<?php

namespace App\Services\Billing;

use App\Models\Customer;
use App\Support\CompanyProfile;

/**
 * Which VAT applies to a customer, and why.
 *
 * Only the two clear-cut cases are decided here:
 *
 *  - a BUSINESS customer whose VERIFIED VAT ID belongs to another EU member
 *    state pays no VAT to us; the liability shifts to them (reverse charge),
 *    and the invoice has to say so. Unverified means domestic rate: a
 *    self-declared string must never be able to zero the tax;
 *  - everyone else is charged the seller's domestic rate.
 *
 * "Business" is the answer the customer gave when they were asked, not
 * something read out of the VAT field. Reverse charge is a business-to-business
 * rule and nothing else, so somebody who has recorded themselves as a private
 * person is charged the domestic rate whatever they have typed into `vat_id` —
 * previously they were not, and a consumer could zero their own VAT by getting
 * a number verified.
 *
 * A customer nobody has asked keeps the treatment they had: a verified VAT ID
 * still earns the reverse charge. That is deliberately NOT the protective
 * default used everywhere else in this codebase, and the reason is that the two
 * questions have different stakes. Withdrawal is a right, and guessing costs a
 * consumer something they cannot get back; VAT is a rate, and flipping every
 * unasked business onto 20 % overnight would put our invoices in disagreement
 * with contracts that are already running. So this file changes no outcome for
 * anyone whose type has never been recorded, and the moment it IS recorded, the
 * recorded answer is the only one that counts.
 *
 * Deliberately NOT handled: cross-border sales to private individuals, which
 * are taxed at the customer's own country's rate under the OSS scheme. Doing
 * that needs a maintained rate table per member state and a tax adviser's sign
 * off, not a guess in a config file — so those customers fall back to the
 * domestic rate, which over-collects rather than under-collects.
 *
 * ## The price at the till is the domestic gross, for everybody
 *
 * The treatment above decides how a DOCUMENT is split, and it decides nothing
 * about the amount taken from the card. That amount is chargedCents(): the net
 * catalogue figure with the domestic rate on it, and it is the same number for a
 * consumer in Vienna and for a reverse-charge business in Rotterdam. The owner's
 * rule is that the figure on the website is the figure charged, and a Stripe
 * Price cannot ask who is buying — it carries one amount for everyone.
 *
 * So a reverse-charge customer pays that same total and their invoice states the
 * whole of it as net at 0 %. That is a real commercial consequence and it is
 * deliberate: see App\Services\Billing\IssueInvoice.
 *
 * Stripe's own `automatic_tax` is deliberately NOT enabled anywhere. This class
 * is the single tax authority here, knowingly over-collecting on cross-border
 * B2C rather than implementing OSS; letting Stripe compute a second rate would
 * have it charge a German consumer 19 % while our document said 20 %.
 */
final readonly class TaxTreatment
{
    /** Reverse charge applies inside the EU only. */
    public const EU_MEMBER_STATES = [
        'AT', 'BE', 'BG', 'CY', 'CZ', 'DE', 'DK', 'EE', 'EL', 'ES', 'FI', 'FR',
        'HR', 'HU', 'IE', 'IT', 'LT', 'LU', 'LV', 'MT', 'NL', 'PL', 'PT', 'RO',
        'SE', 'SI', 'SK',
    ];

    private function __construct(
        public float $rate,
        public bool $reverseCharge,
    ) {}

    /**
     * The seller's own rate, with nobody in particular in mind.
     *
     * From the Finance page, not from config: an operator can change the rate
     * there and cannot change a config value, and two sources for one number is
     * how an invoice ends up disagreeing with the checkout that produced it.
     * CompanyProfile falls back to the config value until a rate has been saved.
     *
     * This is the ONE place the rate is read. Everything that has to form a
     * gross figure — the price sheet, the Stripe catalogue, the module prices —
     * goes through chargedCents() below rather than multiplying by a percentage
     * of its own.
     */
    public static function domestic(): self
    {
        return new self(CompanyProfile::taxRate() / 100, false);
    }

    /**
     * What a net catalogue figure is actually charged as.
     *
     * The number on the website, the amount on the Stripe Price and the total on
     * the invoice are all this one, so it is computed once and here. It takes no
     * customer on purpose: a Stripe Price carries a single amount and cannot ask
     * who is at the checkout, and the owner's rule is that everybody pays the
     * figure they were shown.
     */
    public static function chargedCents(int $netCents): int
    {
        return self::domestic()->grossCents($netCents);
    }

    public static function for(?Customer $customer): self
    {
        $domestic = self::domestic();

        // Unverified is the normal state, and it must cost the customer nothing
        // more than the domestic rate they would pay anyway — but it must not
        // zero the VAT either. Otherwise typing "XX123" is a discount.
        if ($customer === null || ! $customer->hasVerifiedVatId()) {
            return $domestic;
        }

        // Asked of the recorded type, not of the number. Somebody who has said
        // they are a private person is a private person, and a VAT ID on their
        // record — a former sole trader's, a colleague's, a mistake — must not
        // be able to turn that answer over. Only an explicit "consumer" refuses
        // here; an unrecorded type falls through and is treated exactly as it
        // was before this check existed (see the class comment).
        if ($customer->hasRecordedType() && ! $customer->isBusiness()) {
            return $domestic;
        }

        $vatId = $customer->normalisedVatId();
        $country = substr($vatId, 0, 2);
        $seller = strtoupper((string) config('provisioning.tax.seller_country', 'AT'));

        // Reverse charge is an intra-EU business rule: it needs a member state
        // that is not ours, and a number that at least looks like one.
        $eligible = in_array($country, self::EU_MEMBER_STATES, true)
            && $country !== $seller
            && preg_match('/^[A-Z]{2}[0-9A-Z]{8,12}$/', $vatId) === 1;

        return $eligible ? new self(0.0, true) : $domestic;
    }

    public function grossCents(int $netCents): int
    {
        return (int) round($netCents * (1 + $this->rate));
    }

    /**
     * The rate in the form the invoice arithmetic works in — 2000 for 20 %.
     *
     * Here rather than at each call site, because the conversion was written out
     * by hand where an invoice was issued and nowhere else, and a second reader
     * of `rate` doing its own multiplication is how two documents end up at two
     * rates.
     */
    public function basisPoints(): int
    {
        return (int) round($this->rate * 10000);
    }

    public function percentLabel(): string
    {
        return rtrim(rtrim(number_format($this->rate * 100, 1, ',', '.'), '0'), ',');
    }
}