import Foundation

enum RDEPUBNavigationState: Equatable {
    case idle
    case initialLoading
    case restoringLocation
    case preparingChapter(spineIndex: Int)
    case presentingWindow
    case reconcilingFullMap
    case repaginating
}

final class RDEPUBNavigationStateMachine {

    private let lock = NSLock()

    private(set) var state: RDEPUBNavigationState = .idle

    func transition(to newState: RDEPUBNavigationState) {
        lock.lock()
        let oldState = state
        state = newState
        lock.unlock()
        #if DEBUG
        validateTransition(from: oldState, to: newState)
        #endif
    }

    #if DEBUG
    private func validateTransition(from oldState: RDEPUBNavigationState, to newState: RDEPUBNavigationState) {
        let isValid: Bool
        switch (oldState, newState) {
        case (.idle, .initialLoading),
             (.idle, .preparingChapter),
             (.idle, .restoringLocation),
             (.idle, .idle):
            isValid = true
        case (.initialLoading, .preparingChapter),
             (.initialLoading, .presentingWindow),
             (.initialLoading, .idle),
             (.initialLoading, .initialLoading):
            isValid = true
        case (.restoringLocation, .preparingChapter),
             (.restoringLocation, .presentingWindow),
             (.restoringLocation, .idle),
             (.restoringLocation, .restoringLocation):
            isValid = true
        case (.preparingChapter, .presentingWindow),
             (.preparingChapter, .preparingChapter),
             (.preparingChapter, .idle):
            isValid = true
        case (.presentingWindow, .idle),
             (.presentingWindow, .reconcilingFullMap),
             (.presentingWindow, .preparingChapter),
             (.presentingWindow, .presentingWindow),
             (.presentingWindow, .repaginating):
            isValid = true
        case (.reconcilingFullMap, .presentingWindow),
             (.reconcilingFullMap, .idle),
             (.reconcilingFullMap, .reconcilingFullMap):
            isValid = true
        case (.repaginating, .presentingWindow),
             (.repaginating, .idle),
             (.repaginating, .repaginating):
            isValid = true
        default:
            isValid = false
        }
        if !isValid {
            RDEPUBBackgroundTrace.log(
                "NavigationStateMachine",
                "WARNING: unexpected transition \(oldState) → \(newState)"
            )
        }
    }
    #endif
}