platform/tax/price.go

package tax

import (
	"encoding/json"
	"strconv"

	"git.k6n.net/mats/platform/money"
)

// Price is a monetary amount in minor units (money.Cents) with its VAT broken
// down by rate. IncVat is the gross total; VatRates maps each tax rate — basis
// points of a percent, 2500 = 25%, 1250 = 12.5% (the single platform scale,
// matching OrderLine.tax_rate) — to its VAT amount in minor units.
//
// All arithmetic is integer (money.Cents is int64-backed); VAT is only ever
// computed by Compute (ex-VAT-first), always on a real total. Aggregation
// (Add/SumPrices) only sums integers, so totals never drift from the sum of
// their parts. The JSON wire shape is unchanged — Cents marshals as int64.
type Price struct {
	IncVat   money.Cents         `json:"incVat"`
	VatRates map[int]money.Cents `json:"vat,omitempty"`
}

// NewPrice returns a new zero Price.
func NewPrice() *Price {
	return &Price{
		IncVat:   0,
		VatRates: make(map[int]money.Cents),
	}
}

// NewPriceFromIncVat builds a Price from a gross total and a tax rate in basis
// points (2500 = 25%, 1250 = 12.5%). VAT is computed once via Compute, so
// exVat + vat == incVat exactly.
func NewPriceFromIncVat(incVat money.Cents, rateBp int) *Price {
	return &Price{
		IncVat: incVat,
		VatRates: map[int]money.Cents{
			rateBp: money.Cents(Compute(incVat.Int64(), rateBp)),
		},
	}
}

// ValueExVat returns the price excluding VAT.
func (p *Price) ValueExVat() money.Cents {
	exVat := p.IncVat
	for _, amount := range p.VatRates {
		exVat -= amount
	}
	return exVat
}

// TotalVat returns the total VAT amount.
func (p *Price) TotalVat() money.Cents {
	total := money.Cents(0)
	for _, amount := range p.VatRates {
		total += amount
	}
	return total
}

// MultiplyPrice scales a Price to a quantity and returns a new Price.
//
// For a single-rate price (a line item) it recomputes the VAT on the *scaled
// total* via NewPriceFromIncVat — not by multiplying the per-unit VAT — so the
// result matches a payment provider that re-derives tax from the line total, and
// VAT rounding never accumulates per unit. VAT is only ever computed on a real
// total.
//
// A zero-rate price scales its (untaxed) IncVat. A multi-rate price (which a line
// item never is — totals are built by Add, never Multiply) falls back to scaling
// each component, since IncVat can't be split across rates.
func MultiplyPrice(p Price, qty int64) *Price {
	if len(p.VatRates) == 1 {
		for rate := range p.VatRates {
			return NewPriceFromIncVat(p.IncVat.Mul(qty), rate)
		}
	}
	ret := &Price{
		IncVat:   p.IncVat.Mul(qty),
		VatRates: make(map[int]money.Cents, len(p.VatRates)),
	}
	for rate, amount := range p.VatRates {
		ret.VatRates[rate] = amount.Mul(qty)
	}
	return ret
}

// Multiply scales the Price by a quantity in place (see MultiplyPrice).
func (p *Price) Multiply(qty int64) {
	res := MultiplyPrice(*p, qty)
	p.IncVat = res.IncVat
	p.VatRates = res.VatRates
}

// MarshalJSON serializes Price as a wire-friendly object with exVat, incVat, and
// vat (rate-keyed by raw percent as a string, e.g. "25"). Cents marshals as
// int64, so the wire bytes are byte-identical to the pre-money.Cents shape.
func (p Price) MarshalJSON() ([]byte, error) {
	exVat := p.ValueExVat()
	var vat map[string]money.Cents
	if len(p.VatRates) > 0 {
		vat = make(map[string]money.Cents, len(p.VatRates))
		for rate, amount := range p.VatRates {
			vat[strconv.Itoa(rate)] = amount
		}
	}
	type wire struct {
		ExVat  money.Cents            `json:"exVat"`
		IncVat money.Cents            `json:"incVat"`
		Vat    map[string]money.Cents `json:"vat,omitempty"`
	}
	return json.Marshal(wire{ExVat: exVat, IncVat: p.IncVat, Vat: vat})
}

// UnmarshalJSON deserializes Price from its wire format.
func (p *Price) UnmarshalJSON(data []byte) error {
	type wire struct {
		ExVat  money.Cents            `json:"exVat"`
		IncVat money.Cents            `json:"incVat"`
		Vat    map[string]money.Cents `json:"vat,omitempty"`
	}
	var w wire
	if err := json.Unmarshal(data, &w); err != nil {
		return err
	}

	p.IncVat = w.IncVat
	p.VatRates = make(map[int]money.Cents, len(w.Vat))
	for rateStr, amount := range w.Vat {
		// Tolerate legacy float-style keys ("2500", "1250") by truncating to the
		// basis point integer convention.
		rate, err := strconv.Atoi(rateStr)
		if err != nil {
			f, ferr := strconv.ParseFloat(rateStr, 64)
			if ferr != nil {
				return err
			}
			rate = int(f)
		}
		p.VatRates[rate] = amount
	}
	return nil
}

// Add adds another Price's values to this one.
func (p *Price) Add(price Price) {
	p.IncVat += price.IncVat
	for rate, amount := range price.VatRates {
		p.VatRates[rate] += amount
	}
}

// Subtract subtracts another Price's values from this one.
func (p *Price) Subtract(price Price) {
	p.IncVat -= price.IncVat
	for rate, amount := range price.VatRates {
		p.VatRates[rate] -= amount
	}
}

// SumPrices aggregates multiple Prices into one.
func SumPrices(prices ...Price) *Price {
	aggregated := NewPrice()
	for _, price := range prices {
		aggregated.IncVat += price.IncVat
		for rate, amount := range price.VatRates {
			aggregated.VatRates[rate] += amount
		}
	}
	if len(aggregated.VatRates) == 0 {
		aggregated.VatRates = nil
	}
	return aggregated
}