Fix rounding error with deposit products (#9876)

changelog/fix-rounding-error-with-deposit-products

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Ceil product prices after applying currency conversion, but before charm pricing and price rounding from settings is applied.

includes/multi-currency/MultiCurrency.php

@@ -832,7 +832,12 @@ public function get_price( $price, string $type ): float {
 			return (float) $price;
 		}
 
+		// We must ceil the converted price here so that we don't introduce rounding errors when
+		// summing up costs. Consider, e.g. a converted price of 10.003 for a 2-decimal currency.
+		// A single product would cost 10.00, but 2 of them would cost 20.01, _unless_ we round
+		// the individual parts correctly.
 		$converted_price = ( (float) $price ) * $currency->get_rate();
+		$converted_price = $this->ceil_price_for_currency( $converted_price, $currency );
 
 		if ( 'tax' === $type || 'coupon' === $type || 'exchange_rate' === $type ) {
 			return $converted_price;
@@ -1356,6 +1361,39 @@ protected function ceil_price( float $price, float $rounding ): float {
 		return ceil( $price / $rounding ) * $rounding;
 	}
 
+	/**
+	 * Ceils the price to the precision dictated by the number of decimals in the provided currency.
+	 *
+	 * For example: US$10.0091 -> US$10.01, JPY 1001.01 -> JPY 1002.
+	 *
+	 * @param float    $price     The price to be ceiled.
+	 * @param Currency $currency  The currency used to figure out the ceil precision.
+	 *
+	 * @return float  The ceiled price.
+	 */
+	protected function ceil_price_for_currency( float $price, Currency $currency ): float {
+		// phpcs:disable Squiz.PHP.CommentedOutCode.Found, example comments look like code.
+
+		// Example to explain the math:
+		// $price            = 10.003.
+		// expected rounding = 10.01.
+
+		// $num_decimals = 2.
+		// $factor.      = 10^2 = 100.
+		$num_decimals = absint(
+			$this->localization_service->get_currency_format(
+				$currency->get_code()
+			)['num_decimals']
+		);
+		$factor       = 10 ** $num_decimals; // 10^{$num_decimals}.
+
+		// ceil( 10.003 * $factor ) = ceil( 1_000.3 ) = 1_001.
+		// 1_001 / 100 = 10.01.
+		return ceil( $price * $factor ) / $factor; // = 10.01.
+
+		// phpcs:enable Squiz.PHP.CommentedOutCode.Found
+	}
+
 	/**
 	 * Sets up the available currencies, which are alphabetical by name.
 	 *

tests/unit/multi-currency/test-class-multi-currency.php

@@ -620,24 +620,27 @@ public function test_get_price_returns_converted_coupon_price_without_adjustment
 		WC()->session->set( WCPay\MultiCurrency\MultiCurrency::CURRENCY_SESSION_KEY, 'GBP' );
 		add_filter( 'wcpay_multi_currency_apply_charm_only_to_products', '__return_false' );
 
-		// 0.708099 * 10 = 7,08099
-		$this->assertSame( 7.08099, $this->multi_currency->get_price( '10.0', 'coupon' ) );
+		// 0.708099 * 10 = 7.08099.
+		// ceil( 7.08099, 2 ) = 7.09.
+		$this->assertSame( 7.09, $this->multi_currency->get_price( '10.0', 'coupon' ) );
 	}
 
 	public function test_get_price_returns_converted_exchange_rate_without_adjustments() {
 		WC()->session->set( WCPay\MultiCurrency\MultiCurrency::CURRENCY_SESSION_KEY, 'GBP' );
 		add_filter( 'wcpay_multi_currency_apply_charm_only_to_products', '__return_false' );
 
-		// 0.708099 * 10 = 7,08099
-		$this->assertSame( 7.08099, $this->multi_currency->get_price( '10.0', 'exchange_rate' ) );
+		// 0.708099 * 10 = 7.08099.
+		// ceil( 7.08099, 2 ) = 7.09.
+		$this->assertSame( 7.09, $this->multi_currency->get_price( '10.0', 'exchange_rate' ) );
 	}
 
 	public function test_get_price_returns_converted_tax_price() {
 		WC()->session->set( WCPay\MultiCurrency\MultiCurrency::CURRENCY_SESSION_KEY, 'GBP' );
 		add_filter( 'wcpay_multi_currency_apply_charm_only_to_products', '__return_false' );
 
-		// 0.708099 * 10 = 7,08099
-		$this->assertSame( 7.08099, $this->multi_currency->get_price( '10.0', 'tax' ) );
+		// 0.708099 * 10 = 7.08099.
+		// ceil( 7.08099, 2 ) = 7.09.
+		$this->assertSame( 7.09, $this->multi_currency->get_price( '10.0', 'tax' ) );
 	}
 
 	/**
@@ -1014,7 +1017,7 @@ public function test_set_new_customer_currency_meta_does_not_update_user_meta_if
 
 	public function get_price_provider() {
 		return [
-			[ '5.2499', '0.00', 5.2499 ],
+			[ '5.2499', '0.00', 5.25 ], // Even though the precision is 0.00 we make sure the amount is ceiled to the currency's number of digits.
 			[ '5.2499', '0.25', 5.25 ],
 			[ '5.2500', '0.25', 5.25 ],
 			[ '5.2501', '0.25', 5.50 ],