修正

Co-authored-by: Copilot <copilot@github.com>

backend/app/payroll/bonus/service.py

@@ -1,11 +1,26 @@
 """
 賞与計算サービス (Bonus Calculation Service)
 """
-from decimal import Decimal
+import json
+import os
+import functools
+from decimal import Decimal, ROUND_HALF_UP
 from datetime import date
 from typing import Dict, Any
 from ...core.database import get_connection
 
+_BONUS_TAX_RATES_PATH = os.path.join(
+    os.path.dirname(os.path.dirname(__file__)),
+    'config', 'bonus_tax_rates.json'
+)
+
+
+@functools.lru_cache(maxsize=1)
+def _load_bonus_tax_rates():
+    """賞与算出率表をJSONファイルから読み込む（キャッシュ付き）"""
+    with open(_BONUS_TAX_RATES_PATH, encoding='utf-8') as f:
+        return json.load(f)
+
 
 class BonusCalculationService:
     """賞与計算サービス"""
@@ -15,8 +30,10 @@ class BonusCalculationService:
         """保険料率を取得（賞与用、年度がない場合は前年度を参照）"""
         with get_connection() as conn:
             with conn.cursor() as cur:
-                # rate_yearで年度を判定
-                target_year = target_date.year
+                # 翌月払いのため、新年度料率は5月支払分（4月分）から適用
+                # rate_year=XXXX は XXXX年5月支払〜(XXXX+1)年4月支払に適用
+                # 1〜4月支払は前年度（year-1）の料率を使用
+                target_year = target_date.year if target_date.month >= 5 else target_date.year - 1
                 cur.execute(
                     """
                     SELECT * FROM insurance_rates 
@@ -75,12 +92,12 @@ class BonusCalculationService:
         """前月までの3ヶ月平均給与を取得（賞与所得税計算用）"""
         with get_connection() as conn:
             with conn.cursor() as cur:
-                # 前月までの3ヶ月の総支給額を取得
+                # 前月までの3ヶ月の社会保険料等控除後の給与を取得
                 cur.execute(
                     """
-                    SELECT AVG(total_payment - commute_allowance) as avg_salary
+                    SELECT AVG(total_payment - commute_allowance - health_insurance - care_insurance - pension_insurance - employment_insurance - COALESCE(child_support, 0)) as avg_salary
                     FROM (
-                        SELECT total_payment, commute_allowance
+                        SELECT total_payment, commute_allowance, health_insurance, care_insurance, pension_insurance, employment_insurance, child_support
                         FROM monthly_payroll 
                         WHERE employee_id = %s
                         AND payment_date < %s
@@ -102,24 +119,34 @@ class BonusCalculationService:
     ) -> Decimal:
         """
         賞与所得税を計算
-        賞与税率 = (前月給与額 - 社会保険料) × 扶養人数に応じた税率
+        国税庁「賞与に対する源泉徴収税額の算出率の表」甲欄を使用
+        bonus_amount: 社会保険料等控除後の賞与額
+        previous_salary_avg: 前月の社会保険料等控除後の給与額
         """
-        # 簡易計算: 賞与額から社会保険料概算を引いた額の10.21%を所得税とする
-        # 実際は前月給与から税率を求める複雑な計算が必要
         if previous_salary_avg == Decimal("0"):
-            # 前月給与がない場合は賞与額の10.21%
-            return (bonus_amount * Decimal("0.1021")).quantize(Decimal("1"))
-        
-        # 扶養人数による税率軽減（簡略化）
-        rate = Decimal("0.1021")  # 基本税率 10.21%
-        if dependents_count == 1:
-            rate = Decimal("0.0820")  # 8.20%
-        elif dependents_count == 2:
-            rate = Decimal("0.0619")  # 6.19%
-        elif dependents_count >= 3:
-            rate = Decimal("0.0418")  # 4.18%
-        
-        return (bonus_amount * rate).quantize(Decimal("1"))
+            # 前月給与がない場合: 月額表方式が必要だがフォールバックとして10.21%
+            return (bonus_amount * Decimal("0.1021")).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+
+        table = _load_bonus_tax_rates()
+        salary_k = float(previous_salary_avg) / 1000  # 千円単位に変換
+        dep_idx = min(int(dependents_count), 7)
+
+        rate_percent = Decimal("10.21")  # フォールバック
+        for entry in table['entries']:
+            rng = entry['ranges'][dep_idx]
+            from_k = rng[0]
+            to_k = rng[1]
+            if to_k is None:
+                # 上限なし（最高税率帯）
+                if salary_k >= from_k:
+                    rate_percent = Decimal(str(entry['rate']))
+                    break
+            elif from_k <= salary_k < to_k:
+                rate_percent = Decimal(str(entry['rate']))
+                break
+
+        rate = rate_percent / Decimal("100")
+        return (bonus_amount * rate).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
     
     @staticmethod
     def calculate_bonus(
@@ -176,7 +203,7 @@ class BonusCalculationService:
         if health_insurance_rate:
             health_insurance = (
                 health_standard_bonus * Decimal(str(health_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"))
+            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
         
         # 介護保険（40歳以上65歳未満が対象）
         care_insurance = Decimal("0")
@@ -185,7 +212,7 @@ class BonusCalculationService:
             if care_insurance_rate:
                 care_insurance = (
                     health_standard_bonus * Decimal(str(care_insurance_rate["employee_rate"]))
-                ).quantize(Decimal("1"))
+                ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
         
         # 厚生年金
         pension_insurance_rate = BonusCalculationService.get_insurance_rate("厚生年金", payment_date)
@@ -193,7 +220,7 @@ class BonusCalculationService:
         if pension_insurance_rate:
             pension_insurance = (
                 pension_standard_bonus * Decimal(str(pension_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"))
+            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
         
         # 雇用保険
         employment_insurance_rate = BonusCalculationService.get_insurance_rate("雇用保険", payment_date)
@@ -201,35 +228,21 @@ class BonusCalculationService:
         if employment_insurance_rate:
             employment_insurance = (
                 total_bonus * Decimal(str(employment_insurance_rate["employee_rate"]))
-            ).quantize(Decimal("1"))
+            ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
         
         # 子ども・子育て支援金（令和8年4月分以降のみ適用）
-        # 賞与の場合: 標準報酬月額表の淨化率を使い、total_bonus × 子育て率 / 2
+        # 賞与の場合: 社会保険料率設定（insurance_rates）の料率 × 賞与総額で計算
         child_support = Decimal("0")
         is_child_support_applicable = (
-            (bonus_year == 2026 and bonus_month >= 4) or bonus_year >= 2027
+            (bonus_year == 2026 and bonus_month >= 5) or bonus_year >= 2027
         )
         if is_child_support_applicable:
-            with get_connection() as conn:
-                with conn.cursor() as cur:
-                    # 標準報酬月額表から子育て率を取得（最上位等級rowの child_support/monthly_amountで率を計算）
-                    cur.execute(
-                        """
-                        SELECT child_support, monthly_amount
-                        FROM standard_remuneration
-                        WHERE rate_year = %s
-                        AND monthly_amount > 0 AND child_support > 0
-                        ORDER BY monthly_amount DESC
-                        LIMIT 1
-                        """,
-                        (payment_date.year,)
-                    )
-                    sr = cur.fetchone()
-                    if sr and sr["monthly_amount"] and sr["monthly_amount"] > 0:
-                        cs_rate = Decimal(str(sr["child_support"])) / Decimal(str(sr["monthly_amount"]))
-                        # 標準賞与額に封上限なし（法定上賞与には標準賞与額上限なし）
-                        child_support = (total_bonus * cs_rate / Decimal("2")).quantize(Decimal("1"))
-                        print(f"[DEBUG] 賞与子育て支援金: 率={cs_rate}, 賞与総額={total_bonus} → 従業員負担={child_support}")
+            child_support_rate = BonusCalculationService.get_insurance_rate("子ども・子育て支援金", payment_date)
+            if child_support_rate:
+                child_support = (
+                    total_bonus * Decimal(str(child_support_rate["employee_rate"]))
+                ).quantize(Decimal("1"), rounding=ROUND_HALF_UP)
+                print(f"[DEBUG] 賞与子育て支援金: 賞与総額={total_bonus} × 料率={child_support_rate['employee_rate']} = {child_support}")
         
         # 所得税の計算（賞与特有の計算）
         from ..calculation.service import PayrollCalculationService