Jorijn van der Graaf
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322 lines
15 KiB
C++
322 lines
15 KiB
C++
/*
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catcrafts.net
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Copyright (C) 2026 Catcrafts
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The source code of this website is made available for viewing purposes only.
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No permission is granted to copy, modify, distribute, or create derivative works.
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*/
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// Money and VAT arithmetic, in integer minor units. No floats, ever: a double
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// cannot represent 0.01 exactly, and a price that drifts by a cent between the
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// page, the payment request and the invoice is a bookkeeping bug you find at
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// tax time. Everything here is exact integer math with explicit rounding.
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//
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// Lives in Catcrafts.Shared (imports std only) so the same arithmetic renders
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// the price on the page and computes the amount actually charged — one
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// function, so they cannot disagree.
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export module Catcrafts.Shared:Money;
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import std;
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namespace Catcrafts::Money {
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// NL standard VAT rate, in basis points. Prices are stored VAT-inclusive (EU
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// Price Indication Directive: consumers must see the final price), and the net
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// is derived — not the other way round — so the advertised number is exact and
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// the derived one takes the rounding.
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export inline constexpr std::int64_t kVatRateBp = 2100;
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// Net (ex-VAT) amount from a VAT-inclusive gross, rounding half up on the
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// division. gross = net * (1 + rate) exactly when working in real numbers;
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// in minor units the net absorbs the sub-cent remainder.
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export constexpr std::int64_t NetFromGross(std::int64_t grossMinor,
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std::int64_t rateBp = kVatRateBp) {
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// net = gross * 10000 / (10000 + rate), rounded half up.
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const std::int64_t denom = 10000 + rateBp;
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return (grossMinor * 10000 + denom / 2) / denom;
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}
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// The other direction: the VAT-inclusive price that NETS a given ex-VAT cost.
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// This is how "cost plus, eat nothing" survives VAT: a carrier rate of €7.13
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// ex VAT must be charged as €8.63 inclusive, or the remitted VAT comes out of
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// the margin. Half-up like its sibling, and the pair round-trips (GrossFromNet
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// then NetFromGross returns the original cost).
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export constexpr std::int64_t GrossFromNet(std::int64_t netMinor,
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std::int64_t rateBp = kVatRateBp) {
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return (netMinor * (10000 + rateBp) + 5000) / 10000;
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}
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// "580.00" — the wire format both payment providers quote amounts in, and the
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// unambiguous way to show cents. Always two decimals, no thousands separator.
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export std::string FormatMinor(std::int64_t minor) {
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const bool neg = minor < 0;
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if (neg) minor = -minor;
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return std::format("{}{}.{:02}", neg ? "-" : "", minor / 100, minor % 100);
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}
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// Display form: "€580" when the cents are zero, "€479.34" otherwise. Whole
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// prices are chosen deliberately (no .99 games), so showing ".00" everywhere
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// would just add noise to the number that matters.
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export std::string FormatEuro(std::int64_t minor) {
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if (minor % 100 == 0 && minor >= 0) return std::format("€{}", minor / 100);
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return "€" + FormatMinor(minor);
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}
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// EU membership decides VAT treatment: inside the EU the price is charged
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// VAT-inclusive; outside, the sale is a zero-rated export and the buyer's own
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// customs channel collects import VAT and duty. ISO 3166-1 alpha-2, uppercase.
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//
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// Note NIR/GB: the UK left; Northern Ireland's special goods status is not
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// modelled — GB is simply non-EU here, which is the correct default for a
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// consumer parcel.
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export std::span<const std::string_view> EuCountries() {
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static constexpr std::array<std::string_view, 27> eu{
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"AT", "BE", "BG", "HR", "CY", "CZ", "DE", "DK", "EE", "ES", "FI",
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"FR", "GR", "HU", "IE", "IT", "LT", "LU", "LV", "MT", "NL", "PL",
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"PT", "RO", "SE", "SI", "SK",
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};
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return eu;
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}
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export bool IsEuCountry(std::string_view cc) {
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return std::ranges::find(EuCountries(), cc) != EuCountries().end();
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}
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// Destinations this shop refuses outright.
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//
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// Not a carrier problem — parcels reach both fine, and the rate tables price
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// them. It is an insurance boundary: liability cover for a Dutch shop is
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// written "worldwide excluding USA/Canada", and a phone carrying a lithium
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// battery and a replaced OS is precisely the product-liability exposure that
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// exclusion exists for. A sale into either country would therefore be
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// uninsured, with the buyer's own courts deciding the damages, so checkout
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// declines it instead of pricing it. Declining also keeps Catcrafts from
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// having marketed into those jurisdictions at all, which is the other half of
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// why the answer is no rather than a surcharge.
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export std::span<const std::string_view> NoSaleCountries() {
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static constexpr std::array<std::string_view, 2> blocked{ "US", "CA" };
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return blocked;
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}
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// Destinations the law forbids, as opposed to the insurance choice above.
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//
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// EU sanctions — Regulation 833/2014 for Russia, its Belarus mirror, and the
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// North Korea embargo — prohibit exporting consumer electronics to these
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// countries, by customs code and by the luxury-goods value threshold both, and
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// the prohibition covers indirect routes (a forwarder, a reseller) as much as
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// a direct parcel. That binds every EU seller as criminal law; there is no
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// small-shop exemption and no surcharge version of compliance. A separate list
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// rather than more entries in NoSaleCountries because the refusal needs
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// different words: "does not" is a choice, "cannot" is the law, and each gets
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// its own explanation on the terms page.
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export std::span<const std::string_view> SanctionedCountries() {
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static constexpr std::array<std::string_view, 3> blocked{ "RU", "BY", "KP" };
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return blocked;
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}
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export bool IsSanctioned(std::string_view cc) {
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return std::ranges::find(SanctionedCountries(), cc) != SanctionedCountries().end();
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}
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// ISO 3166-1 alpha-2, uppercase, like everything else here. Callers ask this
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// rather than comparing against "US" themselves, so the policy has exactly one
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// definition and adding a country later is a one-line change. Both lists deny:
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// most callers only need "is this destination for sale", and only the checkout
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// error message cares which refusal it is (IsSanctioned above).
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export bool SellsTo(std::string_view cc) {
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return !IsSanctioned(cc) &&
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std::ranges::find(NoSaleCountries(), cc) == NoSaleCountries().end();
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}
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// Delivery-time tiers. NOT a price concept — every rate comes from the carrier
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// (see ShipBracket below). This exists because Sendcloud's method list carries
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// no transit estimate, so the "1-2 / 2-5 / 5-14 days" the listing publishes is
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// ours to state, and distance is the only thing it can reasonably key on.
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export enum class Zone { Nl, Eu, World };
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export Zone ZoneFor(std::string_view cc) {
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if (cc == "NL") return Zone::Nl;
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return IsEuCountry(cc) ? Zone::Eu : Zone::World;
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}
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// ── carrier rates ─────────────────────────────────────────────────────
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//
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// One weight bracket of one carrier method: what a parcel up to
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// `maxWeightGrams` costs to this country, in cents, already grossed up to the
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// consumer price (the server does that once, when it builds the table).
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//
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// Brackets exist because a carrier prices by weight, and Sendcloud lists the
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// same service once per band — so a country's rates arrive as a ladder, not a
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// single number. Nothing here is hardcoded: an empty ladder means the shop
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// cannot ship there, which is a refusal, not a fallback.
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export struct ShipBracket {
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std::int64_t maxWeightGrams = 0;
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std::int64_t minor = 0;
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};
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// One destination's ladder. Kept as a flat vector of these rather than a map
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// so the table is trivially serialisable and the order the carrier gave is
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// preserved.
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export struct ShipRates {
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std::string cc;
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std::vector<ShipBracket> brackets;
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};
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// The rate for a parcel of `grams` to a destination whose ladder this is, or 0
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// when nothing covers it — too heavy, or no rates at all.
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//
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// The cheapest bracket that can CARRY the weight wins, which is not always the
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// tightest one: a carrier's 20 kg band is occasionally priced below its 10 kg
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// band, and quoting the higher of the two would overcharge for a parcel both
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// accept. A band always accepts a parcel lighter than its maximum, so this
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// stays bookable at whatever it quotes.
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export std::int64_t RateFor(std::span<const ShipBracket> ladder, std::int64_t grams) {
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std::int64_t best = 0;
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for (const ShipBracket& b : ladder) {
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if (b.maxWeightGrams < grams) continue;
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if (best == 0 || b.minor < best) best = b.minor;
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}
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return best;
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}
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// How many units of `unitGrams` fit the heaviest bracket this destination has.
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// The quantity ceiling the buy form offers and the checkout enforces: one
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// order is one parcel, so anything above this is an email conversation rather
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// than a quote the shop cannot honour.
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export std::int64_t MaxUnitsFor(std::span<const ShipBracket> ladder,
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std::int64_t unitGrams) {
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if (unitGrams <= 0) return 0;
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std::int64_t heaviest = 0;
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for (const ShipBracket& b : ladder) heaviest = std::max(heaviest, b.maxWeightGrams);
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return heaviest / unitGrams;
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}
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// Ladder lookup across a whole table. Linear because the table is one entry
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// per country the method covers — a couple of hundred at most, walked once per
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// checkout.
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export std::span<const ShipBracket> LadderFor(std::span<const ShipRates> table,
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std::string_view cc) {
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for (const ShipRates& r : table) {
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if (r.cc == cc) return r.brackets;
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}
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return {};
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}
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// One order's money, fully derived. `goods` is what the buyer pays for the
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// device: the VAT-inclusive price inside the EU, the derived net outside it.
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// `vatCharged` is what the total contains in Dutch VAT — zero for exports —
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// kept because the invoice needs it, not because the page shows it.
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export struct Totals {
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std::int64_t goods = 0;
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std::int64_t shipping = 0;
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std::int64_t total = 0;
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std::int64_t vatCharged = 0;
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bool vatIncluded = false; // true when `goods` includes EU VAT
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};
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// The single authority on what an order costs. The checkout handler calls this
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// with the buyer's country; nothing about the amount ever comes from the
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// client. `shippingMinor` arrives already resolved from the carrier table (see
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// RateFor) — an order with no carrier rate is refused before it gets here, so
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// this function never has to invent a price and stays pure.
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//
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// The export net is derived from the LINE total (unit × qty), not per unit —
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// rounding per line is the invoice-correct convention, and it is also the
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// formula the checkout preview script mirrors, so the preview and the charge
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// cannot drift by a cent.
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export Totals ComputeTotals(std::int64_t unitGrossMinor, std::int64_t quantity,
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std::int64_t shippingMinor,
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std::string_view country) {
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Totals t;
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t.shipping = shippingMinor;
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const std::int64_t lineGross = unitGrossMinor * quantity;
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if (IsEuCountry(country)) {
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t.goods = lineGross;
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t.vatIncluded = true;
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// VAT applies to the shipping too — it is part of the taxable supply.
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const std::int64_t taxable = t.goods + t.shipping;
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t.vatCharged = taxable - NetFromGross(taxable);
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} else {
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t.goods = NetFromGross(lineGross);
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t.vatIncluded = false;
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t.vatCharged = 0;
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}
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t.total = t.goods + t.shipping;
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return t;
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}
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// ── indicative currency display ───────────────────────────────────────
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//
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// Orders are charged in euros, always — both rails collect EUR (crypto is
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// accepted in EURC, a euro stablecoin, so the token amount IS the euro
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// amount) and the invoice is EUR. But a Canadian reading "€614" has to do mental arithmetic to know what
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// their card will actually take, so the order page also shows an INDICATIVE
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// conversion in the buyer's national currency, from ECB reference rates baked
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// in at build time. Indicative is the whole contract: the buyer's bank sets
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// the real conversion rate, and the page says so next to the number.
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export struct Currency {
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std::string_view code; // ISO 4217
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std::string_view symbol; // display prefix, e.g. "CA$"
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};
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// One supported non-euro display currency and its representative country.
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// `cc` matters beyond lookup: IsEuCountry(cc) decides which euro amount a
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// conversion starts from — an EU member's currency (SEK, PLN, …) converts the
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// VAT-inclusive price, everyone else's converts the ex-VAT export price.
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export struct CurrencyRow {
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std::string_view cc;
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Currency cur;
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};
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// Only currencies the ECB publishes reference rates for; anywhere else shows
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// plain euros. Euro countries are deliberately absent — converting EUR to EUR
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// is noise. So are USD and CAD: NoSaleCountries means no order can ever be
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// charged from those countries, and quoting a visitor a friendly price in
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// their own currency before refusing them at checkout is both a worse
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// experience and the kind of localisation that reads as marketing there.
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export std::span<const CurrencyRow> AllCurrencies() {
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static constexpr std::array<CurrencyRow, 14> rows{{
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{ "GB", { "GBP", "£" } },
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{ "CH", { "CHF", "CHF " } },
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{ "NO", { "NOK", "kr " } },
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{ "SE", { "SEK", "kr " } },
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{ "DK", { "DKK", "kr " } },
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{ "PL", { "PLN", "zł " } },
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{ "CZ", { "CZK", "Kč " } },
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{ "HU", { "HUF", "Ft " } },
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{ "RO", { "RON", "lei " } },
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{ "BG", { "BGN", "лв " } },
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{ "AU", { "AUD", "A$" } },
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{ "NZ", { "NZD", "NZ$" } },
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{ "JP", { "JPY", "¥" } },
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{ "IS", { "ISK", "kr " } },
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}};
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return rows;
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}
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// Currency for a destination country, or nullopt for euro countries and
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// anywhere unsupported.
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export std::optional<Currency> CurrencyFor(std::string_view cc) {
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for (const CurrencyRow& r : AllCurrencies()) {
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if (r.cc == cc) return r.cur;
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}
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return std::nullopt;
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}
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// Convert cents-EUR to WHOLE units of the target currency, half-up. Whole
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// units on purpose: a number that is explicitly approximate should not carry
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// two decimals of false precision. `rateMicro` is target-per-euro in millionths
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// (1 EUR = 1.0834 USD -> 1'083'400).
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export constexpr std::int64_t ConvertIndicative(std::int64_t minorEur,
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std::int64_t rateMicro) {
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// units = minorEur/100 * rateMicro/1e6, rounded half up.
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return (minorEur * rateMicro + 50'000'000) / 100'000'000;
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}
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// "≈ CA$920" — the display form of an indicative conversion.
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export std::string FormatIndicative(const Currency& cur, std::int64_t wholeUnits) {
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return std::format("≈ {}{}", cur.symbol, wholeUnits);
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}
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} // namespace Catcrafts::Money
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