Refining rounding for EN16931 in line discounts and charges

CHANGELOG.md

@@ -6,10 +6,15 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/) and this p
 
 ## [Unreleased]
 
+### Added
+
+- `bill`: Line discount and charge `base` property, to use instead of the line sum in order to comply with EN16931.
+
 ### Changed
 
 - `bill`: line totals will be rounded to currency precision for presentation only
-- `bill`: Discount and Charge amounts always rounded to currency's precision
+- `bill`: document Discount and Charge base and amounts always rounded to currency's precision
+- `bill`: line Discount and Charge base and amounts always rounded to currency's precision
 - `bill`: `round-then-sum` rounding rule _now_ implies that line totals will not be calculated with additional precision, this brings closer aliance with EN16931 requirements.
 
 ## [v0.211.0] - 2025-02-28

bill/invoice_test.go

@@ -211,7 +211,7 @@ func TestRemoveIncludedTax(t *testing.T) {
 	assert.Equal(t, "826.4463", l0.Item.Price.String())
 	assert.Equal(t, "826.4463", l0.Sum.String())
 	assert.Equal(t, "826.4463", l0.Sum.String())
-	assert.Equal(t, "82.6446", l0.Discounts[0].Amount.String())
+	assert.Equal(t, "82.64", l0.Discounts[0].Amount.String())
 	assert.Equal(t, "743.80", l0.Total.String())
 
 	assert.Equal(t, "743.80", i2.Totals.Sum.String())
@@ -589,7 +589,7 @@ func TestRemoveIncludedTaxQuantity(t *testing.T) {
 	l0 := i2.Lines[0]
 	assert.Equal(t, "8.2645", l0.Item.Price.String())
 	assert.Equal(t, "826.4500", l0.Sum.String())
-	assert.Equal(t, "82.6450", l0.Discounts[0].Amount.String())
+	assert.Equal(t, "82.65", l0.Discounts[0].Amount.String())
 	assert.Equal(t, "743.81", l0.Total.String())
 	assert.Equal(t, "10.00", i.Lines[0].Item.Price.String())
 

bill/line_calculate.go

@@ -100,9 +100,9 @@ func calculateLine(l *Line, cur currency.Code, rates []*currency.ExchangeRate, r
 
 	// Calculate the line sum and total
 	sum := price.Multiply(l.Quantity)
-	total := sum
-	total = calculateLineDiscounts(l.Discounts, *l.Item.Price, sum, total, cur, rr)
-	total = calculateLineCharges(l.Charges, *l.Item.Price, sum, total, cur, rr)
+	total := tax.ApplyRoundingRule(rr, cur, sum)
+	total = calculateLineDiscounts(l.Discounts, sum, total, cur)
+	total = calculateLineCharges(l.Charges, sum, total, cur)
 
 	// Rescale the final sum to match item's price
 	sum = sum.Rescale(l.Item.Price.Exp())
@@ -114,7 +114,9 @@ func calculateLine(l *Line, cur currency.Code, rates []*currency.ExchangeRate, r
 	return nil
 }
 
-// calculate figures out the totals according to quantity and discounts.
+// calculateSubline figures out the totals according to quantity and discounts.
+// We don't apply rounding rules here, as the objective is to have
+// maximum precision to determine the final line item price.
 func calculateSubLine(sl *SubLine, cur currency.Code, rates []*currency.ExchangeRate) error {
 	if sl.Item == nil {
 		return nil
@@ -139,8 +141,8 @@ func calculateSubLine(sl *SubLine, cur currency.Code, rates []*currency.Exchange
 	// Calculate the line sum and total
 	sum := price.Multiply(sl.Quantity)
 	total := sum
-	total = calculateLineDiscounts(sl.Discounts, *sl.Item.Price, sum, total, cur, tax.RoundingRuleSumThenRound)
-	total = calculateLineCharges(sl.Charges, *sl.Item.Price, sum, total, cur, tax.RoundingRuleSumThenRound)
+	total = calculateLineDiscounts(sl.Discounts, sum, total, cur)
+	total = calculateLineCharges(sl.Charges, sum, total, cur)
 
 	// Rescale the final sum and total
 	sl.total = total
@@ -152,26 +154,40 @@ func calculateSubLine(sl *SubLine, cur currency.Code, rates []*currency.Exchange
 	return nil
 }
 
-func calculateLineDiscounts(discounts []*LineDiscount, price, sum, total num.Amount, cur currency.Code, rr cbc.Key) num.Amount {
+func calculateLineDiscounts(discounts []*LineDiscount, sum, total num.Amount, cur currency.Code) num.Amount {
+	cd := cur.Def()
 	for _, d := range discounts {
 		if d.Percent != nil && !d.Percent.IsZero() {
-			d.Amount = d.Percent.Of(sum) // always override
+			base := sum
+			if d.Base != nil {
+				base = cd.Rescale(*d.Base)
+				d.Base = &base
+			}
+			d.Amount = d.Percent.Of(base) // always override
 		}
-		d.Amount = d.Amount.MatchPrecision(price)
-		d.Amount = tax.ApplyRoundingRule(rr, cur, d.Amount)
 		total = total.Subtract(d.Amount)
+		// As per EN16931 specs, discount amounts have same number of
+		// decimal places as the currency.
+		d.Amount = cd.Rescale(d.Amount)
 	}
 	return total
 }
 
-func calculateLineCharges(charges []*LineCharge, price, sum, total num.Amount, cur currency.Code, rr cbc.Key) num.Amount {
+func calculateLineCharges(charges []*LineCharge, sum, total num.Amount, cur currency.Code) num.Amount {
+	cd := cur.Def()
 	for _, c := range charges {
 		if c.Percent != nil && !c.Percent.IsZero() {
+			base := sum
+			if c.Base != nil {
+				base = cd.Rescale(*c.Base)
+				c.Base = &base
+			}
 			c.Amount = c.Percent.Of(sum) // always override
 		}
-		c.Amount = c.Amount.MatchPrecision(price)
-		c.Amount = tax.ApplyRoundingRule(rr, cur, c.Amount)
 		total = total.Add(c.Amount)
+		// As per EN16931 specs, charge amounts have same number of
+		// decimal places as the currency.
+		c.Amount = cd.Rescale(c.Amount)
 	}
 	return total
 }

bill/line_calculate_test.go

@@ -286,7 +286,7 @@ func TestLineCalculate(t *testing.T) {
 		sum := calculateLineSum(lines, currency.EUR)
 		assert.Equal(t, "35.1356", sum.String())
 		assert.Equal(t, "37.7802", lines[0].Sum.String())
-		assert.Equal(t, "2.6446", lines[0].Discounts[0].Amount.String())
+		assert.Equal(t, "2.64", lines[0].Discounts[0].Amount.String())
 		assert.Equal(t, "35.1356", lines[0].total.String())
 		assert.Equal(t, "35.14", lines[0].Total.String())
 	})
@@ -312,4 +312,55 @@ func TestLineCalculate(t *testing.T) {
 		assert.Equal(t, "35.13", lines[0].Total.String())
 		assert.Equal(t, "35.1261", lines[0].total.String())
 	})
+
+	t.Run("lines with discount base", func(t *testing.T) {
+		lines := []*Line{
+			{
+				Quantity: num.MakeAmount(3, 0),
+				Item: &org.Item{
+					Name:  "Test Item",
+					Price: num.NewAmount(1259, 2),
+				},
+				Discounts: []*LineDiscount{
+					{
+						Base:    num.NewAmount(51256, 3),
+						Percent: num.NewPercentage(7, 2),
+					},
+				},
+			},
+		}
+		err := calculateLines(lines, currency.EUR, nil, tax.RoundingRuleSumThenRound)
+		assert.NoError(t, err)
+		sum := calculateLineSum(lines, currency.EUR)
+		assert.Equal(t, "34.1800", sum.String())
+		assert.Equal(t, "51.26", lines[0].Discounts[0].Base.String())
+		assert.Equal(t, "3.59", lines[0].Discounts[0].Amount.String())
+		assert.Equal(t, "34.18", lines[0].Total.String())
+		assert.Equal(t, "34.1800", lines[0].total.String())
+	})
+
+	t.Run("lines with charge base", func(t *testing.T) {
+		lines := []*Line{
+			{
+				Quantity: num.MakeAmount(3, 0),
+				Item: &org.Item{
+					Name:  "Test Item",
+					Price: num.NewAmount(1259, 2),
+				},
+				Charges: []*LineCharge{
+					{
+						Base:    num.NewAmount(512, 2),
+						Percent: num.NewPercentage(7, 2),
+					},
+				},
+			},
+		}
+		err := calculateLines(lines, currency.EUR, nil, tax.RoundingRuleSumThenRound)
+		assert.NoError(t, err)
+		sum := calculateLineSum(lines, currency.EUR)
+		assert.Equal(t, "40.4139", sum.String())
+		assert.Equal(t, "2.64", lines[0].Charges[0].Amount.String())
+		assert.Equal(t, "40.41", lines[0].Total.String())
+		assert.Equal(t, "40.4139", lines[0].total.String())
+	})
 }

bill/line_charge.go

@@ -19,7 +19,9 @@ type LineCharge struct {
 	Code cbc.Code `json:"code,omitempty" jsonschema:"title=Code"`
 	// Text description as to why the charge was applied
 	Reason string `json:"reason,omitempty" jsonschema:"title=Reason"`
-	// Percentage if fixed amount not applied
+	// Base for percent calculations instead of the line's sum
+	Base *num.Amount `json:"base,omitempty" jsonschema:"title=Base"`
+	// Percentage of base or parent line's sum
 	Percent *num.Percentage `json:"percent,omitempty" jsonschema:"title=Percent"`
 	// Fixed or resulting charge amount to apply (calculated if percent present).
 	Amount num.Amount `json:"amount" jsonschema:"title=Amount" jsonschema_extras:"calculated=true"`
@@ -40,7 +42,13 @@ func (lc *LineCharge) Validate() error {
 	return validation.ValidateStruct(lc,
 		validation.Field(&lc.Key),
 		validation.Field(&lc.Code),
-		validation.Field(&lc.Percent),
+		validation.Field(&lc.Base),
+		validation.Field(&lc.Percent,
+			validation.When(
+				lc.Base != nil,
+				validation.Required,
+			),
+		),
 		validation.Field(&lc.Amount, validation.Required, num.NotZero),
 		validation.Field(&lc.Ext),
 	)

bill/line_discount.go

@@ -17,7 +17,9 @@ type LineDiscount struct {
 	Code cbc.Code `json:"code,omitempty" jsonschema:"title=Code"`
 	// Text description as to why the discount was applied
 	Reason string `json:"reason,omitempty" jsonschema:"title=Reason"`
-	// Percentage to apply to the line total to calcaulte the discount amount
+	// Base for percent calculations instead of the line's sum.
+	Base *num.Amount `json:"base,omitempty" jsonschema:"title=Base"`
+	// Percentage to apply to the base or line sum to calculate the discount amount
 	Percent *num.Percentage `json:"percent,omitempty" jsonschema:"title=Percent"`
 	// Fixed discount amount to apply (calculated if percent present)
 	Amount num.Amount `json:"amount" jsonschema:"title=Amount" jsonschema_extras:"calculated=true"`
@@ -38,7 +40,13 @@ func (ld *LineDiscount) Validate() error {
 	return validation.ValidateStruct(ld,
 		validation.Field(&ld.Key),
 		validation.Field(&ld.Code),
-		validation.Field(&ld.Percent),
+		validation.Field(&ld.Base),
+		validation.Field(&ld.Percent,
+			validation.When(
+				ld.Base != nil,
+				validation.Required,
+			),
+		),
 		validation.Field(&ld.Amount, validation.Required, num.NotZero),
 		validation.Field(&ld.Ext),
 	)

cmd/gobl/testdata/Test_build_explicit_stdout

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "7a28e3ac52dc87a13f13ad9af168f9d37379c6795b082a700d9c462a176c8e93"
+			"val": "87d4a01626fd62e7d19ecefc072fbd966ae1837e9d81e84eb68d0d452f58bb68"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [

cmd/gobl/testdata/Test_build_input_file

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "7a28e3ac52dc87a13f13ad9af168f9d37379c6795b082a700d9c462a176c8e93"
+			"val": "87d4a01626fd62e7d19ecefc072fbd966ae1837e9d81e84eb68d0d452f58bb68"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [

cmd/gobl/testdata/Test_build_merge_values

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "aa4ce3a998bd7a6523730ae5e08e1c791f504121263499e51598499e5a0f6ff6"
+			"val": "1f2afbcbdda51a5fc5b81300a406d27cf31f48f6a6ecd269b2cb8810f42cf1a2"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [

cmd/gobl/testdata/Test_build_output_file_outfile

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "7a28e3ac52dc87a13f13ad9af168f9d37379c6795b082a700d9c462a176c8e93"
+			"val": "87d4a01626fd62e7d19ecefc072fbd966ae1837e9d81e84eb68d0d452f58bb68"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [

cmd/gobl/testdata/Test_build_overwrite_output_file_outfile

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "7a28e3ac52dc87a13f13ad9af168f9d37379c6795b082a700d9c462a176c8e93"
+			"val": "87d4a01626fd62e7d19ecefc072fbd966ae1837e9d81e84eb68d0d452f58bb68"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [

cmd/gobl/testdata/Test_sign_explicit_stdout

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "7a28e3ac52dc87a13f13ad9af168f9d37379c6795b082a700d9c462a176c8e93"
+			"val": "87d4a01626fd62e7d19ecefc072fbd966ae1837e9d81e84eb68d0d452f58bb68"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [

cmd/gobl/testdata/Test_sign_input_file

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "7a28e3ac52dc87a13f13ad9af168f9d37379c6795b082a700d9c462a176c8e93"
+			"val": "87d4a01626fd62e7d19ecefc072fbd966ae1837e9d81e84eb68d0d452f58bb68"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [

cmd/gobl/testdata/Test_sign_merge_values

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "63a34456e8508328c8706544554b79a1ec37fb5bbcc86797f58f232796565784"
+			"val": "2dff8279b81655599c7a82b8991c68e93cf327f152720cb74ae57b46a4927b40"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [

cmd/gobl/testdata/Test_sign_output_file_outfile

@@ -4,7 +4,7 @@
 		"uuid": "4905f174-0384-11ed-9fa6-b24117999d50",
 		"dig": {
 			"alg": "sha256",
-			"val": "7a28e3ac52dc87a13f13ad9af168f9d37379c6795b082a700d9c462a176c8e93"
+			"val": "87d4a01626fd62e7d19ecefc072fbd966ae1837e9d81e84eb68d0d452f58bb68"
 		}
 	},
 	"doc": {
@@ -67,7 +67,7 @@
 					{
 						"reason": "Special discount",
 						"percent": "10%",
-						"amount": "180.0000"
+						"amount": "180.00"
 					}
 				],
 				"taxes": [