import Foundation
import MetricKit
import os

/// On-device replacement for Crashlytics.
///
/// `MXMetricManager` hands the app a daily payload with crash, hang and
/// disk-write diagnostics that Apple already collected for it. This collector
/// writes those payloads to the app container as JSON and **never sends them
/// anywhere** — no network code, no third-party SDK, no identifiers. Crash
/// reports still reach Xcode Organizer through Apple's own pipeline, which is
/// the channel that was actually useful about Crashlytics.
///
/// The user can share the stored reports from Settings; that is the only way
/// one ever leaves the device, and it is an explicit action.
final class DiagnosticsCollector: NSObject {
    static let shared = DiagnosticsCollector()

    /// Reports older than this are pruned on every new payload.
    private static let retentionCount = 5

    private let logger = Logger(subsystem: AppConstants.bundleIdentifier, category: "diagnostics")
    private let fileManager = FileManager.default
    private let dateFormatter: DateFormatter = {
        let formatter = DateFormatter()
        formatter.dateFormat = "yyyyMMdd-HHmmss"
        formatter.locale = Locale(identifier: "en_US_POSIX")
        return formatter
    }()

    private override init() {
        super.init()
    }

    /// Subscribes to MetricKit. Safe to call more than once.
    func start() {
        MXMetricManager.shared.add(self)
    }

    // MARK: - Storage

    /// `Application Support/Diagnostics`, created on demand.
    private var directory: URL? {
        guard let base = fileManager.urls(for: .applicationSupportDirectory, in: .userDomainMask).first else {
            return nil
        }
        let url = base.appendingPathComponent("Diagnostics", isDirectory: true)
        if !fileManager.fileExists(atPath: url.path) {
            try? fileManager.createDirectory(at: url, withIntermediateDirectories: true)
        }
        return url
    }

    /// Stored reports, newest first. Used by Settings to offer sharing them.
    func storedReports() -> [URL] {
        guard let directory else { return [] }
        let contents = (try? fileManager.contentsOfDirectory(
            at: directory,
            includingPropertiesForKeys: [.contentModificationDateKey]
        )) ?? []
        return contents
            .filter { $0.pathExtension == "json" }
            .sorted { lhs, rhs in
                let l = (try? lhs.resourceValues(forKeys: [.contentModificationDateKey]).contentModificationDate) ?? .distantPast
                let r = (try? rhs.resourceValues(forKeys: [.contentModificationDateKey]).contentModificationDate) ?? .distantPast
                return l > r
            }
    }

    func deleteStoredReports() {
        for url in storedReports() {
            try? fileManager.removeItem(at: url)
        }
    }

    private func write(_ data: Data, prefix: String) {
        guard let directory else { return }
        let name = "\(prefix)-\(dateFormatter.string(from: Date())).json"
        let url = directory.appendingPathComponent(name)
        do {
            try data.write(to: url, options: .atomic)
            // Diagnostics are debugging aids, not user data: keep them out of
            // iCloud/iTunes backups.
            var resourceValues = URLResourceValues()
            resourceValues.isExcludedFromBackup = true
            var mutableURL = url
            try? mutableURL.setResourceValues(resourceValues)
            logger.info("Stored diagnostic report \(name, privacy: .public)")
        } catch {
            logger.error("Could not store diagnostic report: \(error.localizedDescription, privacy: .public)")
        }
        prune()
    }

    private func prune() {
        let reports = storedReports()
        guard reports.count > Self.retentionCount else { return }
        for url in reports.dropFirst(Self.retentionCount) {
            try? fileManager.removeItem(at: url)
        }
    }
}

// MARK: - MXMetricManagerSubscriber

extension DiagnosticsCollector: MXMetricManagerSubscriber {
    func didReceive(_ payloads: [MXMetricPayload]) {
        for payload in payloads {
            write(payload.jsonRepresentation(), prefix: "metrics")
        }
    }

    func didReceive(_ payloads: [MXDiagnosticPayload]) {
        for payload in payloads {
            let crashes = payload.crashDiagnostics?.count ?? 0
            let hangs = payload.hangDiagnostics?.count ?? 0
            logger.info("Diagnostic payload: \(crashes, privacy: .public) crash(es), \(hangs, privacy: .public) hang(s)")
            write(payload.jsonRepresentation(), prefix: "diagnostics")
        }
    }
}