namespace Prices;

/// <summary>Turns a list price into what the customer actually pays.</summary>
public static class PriceCalculator
{
    /// <summary>Applies a percentage discount to a price, e.g. 25 for a quarter off.</summary>
    public static decimal ApplyDiscount(decimal price, decimal discountPercent)
    {
        if (price < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(price), "A price cannot be negative.");
        }

        if (discountPercent is < 0 or > 100)
        {
            throw new ArgumentOutOfRangeException(nameof(discountPercent), "A discount is between 0 and 100 percent.");
        }

        var raw = price * (100 - discountPercent) / 100;
        return Math.Round(raw / 0.05m, MidpointRounding.AwayFromZero) * 0.05m;
    }
}