<?php
// SPDX-License-Identifier: GPL-3.0-or-later
declare(strict_types=1);

namespace Aiwend\Pricing;

/**
 * An exact amount of money in one currency: `micro` whole millionths of the currency unit plus `rem` millionths of a millionth
 * (0 to 999,999). Integer arithmetic only; a sum that would not fit raises instead of wrapping. The one rounding rule (half up) is
 * applied only when an amount is turned into text for display.
 */
final class Amount
{
    private const MILLION = 1_000_000;

    public function __construct(public readonly int $micro = 0, public readonly int $rem = 0)
    {
    }

    public static function zero(): self
    {
        return new self();
    }

    /** tokens x (rate in millionths of a unit per million tokens), exactly, without a product that could overflow. */
    public static function ofTokens(int $tokens, int $rateMicro): self
    {
        $whole = intdiv($tokens, self::MILLION);
        $part = ($tokens % self::MILLION) * $rateMicro;
        return new self($whole * $rateMicro + intdiv($part, self::MILLION), $part % self::MILLION);
    }

    public function plus(self $other): self
    {
        $rem = $this->rem + $other->rem;
        $carry = intdiv($rem, self::MILLION);
        if ($this->micro > PHP_INT_MAX - $other->micro - $carry) {
            throw new \OverflowException('The amount is too large to add exactly.');
        }
        return new self($this->micro + $other->micro + $carry, $rem % self::MILLION);
    }

    public function isZero(): bool
    {
        return $this->micro === 0 && $this->rem === 0;
    }

    /** The amount as a decimal string with $decimals places (0 to 6), rounded half up. */
    public function toDecimalString(int $decimals): string
    {
        $decimals = max(0, min(6, $decimals));
        $scale = 10 ** (6 - $decimals);
        $q = $scale === 1 ? $this->micro + ($this->rem >= 500_000 ? 1 : 0) : intdiv($this->micro + intdiv($scale, 2), $scale);
        if ($decimals === 0) {
            return (string) $q;
        }
        $digits = str_pad((string) $q, $decimals + 1, '0', STR_PAD_LEFT);
        return substr($digits, 0, -$decimals) . '.' . substr($digits, -$decimals);
    }
}