feat(fusion_accounting_bank_rec): exchange_diff helper for FX gain/loss pre-check

Made-with: Cursor

fusion_accounting_bank_rec/tests/test_exchange_diff.py

@@ -0,0 +1,56 @@
+from odoo.tests.common import TransactionCase, tagged
+from odoo.addons.fusion_accounting_bank_rec.services.exchange_diff import (
+    compute_exchange_diff, ExchangeDiffResult,
+)
+
+
+@tagged('post_install', '-at_install')
+class TestExchangeDiff(TransactionCase):
+
+    def test_no_diff_when_currencies_match_and_rates_match(self):
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='CAD',
+            against_amount=100.00, against_currency_code='CAD',
+            line_rate=1.0, against_rate=1.0,
+        )
+        self.assertFalse(result.needs_diff_move)
+        self.assertEqual(result.diff_amount, 0.0)
+
+    def test_diff_when_rates_differ_same_currency(self):
+        """USD invoice posted at 1.35, USD bank line settled at 1.40 -> diff exists.
+        100 USD at 1.40 = 140 CAD; same at 1.35 = 135 CAD; diff = 5 CAD gain."""
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='USD',
+            against_amount=100.00, against_currency_code='USD',
+            line_rate=1.40, against_rate=1.35,
+        )
+        self.assertTrue(result.needs_diff_move)
+        self.assertAlmostEqual(result.diff_amount, 5.00, places=2)
+
+    def test_diff_negative_when_rate_dropped(self):
+        """USD invoice at 1.40, settled at 1.35 -> loss"""
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='USD',
+            against_amount=100.00, against_currency_code='USD',
+            line_rate=1.35, against_rate=1.40,
+        )
+        self.assertTrue(result.needs_diff_move)
+        self.assertAlmostEqual(result.diff_amount, -5.00, places=2)
+
+    def test_company_amounts_computed_correctly(self):
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='USD',
+            against_amount=100.00, against_currency_code='USD',
+            line_rate=1.40, against_rate=1.35,
+        )
+        self.assertAlmostEqual(result.line_company_amount, 140.00, places=2)
+        self.assertAlmostEqual(result.against_company_amount, 135.00, places=2)
+
+    def test_tolerance_handles_rounding_noise(self):
+        """Tiny FX rounding under 0.005 should NOT trigger a diff move."""
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='USD',
+            against_amount=100.00, against_currency_code='USD',
+            line_rate=1.40000, against_rate=1.40003,  # 0.003 cent diff
+        )
+        self.assertFalse(result.needs_diff_move)