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Fix rounding inaccuracies with quantity #156

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merged 2 commits into from
Nov 21, 2018
Merged

Fix rounding inaccuracies with quantity #156

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80aa35b
Remove homebrewed Round function since it is in standard since 1.10
mraerino Nov 2, 2018
61a8e7b
Fix rounding inaccuracies with quantity
mraerino Nov 2, 2018
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112 changes: 41 additions & 71 deletions calculator/calculator.go
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Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand All @@ -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{
Expand All @@ -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)
Expand Down Expand Up @@ -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))
}
10 changes: 3 additions & 7 deletions calculator/calculator_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
})
}
Expand Down