第16章: 給与計算システム¶
はじめに¶
本章では、給与計算システムを通じて、関数型デザインパターンを実践的に適用します。従業員の種類、支払い方法、控除など、複雑なビジネスロジックを純粋関数と不変データ構造で表現します。
1. データ構造¶
従業員タイプ¶
use rust_decimal::Decimal;
/// 支払い区分
#[derive(Debug, Clone, PartialEq)]
pub enum PayClassification {
Hourly { hourly_rate: Decimal },
Salaried { monthly_salary: Decimal },
Commissioned { base_salary: Decimal, commission_rate: Decimal },
}
/// 支払いスケジュール
#[derive(Debug, Clone, PartialEq)]
pub enum PaySchedule {
Weekly,
BiWeekly,
Monthly,
}
/// 支払い方法
#[derive(Debug, Clone, PartialEq)]
pub enum PaymentMethod {
Hold,
Direct { bank: String, account: String },
Mail { address: String },
}
/// 従業員
#[derive(Debug, Clone)]
pub struct Employee {
pub id: String,
pub name: String,
pub classification: PayClassification,
pub schedule: PaySchedule,
pub payment_method: PaymentMethod,
}
タイムカードと売上¶
/// タイムカード
#[derive(Debug, Clone)]
pub struct TimeCard {
pub employee_id: String,
pub date: NaiveDate,
pub hours: Decimal,
}
/// 売上レコード
#[derive(Debug, Clone)]
pub struct SalesReceipt {
pub employee_id: String,
pub date: NaiveDate,
pub amount: Decimal,
}
2. 給与計算ロジック¶
/// 総支給額を計算
pub fn calculate_gross_pay(
employee: &Employee,
period_start: NaiveDate,
period_end: NaiveDate,
time_cards: &[TimeCard],
sales_receipts: &[SalesReceipt],
) -> Decimal {
match &employee.classification {
PayClassification::Hourly { hourly_rate } => {
let total_hours: Decimal = time_cards
.iter()
.filter(|tc| {
tc.employee_id == employee.id
&& tc.date >= period_start
&& tc.date <= period_end
})
.map(|tc| tc.hours)
.sum();
// 残業計算（40時間超過分は1.5倍）
let regular_hours = total_hours.min(Decimal::from(40));
let overtime_hours = (total_hours - Decimal::from(40)).max(Decimal::ZERO);
regular_hours * hourly_rate + overtime_hours * hourly_rate * Decimal::from_str("1.5").unwrap()
}
PayClassification::Salaried { monthly_salary } => {
*monthly_salary
}
PayClassification::Commissioned { base_salary, commission_rate } => {
let total_sales: Decimal = sales_receipts
.iter()
.filter(|sr| {
sr.employee_id == employee.id
&& sr.date >= period_start
&& sr.date <= period_end
})
.map(|sr| sr.amount)
.sum();
*base_salary + total_sales * commission_rate
}
}
}
3. 控除計算¶
/// 控除タイプ
#[derive(Debug, Clone)]
pub enum Deduction {
Tax { rate: Decimal },
Insurance { amount: Decimal },
Union { amount: Decimal },
ServiceCharge { amount: Decimal },
}
/// 控除を適用
pub fn apply_deductions(gross_pay: Decimal, deductions: &[Deduction]) -> (Decimal, Vec<(String, Decimal)>) {
let mut details = Vec::new();
let mut net_pay = gross_pay;
for deduction in deductions {
let (name, amount) = match deduction {
Deduction::Tax { rate } => {
let tax = gross_pay * rate;
("Tax".to_string(), tax)
}
Deduction::Insurance { amount } => {
("Insurance".to_string(), *amount)
}
Deduction::Union { amount } => {
("Union".to_string(), *amount)
}
Deduction::ServiceCharge { amount } => {
("Service Charge".to_string(), *amount)
}
};
net_pay -= amount;
details.push((name, amount));
}
(net_pay, details)
}
4. 給与明細¶
/// 給与明細
#[derive(Debug, Clone)]
pub struct Payslip {
pub employee_id: String,
pub employee_name: String,
pub period_start: NaiveDate,
pub period_end: NaiveDate,
pub gross_pay: Decimal,
pub deductions: Vec<(String, Decimal)>,
pub net_pay: Decimal,
}
/// 給与明細を生成
pub fn generate_payslip(
employee: &Employee,
period_start: NaiveDate,
period_end: NaiveDate,
time_cards: &[TimeCard],
sales_receipts: &[SalesReceipt],
deductions: &[Deduction],
) -> Payslip {
let gross_pay = calculate_gross_pay(
employee, period_start, period_end, time_cards, sales_receipts
);
let (net_pay, deduction_details) = apply_deductions(gross_pay, deductions);
Payslip {
employee_id: employee.id.clone(),
employee_name: employee.name.clone(),
period_start,
period_end,
gross_pay,
deductions: deduction_details,
net_pay,
}
}
5. 支払い日判定¶
/// 支払い日かどうかを判定
pub fn is_pay_day(employee: &Employee, date: NaiveDate) -> bool {
match employee.schedule {
PaySchedule::Weekly => date.weekday() == Weekday::Fri,
PaySchedule::BiWeekly => {
date.weekday() == Weekday::Fri && date.iso_week().week() % 2 == 0
}
PaySchedule::Monthly => {
let next_day = date + chrono::Duration::days(1);
next_day.month() != date.month()
}
}
}
/// 支払い期間を計算
pub fn calculate_pay_period(employee: &Employee, pay_date: NaiveDate) -> (NaiveDate, NaiveDate) {
match employee.schedule {
PaySchedule::Weekly => {
let start = pay_date - chrono::Duration::days(6);
(start, pay_date)
}
PaySchedule::BiWeekly => {
let start = pay_date - chrono::Duration::days(13);
(start, pay_date)
}
PaySchedule::Monthly => {
let start = NaiveDate::from_ymd_opt(pay_date.year(), pay_date.month(), 1).unwrap();
(start, pay_date)
}
}
}
6. ペイロール実行¶
/// ペイロールコンテキスト
pub struct PayrollContext {
pub employees: Vec<Employee>,
pub time_cards: Vec<TimeCard>,
pub sales_receipts: Vec<SalesReceipt>,
pub deductions: HashMap<String, Vec<Deduction>>,
}
/// ペイロールを実行
pub fn run_payroll(context: &PayrollContext, date: NaiveDate) -> Vec<Payslip> {
context.employees
.iter()
.filter(|emp| is_pay_day(emp, date))
.map(|emp| {
let (start, end) = calculate_pay_period(emp, date);
let emp_deductions = context.deductions.get(&emp.id).cloned().unwrap_or_default();
generate_payslip(
emp,
start,
end,
&context.time_cards,
&context.sales_receipts,
&emp_deductions,
)
})
.collect()
}
7. パターンの適用¶
この問題では以下のパターンが適用されています：
- ADT (enum): PayClassification, PaySchedule, PaymentMethod
- 不変データ: Employee, TimeCard, Payslip
- 純粋関数: calculate_gross_pay, apply_deductions
- Strategy: 支払い区分ごとの計算ロジック
まとめ¶
本章では、給与計算システムを通じて：
- enum による多様な支払いタイプの表現
- 純粋関数によるビジネスロジック
- 不変データ構造による安全な状態管理
- 高階関数によるデータ処理
を学びました。
参考コード¶
- ソースコード:
apps/rust/part6/src/chapter16.rs
次章予告¶
次章では、レンタルビデオシステムを通じて、さらに複雑なドメインモデリングを学びます。