taking `SalaryAdjustment` events into account.
    ///
    /// If the latest qualifying change became effective before `period_start`,
    /// the new salary applies for the full period.  If it became effective
    /// mid-period, the salary is prorated by calendar days.
    fn apply_salary_changes(
        employee_id: &str,
        base_annual_salary: Decimal,
        period_start: NaiveDate,
        period_end: NaiveDate,
        changes: &[datasynth_core::models::EmployeeChangeEvent],
    ) -> Decimal {
        use datasynth_core::models::EmployeeEventType;

        // Filter: only SalaryAdjustment events for this employee that are on or
        // before period_end (i.e. they could affect this payroll period).
        let relevant: Vec<&datasynth_core::models::EmployeeChangeEvent> = changes
            .iter()
            .filter(|c| {
                c.employee_id == employee_id
                    && c.event_type == EmployeeEventType::SalaryAdjustment
                    && c.effective_date <= period_end
            })
            .collect();

        if relevant.is_empty() {
            return base_annual_salary;
        }

        // Find the latest change by effective_date.
        let latest = relevant
            .iter()
            .max_by_key(|c| c.effective_date)
            .expect("non-empty slice always has a max");

        // Parse the new annual salary from new_value.
        let new_salary = match latest
            .new_value
            .as_deref()
            .and_then(|v| v.parse::<Decimal>().ok())
        {
            Some(s) => s,
            None => return base_annual_salary,
        };

        let effective = latest.effective_date;

        if effective <= period_start {
            // Change took effect before or on period start → full period at new rate.
            new_salary
        } else {
            // Change is mid-period → prorate.
            let total_days = (period_end - period_start).num_days() + 1;
            let days_at_old = (effective - period_start).num_days();
            let days_at_new = total_days - days_at_old;

            let total = Decimal::from(total_days);
            let old_fraction = Decimal::from(days_at_old) / total;
            let new_fraction = Decimal::from(days_at_new) / total;

            (base_annual_salary * old_fraction + new_salary * new_fraction).round_dp(2)
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn test_employees() -> Vec<(String, Decimal, Option<String>, Option<String>)> {
        vec![
            (
                "EMP-001".to_string(),
                Decimal::from(60_000),
                Some("CC-100".to_string()),
                Some("Engineering".to_string()),
            ),
            (
                "EMP-002".to_string(),
                Decimal::from(85_000),
                Some("CC-200".to_string()),
                Some("Finance".to_string()),
            ),
            (
                "EMP-003".to_string(),
                Decimal::from(120_000),
                None,
                Some("Sales".to_string()),
            ),
        ]
    }

    #[test]
    fn test_basic_payroll_generation() {
        let mut gen = PayrollGenerator::new(42);
        let employees = test_employees();
        let period_start = NaiveDate::from_ymd_opt(2024, 1, 1).unwrap();
        let period_end = NaiveDate::from_ymd_opt(2024, 1, 31).unwrap();

        let (run, items) = gen.generate("C001", &employees, period_start, period_end, "USD");

        assert_eq!(run.company_code, "C001");
        assert_eq!(run.currency, "USD");
        assert_eq!(run.employee_count, 3);
        assert_eq!(items.len(), 3);
        assert!(run.total_gross > Decimal::ZERO);
        assert!(run.total_deductions > Decimal::ZERO);
        assert!(run.total_net > Decimal::ZERO);
        assert!(run.total_employer_cost > run.total_gross);
        // net = gross - deductions
        assert_eq!(run.total_net, run.total_gross - run.total_deductions);

        for item in &items {
            assert_eq!(item.payroll_id, run.payroll_id);
            assert!(item.gross_pay > Decimal::ZERO);
            assert!(item.net_pay > Decimal::ZERO);
            assert!(item.net_pay < item.gross_pay);
            assert!(item.base_salary > Decimal::ZERO);
            assert_eq!(item.pay_date, period_end);
            // Without country pack, labels should be None
            assert!(item.tax_withholding_label.is_none());
            assert!(item.social_security_label.is_none());
        }
    }

    #[test]
    fn test_deterministic_payroll() {
        let employees = test_employees();
        let period_start = NaiveDate::from_ymd_opt(2024, 3, 1).unwrap();
        let period_end = NaiveDate::from_ymd_opt(2024, 3, 31).unwrap();

        let mut gen1 = PayrollGenerator::new(42);
        let (run1, items1) = gen1.generate("C001", &employees, period_start, period_end, "USD");

        let mut gen2 = PayrollGenerator::new(42);
        let (run2, items2) = gen2.generate("C001", &employees, period_start, period_end, "USD");

        assert_eq!(run1.payroll_id, run2.payroll_id);
        assert_eq!(run1.total_gross, run2.total_gross);
        assert_eq!(run1.total_net, run2.total_net);
        assert_eq!(run1.status, run2.status);
        assert_eq!(items1.len(), items2.len());
        for (a, b) in items1.iter().zip(items2.iter()) {
            assert_eq!(a.line_id, b.line_id);
            assert_eq!(a.gross_pay, b.gross_pay);
            assert_eq!(a.net_pay, b.net_pay);
        }
    }

    #[test]
    fn test_payroll_deduction_components() {
        let mut gen = PayrollGenerator::new(99);
        let employees = vec![(
            "EMP-010".to_string(),
            Decimal::from(100_000),
            Some("CC-300".to_string()),
            Some("HR".to_string()),
        )];
        let period_start = NaiveDate::from_ymd_opt(2024, 6, 1).unwrap();
        let period_end = NaiveDate::from_ymd_opt(2024, 6, 30).unwrap();

        let (_run, items) = gen.generate("C001", &employees, period_start, period_end, "USD");
        assert_eq!(items.len(), 1);

        let item = &items[0];
        // base_salary should be approximately 100000/12 = 8333.33
        let expected_monthly = (Decimal::from(100_000) / Decimal::from(12)).round_dp(2);
        assert_eq!(item.base_salary, expected_monthly);

        // Deductions should sum correctly
        let deduction_sum = item.tax_withholding
            + item.social_security
            + item.health_insurance
            + item.retirement_contribution
            + item.other_deductions;
        let expected_net = item.gross_pay - deduction_sum;
        assert_eq!(item.net_pay, expected_net);

        // Tax withholding should be reasonable (22% federal + 5% state = 27% of gross)
        assert!(item.tax_withholding > Decimal::ZERO);
        assert!(item.social_security > Decimal::ZERO);
    }

    // ---------------------------------------------------------------
    // Country-pack tests
    // ---------------------------------------------------------------

    /// Helper: build a US-like country pack with explicit statutory deductions.
    fn us_country_pack() -> CountryPack {
        use datasynth_core::country::schema::{PayrollCountryConfig, PayrollDeduction};
        CountryPack {
            country_code: "US".to_string(),
            payroll: PayrollCountryConfig {
                statutory_deductions: vec![
                    PayrollDeduction {
                        code: "FICA".to_string(),
                        name_en: "Federal Insurance Contributions Act".to_string(),
                        deduction_type: "percentage".to_string(),
                        rate: 0.0765,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "FIT".to_string(),
                        name_en: "Federal Income Tax".to_string(),
                        deduction_type: "progressive".to_string(),
                        rate: 0.0, // progressive placeholder
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "SIT".to_string(),
                        name_en: "State Income Tax".to_string(),
                        deduction_type: "percentage".to_string(),
                        rate: 0.05,
                        ..Default::default()
                    },
                ],
                ..Default::default()
            },
            ..Default::default()
        }
    }

    /// Helper: build a DE-like country pack.
    fn de_country_pack() -> CountryPack {
        use datasynth_core::country::schema::{PayrollCountryConfig, PayrollDeduction};
        CountryPack {
            country_code: "DE".to_string(),
            payroll: PayrollCountryConfig {
                pay_frequency: "monthly".to_string(),
                currency: "EUR".to_string(),
                statutory_deductions: vec![
                    PayrollDeduction {
                        code: "LOHNST".to_string(),
                        name_en: "Income Tax".to_string(),
                        type_field: "progressive".to_string(),
                        rate: 0.0, // progressive placeholder
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "SOLI".to_string(),
                        name_en: "Solidarity Surcharge".to_string(),
                        type_field: "percentage".to_string(),
                        rate: 0.055,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "KiSt".to_string(),
                        name_en: "Church Tax".to_string(),
                        type_field: "percentage".to_string(),
                        rate: 0.08,
                        optional: true,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "RV".to_string(),
                        name_en: "Pension Insurance".to_string(),
                        type_field: "percentage".to_string(),
                        rate: 0.093,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "KV".to_string(),
                        name_en: "Health Insurance".to_string(),
                        type_field: "percentage".to_string(),
                        rate: 0.073,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "AV".to_string(),
                        name_en: "Unemployment Insurance".to_string(),
                        type_field: "percentage".to_string(),
                        rate: 0.013,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "PV".to_string(),
                        name_en: "Long-Term Care Insurance".to_string(),
                        type_field: "percentage".to_string(),
                        rate: 0.017,
                        ..Default::default()
                    },
                ],
                employer_contributions: vec![
                    PayrollDeduction {
                        code: "AG-RV".to_string(),
                        name_en: "Employer Pension Insurance".to_string(),
                        type_field: "percentage".to_string(),
                        rate: 0.093,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "AG-KV".to_string(),
                        name_en: "Employer Health Insurance".to_string(),
                        type_field: "percentage".to_string(),
                        rate: 0.073,
                        ..Default::default()
                    },
                ],
                ..Default::default()
            },
            ..Default::default()
        }
    }

    #[test]
    fn test_generate_with_us_country_pack() {
        let mut gen = PayrollGenerator::new(42);
        let employees = test_employees();
        let period_start = NaiveDate::from_ymd_opt(2024, 1, 1).unwrap();
        let period_end = NaiveDate::from_ymd_opt(2024, 1, 31).unwrap();
        let pack = us_country_pack();

        let (run, items) = gen.generate_with_country_pack(
            "C001",
            &employees,
            period_start,
            period_end,
            "USD",
            &pack,
        );

        assert_eq!(run.company_code, "C001");
        assert_eq!(run.employee_count, 3);
        assert_eq!(items.len(), 3);
        assert_eq!(run.total_net, run.total_gross - run.total_deductions);

        for item in &items {
            assert!(item.gross_pay > Decimal::ZERO);
            assert!(item.net_pay > Decimal::ZERO);
            assert!(item.net_pay < item.gross_pay);
            // FICA deduction should be present
            assert!(item.social_security > Decimal::ZERO);
            // US pack should produce labels
            assert!(item.tax_withholding_label.is_some());
            assert!(item.social_security_label.is_some());
        }
    }

    #[test]
    fn test_generate_with_de_country_pack() {
        let mut gen = PayrollGenerator::new(42);
        let employees = test_employees();
        let period_start = NaiveDate::from_ymd_opt(2024, 1, 1).unwrap();
        let period_end = NaiveDate::from_ymd_opt(2024, 1, 31).unwrap();
        let pack = de_country_pack();

        let (run, items) = gen.generate_with_country_pack(
            "DE01",
            &employees,
            period_start,
            period_end,
            "EUR",
            &pack,
        );

        assert_eq!(run.company_code, "DE01");
        assert_eq!(items.len(), 3);
        assert_eq!(run.total_net, run.total_gross - run.total_deductions);

        // DE pack should use pension rate 0.093 for retirement
        // and health insurance rate 0.073
        let rates = gen.rates_from_country_pack(&pack);
        assert_eq!(
            rates.retirement_rate,
            Decimal::from_f64_retain(0.093).unwrap()
        );
        assert_eq!(rates.health_rate, Decimal::from_f64_retain(0.073).unwrap());

        // Check DE labels are populated
        let item = &items[0];
        assert_eq!(
            item.health_insurance_label.as_deref(),
            Some("Health Insurance")
        );
        assert_eq!(
            item.retirement_contribution_label.as_deref(),
            Some("Pension Insurance")
        );
        // Employer contribution labels should include both AG-RV and AG-KV
        assert!(item.employer_contribution_label.is_some());
        let ec = item.employer_contribution_label.as_ref().unwrap();
        assert!(ec.contains("Employer Pension Insurance"));
        assert!(ec.contains("Employer Health Insurance"));
    }

    #[test]
    fn test_country_pack_falls_back_to_config_for_missing_categories() {
        // Empty pack: no statutory deductions => all rates fall back to config
        let pack = CountryPack::default();
        let gen = PayrollGenerator::new(42);
        let rates_pack = gen.rates_from_country_pack(&pack);
        let rates_cfg = gen.rates_from_config();

        assert_eq!(rates_pack.income_tax_rate, rates_cfg.income_tax_rate);
        assert_eq!(rates_pack.fica_rate, rates_cfg.fica_rate);
        assert_eq!(rates_pack.health_rate, rates_cfg.health_rate);
        assert_eq!(rates_pack.retirement_rate, rates_cfg.retirement_rate);
    }

    #[test]
    fn test_country_pack_deterministic() {
        let employees = test_employees();
        let period_start = NaiveDate::from_ymd_opt(2024, 3, 1).unwrap();
        let period_end = NaiveDate::from_ymd_opt(2024, 3, 31).unwrap();
        let pack = de_country_pack();

        let mut gen1 = PayrollGenerator::new(42);
        let (run1, items1) = gen1.generate_with_country_pack(
            "DE01",
            &employees,
            period_start,
            period_end,
            "EUR",
            &pack,
        );

        let mut gen2 = PayrollGenerator::new(42);
        let (run2, items2) = gen2.generate_with_country_pack(
            "DE01",
            &employees,
            period_start,
            period_end,
            "EUR",
            &pack,
        );

        assert_eq!(run1.payroll_id, run2.payroll_id);
        assert_eq!(run1.total_gross, run2.total_gross);
        assert_eq!(run1.total_net, run2.total_net);
        for (a, b) in items1.iter().zip(items2.iter()) {
            assert_eq!(a.net_pay, b.net_pay);
        }
    }

    #[test]
    fn test_de_rates_differ_from_default() {
        // With the DE pack, the resolved rates should differ from config defaults
        let gen = PayrollGenerator::new(42);
        let pack = de_country_pack();
        let rates_cfg = gen.rates_from_config();
        let rates_de = gen.rates_from_country_pack(&pack);

        // DE has no non-progressive income tax in pack -> income_tax_rate falls
        // back to config default for federal part.
        // But health (0.073 vs 0.03) and retirement (0.093 vs 0.05) should differ.
        assert_ne!(rates_de.health_rate, rates_cfg.health_rate);
        assert_ne!(rates_de.retirement_rate, rates_cfg.retirement_rate);
    }

    #[test]
    fn test_set_country_pack_uses_labels() {
        let mut gen = PayrollGenerator::new(42);
        let pack = de_country_pack();
        gen.set_country_pack(pack);

        let employees = test_employees();
        let period_start = NaiveDate::from_ymd_opt(2024, 1, 1).unwrap();
        let period_end = NaiveDate::from_ymd_opt(2024, 1, 31).unwrap();

        // generate() should now use the stored pack for rates + labels
        let (_run, items) = gen.generate("DE01", &employees, period_start, period_end, "EUR");

        let item = &items[0];
        // Labels should be populated from the DE pack
        assert!(item.tax_withholding_label.is_some());
        assert!(item.health_insurance_label.is_some());
        assert!(item.retirement_contribution_label.is_some());
        assert!(item.employer_contribution_label.is_some());
    }

    #[test]
    fn test_compute_progressive_tax_us_brackets() {
        // Simplified US-style brackets for testing
        let brackets = vec![
            TaxBracket {
                above: Some(0.0),
                up_to: Some(11_000.0),
                rate: 0.10,
            },
            TaxBracket {
                above: Some(11_000.0),
                up_to: Some(44_725.0),
                rate: 0.12,
            },
            TaxBracket {
                above: Some(44_725.0),
                up_to: Some(95_375.0),
                rate: 0.22,
            },
            TaxBracket {
                above: Some(95_375.0),
                up_to: None,
                rate: 0.24,
            },
        ];

        // $60,000 income
        let tax = PayrollGenerator::compute_progressive_tax(Decimal::from(60_000), &brackets);
        // 11,000 * 0.10 = 1,100
        // (44,725 - 11,000) * 0.12 = 4,047
        // (60,000 - 44,725) * 0.22 = 3,360.50
        // Total = 8,507.50
        assert_eq!(tax, Decimal::from_f64_retain(8507.50).unwrap());

        // $11,000 income — only first bracket
        let tax = PayrollGenerator::compute_progressive_tax(Decimal::from(11_000), &brackets);
        assert_eq!(tax, Decimal::from_f64_retain(1100.0).unwrap());
    }

    #[test]
    fn test_progressive_tax_zero_income() {
        let brackets = vec![TaxBracket {
            above: Some(0.0),
            up_to: Some(10_000.0),
            rate: 0.10,
        }];
        let tax = PayrollGenerator::compute_progressive_tax(Decimal::ZERO, &brackets);
        assert_eq!(tax, Decimal::ZERO);
    }

    #[test]
    fn test_us_pack_employees_have_varying_rates() {
        use datasynth_core::country::schema::{
            CountryTaxConfig, PayrollCountryConfig, PayrollDeduction, PayrollTaxBracketsConfig,
        };

        let brackets = vec![
            TaxBracket {
                above: Some(0.0),
                up_to: Some(11_000.0),
                rate: 0.10,
            },
            TaxBracket {
                above: Some(11_000.0),
                up_to: Some(44_725.0),
                rate: 0.12,
            },
            TaxBracket {
                above: Some(44_725.0),
                up_to: None,
                rate: 0.22,
            },
        ];
        let pack = CountryPack {
            country_code: "US".to_string(),
            payroll: PayrollCountryConfig {
                statutory_deductions: vec![
                    PayrollDeduction {
                        code: "FIT".to_string(),
                        name_en: "Federal Income Tax".to_string(),
                        deduction_type: "progressive".to_string(),
                        rate: 0.0,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "FICA".to_string(),
                        name_en: "Social Security".to_string(),
                        deduction_type: "percentage".to_string(),
                        rate: 0.0765,
                        ..Default::default()
                    },
                ],
                ..Default::default()
            },
            tax: CountryTaxConfig {
                payroll_tax: PayrollTaxBracketsConfig {
                    income_tax_brackets: brackets,
                    ..Default::default()
                },
                ..Default::default()
            },
            ..Default::default()
        };

        let mut gen = PayrollGenerator::new(42);
        gen.set_country_pack(pack);

        // Low earner vs high earner
        let low_earner = vec![("LOW".to_string(), Decimal::from(30_000), None, None)];
        let high_earner = vec![("HIGH".to_string(), Decimal::from(200_000), None, None)];

        let start = NaiveDate::from_ymd_opt(2024, 1, 1).unwrap();
        let end = NaiveDate::from_ymd_opt(2024, 1, 31).unwrap();

        let (_, low_items) = gen.generate("C001", &low_earner, start, end, "USD");
        let mut gen2 = PayrollGenerator::new(42);
        gen2.set_country_pack(CountryPack {
            country_code: "US".to_string(),
            payroll: PayrollCountryConfig {
                statutory_deductions: vec![
                    PayrollDeduction {
                        code: "FIT".to_string(),
                        name_en: "Federal Income Tax".to_string(),
                        deduction_type: "progressive".to_string(),
                        rate: 0.0,
                        ..Default::default()
                    },
                    PayrollDeduction {
                        code: "FICA".to_string(),
                        name_en: "Social Security".to_string(),
                        deduction_type: "percentage".to_string(),
                        rate: 0.0765,
                        ..Default::default()
                    },
                ],
                ..Default::default()
            },
            tax: CountryTaxConfig {
                payroll_tax: PayrollTaxBracketsConfig {
                    income_tax_brackets: vec![
                        TaxBracket {
                            above: Some(0.0),
                            up_to: Some(11_000.0),
                            rate: 0.10,
                        },
                        TaxBracket {
                            above: Some(11_000.0),
                            up_to: Some(44_725.0),
                            rate: 0.12,
                        },
                        TaxBracket {
                            above: Some(44_725.0),
                            up_to: None,
                            rate: 0.22,
                        },
                    ],
                    ..Default::default()
                },
                ..Default::default()
            },
            ..Default::default()
        });
        let (_, high_items) = gen2.generate("C001", &high_earner, start, end, "USD");

        let low_eff = low_items[0].tax_withholding / low_items[0].gross_pay;
        let high_eff = high_items[0].tax_withholding / high_items[0].gross_pay;

        // High earner should have a higher effective tax rate
        assert!(
            high_eff > low_eff,
            "High earner effective rate ({high_eff}) should exceed low earner ({low_eff})"
        );
    }

    #[test]
    fn test_empty_pack_labels_are_none() {
        let pack = CountryPack::default();
        let labels = PayrollGenerator::labels_from_country_pack(&pack);
        assert!(labels.tax_withholding.is_none());
        assert!(labels.social_security.is_none());
        assert!(labels.health_insurance.is_none());
        assert!(labels.retirement_contribution.is_none());
        assert!(labels.employer_contribution.is_none());
    }

    #[test]
    fn test_us_pack_labels() {
        let pack = us_country_pack();
        let labels = PayrollGenerator::labels_from_country_pack(&pack);
        // FIT is a progressive placeholder but label is still captured
        assert!(labels.tax_withholding.is_some());
        let tw = labels.tax_withholding.unwrap();
        assert!(tw.contains("Federal Income Tax"));
        assert!(tw.contains("State Income Tax"));
        // FICA label
        assert!(labels.social_security.is_some());
        assert!(labels
            .social_security
            .unwrap()
            .contains("Federal Insurance Contributions Act"));
    }
}