catcrafts.net/server/implementations/Catcrafts.Server-Transfer.cpp

/*
catcrafts.net
Copyright (C) 2026 Catcrafts

The source code of this website is made available for viewing purposes only.
No permission is granted to copy, modify, distribute, or create derivative works.
*/

// The bank-transfer rail: the buyer sends a plain SEPA transfer to our own
// IBAN quoting the order's creditor reference, and the order settles when a
// credit carrying that reference shows up on the account.
//
// Why this rail exists at all: after the bank payment provider closed the
// shop's account on 2026-08-20 with no appeal, the lesson taken was not "find
// a better provider" but "stop putting a party with unilateral offboarding
// power in the payment path". A transfer to our own account has no such party.
// The bank can still close the account — that is unavoidable, every euro has
// to land somewhere — but it cannot decline a payment method while leaving the
// business running, which is what actually happened.
//
// Shape: self-hosted, like the EURC rail and unlike the hosted ones. There is
// no checkout to redirect to, so CreateLink makes no network call at all and
// Instructions() is what the buyer actually acts on. Three facts go on the
// order page — IBAN, amount, reference — and the beneficiary name, which is
// load-bearing rather than decorative (see PayInstructions).
//
// Trust direction is the same rule as every other rail: an order becomes paid
// only when the rail's own authenticated read of the account says a covering
// credit arrived. Nothing the buyer tells us is evidence, including "I paid".
//
// The bank is behind a CreditSource so it is swappable. That is a deliberate
// hedge and not speculative generality: the account being reconciled is also
// the shop's own bank account, so the day that relationship ends, the rail
// must survive with one new adapter rather than a rewrite.

module;
module Catcrafts.Server;

import std;
import Catcrafts.Shared;

namespace Catcrafts::Server {

namespace {

// Reduce remittance text to the alphabet a reference lives in: upper-case
// letters and digits, everything else dropped. Banks are free to reformat the
// field — spacing, punctuation, case — and a payer types it by hand, so
// comparing raw strings would fail on cosmetics. Dropping separators is also
// what makes one needle match every form the payer might have used.
std::string Fold(std::string_view s) {
    std::string out;
    out.reserve(s.size());
    for (const char c : s) {
        if (c >= '0' && c <= '9') out += c;
        else if (c >= 'A' && c <= 'Z') out += c;
        else if (c >= 'a' && c <= 'z') out += static_cast<char>(c - 'a' + 'A');
    }
    return out;
}

// Minimal JSON string escaping for the credits file. The reference field is
// text a stranger typed into their banking app, so a raw quote or newline in
// it would corrupt the line-per-record format and silently truncate the
// evidence a settlement decision reads.
std::string EscT(std::string_view s) {
    std::string out;
    out.reserve(s.size() + 8);
    for (const char c : s) {
        switch (c) {
            case '"':  out += "\\\""; break;
            case '\\': out += "\\\\"; break;
            case '\n': out += "\\n";  break;
            case '\r': out += "\\r";  break;
            case '\t': out += "\\t";  break;
            default:
                if (static_cast<unsigned char>(c) < 0x20) {
                    out += std::format("\\u{:04x}", static_cast<unsigned char>(c));
                } else {
                    out += c;
                }
        }
    }
    return out;
}

// Does the folded remittance text carry a SECOND thing shaped like one of our
// references, beyond the one at `foundAt`? "CC" followed by six characters of
// the token alphabet is the shape. Used only to raise the advisory ambiguous
// flag: one transfer quoting two orders cannot be attributed per-order, and a
// human should look rather than two orders settling on the same money.
bool HasOtherReferenceShape(std::string_view folded, std::size_t foundAt,
                            std::size_t needleLen) {
    for (std::size_t i = 0; i + 8 <= folded.size(); ++i) {
        if (i >= foundAt && i < foundAt + needleLen) continue;   // the known one
        if (folded[i] != 'C' || folded[i + 1] != 'C') continue;
        bool hex = true;
        for (std::size_t j = i + 2; j < i + 8; ++j) {
            const char c = folded[j];
            const bool isHex = (c >= '0' && c <= '9') || (c >= 'A' && c <= 'F');
            if (!isHex) { hex = false; break; }
        }
        if (hex) return true;
    }
    return false;
}

} // namespace

TransferMatch MatchCredits(std::span<const BankCredit> credits,
                           std::string_view reference) {
    TransferMatch out;
    const std::string needle = Fold(reference);
    // An empty needle would match every credit on the account. Refuse rather
    // than settle the whole ledger from one payment.
    if (needle.size() < 4) return out;

    for (const BankCredit& c : credits) {
        // Only money coming IN can pay for something. A negative amount is an
        // outgoing payment that happens to quote the reference — a refund we
        // sent, most likely — and counting it would be a refund paying for
        // the order it refunded.
        if (c.amountMinor <= 0) continue;
        const std::string folded = Fold(c.reference);
        const std::size_t at = folded.find(needle);
        if (at == std::string::npos) continue;
        out.paidMinor += c.amountMinor;
        ++out.count;
        out.method = c.method;
        if (HasOtherReferenceShape(folded, at, needle.size())) out.ambiguous = true;
    }
    return out;
}

namespace {

class FileCreditSource final : public CreditSource {
public:
    explicit FileCreditSource(std::filesystem::path path) : path_(std::move(path)) {}

    std::optional<std::vector<BankCredit>> Recent() override {
        std::error_code ec;
        if (!std::filesystem::exists(path_, ec)) {
            // Not an error: a shop that has taken no transfers yet has no
            // file. Distinct from a read failure below, which IS unknown.
            return std::vector<BankCredit>{};
        }
        std::ifstream in(path_, std::ios::binary);
        if (!in) {
            std::println(std::cerr, "transfer: cannot read credits file {}",
                         path_.string());
            return std::nullopt;
        }
        std::vector<BankCredit> out;
        std::string line;
        int lineNo = 0;
        while (std::getline(in, line)) {
            ++lineNo;
            if (line.empty()) continue;
            const auto doc = Json::Parse(line);
            if (!doc || !doc->IsObject()) {
                // One malformed line must not silently shrink the evidence a
                // settlement decision rests on, so say so and keep the rest:
                // dropping the whole file would strand every paid order.
                std::println(std::cerr, "transfer: {}:{} is not a JSON object, skipped",
                             path_.string(), lineNo);
                continue;
            }
            BankCredit c;
            c.id = std::string(doc->Str("id"));
            c.reference = std::string(doc->Str("reference"));
            c.method = std::string(doc->Str("method"));
            if (c.method.empty()) c.method = "sepa";
            c.amountMinor = doc->Int("amount_minor");
            out.push_back(std::move(c));
        }
        return out;
    }

    std::string_view Name() const override { return "file"; }

private:
    std::filesystem::path path_;
};

class TransferRail final : public PaymentRail {
public:
    TransferRail(RailConfig cfg, std::unique_ptr<CreditSource> credits)
        : cfg_(std::move(cfg)), credits_(std::move(credits)) {}

    // No network call, and nothing to reserve: unlike the EURC rail, which
    // burns a receiving address per order, a transfer reuses one IBAN forever
    // and the reference is what separates orders. So this cannot fail, which
    // is worth noticing — checkout can never lose a sale to a provider being
    // down, because there is no provider.
    std::optional<PaymentLink> CreateLink(std::int64_t,
                                          const std::string& description,
                                          const std::string& redirectUrl) override {
        PaymentLink link;
        // payId carries the reference AND the deadline, the same trick the
        // EURC rail uses: CheckPaid is given only the id and the amount, and
        // this rail has to know its own window. Both halves are wanted in the
        // ledger anyway.
        const std::int64_t deadline = NowUnix() + WindowSeconds();
        link.payId = std::format("{}@{}", ReferenceOf(description), deadline);
        // Self-hosted: there is nowhere to send the buyer but the order page,
        // which is where Instructions() renders.
        link.payUrl = redirectUrl;
        return link;
    }

    std::optional<PaidStatus> CheckPaid(const std::string& payId,
                                        std::int64_t expectedMinor) override {
        const auto [reference, deadline] = SplitPayId(payId);
        if (reference.empty()) {
            // Not one of ours. This is the shape a ledger line from the OLD
            // hosted provider has, and this rail genuinely cannot decide it:
            // Pending lets the reconciler age it out instead of pretending to
            // know it is dead.
            return PaidStatus{ PayState::Pending, {} };
        }

        const std::optional<std::vector<BankCredit>> all = Fetch();
        if (!all) return std::nullopt;   // unknown, retry — never "unpaid"

        const TransferMatch m = MatchCredits(*all, reference);
        if (m.ambiguous) {
            std::println(std::cerr,
                         "transfer: {} matched a credit that also quotes another "
                         "order reference — settle this one by hand", reference);
        }

        PaidStatus out;
        out.method = m.method.empty() ? std::string("sepa") : m.method;
        if (m.paidMinor >= expectedMinor) {
            out.state = PayState::Paid;
            return out;
        }
        // Money arrived but does not cover the order. The buyer is told to
        // send the difference to the same IBAN with the same reference, which
        // is why partials stay Pending and keep accumulating rather than
        // failing: the next credit adds to this sum.
        out.seen = m.paidMinor > 0;
        if (deadline > 0 && NowUnix() > deadline) {
            // The window closed. NOT bounced money: the IBAN is ours and a
            // late transfer still lands there — say so in the log, because the
            // operator settling it by hand is the one who needs to know.
            std::println(std::cerr,
                         "transfer: {} lapsed with {} of {} received; the IBAN "
                         "remains ours, a late payment still arrives and is "
                         "settled with --mark-paid", reference,
                         Money::FormatMinor(m.paidMinor),
                         Money::FormatMinor(expectedMinor));
            out.state = PayState::Dead;
            return out;
        }
        out.state = PayState::Pending;
        return out;
    }

    std::optional<PayInstructions> Instructions(const std::string& payId,
                                               std::int64_t totalMinor) const override {
        const auto [reference, deadline] = SplitPayId(payId);
        if (reference.empty() || totalMinor <= 0) return std::nullopt;
        PayInstructions out;
        out.address = cfg_.transferIban;
        out.beneficiary = cfg_.transferBeneficiary;
        out.bic = cfg_.transferBic;
        out.reference = reference;
        out.amount = Money::FormatMinor(totalMinor);
        out.deadlineUnix = deadline;
        return out;
    }

    std::string_view Name() const override { return "transfer"; }

    // A transfer does not arrive in seconds even under instant payments, and
    // the bank's API is rate limited per method — the shared cache below is
    // what keeps a sweep of many orders down to one request, but a slow
    // cadence is the other half of being a good citizen there. Configurable so
    // a suite reading a local file can run at a speed a test can wait for.
    std::chrono::seconds PollInterval() const override {
        return std::chrono::seconds(cfg_.transferPollSeconds > 0
                                        ? cfg_.transferPollSeconds : 60);
    }

private:
    static std::int64_t NowUnix() {
        return std::chrono::duration_cast<std::chrono::seconds>(
                   std::chrono::system_clock::now().time_since_epoch()).count();
    }

    std::int64_t WindowSeconds() const {
        const int hours = cfg_.transferWindowHours > 0 ? cfg_.transferWindowHours
                                                       : 14 * 24;
        return static_cast<std::int64_t>(hours) * 3600;
    }

    // "CC-2B6457 catcrafts.net" -> "CC-2B6457". The description checkout
    // builds starts with the reference; take the first token so the payId
    // stays short and the '@' split below cannot be confused by a space.
    static std::string ReferenceOf(std::string_view description) {
        const std::size_t sp = description.find(' ');
        return std::string(sp == std::string_view::npos ? description
                                                       : description.substr(0, sp));
    }

    static std::pair<std::string, std::int64_t> SplitPayId(std::string_view payId) {
        const std::size_t at = payId.rfind('@');
        if (at == std::string_view::npos) return { {}, 0 };
        const std::string_view ref = payId.substr(0, at);
        // The reference must look like ours before this rail claims the order.
        if (ref.size() < 4 || !ref.starts_with("CC")) return { {}, 0 };
        std::int64_t deadline = 0;
        const std::string_view tail = payId.substr(at + 1);
        const auto [ptr, ec] = std::from_chars(tail.data(), tail.data() + tail.size(),
                                               deadline);
        if (ec != std::errc{} || ptr != tail.data() + tail.size()) return { {}, 0 };
        return { std::string(ref), deadline };
    }

    // ONE bank read per sweep, shared by every order in it. Not an
    // optimisation: the bank rate-limits reads to a few per second, and the
    // reconciler asks per order, so without this a shop with a dozen open
    // orders would throttle itself and the answers would start coming back as
    // "unknown" — which is indistinguishable, from the outside, from a shop
    // whose payments have stopped working.
    //
    // The window only has to span ONE sweep, not one poll interval. Setting it
    // to PollInterval() is the tempting mistake and it doubles the worst-case
    // wait: a sweep that lands just after a fetch would answer every order
    // from data already a minute old, so money could sit visible at the bank
    // for two minutes before any order noticed. A few seconds is enough to
    // collapse a sweep into a single request, and at one request per window
    // the rate limit is nowhere in sight.
    // Never longer than the poll interval itself: at the default 60 s cadence
    // five seconds comfortably spans one sweep, but a rail polling every
    // second would otherwise answer from data older than its own interval.
    std::chrono::seconds CacheWindow() const {
        return std::min(std::chrono::seconds(5), PollInterval());
    }

    std::optional<std::vector<BankCredit>> Fetch() {
        std::lock_guard lock(mutex_);
        const auto now = std::chrono::steady_clock::now();
        if (cached_ && now - fetchedAt_ < CacheWindow()) return cached_;
        std::optional<std::vector<BankCredit>> fresh = credits_->Recent();
        if (!fresh) {
            // Keep serving the last good answer rather than turning a blip
            // into "unknown" for every order at once. Only when there has
            // never been one does the caller get nullopt.
            return cached_ ? cached_ : std::nullopt;
        }
        cached_ = std::move(fresh);
        fetchedAt_ = now;
        return cached_;
    }

    RailConfig cfg_;
    std::unique_ptr<CreditSource> credits_;
    std::mutex mutex_;
    std::optional<std::vector<BankCredit>> cached_;
    std::chrono::steady_clock::time_point fetchedAt_{};
};

} // namespace

std::unique_ptr<CreditSource> MakeFileCreditSource(std::filesystem::path path) {
    return std::make_unique<FileCreditSource>(std::move(path));
}

std::optional<int> PullCreditsInto(CreditSource& source,
                                   const std::filesystem::path& creditsPath) {
    const std::optional<std::vector<BankCredit>> fresh = source.Recent();
    if (!fresh) return std::nullopt;

    // Which ids the file already holds. Append-only and deduplicated by the
    // bank's own payment id, so running this twice — or on an overlapping
    // window, which every run does — adds nothing the second time. That
    // matters more than it sounds: the file is evidence for settling money,
    // and a duplicated credit would double a payment and settle an order
    // nobody paid twice for.
    std::set<std::string> known;
    {
        std::ifstream in(creditsPath, std::ios::binary);
        std::string line;
        while (std::getline(in, line)) {
            if (line.empty()) continue;
            const auto doc = Json::Parse(line);
            if (!doc || !doc->IsObject()) continue;
            if (const std::string_view id = doc->Str("id"); !id.empty()) {
                known.emplace(id);
            }
        }
    }

    std::string add;
    int appended = 0;
    for (const BankCredit& c : *fresh) {
        if (c.id.empty() || known.contains(c.id)) continue;
        add += std::format(
            R"({{"id":"{}","reference":"{}","amount_minor":{},"method":"{}"}})"
            "\n",
            EscT(c.id), EscT(c.reference), c.amountMinor, EscT(c.method));
        ++appended;
    }
    if (appended == 0) return 0;

    // Append rather than rewrite: the file may also hold lines an operator
    // added by hand to settle something, and a rewrite would lose them.
    std::ofstream out(creditsPath, std::ios::app | std::ios::binary);
    if (!out) {
        std::println(std::cerr, "transfer: cannot append to {}", creditsPath.string());
        return std::nullopt;
    }
    out << add;
    out.flush();
    if (!out) {
        std::println(std::cerr, "transfer: write to {} failed", creditsPath.string());
        return std::nullopt;
    }
    return appended;
}

std::unique_ptr<PaymentRail> MakeTransferRail(const RailConfig& config,
                                             std::unique_ptr<CreditSource> credits) {
    if (!credits) return nullptr;
    // The two facts the buyer is told. Without them the order page would
    // render an incomplete instruction, which loses the money rather than the
    // sale — refuse to start instead, the same way the EURC rail refuses
    // without its chains file.
    if (config.transferIban.empty()) {
        std::println(std::cerr,
                     "transfer: TRANSFER_IBAN is not set — the order page would "
                     "have no account to name");
        return nullptr;
    }
    if (config.transferBeneficiary.empty()) {
        std::println(std::cerr,
                     "transfer: TRANSFER_BENEFICIARY is not set — Verification of "
                     "Payee shows the payer a mismatch warning without the exact "
                     "account-holder name");
        return nullptr;
    }
    return std::make_unique<TransferRail>(config, std::move(credits));
}

} // namespace Catcrafts::Server