<?php
/**
 * Margin-scheme VAT, as arithmetic (EU VAT Directive 2006/112/EC, Arts. 311-325).
 *
 * A dealer in second-hand goods bought from consumers (or other margin-scheme
 * sellers) pays VAT on the profit margin only. The selling price includes that VAT, so
 * the VAT is the margin times rate / (100 + rate) (Art. 315: the taxable amount is the
 * margin less the VAT on it). A loss on an item is no VAT.
 *
 * Per item is the rule. Member states may allow the "global" method instead
 * (Art. 318): the period's margin-scheme sales less its margin-scheme purchases, sold
 * or not; a negative total is no VAT that period and -- where the state allows, as
 * Finland does ("yksinkertaistettu menettely") -- carried into the next. Both are here;
 * which one applies, and the rate, are the shop's to set with its accountant.
 */

if (!defined('ABSPATH')) {
    exit;
}

/** The VAT in one margin-scheme sale. Prices include VAT; amounts rounded to cents. */
function usedkit_margin_vat($sale, $cost, $rate) {
    $margin = $sale - $cost;
    if ($margin <= 0) {
        return 0.0;
    }
    return round($margin * $rate / (100 + $rate), 2);
}

/**
 * A period's totals from its rows, [[sale, cost], ...].
 * 'item': one row per sale with its item's purchase price; losses count as nothing.
 * 'global': rows for the period's sales and, separately, its purchases ([0, cost]),
 * all added up with `$carry_in` (a negative total carried from the
 * previous period); a negative result is no VAT and becomes the carry out.
 */
function usedkit_margin_period($sales, $rate, $method = 'item', $carry_in = 0.0) {
    $total_sale = $total_cost = $margin = 0.0;
    foreach ($sales as $row) {
        list($sale, $cost) = $row;
        $total_sale += $sale;
        $total_cost += $cost;
        $margin += ($method === 'item') ? max(0.0, $sale - $cost) : ($sale - $cost);
    }
    $carry = 0.0;
    if ($method !== 'item') {
        $margin += min(0.0, $carry_in);
        if ($margin < 0) {
            $carry = $margin;
        }
    }
    $taxable = max(0.0, $margin);
    $vat = round($taxable * $rate / (100 + $rate), 2);
    return array('sales' => round($total_sale, 2), 'costs' => round($total_cost, 2),
                 'margin' => round($margin, 2), 'vat' => $vat,
                 'net_margin' => round($taxable - $vat, 2), 'carry' => round($carry, 2));
}