//
//  DataContextValidator.swift
//  Yattee
//
//  Decides whether the selected data context can be used right now.
//

import Foundation

/// Why the selected context cannot be used at the moment.
enum DataContextUnavailability: Hashable, Sendable {
    /// Nothing is known yet: the Apple account the credentials are keyed by is
    /// still being established, so the Keychain cannot be read under the right
    /// scope. Never render this as "sign in required".
    case checking
    /// The account exists but its instance hasn't arrived from iCloud yet.
    case instancePending
    /// The account's instance has been switched off. The selection is kept —
    /// the user can turn the instance back on — but nothing may be served from
    /// or written to a server they have disabled.
    case instanceDisabled
    /// The user set the account itself to inactive. Same shape as a disabled
    /// instance: the selection is kept, the credential is untouched, and
    /// nothing may be served from or written to an account they switched off.
    case accountInactive
    /// No credential on this device yet. On tvOS this never resolves on its own
    /// — app Keychain items are not synced there — so the UI offers a local
    /// sign-in rather than implying a wait.
    case credentialMissing
    /// The server rejected the credential; the user must sign in again.
    case credentialRejected
    /// The server could not be reached.
    case unreachable
}

/// The three states a context can be in.
///
/// The distinction that matters is between *unknown* and *gone*. Anything that
/// might still resolve — a slow iCloud Keychain, an instance that hasn't synced
/// — is `temporarilyUnavailable` and leaves the user's choice untouched. Only
/// a deletion the app can point at turns into `removed`.
enum DataContextValidity: Hashable, Sendable {
    case valid
    case temporarilyUnavailable(DataContextUnavailability)
    case removed

    var isValid: Bool { self == .valid }
}

/// Validates the selected `DataContext` and supplies the list of switchable
/// contexts.
///
/// Replaces `SubscriptionAccountValidator`, which auto-corrected on any
/// selection it could not immediately verify. That was safe when a context
/// could only be "an instance I am logged into", but with credentials arriving
/// asynchronously it would silently discard the user's choice during a slow
/// Keychain sync — so correction is now restricted to deletions.
@MainActor
@Observable
final class DataContextValidator {
    private let accountsManager: AccountsManager
    private let instancesManager: InstancesManager
    private let toastManager: ToastManager
    private weak var appEnvironment: AppEnvironment?

    init(
        accountsManager: AccountsManager,
        instancesManager: InstancesManager,
        toastManager: ToastManager
    ) {
        self.accountsManager = accountsManager
        self.instancesManager = instancesManager
        self.toastManager = toastManager
    }

    func setAppEnvironment(_ environment: AppEnvironment) {
        appEnvironment = environment
    }

    // MARK: - Validity

    var validity: DataContextValidity {
        validity(of: accountsManager.currentContext)
    }

    func validity(of context: DataContext) -> DataContextValidity {
        switch context {
        case .local:
            // Local data is always present. The feed additionally needs a
            // Yattee Server, but that is a feed-level concern, not a reason to
            // reject the context.
            return .valid

        case let .account(id):
            guard let account = accountsManager.account(id: id) else {
                // Absence is not proof. An account can be missing because it
                // hasn't synced yet; only an actual deletion removes it, and
                // that path corrects the context directly.
                return .temporarilyUnavailable(.instancePending)
            }

            guard let instance = accountsManager.instance(for: account) else {
                return .temporarilyUnavailable(.instancePending)
            }

            // The account's own status says nothing about the instance being
            // switched off, and the switcher only *lists* a disabled source's
            // contexts — it refuses to switch to them — so a context that
            // stayed valid here would keep sending requests to a server the
            // user has turned off.
            guard instance.isEnabled else {
                return .temporarilyUnavailable(.instanceDisabled)
            }

            // Like the disabled source above: not a credential state, and the
            // selection is kept so activating the account restores it.
            guard account.isActive else {
                return .temporarilyUnavailable(.accountInactive)
            }

            switch accountsManager.status(for: id) {
            case .signedIn: return .valid
            case .checking: return .temporarilyUnavailable(.checking)
            case .credentialMissing: return .temporarilyUnavailable(.credentialMissing)
            case .credentialRejected: return .temporarilyUnavailable(.credentialRejected)
            case .instanceDisabled: return .temporarilyUnavailable(.instanceDisabled)
            case .inactive: return .temporarilyUnavailable(.accountInactive)
            case .unreachable: return .temporarilyUnavailable(.unreachable)
            }
        }
    }

    /// The context reads should actually use.
    ///
    /// Falls back to `.local` while the selected context is unusable, but the
    /// fallback is **never persisted** — the moment the credential or instance
    /// arrives, the user's choice takes effect again without them having to
    /// reselect it.
    var effectiveContext: DataContext {
        validity.isValid ? accountsManager.currentContext : .local
    }

    /// Whether writes may be sent to the current context's server.
    ///
    /// Deliberately not the same as having a read fallback: a subscribe or a
    /// playlist edit must fail visibly rather than being redirected into local
    /// data the user did not choose.
    var allowsServerMutations: Bool {
        validity.isValid
    }

    /// Set while the selected context is unusable, for the banner.
    var unavailability: DataContextUnavailability? {
        if case let .temporarilyUnavailable(reason) = validity { return reason }
        return nil
    }

    // MARK: - Available contexts

    /// Every context the switcher *lists*, usable or not.
    ///
    /// A source the user switched off does not make its logins disappear. The
    /// switcher listing only what can be switched to meant the badge on the
    /// toolbar icon pointed at a menu the account had dropped out of — the
    /// selected account included, so the one control that explains the badge
    /// stopped naming the thing it was badging for. The rows stay, carrying
    /// their status, and `isSwitchable(_:)` decides which of them can be picked.
    ///
    /// The selection is the row that may never be missing, so it is appended
    /// even when it is no longer among the selectable accounts — as long as the
    /// account still exists. A context whose account is gone is not listed:
    /// there is nothing to name it with, and correction handles deletions.
    var listedContexts: [DataContext] {
        var contexts: [DataContext] = [.local]
        contexts += accountsManager.selectableAccounts.map { DataContext.account($0.id) }

        // An inactive account is the exception to "the selection is always
        // listed": the user set it aside, and deactivating the selected
        // account switches to local anyway — this covers the window where
        // another device's deactivation has arrived ahead of its context
        // switch.
        let current = accountsManager.currentContext
        if case let .account(id) = current,
           let account = accountsManager.account(id: id),
           account.isActive,
           !contexts.contains(current) {
            contexts.append(current)
        }

        return contexts
    }

    /// Every context the user can switch to.
    ///
    /// Includes accounts that are currently signed out: hiding them would make
    /// a temporarily unreachable account look deleted, and the switcher is
    /// where the user goes to fix exactly that. A disabled source is the one
    /// exception — nothing may be read from or written to a server the user has
    /// turned off — and it is expressed by refusing the switch, not by hiding
    /// the row.
    var availableContexts: [DataContext] {
        listedContexts.filter { isSwitchable($0) }
    }

    /// Whether picking this context would land the user somewhere usable.
    ///
    /// The line runs between states that heal on their own and states only the
    /// user can end. An unreachable account may be back in a minute — and
    /// while the device itself is offline *every* account is unreachable, so
    /// refusing those would lock the whole switcher — and a context still
    /// resolving is refused nothing it may turn out to deserve. But a rejected
    /// or missing credential and a switched-off source stay that way until the
    /// user acts, in Settings, not here: picking one only lands on the local
    /// fallback with a banner, so the row is listed — status in its title —
    /// and refused.
    func isSwitchable(_ context: DataContext) -> Bool {
        switch context {
        case .local:
            return true

        case .account(let id):
            guard accountsManager.selectableAccounts.contains(where: { $0.id == id }) else {
                return false
            }

            switch validity(of: context) {
            case .valid:
                return true
            case .removed:
                return false
            case let .temporarilyUnavailable(reason):
                switch reason {
                case .instanceDisabled, .accountInactive, .credentialRejected, .credentialMissing:
                    return false
                case .checking, .instancePending, .unreachable:
                    return true
                }
            }
        }
    }

    // MARK: - Correction

    /// Corrects the context only when the selected account is provably gone.
    ///
    /// Called after deletions and on foreground activation. A timeout, a failed
    /// fetch or a record that simply has not arrived must never move anything —
    /// those are all "unknown", and treating them as removal is how a user
    /// loses their selection during a slow sync. Only the deletion tombstone
    /// counts, and the repair it triggers is never published.
    func correctIfContextRemoved() {
        // An unresolved identity has not loaded anybody's accounts yet, so
        // neither the selection nor the tombstones read here are the ones this
        // device will end up with.
        guard accountsManager.isIdentityResolved else { return }

        // The manager owns the decision: it holds the tombstones, and it
        // repairs the same way an account deleted right now would be — onto a
        // signed-in login on the same server when there is one, keeping the
        // replaced choice's timestamp and publishing nothing.
        guard accountsManager.correctContextIfAccountDeleted() else { return }

        toastManager.showInfo(
            String(localized: "subscriptions.accountSwitched.title"),
            subtitle: displayName(for: accountsManager.currentContext)
        )

        LoggingService.shared.info(
            "Selected account was deleted — data context repaired",
            category: .general
        )
    }

    // MARK: - Display

    func displayName(for context: DataContext) -> String {
        switch context {
        case .local:
            return localContextName

        case let .account(id):
            guard let account = accountsManager.account(id: id) else {
                return localContextName
            }
            return displayName(for: account)
        }
    }

    /// "(iCloud)" only when the data actually syncs; otherwise it lives on
    /// this device alone and the label says so.
    var localContextName: String {
        if appEnvironment?.settingsManager.iCloudSyncEnabled == true {
            return String(localized: "subscriptions.account.local")
        }
        return String(localized: "subscriptions.account.local.device")
    }

    /// User-set label, else whatever the account falls back to.
    func displayName(for account: Account) -> String {
        if let name = account.displayName, !name.isEmpty { return name }
        return defaultDisplayName(for: account)
    }

    /// What the account is called with no user-set label: the stored login
    /// identifier, else the source's name, else its host.
    ///
    /// After the `@` comes the source's *title* when it has one — the host is
    /// the fallback, not the name: `arek@nerdvpn.de` reads, while
    /// `arek@invidious.nerdvpn.de` overflows every menu row it appears in.
    ///
    /// Device-local by nature. The username lives in the Keychain beside the
    /// credential rather than in synced metadata, so it is only available on a
    /// device that is signed in — two devices holding the very same account can
    /// legitimately fall back to different names for it.
    func defaultDisplayName(for account: Account) -> String {
        if let username = appEnvironment?
            .accountCredentialsManager(for: account)?
            .username(for: account),
            !username.isEmpty {
            if let label = accountsManager.instance(for: account)?.shortLabel {
                return "\(username)@\(label)"
            }
            return username
        }

        if let instance = accountsManager.instance(for: account) {
            return instance.shortLabel ?? instance.type.displayName
        }

        return String(localized: "accounts.status.instanceMissing")
    }

    /// What a rename field's contents mean once the user is done typing.
    ///
    /// `nil` clears the user-set label and hands the account back to
    /// `defaultDisplayName`; anything else is stored as typed.
    ///
    /// The question the field cannot answer on its own is what an *untouched*
    /// save means. The field starts prefilled with the label the row shows, and
    /// on an account with no user-set label that label is device-local — the
    /// login identifier only exists where the credential does. Saving it
    /// unedited must not turn one device's fallback into a synced label every
    /// other device is then made to display, so that case clears instead.
    ///
    /// The test is editedness, never equality with the default: on a second
    /// device the string that device happens to fall back to is a perfectly
    /// ordinary name to *choose*, and typing it has to store and sync like any
    /// other. An unedited save on an account that already carries a label
    /// re-states that label, which the manager then drops as a no-op — the name
    /// stays, rather than being silently cleared by a tap that changed nothing.
    static func renamedDisplayName(
        _ input: String,
        prefilledWith prefill: String,
        hasUserSetLabel: Bool
    ) -> String? {
        let trimmed = input.trimmingCharacters(in: .whitespacesAndNewlines)
        guard !trimmed.isEmpty else { return nil }

        let wasEdited = trimmed != prefill.trimmingCharacters(in: .whitespacesAndNewlines)
        guard wasEdited || hasUserSetLabel else { return nil }

        return trimmed
    }
}