Merge pull request #156 from mraerino/fix/calculation-rounding-amounts

Fix rounding inaccuracies with quantity

Author: bcomnes

calculator/calculator.go

@@ -173,6 +173,38 @@ func (t *Tax) AppliesTo(country, productType string) bool {
 	return applies
 }
 
+func calculateAmountsForSingleItem(settings *Settings, lineLogger logrus.FieldLogger, jwtClaims map[string]interface{}, params PriceParameters, item Item, multiplier uint64) ItemPrice {
+	itemPrice := ItemPrice{Quantity: item.GetQuantity()}
+
+	singlePrice := item.PriceInLowestUnit() * multiplier
+	_, itemPrice.Subtotal = calculateTaxes(singlePrice, item, params, settings)
+
+	// apply discount to original price
+	coupon := params.Coupon
+	if coupon != nil && coupon.ValidForType(item.ProductType()) && coupon.ValidForProduct(item.ProductSku()) {
+		itemPrice.Discount = calculateDiscount(singlePrice, coupon.PercentageDiscount(), coupon.FixedDiscount(params.Currency)*multiplier)
+	}
+	if settings != nil && settings.MemberDiscounts != nil {
+		for _, discount := range settings.MemberDiscounts {
+
+			if jwtClaims != nil && claims.HasClaims(jwtClaims, discount.Claims) && discount.ValidForType(item.ProductType()) && discount.ValidForProduct(item.ProductSku()) {
+				lineLogger = lineLogger.WithField("discount", discount.Claims)
+				itemPrice.Discount += calculateDiscount(singlePrice, discount.Percentage, discount.FixedDiscount(params.Currency)*multiplier)
+			}
+		}
+	}
+
+	discountedPrice := uint64(0)
+	if itemPrice.Discount < singlePrice {
+		discountedPrice = singlePrice - itemPrice.Discount
+	}
+
+	itemPrice.Taxes, itemPrice.NetTotal = calculateTaxes(discountedPrice, item, params, settings)
+	itemPrice.Total = int64(itemPrice.NetTotal + itemPrice.Taxes)
+
+	return itemPrice
+}
+
 // CalculatePrice will calculate the final total price. It takes into account
 // currency, country, coupons, and discounts.
 func CalculatePrice(settings *Settings, jwtClaims map[string]interface{}, params PriceParameters, log logrus.FieldLogger) Price {
@@ -193,34 +225,7 @@ func CalculatePrice(settings *Settings, jwtClaims map[string]interface{}, params
 			"product_sku":  item.ProductSku(),
 		})
 
-		itemPrice := ItemPrice{Quantity: item.GetQuantity()}
-
-		singlePrice := item.PriceInLowestUnit()
-		_, itemPrice.Subtotal = calculateTaxes(singlePrice, item, params, settings)
-
-		// apply discount to original price
-		coupon := params.Coupon
-		if coupon != nil && coupon.ValidForType(item.ProductType()) && coupon.ValidForProduct(item.ProductSku()) {
-			itemPrice.Discount = calculateDiscount(singlePrice, coupon.PercentageDiscount(), coupon.FixedDiscount(params.Currency))
-		}
-		if settings != nil && settings.MemberDiscounts != nil {
-			for _, discount := range settings.MemberDiscounts {
-
-				if jwtClaims != nil && claims.HasClaims(jwtClaims, discount.Claims) && discount.ValidForType(item.ProductType()) && discount.ValidForProduct(item.ProductSku()) {
-					lineLogger = lineLogger.WithField("discount", discount.Claims)
-					itemPrice.Discount += calculateDiscount(singlePrice, discount.Percentage, discount.FixedDiscount(params.Currency))
-				}
-			}
-		}
-
-		discountedPrice := uint64(0)
-		if itemPrice.Discount < singlePrice {
-			discountedPrice = singlePrice - itemPrice.Discount
-		}
-
-		itemPrice.Taxes, itemPrice.NetTotal = calculateTaxes(discountedPrice, item, params, settings)
-
-		itemPrice.Total = int64(itemPrice.NetTotal + itemPrice.Taxes)
+		itemPrice := calculateAmountsForSingleItem(settings, lineLogger, jwtClaims, params, item, 1)
 
 		lineLogger.WithFields(
 			logrus.Fields{
@@ -233,11 +238,13 @@ func CalculatePrice(settings *Settings, jwtClaims map[string]interface{}, params
 
 		price.Items = append(price.Items, itemPrice)
 
-		price.Subtotal += (itemPrice.Subtotal * itemPrice.Quantity)
-		price.Discount += (itemPrice.Discount * itemPrice.Quantity)
-		price.NetTotal += (itemPrice.NetTotal * itemPrice.Quantity)
-		price.Taxes += (itemPrice.Taxes * itemPrice.Quantity)
-		price.Total += (itemPrice.Total * int64(itemPrice.Quantity))
+		// avoid issues with rounding when multiplying by quantity before taxation
+		itemPriceMultiple := calculateAmountsForSingleItem(settings, lineLogger, jwtClaims, params, item, item.GetQuantity())
+		price.Subtotal += itemPriceMultiple.Subtotal
+		price.Discount += itemPriceMultiple.Discount
+		price.NetTotal += itemPriceMultiple.NetTotal
+		price.Taxes += itemPriceMultiple.Taxes
+		price.Total += itemPriceMultiple.Total
 	}
 
 	price.Total = int64(price.NetTotal + price.Taxes)
@@ -325,43 +332,6 @@ const (
 	fracMask = 1<<shift - 1
 )
 
-// Round returns the nearest integer, rounding half away from zero.
-//
-// Special cases are:
-//	Round(±0) = ±0
-//	Round(±Inf) = ±Inf
-//	Round(NaN) = NaN
-func Round(x float64) float64 {
-	// Round is a faster implementation of:
-	//
-	// func Round(x float64) float64 {
-	//   t := Trunc(x)
-	//   if Abs(x-t) >= 0.5 {
-	//     return t + Copysign(1, x)
-	//   }
-	//   return t
-	// }
-	bits := math.Float64bits(x)
-	e := uint(bits>>shift) & mask
-	if e < bias {
-		// Round abs(x) < 1 including denormals.
-		bits &= signMask // +-0
-		if e == bias-1 {
-			bits |= uvone // +-1
-		}
-	} else if e < bias+shift {
-		// Round any abs(x) >= 1 containing a fractional component [0,1).
-		//
-		// Numbers with larger exponents are returned unchanged since they
-		// must be either an integer, infinity, or NaN.
-		const half = 1 << (shift - 1)
-		e -= bias
-		bits += half >> e
-		bits &^= fracMask >> e
-	}
-	return math.Float64frombits(bits)
-}
-
 func rint(x float64) uint64 {
-	return uint64(Round(x))
+	return uint64(math.Round(x))
 }

calculator/calculator_test.go

@@ -208,15 +208,11 @@ func TestCouponWithVATWhenPRiceIncludeTaxesWithQuantity(t *testing.T) {
 	params := PriceParameters{"USA", "USD", coupon, []Item{&TestItem{quantity: 2, price: 100, itemType: "test", vat: 9}}}
 	price := CalculatePrice(settings, nil, params, testLogger)
 
-	// todo: This result is wrong because a rounding inaccuracy is quantified
-	// Therefore the tax amount is not 9% of the net total
-	// Correct net total: 165
-	// Correct tax amount: 15
 	validatePrice(t, price, Price{
-		Subtotal: 184,
+		Subtotal: 183,
 		Discount: 20,
-		NetTotal: 166,
-		Taxes:    14,
+		NetTotal: 165,
+		Taxes:    15,
 		Total:    180,
 	})
 }