import Foundation

/// Produces filenames safe for an iPod's volume, and unique within one export.
///
/// Click-wheel iPods are usually FAT32 and their Contacts/Calendars browsers
/// handle non-ASCII filenames poorly, so names are folded to ASCII rather than
/// passed through. Accents fold to their base letter (`Renée` → `Renee`);
/// anything still outside the safe set is dropped.
public struct FileNameAllocator {
    /// Lowercased names already handed out, so collisions can be numbered.
    private var used: Set<String> = []

    public static let maximumLength = 60

    public init() {}

    /// Returns a unique filename with `fileExtension`, based on `preferred`.
    public mutating func allocate(
        preferred: String,
        fallback: String = "item",
        fileExtension: String
    ) -> String {
        let base = Self.sanitize(preferred, fallback: fallback)

        var candidate = base
        var counter = 1
        while used.contains(candidate.lowercased()) {
            counter += 1
            let suffix = "_\(counter)"
            let trimmed = String(base.prefix(Self.maximumLength - suffix.count))
            candidate = trimmed + suffix
        }
        used.insert(candidate.lowercased())

        return "\(candidate).\(fileExtension)"
    }

    /// Folds to ASCII, keeps `[A-Za-z0-9 ._-]`, collapses runs of whitespace.
    static func sanitize(_ name: String, fallback: String) -> String {
        let folded = name.folding(
            options: [.diacriticInsensitive, .widthInsensitive],
            locale: Locale(identifier: "en_US_POSIX")
        )

        let allowed = folded.map { character -> Character in
            character.isASCII && (character.isLetter || character.isNumber
                || character == " " || character == "." || character == "_"
                || character == "-")
                ? character
                : " "
        }

        let collapsed = String(allowed)
            .split(separator: " ", omittingEmptySubsequences: true)
            .joined(separator: " ")

        // A name of only dots would collide with "." / ".." on the device.
        let trimmed = collapsed
            .trimmingCharacters(in: CharacterSet(charactersIn: " ."))
            .prefix(maximumLength)

        return trimmed.isEmpty ? fallback : String(trimmed)
    }
}