import Foundation

enum GoogleClassroomClientError: Error {
    case invalidResponse
    case httpStatus(Int, String)
    case decodeFailed(String)
}

/// Minimal Google Classroom REST wrapper for a student's to-do list.
final class GoogleClassroomClient {
    struct Options: Sendable {
        var maxCourses: Int
        var maxCourseWorkPerCourse: Int

        init(maxCourses: Int = 20, maxCourseWorkPerCourse: Int = 50) {
            self.maxCourses = maxCourses
            self.maxCourseWorkPerCourse = maxCourseWorkPerCourse
        }
    }

    private let session: URLSession

    init(session: URLSession = .shared) {
        self.session = session
    }

    func fetchTodo(accessToken: String, options: Options? = nil) async throws -> [ClassroomTodoItem] {
        let resolvedOptions = options ?? Options()
        let courses = try await listActiveCourses(accessToken: accessToken, pageSize: resolvedOptions.maxCourses)
        if courses.isEmpty { return [] }

        var todos: [ClassroomTodoItem] = []
        todos.reserveCapacity(courses.count * 8)

        for course in courses.prefix(max(1, resolvedOptions.maxCourses)) {
            let courseWork = try await listPublishedCourseWork(
                accessToken: accessToken,
                courseId: course.id,
                pageSize: resolvedOptions.maxCourseWorkPerCourse
            )
            for work in courseWork {
                let due = work.resolvedDueDate
                let link = work.alternateLink.flatMap(URL.init(string:))
                let workType = ClassroomTodoWorkType(rawValue: work.workType ?? "") ?? .unspecified
                todos.append(
                    ClassroomTodoItem(
                        id: "\(course.id)::\(work.id ?? UUID().uuidString)",
                        courseId: course.id,
                        courseName: course.name ?? "Course",
                        title: (work.title?.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty == false) ? (work.title ?? "Untitled") : "Untitled",
                        dueDate: due,
                        alternateLink: link,
                        workType: workType
                    )
                )
            }
        }

        // Keep only items with a due date near-term first; undated items at end.
        todos.sort {
            switch ($0.dueDate, $1.dueDate) {
            case let (a?, b?):
                if a != b { return a < b }
                return $0.courseName < $1.courseName
            case (_?, nil):
                return true
            case (nil, _?):
                return false
            case (nil, nil):
                if $0.courseName != $1.courseName { return $0.courseName < $1.courseName }
                return $0.title < $1.title
            }
        }
        return todos
    }

    func fetchEnrolledCourses(accessToken: String, pageSize: Int = 50) async throws -> [ClassroomCourse] {
        let courses = try await listActiveCourses(accessToken: accessToken, pageSize: pageSize)
        if courses.isEmpty { return [] }

        var enrolled: [ClassroomCourse] = []
        enrolled.reserveCapacity(courses.count)
        for course in courses {
            let teachers = try await listCourseTeachers(accessToken: accessToken, courseId: course.id)
            let teacherNames = teachers.compactMap { teacher -> String? in
                let profileName = teacher.profile?.name?.fullName?.trimmingCharacters(in: .whitespacesAndNewlines)
                if let profileName, profileName.isEmpty == false { return profileName }
                let email = teacher.profile?.emailAddress?.trimmingCharacters(in: .whitespacesAndNewlines)
                if let email, email.isEmpty == false { return email }
                return nil
            }

            enrolled.append(
                ClassroomCourse(
                    id: course.id,
                    name: (course.name?.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty == false) ? (course.name ?? "Course") : "Course",
                    section: course.section,
                    room: course.room,
                    teacherNames: teacherNames,
                    enrollmentCode: course.enrollmentCode,
                    courseState: course.courseState ?? "ACTIVE"
                )
            )
        }

        enrolled.sort {
            if $0.name != $1.name { return $0.name.localizedCaseInsensitiveCompare($1.name) == .orderedAscending }
            return $0.id < $1.id
        }
        return enrolled
    }

    func fetchClassDetails(accessToken: String, courseId: String, courseName: String) async throws -> ClassroomClassDetails {
        async let announcementsTask = listCourseAnnouncements(accessToken: accessToken, courseId: courseId, pageSize: 30)
        async let courseworkTask = listPublishedCourseWork(accessToken: accessToken, courseId: courseId, pageSize: 30)
        let (announcements, coursework) = try await (announcementsTask, courseworkTask)

        let mappedAnnouncements = announcements.map { item in
            let text = item.text?.trimmingCharacters(in: .whitespacesAndNewlines)
            return ClassroomAnnouncement(
                id: item.id ?? UUID().uuidString,
                text: (text?.isEmpty == false) ? text! : "Announcement",
                postedAt: item.createdDate,
                alternateLink: item.alternateLink.flatMap(URL.init(string:)),
                attachments: mapMaterialsToAttachments(item.materials)
            )
        }.sorted { ($0.postedAt ?? .distantPast) > ($1.postedAt ?? .distantPast) }

        let mappedCourseWork = coursework.map { work in
            let title = work.title?.trimmingCharacters(in: .whitespacesAndNewlines)
            let workType = ClassroomTodoWorkType(rawValue: work.workType ?? "") ?? .unspecified
            return ClassroomCourseWork(
                id: work.id ?? UUID().uuidString,
                title: (title?.isEmpty == false) ? title! : "Untitled",
                subtitle: workType.displayName,
                dueDate: work.resolvedDueDate,
                alternateLink: work.alternateLink.flatMap(URL.init(string:)),
                workType: workType,
                attachments: mapMaterialsToAttachments(work.materials)
            )
        }.sorted { ($0.dueDate ?? .distantFuture) < ($1.dueDate ?? .distantFuture) }

        return ClassroomClassDetails(
            courseId: courseId,
            courseName: courseName,
            courseLink: URL(string: "https://classroom.google.com/c/\(courseId)"),
            announcements: mappedAnnouncements,
            coursework: mappedCourseWork
        )
    }

    // MARK: - Courses

    private func listActiveCourses(accessToken: String, pageSize: Int) async throws -> [Course] {
        var components = URLComponents(string: "https://classroom.googleapis.com/v1/courses")!
        components.queryItems = [
            URLQueryItem(name: "studentId", value: "me"),
            URLQueryItem(name: "courseStates", value: "ACTIVE"),
            URLQueryItem(name: "pageSize", value: String(max(1, min(100, pageSize))))
        ]

        var request = URLRequest(url: components.url!)
        request.httpMethod = "GET"
        request.setValue("Bearer \(accessToken)", forHTTPHeaderField: "Authorization")

        let (data, response) = try await session.data(for: request)
        guard let http = response as? HTTPURLResponse else { throw GoogleClassroomClientError.invalidResponse }
        guard (200..<300).contains(http.statusCode) else {
            let body = String(data: data, encoding: .utf8) ?? "<no body>"
            throw GoogleClassroomClientError.httpStatus(http.statusCode, body)
        }

        let decoded: CoursesList
        do {
            decoded = try JSONDecoder().decode(CoursesList.self, from: data)
        } catch {
            let raw = String(data: data, encoding: .utf8) ?? "<unreadable body>"
            throw GoogleClassroomClientError.decodeFailed(raw)
        }
        return decoded.courses ?? []
    }

    private func listCourseTeachers(accessToken: String, courseId: String) async throws -> [Teacher] {
        var components = URLComponents(string: "https://classroom.googleapis.com/v1/courses/\(courseId)/teachers")!
        components.queryItems = [
            URLQueryItem(name: "pageSize", value: "30")
        ]

        var request = URLRequest(url: components.url!)
        request.httpMethod = "GET"
        request.setValue("Bearer \(accessToken)", forHTTPHeaderField: "Authorization")

        let (data, response) = try await session.data(for: request)
        guard let http = response as? HTTPURLResponse else { throw GoogleClassroomClientError.invalidResponse }
        guard (200..<300).contains(http.statusCode) else {
            let body = String(data: data, encoding: .utf8) ?? "<no body>"
            throw GoogleClassroomClientError.httpStatus(http.statusCode, body)
        }

        let decoded: TeachersList
        do {
            decoded = try JSONDecoder().decode(TeachersList.self, from: data)
        } catch {
            let raw = String(data: data, encoding: .utf8) ?? "<unreadable body>"
            throw GoogleClassroomClientError.decodeFailed(raw)
        }
        return decoded.teachers ?? []
    }

    private func listCourseAnnouncements(accessToken: String, courseId: String, pageSize: Int) async throws -> [CourseAnnouncement] {
        var components = URLComponents(string: "https://classroom.googleapis.com/v1/courses/\(courseId)/announcements")!
        components.queryItems = [
            URLQueryItem(name: "announcementStates", value: "PUBLISHED"),
            URLQueryItem(name: "orderBy", value: "updateTime desc"),
            URLQueryItem(name: "pageSize", value: String(max(1, min(100, pageSize))))
        ]

        var request = URLRequest(url: components.url!)
        request.httpMethod = "GET"
        request.setValue("Bearer \(accessToken)", forHTTPHeaderField: "Authorization")

        let (data, response) = try await session.data(for: request)
        guard let http = response as? HTTPURLResponse else { throw GoogleClassroomClientError.invalidResponse }
        guard (200..<300).contains(http.statusCode) else {
            let body = String(data: data, encoding: .utf8) ?? "<no body>"
            throw GoogleClassroomClientError.httpStatus(http.statusCode, body)
        }

        let decoded: CourseAnnouncementList
        do {
            decoded = try JSONDecoder().decode(CourseAnnouncementList.self, from: data)
        } catch {
            let raw = String(data: data, encoding: .utf8) ?? "<unreadable body>"
            throw GoogleClassroomClientError.decodeFailed(raw)
        }
        return decoded.announcements ?? []
    }

    // MARK: - CourseWork

    private func listPublishedCourseWork(accessToken: String, courseId: String, pageSize: Int) async throws -> [CourseWork] {
        var components = URLComponents(string: "https://classroom.googleapis.com/v1/courses/\(courseId)/courseWork")!
        components.queryItems = [
            URLQueryItem(name: "courseWorkStates", value: "PUBLISHED"),
            URLQueryItem(name: "orderBy", value: "dueDate desc"),
            URLQueryItem(name: "pageSize", value: String(max(1, min(100, pageSize))))
        ]

        var request = URLRequest(url: components.url!)
        request.httpMethod = "GET"
        request.setValue("Bearer \(accessToken)", forHTTPHeaderField: "Authorization")

        let (data, response) = try await session.data(for: request)
        guard let http = response as? HTTPURLResponse else { throw GoogleClassroomClientError.invalidResponse }
        guard (200..<300).contains(http.statusCode) else {
            let body = String(data: data, encoding: .utf8) ?? "<no body>"
            throw GoogleClassroomClientError.httpStatus(http.statusCode, body)
        }

        let decoded: CourseWorkList
        do {
            decoded = try JSONDecoder().decode(CourseWorkList.self, from: data)
        } catch {
            let raw = String(data: data, encoding: .utf8) ?? "<unreadable body>"
            throw GoogleClassroomClientError.decodeFailed(raw)
        }
        return decoded.courseWork ?? []
    }

    private func mapMaterialsToAttachments(_ materials: [Material]?) -> [ClassroomAttachment] {
        guard let materials else { return [] }
        var attachments: [ClassroomAttachment] = []
        attachments.reserveCapacity(materials.count)

        for (index, material) in materials.enumerated() {
            if let drive = material.driveFile?.driveFile,
               let linkString = drive.alternateLink,
               let url = URL(string: linkString) {
                attachments.append(
                    ClassroomAttachment(
                        id: drive.id ?? "drive-\(index)",
                        title: drive.title?.nonEmptyOr("Drive file") ?? "Drive file",
                        url: url,
                        mimeType: drive.mimeType,
                        sourceType: "driveFile"
                    )
                )
            } else if let link = material.link,
                      let linkString = link.url,
                      let url = URL(string: linkString) {
                attachments.append(
                    ClassroomAttachment(
                        id: "link-\(index)",
                        title: link.title?.nonEmptyOr("Link") ?? "Link",
                        url: url,
                        mimeType: nil,
                        sourceType: "link"
                    )
                )
            } else if let form = material.form,
                      let linkString = form.formUrl,
                      let url = URL(string: linkString) {
                attachments.append(
                    ClassroomAttachment(
                        id: "form-\(index)",
                        title: form.title?.nonEmptyOr("Google Form") ?? "Google Form",
                        url: url,
                        mimeType: nil,
                        sourceType: "form"
                    )
                )
            } else if let yt = material.youtubeVideo,
                      let linkString = yt.alternateLink,
                      let url = URL(string: linkString) {
                attachments.append(
                    ClassroomAttachment(
                        id: "youtube-\(index)",
                        title: yt.title?.nonEmptyOr("YouTube") ?? "YouTube",
                        url: url,
                        mimeType: nil,
                        sourceType: "youtubeVideo"
                    )
                )
            }
        }

        return attachments
    }
}

extension GoogleClassroomClientError: LocalizedError {
    var errorDescription: String? {
        switch self {
        case .invalidResponse:
            return "Google Classroom returned an invalid response."
        case let .httpStatus(status, body):
            return "Google Classroom API error (\(status)): \(body)"
        case let .decodeFailed(raw):
            return "Failed to parse Google Classroom response: \(raw)"
        }
    }
}

// MARK: - REST models (minimal)

private struct CoursesList: Decodable {
    let courses: [Course]?
    let nextPageToken: String?
}

private struct Course: Decodable {
    let id: String
    let name: String?
    let section: String?
    let room: String?
    let enrollmentCode: String?
    let courseState: String?
}

private struct CourseWorkList: Decodable {
    let courseWork: [CourseWork]?
    let nextPageToken: String?
}

private struct CourseWork: Decodable {
    let id: String?
    let title: String?
    let alternateLink: String?
    let workType: String?
    let dueDate: DateParts?
    let dueTime: TimeOfDay?
    let materials: [Material]?

    var resolvedDueDate: Date? {
        guard let dueDate else { return nil }
        var comps = DateComponents()
        comps.calendar = Calendar.current
        comps.timeZone = TimeZone.current
        comps.year = dueDate.year
        comps.month = dueDate.month
        comps.day = dueDate.day
        if let dueTime {
            comps.hour = dueTime.hours
            comps.minute = dueTime.minutes
            comps.second = dueTime.seconds
        } else {
            comps.hour = 23
            comps.minute = 59
            comps.second = 0
        }
        return comps.date
    }
}

private struct CourseAnnouncementList: Decodable {
    let announcements: [CourseAnnouncement]?
    let nextPageToken: String?
}

private struct CourseAnnouncement: Decodable {
    let id: String?
    let text: String?
    let alternateLink: String?
    let creationTime: String?
    let materials: [Material]?

    var createdDate: Date? {
        guard let creationTime else { return nil }
        return Date.parseRFC3339(creationTime)
    }
}

private struct DateParts: Decodable {
    let year: Int?
    let month: Int?
    let day: Int?
}

private struct TimeOfDay: Decodable {
    let hours: Int?
    let minutes: Int?
    let seconds: Int?
    let nanos: Int?
}

private struct Material: Decodable {
    let driveFile: SharedDriveFileContainer?
    let link: SharedLink?
    let form: SharedForm?
    let youtubeVideo: SharedYoutubeVideo?
}

private struct SharedDriveFileContainer: Decodable {
    let driveFile: SharedDriveFile?
}

private struct SharedDriveFile: Decodable {
    let id: String?
    let title: String?
    let alternateLink: String?
    let mimeType: String?
}

private struct SharedLink: Decodable {
    let url: String?
    let title: String?
}

private struct SharedForm: Decodable {
    let formUrl: String?
    let title: String?
}

private struct SharedYoutubeVideo: Decodable {
    let alternateLink: String?
    let title: String?
}

private struct TeachersList: Decodable {
    let teachers: [Teacher]?
    let nextPageToken: String?
}

private struct Teacher: Decodable {
    let profile: UserProfile?
}

private struct UserProfile: Decodable {
    let name: UserName?
    let emailAddress: String?
}

private struct UserName: Decodable {
    let fullName: String?
}

private extension String {
    func nonEmptyOr(_ fallback: String) -> String {
        let trimmed = trimmingCharacters(in: .whitespacesAndNewlines)
        return trimmed.isEmpty ? fallback : trimmed
    }
}

private extension Date {
    static func parseRFC3339(_ text: String) -> Date? {
        let formatterWithFractional = ISO8601DateFormatter()
        formatterWithFractional.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
        if let value = formatterWithFractional.date(from: text) { return value }
        let formatter = ISO8601DateFormatter()
        formatter.formatOptions = [.withInternetDateTime]
        return formatter.date(from: text)
    }
}