From 273d769836bb11465ca511407a1ea58b2c1f53f8 Mon Sep 17 00:00:00 2001
From: baaakgun4543 <baaakgun4543@gmail.com>
Date: Wed, 14 Oct 2026 09:45:42 +0900
Subject: [PATCH] fix(sdk): convert payment.budget max to minor units per ISO
 4217

BudgetEvaluator converted the major-unit budget with int(max * 100). For zero-decimal currencies this raised the cap 100x (a 1000 JPY budget admitted 100000 JPY), and binary-float truncation rejected payments exactly at the cap (19.99 USD -> 1998 cents). Convert with exact decimal arithmetic and the ISO 4217 minor-unit exponent, flooring fractions so rounding can never raise the cap.
---
 code/sdk/python/ap2/sdk/constraints.py        | 69 ++++++++++++++++++-
 .../sdk/python/ap2/tests/constraints_tests.py | 42 +++++++++++
 2 files changed, 108 insertions(+), 3 deletions(-)

diff --git a/code/sdk/python/ap2/sdk/constraints.py b/code/sdk/python/ap2/sdk/constraints.py
index 48aed798..aaa93eb1 100644
--- a/code/sdk/python/ap2/sdk/constraints.py
+++ b/code/sdk/python/ap2/sdk/constraints.py
@@ -9,6 +9,7 @@
 
 from abc import ABC, abstractmethod
 from datetime import datetime, timedelta
+from decimal import ROUND_FLOOR, Decimal
 
 from ap2.sdk.generated.open_checkout_mandate import (
     AllowedMerchants,
@@ -34,6 +35,66 @@
 from pydantic import BaseModel
 
 
+# ISO 4217 currencies whose minor-unit exponent differs from the default 2.
+_ZERO_DECIMAL_CURRENCIES = frozenset(
+    {
+        'BIF',
+        'CLP',
+        'DJF',
+        'GNF',
+        'ISK',
+        'JPY',
+        'KMF',
+        'KRW',
+        'PYG',
+        'RWF',
+        'UGX',
+        'UYI',
+        'VND',
+        'VUV',
+        'XAF',
+        'XOF',
+        'XPF',
+    }
+)
+_THREE_DECIMAL_CURRENCIES = frozenset(
+    {
+        'BHD',
+        'IQD',
+        'JOD',
+        'KWD',
+        'LYD',
+        'OMR',
+        'TND',
+    }
+)
+_FOUR_DECIMAL_CURRENCIES = frozenset({'CLF', 'UYW'})
+
+
+def minor_unit_exponent(currency: str) -> int:
+    """Return the ISO 4217 minor-unit exponent for ``currency``."""
+    code = currency.upper()
+    if code in _ZERO_DECIMAL_CURRENCIES:
+        return 0
+    if code in _THREE_DECIMAL_CURRENCIES:
+        return 3
+    if code in _FOUR_DECIMAL_CURRENCIES:
+        return 4
+    return 2
+
+
+def major_to_minor_units(value: float | int | str, currency: str) -> int:
+    """Convert a major-unit amount to integer minor units for ``currency``.
+
+    Uses exact decimal arithmetic (``19.99`` USD -> ``1999``, not ``1998``)
+    and the ISO 4217 minor-unit exponent (``1000`` JPY -> ``1000``,
+    ``1.5`` KWD -> ``1500``). Fractions of a minor unit are floored so a
+    budget can never be exceeded through rounding.
+    """
+    scaled = Decimal(str(value)) * Decimal(10) ** minor_unit_exponent(currency)
+    return int(scaled.to_integral_value(rounding=ROUND_FLOOR))
+
+
 class MandateContext(BaseModel):
     """Aggregated usage context for a mandate."""
 
@@ -304,11 +365,13 @@ def evaluate(
         past_spend = self.mandate_context.total_amount
         total_spend = past_spend + closed_mandate.payment_amount.amount
 
-        budget_max_cents = int(self.constraint.max * 100)
-        if total_spend > budget_max_cents:
+        budget_max_minor = major_to_minor_units(
+            self.constraint.max, self.constraint.currency
+        )
+        if total_spend > budget_max_minor:
             return [
                 f'Cumulative spend {total_spend} exceeds '
-                f'budget limit {budget_max_cents} (past spend: {past_spend})'
+                f'budget limit {budget_max_minor} (past spend: {past_spend})'
             ]
         return []
 
diff --git a/code/sdk/python/ap2/tests/constraints_tests.py b/code/sdk/python/ap2/tests/constraints_tests.py
index 910c97fc..5ce1692e 100644
--- a/code/sdk/python/ap2/tests/constraints_tests.py
+++ b/code/sdk/python/ap2/tests/constraints_tests.py
@@ -492,6 +492,48 @@ def test_payment_budget_missing_context():
     )
 
 
+def test_payment_budget_decimal_boundary_usd():
+    """A budget of 19.99 USD admits a 1999-cent spend (no float truncation)."""
+    violations = check_payment_constraints(
+        _open_payment(
+            constraints=[
+                Budget(max=19.99, currency='USD'),  # 1999 cents
+            ]
+        ),
+        _closed_payment(payment_amount=Amount(amount=1999, currency='USD')),
+        mandate_context=MandateContext(total_uses=0, total_amount=0),
+    )
+    assert violations == []
+
+
+@pytest.mark.parametrize(
+    ('budget_max', 'currency', 'amount', 'allowed'),
+    [
+        pytest.param(1000, 'JPY', 1000, True, id='jpy_zero_decimal_at_limit'),
+        pytest.param(1000, 'JPY', 1001, False, id='jpy_zero_decimal_over'),
+        pytest.param(50000, 'KRW', 50000, True, id='krw_zero_decimal_at_limit'),
+        pytest.param(1.5, 'KWD', 1500, True, id='kwd_three_decimal_at_limit'),
+        pytest.param(1.5, 'KWD', 1501, False, id='kwd_three_decimal_over'),
+        pytest.param(0.29, 'EUR', 29, True, id='eur_float_truncation'),
+    ],
+)
+def test_payment_budget_iso4217_minor_units(
+    budget_max, currency, amount, allowed
+):
+    """Budget max (major units) is converted with the currency's exponent."""
+    violations = check_payment_constraints(
+        _open_payment(constraints=[Budget(max=budget_max, currency=currency)]),
+        _closed_payment(
+            payment_amount=Amount(amount=amount, currency=currency)
+        ),
+        mandate_context=MandateContext(total_uses=0, total_amount=0),
+    )
+    if allowed:
+        assert violations == []
+    else:
+        assert any('exceeds budget limit' in v for v in violations)
+
+
 # ── check_payment_constraints – execution_date ───────────────────────────