import Foundation

/// A validator reducing a submitted email address to its canonical form.
///
/// Trims surrounding whitespace, lowercases the address (so one mailbox cannot register once per spelling), caps it at the 254 bytes an address can
/// be, and checks its shape: something before the `@`, and a domain with a dot. Control characters are rejected separately: the shape only excludes
/// whitespace, which would let non-whitespace controls (such as `NUL`) into the stored address.
public struct NormalizeEmail: Sendable {

    // MARK: Initializers

    /// Creates an email normalization method.
    public init() {}

    // MARK: Functions

    /// Validates and normalizes a submitted email address.
    /// - Parameter email: the submitted email address, if any.
    /// - Returns: the normalized address, or `nil` when the submission is missing or invalid.
    public func callAsFunction(
        _ email: String?
    ) -> String? {
        guard
            let email = email?
                .trimmingCharacters(in: .whitespacesAndNewlines)
                .lowercased(),
            email.utf8.count <= Constant.Length.maxEmail,
            !email.unicodeScalars.contains(where: {
                $0.properties.generalCategory == .control
            }),
            email.wholeMatch(of: /[^\s@]+@[^\s@]+\.[^\s@]+/) != nil
        else {
            return nil
        }

        return email
    }

}

// MARK: - Constants

private enum Constant {
    /// A namespace for the length constants.
    enum Length {
        /// The longest an email address can be, in bytes, per RFC 5321's path limit.
        static let maxEmail = 254
    }
}