import Foundation

final class SentenceExplanationStore {
    static let shared = SentenceExplanationStore()

    private let root: URL

    init() {
        let documents = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        root = documents.appendingPathComponent("SentenceExplanations", isDirectory: true)
        try? FileManager.default.createDirectory(at: root, withIntermediateDirectories: true)
    }

    func load(mediaId: String, sentenceIndex: Int) -> String? {
        let url = fileURL(mediaId: mediaId, sentenceIndex: sentenceIndex)
        guard FileManager.default.fileExists(atPath: url.path) else { return nil }
        let text = try? String(contentsOf: url, encoding: .utf8)
        let trimmed = text?.trimmingCharacters(in: .whitespacesAndNewlines) ?? ""
        return trimmed.isEmpty ? nil : trimmed
    }

    @discardableResult
    func save(mediaId: String, sentenceIndex: Int, explanation: String) -> Bool {
        let trimmed = explanation.trimmingCharacters(in: .whitespacesAndNewlines)
        guard !trimmed.isEmpty else { return false }
        let url = fileURL(mediaId: mediaId, sentenceIndex: sentenceIndex)
        try? FileManager.default.createDirectory(at: url.deletingLastPathComponent(), withIntermediateDirectories: true)
        do {
            try trimmed.write(to: url, atomically: true, encoding: .utf8)
            return true
        } catch {
            return false
        }
    }

    func sync(mediaId: String, sentences: [SentenceBoundary]) {
        guard !sentences.isEmpty else { return }
        let directory = root.appendingPathComponent(Self.sanitize(mediaId), isDirectory: true)
        let revision = Self.revision(for: sentences)
        let marker = directory.appendingPathComponent("revision.txt")
        let previous = (try? String(contentsOf: marker, encoding: .utf8))?
            .trimmingCharacters(in: .whitespacesAndNewlines) ?? ""
        guard previous != revision else { return }
        try? FileManager.default.removeItem(at: directory)
        try? FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
        try? revision.write(to: marker, atomically: true, encoding: .utf8)
    }

    private func fileURL(mediaId: String, sentenceIndex: Int) -> URL {
        let directory = root.appendingPathComponent(Self.sanitize(mediaId), isDirectory: true)
        return directory.appendingPathComponent("\(sentenceIndex).txt")
    }

    static func revision(for sentences: [SentenceBoundary]) -> String {
        sentences.map { sentence in
            let text = (sentence.text ?? "").trimmingCharacters(in: .whitespacesAndNewlines)
            return "\(sentence.index)\t\(text)"
        }.joined(separator: "\n")
    }

    static func sanitize(_ mediaId: String) -> String {
        let allowed = CharacterSet.alphanumerics.union(CharacterSet(charactersIn: "._-"))
        let filtered = mediaId.lowercased().unicodeScalars.map { allowed.contains($0) ? Character($0) : "_" }
        let value = String(filtered).trimmingCharacters(in: CharacterSet(charactersIn: "_"))
        return value.isEmpty || value == "." || value == ".." ? "unknown" : value
    }
}