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.");
        }

        return price * (1m - discountPercent / 100m);
    }

    /// <summary>Rounds an amount to the nearest 0.05, rounding halves away from zero.</summary>
    public static decimal RoundToFiveRappen(decimal amount)
    {
        return Math.Round(amount * 20m, MidpointRounding.AwayFromZero) / 20m;
    }

    /// <summary>Returns the quantity discount percent for a quantity: 0, 5 or 10.</summary>
    public static int GetQuantityDiscountPercent(int quantity)
    {
        if (quantity < 1)
        {
            throw new ArgumentOutOfRangeException(nameof(quantity), "A quantity must be at least 1.");
        }

        return quantity switch
        {
            >= 50 => 10,
            >= 10 => 5,
            _ => 0,
        };
    }

    /// <summary>
    /// Calculates the total: percentage discount on the unit price first, then the quantity
    /// discount on the total, then rounds the end amount to the nearest 0.05.
    /// </summary>
    public static decimal CalculateTotal(decimal unitPrice, int quantity, decimal discountPercent)
    {
        if (unitPrice < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(unitPrice), "A price cannot be negative.");
        }

        if (quantity < 1)
        {
            throw new ArgumentOutOfRangeException(nameof(quantity), "A quantity must be at least 1.");
        }

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

        var discountedUnitPrice = ApplyDiscount(unitPrice, discountPercent);
        var totalBeforeQuantityDiscount = discountedUnitPrice * quantity;
        var quantityDiscountPercent = GetQuantityDiscountPercent(quantity);
        var totalBeforeRounding = totalBeforeQuantityDiscount * (1m - quantityDiscountPercent / 100m);

        return RoundToFiveRappen(totalBeforeRounding);
    }
}