110 lines
3.9 KiB
PHP
110 lines
3.9 KiB
PHP
<?php
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namespace App\Services\Billing;
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/**
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* The arithmetic on an invoice, in cents, rounded once.
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*
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* Everything is integer cents from end to end. Money in floats accumulates a
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* cent here and there and then the sum printed under a column does not match
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* the column — which on an invoice is not a rounding artefact, it is a document
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* that does not add up.
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*
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* Rounding happens ONCE, at the point a figure is shown or stored, never per
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* line and then again on the total. A percentage discount applied line by line
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* and rounded each time drifts from the same discount applied to the sum, and
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* the customer's calculator is the one that will find it.
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*
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* Net is the basis for everything. Gross is derived and never the other way
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* round: taking VAT out of a gross figure and putting it back gives a different
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* number often enough to matter.
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*/
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final class InvoiceMath
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{
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/** A discount or surcharge as a percentage of something. */
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public const PERCENT = 'percent';
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/** A discount or surcharge as a fixed amount in cents. */
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public const AMOUNT = 'amount';
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/**
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* One line's net total before any invoice-level adjustment.
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*
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* Quantity carries three decimals — hours, gigabytes and part-months are
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* all real — so it is held as thousandths and divided out at the end,
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* rather than multiplying a float by a price.
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*/
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public static function lineNet(int $unitNetCents, int $quantityMilli): int
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{
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return (int) round($unitNetCents * $quantityMilli / 1000);
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}
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/**
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* A discount or surcharge, as a signed amount in cents.
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*
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* Positive raises the total, negative lowers it — so a discount is passed
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* as a negative value and a surcharge as a positive one, and nothing has to
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* remember which of two flags means which.
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*
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* @param string $type PERCENT (value in hundredths of a percent) or
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* AMOUNT (value in cents)
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*/
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public static function adjustment(int $baseCents, string $type, int $value): int
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{
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return $type === self::PERCENT
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? (int) round($baseCents * $value / 10000)
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: $value;
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}
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/** VAT on a net figure. `$rateBasisPoints` is 2000 for 20 %. */
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public static function tax(int $netCents, int $rateBasisPoints): int
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{
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return (int) round($netCents * $rateBasisPoints / 10000);
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}
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/**
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* Every figure the document shows, from the lines and one adjustment.
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*
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* @param array<int, array{unit_net_cents:int, quantity_milli:int}> $lines
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* @param array{type:string, value:int}|null $adjustment
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* @return array{lines_net:int, adjustment:int, net:int, tax:int, gross:int}
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*/
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public static function totals(array $lines, ?array $adjustment, int $rateBasisPoints): array
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{
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$linesNet = 0;
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foreach ($lines as $line) {
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$linesNet += self::lineNet((int) $line['unit_net_cents'], (int) $line['quantity_milli']);
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}
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$adjusted = $adjustment === null
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? 0
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: self::adjustment($linesNet, (string) $adjustment['type'], (int) $adjustment['value']);
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$net = $linesNet + $adjusted;
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$tax = self::tax($net, $rateBasisPoints);
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return [
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'lines_net' => $linesNet,
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'adjustment' => $adjusted,
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'net' => $net,
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'tax' => $tax,
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'gross' => $net + $tax,
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];
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}
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/** "1.234,56" — Austrian formatting, done here so no view invents its own. */
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public static function money(int $cents): string
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{
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return number_format($cents / 100, 2, ',', '.');
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}
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/** "12", "1,5", "0,25" — trailing zeroes dropped, because "12,000 Std." reads as a defect. */
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public static function quantity(int $quantityMilli): string
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{
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$formatted = number_format($quantityMilli / 1000, 3, ',', '.');
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return rtrim(rtrim($formatted, '0'), ',');
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}
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}
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