import Foundation

/// Identifies a cached evaluation — FC-060 §5: "environment + subjectId + keys(sorted) +
/// platform + appVersion". `sortedKeys` means caller `keys` order never affects hits.
struct FeatureControlCacheKey: Hashable, Sendable {
    let environment: String
    let subjectId: String
    let sortedKeys: [String]
    let platform: String?
    let appVersion: String?
}

/// In-memory-only TTL cache (no disk persistence — see SDD §2 Non-Goals). The clock is
/// injectable so tests can force expiry deterministically instead of sleeping.
actor FeatureControlCache {
    private struct Entry {
        let snapshot: FeatureControlSnapshot
        let storedAt: Date
        let ttl: TimeInterval
    }

    private var entries: [FeatureControlCacheKey: Entry] = [:]
    private let now: @Sendable () -> Date

    init(now: @escaping @Sendable () -> Date = { Date() }) {
        self.now = now
    }

    /// `allowStale: true` returns an expired entry rather than `nil` — used by the
    /// safe-degrade fallback path (SDD §4.4/§4.5).
    func get(_ key: FeatureControlCacheKey, allowStale: Bool) -> FeatureControlSnapshot? {
        guard let entry = entries[key] else { return nil }
        let age = now().timeIntervalSince(entry.storedAt)
        if age <= entry.ttl { return entry.snapshot }
        return allowStale ? entry.snapshot : nil
    }

    /// Always overwrites — a fresh fetch's `configVersion` simply replaces whatever was there.
    func set(_ key: FeatureControlCacheKey, snapshot: FeatureControlSnapshot, ttl: TimeInterval) {
        entries[key] = Entry(snapshot: snapshot, storedAt: now(), ttl: ttl)
    }

    func invalidateAll() {
        entries.removeAll()
    }
}