import Foundation
import Vision

/// 1ファイル分のキャッシュ。ファイルサイズと更新日時が一致するときだけ再利用する。
struct FingerprintRecord: Codable, Sendable {
    var fileSize: Int64
    var modificationDate: Date
    var basics: ImageBasics?
    var featurePrint: FeaturePrintObservation?

    func matches(_ file: ScannedFile) -> Bool {
        fileSize == file.fileSize
            && abs(modificationDate.timeIntervalSince(file.modificationDate)) < 1
    }
}

/// dHash と feature print をディスクに保存し、同じフォルダの再スキャンを速くする。
///
/// feature print は1枚あたり数 KB あるので、JSON ではなくバイナリ plist で持つ。
actor FingerprintCache {
    private var records: [String: FingerprintRecord] = [:]
    private var isDirty = false
    private let fileURL: URL?

    init(fileURL: URL? = FingerprintCache.defaultFileURL()) {
        self.fileURL = fileURL
        self.records = Self.loadRecords(from: fileURL)
    }

    static func defaultFileURL() -> URL? {
        guard let caches = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask).first
        else { return nil }
        let directory = caches.appendingPathComponent("Photochk", isDirectory: true)
        try? FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
        return directory.appendingPathComponent("fingerprints.plist")
    }

    // MARK: - 読み出し

    func basics(for files: [ScannedFile]) -> [URL: ImageBasics] {
        var found: [URL: ImageBasics] = [:]
        for file in files {
            if let record = records[file.url.path], record.matches(file), let basics = record.basics {
                found[file.url] = basics
            }
        }
        return found
    }

    func featurePrints(for files: [ScannedFile]) -> [URL: FeaturePrintObservation] {
        var found: [URL: FeaturePrintObservation] = [:]
        for file in files {
            if let record = records[file.url.path], record.matches(file),
               let print = record.featurePrint {
                found[file.url] = print
            }
        }
        return found
    }

    // MARK: - 書き込み

    func store(basics: [URL: ImageBasics], for files: [ScannedFile]) {
        for file in files {
            guard let value = basics[file.url] else { continue }
            update(file) { $0.basics = value }
        }
    }

    func store(featurePrints: [URL: FeaturePrintObservation], for files: [ScannedFile]) {
        for file in files {
            guard let value = featurePrints[file.url] else { continue }
            update(file) { $0.featurePrint = value }
        }
    }

    private func update(_ file: ScannedFile, _ body: (inout FingerprintRecord) -> Void) {
        var record = records[file.url.path] ?? FingerprintRecord(
            fileSize: file.fileSize,
            modificationDate: file.modificationDate
        )
        if !record.matches(file) {
            // ファイルが変わっていたら作り直す
            record = FingerprintRecord(
                fileSize: file.fileSize,
                modificationDate: file.modificationDate
            )
        }
        body(&record)
        records[file.url.path] = record
        isDirty = true
    }

    /// 消えたファイルのエントリを捨てる。スキャンのたびに肥大化するのを防ぐ。
    func prune(keeping files: [ScannedFile]) {
        let alive = Set(files.map(\.url.path))
        let before = records.count
        records = records.filter { alive.contains($0.key) }
        if records.count != before { isDirty = true }
    }

    // MARK: - 永続化

    func save() {
        guard isDirty, let fileURL else { return }
        let encoder = PropertyListEncoder()
        encoder.outputFormat = .binary
        guard let data = try? encoder.encode(records) else { return }
        try? data.write(to: fileURL, options: .atomic)
        isDirty = false
    }

    private static func loadRecords(from fileURL: URL?) -> [String: FingerprintRecord] {
        guard let fileURL, let data = try? Data(contentsOf: fileURL) else { return [:] }
        // 形式が変わっていたら黙って捨てる。キャッシュなので作り直せばよい。
        return (try? PropertyListDecoder().decode([String: FingerprintRecord].self, from: data)) ?? [:]
    }
}