import AgentKit
import Foundation

/// The skills the user has installed.
///
/// Its own store rather than more fields on `AIConfigurationModel`: that one is
/// about endpoints you spend a key at, and this is a library of documents. They
/// share nothing but the Settings page they appear on.
///
/// The only writer. A skill reaches this model from the editor or from an import
/// the user chose, and from nowhere else — no tool, no MCP server and no fetched
/// page can reach it. `LoadSkillTool` depends on that: it is why a skill body is
/// the one tool result that is not framed as untrusted data.
@Observable
public final class SkillLibraryModel {
    public private(set) var skills: [AgentSkill] = []
    public private(set) var storageError: String?

    private var persistence: (any SkillLibraryPersisting)?
    private let seedDefaults: UserDefaults?

    public init(
        skills: [AgentSkill] = [],
        persistence: (any SkillLibraryPersisting)? = nil,
        seedDefaults: UserDefaults? = nil
    ) {
        self.skills = skills
        self.persistence = persistence
        self.seedDefaults = seedDefaults
        guard let persistence else { return }
        do {
            self.skills = try persistence.load()
        } catch {
            storageError = error.localizedDescription
        }
        seedBuiltInSkills()
    }

    /// Copies in the starter skills a release added, once.
    ///
    /// The generation mark is the whole mechanism, and it is what makes the
    /// built-ins genuinely the user's: seeding only ever installs skills newer
    /// than the mark, so deleting one is permanent and editing one is never
    /// undone. A release that adds a starter skill raises
    /// `AgentSkillLibrary.currentGeneration` and only that skill arrives.
    ///
    /// Skipped entirely without `seedDefaults` — previews, tests and the
    /// screenshot build get the library they were handed and nothing else.
    private func seedBuiltInSkills() {
        guard let seedDefaults else { return }
        let seeded = seedDefaults.integer(forKey: SharedSettingsKey.seededSkillGeneration)
        guard seeded < AgentSkillLibrary.currentGeneration else { return }
        for skill in AgentSkillLibrary.builtIns(after: seeded) {
            var installed = skill
            installed.name = availableName(basedOn: skill.name)
            upsert(installed)
        }
        // Written even when nothing was installed, so a failed save is the only
        // thing that can make this run twice.
        guard storageError == nil else { return }
        seedDefaults.set(
            AgentSkillLibrary.currentGeneration, forKey: SharedSettingsKey.seededSkillGeneration
        )
    }

    // MARK: - Reads

    /// What a run may be offered. Incomplete drafts are excluded rather than
    /// listed as names with nothing to match on — see `AgentSkill.isComplete`.
    public var enabledSkills: [AgentSkill] {
        let conflicts = conflictingSkillIDs
        return skills.filter { $0.isEnabled && $0.isComplete && !conflicts.contains($0.id) }
    }

    /// The snapshot a run is given. Empty when nothing is enabled, which is what
    /// removes the tool and the catalog block entirely rather than sending an
    /// empty list.
    public var catalog: AgentSkillCatalog { AgentSkillCatalog(enabledSkills) }

    public func skill(_ id: AgentSkill.ID) -> AgentSkill? {
        skills.first { $0.id == id }
    }

    public func skill(named name: String) -> AgentSkill? {
        skills.first { $0.effectiveName == name }
    }

    public func hasNameConflict(_ name: String, excluding id: AgentSkill.ID) -> Bool {
        !name.isEmpty && skills.contains { $0.id != id && $0.effectiveName == name }
    }

    public var conflictingSkillIDs: Set<UUID> {
        var names = Set<String>()
        var conflicts = Set<UUID>()
        for skill in skills.sorted(by: { $0.id.uuidString < $1.id.uuidString }) {
            if !skill.effectiveName.isEmpty, !names.insert(skill.effectiveName).inserted {
                conflicts.insert(skill.id)
            }
        }
        return conflicts
    }

    /// Only the trusted persistence/sync composition root supplies this snapshot.
    public func applyRemoteSnapshot(_ values: [AgentSkill]) {
        skills = values
        storageError = nil
    }

    // MARK: - Writes

    public func upsert(_ skill: AgentSkill) {
        mutatePersistedState {
            var updated = skill
            updated.updatedAt = .now
            if let index = skills.firstIndex(where: { $0.id == skill.id }) {
                skills[index] = updated
            } else {
                skills.append(updated)
            }
        }
    }

    /// Locks a valid, unique derived name without changing the UUID identity.
    /// The same move `AIConfigurationModel.lockMCPNamespace` makes, and for the
    /// same reason: the model-facing identifier must stop moving once anything
    /// could have referred to it.
    @discardableResult
    public func lockName(for id: AgentSkill.ID) -> Bool {
        guard let index = skills.firstIndex(where: { $0.id == id }) else { return false }
        if !skills[index].name.isEmpty { return true }
        let name = skills[index].effectiveName
        guard AgentSkillNaming.isValid(name), !hasNameConflict(name, excluding: id) else {
            return false
        }
        return mutatePersistedState { skills[index].name = name }
    }

    public func delete(id: AgentSkill.ID) {
        mutatePersistedState { skills.removeAll { $0.id == id } }
    }

    public func setEnabled(_ enabled: Bool, for id: AgentSkill.ID) {
        mutatePersistedState {
            guard let index = skills.firstIndex(where: { $0.id == id }) else { return }
            skills[index].isEnabled = enabled
            skills[index].updatedAt = .now
        }
    }

    /// Puts the library back to exactly the skills Termind ships with.
    ///
    /// A replacement, not a merge: everything the user wrote, imported or edited
    /// is discarded, and what is left is the starter set as released. That is
    /// the destructive reading of "reset", and it is the one the caller has to
    /// confirm out loud — nothing here is recoverable afterwards, because a
    /// skill's only copy lives in this store.
    ///
    /// It exists because seeding installs each starter skill exactly once and a
    /// deleted one never returns on its own. Without this there is no way back
    /// to a known state, only forward from whatever the library became.
    @discardableResult
    public func resetToBuiltInSkills() -> Int {
        let shipped = AgentSkillLibrary.builtIns(after: 0)
        let changed = mutatePersistedState { skills = shipped }
        guard changed else { return 0 }
        // Nothing left for seeding to add, whatever generation this store was
        // written by.
        seedDefaults?.set(
            AgentSkillLibrary.currentGeneration, forKey: SharedSettingsKey.seededSkillGeneration
        )
        return shipped.count
    }

    /// Adds an imported document under a name that is free.
    ///
    /// A collision is resolved rather than refused: someone importing a second
    /// `pdf-processing` from another collection wants both, and a numbered name
    /// is a smaller surprise than a silent overwrite of the one they had.
    @discardableResult
    public func install(_ document: AgentSkillDocument, importedFrom: String) -> AgentSkill {
        let skill = AgentSkill(
            name: availableName(basedOn: document.name),
            title: Self.title(of: document),
            summary: document.description,
            body: document.body,
            importedFrom: importedFrom
        )
        upsert(skill)
        return skill
    }

    private func availableName(basedOn name: String) -> String {
        var candidate = name
        var suffix = 2
        while skills.contains(where: { $0.effectiveName == candidate }) {
            let room = AgentSkillNaming.maximumLength - "-\(suffix)".count
            candidate = "\(name.prefix(room))-\(suffix)"
            suffix += 1
        }
        return candidate
    }

    /// The document's first Markdown heading, which is where a skill written for
    /// the standard puts its human-facing name — the format itself has no title
    /// field. Falls back to the name.
    private static func title(of document: AgentSkillDocument) -> String {
        for line in document.body.split(separator: "\n", omittingEmptySubsequences: false) {
            let trimmed = line.trimmingCharacters(in: .whitespaces)
            guard trimmed.hasPrefix("# ") else {
                if trimmed.isEmpty { continue }
                break
            }
            return String(trimmed.dropFirst(2)).trimmingCharacters(in: .whitespaces)
        }
        return document.name
    }

    // MARK: - Persistence

    /// Mutate, save, and put it back if the save failed. Lifted from
    /// `AIConfigurationModel.mutatePersistedState`: observable state that
    /// survived a failed write is state the user would go on editing.
    @discardableResult
    private func mutatePersistedState(_ mutation: () -> Void) -> Bool {
        let previous = skills
        mutation()
        guard let persistence else { return true }
        do {
            try persistence.save(skills)
            storageError = nil
            return true
        } catch {
            skills = previous
            storageError = error.localizedDescription
            return false
        }
    }
}