//
//  RaPlaFetcher.swift
//  DHBW-Service
//
//  Created by Patrick Müller on 29.01.21.
//

import Foundation
import CoreData

class RaPlaFetcher {
    public class iCalEvent {
        var startDate: Date = Date()    // DTSTART
        var endDate: Date = Date()      // DTEND
        var summary: String = ""        // SUMMARY
        var description: String = ""    // DESCRIPTION
        var location: String = ""       // LOCATION
        var category: String = ""       // CATEGORIES
        var uid: String = ""            // UID
        var lecturers: [LecturerObj] = []  // ATTENDEE
        var excludedDates: [Date] = []  // EXDATE
        var isRecurring: Bool = false   // If the event is recurring
        var frequency: String = ""      // Frequence in case of recurring events, e.g. DAILY or WEEKLY
        var recCount: Int = 0           // How often the event occurs in case of recurring events
        var recInterval: Int = 0        // Interval of the recurring event, e.g. 1 for every week / day / ...
        var recDay: String = ""         // The day of the recurrence, e.g. FR for friday
        var recUntil: Date = Date()     // Until when the event has to be repeated
    }
    
    public class LecturerObj {
        var name: String = ""
        var email: String = ""
    }
    
    // Get the RaPla file from the given URL and save the events to CoreData
    public class func getRaplaFileAndSaveToCoreData(from urlString: String) -> Bool {
        let file = getFileAsString(from: urlString)
        
        let eventStrings = splitIntoEvents(file: file)
        
        let eventObjects = convertStringsToObjects(eventStrings: eventStrings)
        
        return saveToCoreData(eventObjects: eventObjects)
    }
    
    // Get the RaPla files from the given URL and return the event objects
    public class func getRaplaFileAndReturnEvents(from urlString: String) -> [iCalEvent] {
        let file = getFileAsString(from: urlString)
        
        let eventStrings = splitIntoEvents(file: file)
        
        return convertStringsToObjects(eventStrings: eventStrings)
    }
    
    // GET the file from the given URL and convert it to a String that is then returned
    private class func getFileAsString(from urlString: String) -> String {
        let url = URL(string: urlString)!
        var file: String = ""
        
        do {
            file = try String(contentsOf: url, encoding: .utf8)
        } catch let error {
            print(error.localizedDescription)
        }
        
        return file
    }
    
    // Split the given ical file string into individual event strings and return them as a list
    private class func splitIntoEvents(file: String) -> [String] {
        let regexOptions: NSRegularExpression.Options = [.dotMatchesLineSeparators]
        // Regex explanation: Matches BEGIN:VEVENT "Any character" END:VEVENT across multiple lines. The *? assures that we receive the
        // maximum amount of matches, i.e. it makes the regex non-greedy as we would otherwise just receive one giant match
        let eventStrings = UtilityFunctions.regexMatches(for: "BEGIN:VEVENT.*?END:VEVENT", with: regexOptions, in: file)
        
        return eventStrings
    }
    
    // Convert an ical event String into an iCalEvent Codable object as defined above
    private class func convertStringsToObjects(eventStrings: [String]) -> [iCalEvent] {
        var events: [iCalEvent] = []
        
        for eventString in eventStrings {
            var lines = eventString.components(separatedBy: .newlines)
            // Remove all blank lines that somehow are generated by the .components function
            lines = removeBlankLines(lines: lines)
            
            let evt = iCalEvent()
            
            // Iterate over all lines and merge lines that have been split by rapla first
            for lineNr in 0...lines.count-1 {
                if(lines[lineNr].hasPrefix(" ")){
                    lines[lineNr] = String(lines[lineNr].dropFirst())
                    
                    // If there are more than 2 lines that have to be merged, we need to find out how many lines we have to go up
                    var goUp = 1
                    while(lines[lineNr-goUp] == ""){
                        goUp += 1
                    }
                    
                    lines[lineNr-goUp].append(lines[lineNr])
                    lines[lineNr] = ""
                }
            }
            
            // Remove all blank lines again as we produced some while merging the lines
            lines = removeBlankLines(lines: lines)
            
            for line in lines {
                var lineWithoutPrefix = line
                if(!line.contains(":")) {
                    continue
                }
                lineWithoutPrefix.removeSubrange(lineWithoutPrefix.startIndex...lineWithoutPrefix.firstIndex(of: ":")!)
                
                if(line.hasPrefix("DTSTART")){
                    //Date format: 20181101T080000
                    let dateFormatter = DateFormatter()
                    if(lineWithoutPrefix.contains("Z")){
                        dateFormatter.dateFormat = "yyyyMMdd'T'HHmmss'Z'"
                    } else {
                        dateFormatter.dateFormat = "yyyyMMdd'T'HHmmss"
                    }
                    let date = dateFormatter.date(from: lineWithoutPrefix)!
                    evt.startDate = date
                } else if(line.hasPrefix("DTEND")){
                    let dateFormatter = DateFormatter()
                    if(lineWithoutPrefix.contains("Z")){
                        dateFormatter.dateFormat = "yyyyMMdd'T'HHmmss'Z'"
                    } else {
                        dateFormatter.dateFormat = "yyyyMMdd'T'HHmmss"
                    }
                    let date = dateFormatter.date(from: lineWithoutPrefix)!
                    evt.endDate = date
                } else if(line.hasPrefix("SUMMARY")){
                    evt.summary = lineWithoutPrefix
                } else if(line.hasPrefix("DESCRIPTION")){
                    evt.description = lineWithoutPrefix
                } else if(line.hasPrefix("LOCATION")){
                    evt.location = lineWithoutPrefix
                } else if(line.hasPrefix("CATEGORIES")){
                    evt.category = lineWithoutPrefix
                } else if(line.hasPrefix("UID")){
                    evt.uid = lineWithoutPrefix
                } else if(line.hasPrefix("ATTENDEE;ROLE=REQ-PARTICIPANT;")) {
                    var lecturerName = line
                    
                    let begin = lecturerName.firstIndex(of: "\"")!
                    lecturerName.removeSubrange(lecturerName.startIndex...begin)
                    
                    let end = lecturerName.lastIndex(of: "\"")!
                    lecturerName = String(lecturerName[..<end])
                    
                    let lecturerEmail = String(lineWithoutPrefix[String.Index(utf16Offset: 7, in: lineWithoutPrefix)..<lineWithoutPrefix.endIndex])
                    
                    let lecturer = LecturerObj()
                    lecturer.name = lecturerName
                    lecturer.email = lecturerEmail
                    
                    evt.lecturers.append(lecturer)
                } else if(line.hasPrefix("RRULE")) {
                    // This line normally looks like this:  RRULE:FREQ=WEEKLY;COUNT=12;INTERVAL=1;BYDAY=TU
                    // Can also look smth like this though: RRULE:FREQ=WEEKLY;COUNT=12
                    
                    evt.isRecurring = true
                    
                    let params = lineWithoutPrefix.components(separatedBy: ";")
                    
                    for param in params {
                        let keyword = param[param.startIndex..<param.firstIndex(of: "=")!]
                        let value = param[param.index(param.firstIndex(of: "=")!, offsetBy: 1)..<param.endIndex]
                        
                        switch keyword {
                        case "FREQ":
                            evt.frequency = String(value)
                            break
                        case "COUNT":
                            evt.recCount = Int(value)!
                            break
                        case "INTERVAL":
                            evt.recInterval = Int(value)!
                            break
                        case "BYDAY":
                            evt.recDay = String(value)
                            break
                        case "UNTIL":
                            let dateFormatter = DateFormatter()
                            dateFormatter.dateFormat = "yyyyMMdd"
                            let date = dateFormatter.date(from: String(value))!
                            evt.recUntil = date
                            break
                        default:
                            print("Unknown parameter found: " + line)
                        }
                    }
                } else if (line.hasPrefix("EXDATE")) {
                    // Excluded from recurring events
                    // Format: 20210401T133000,20210408T133000,20210513T133000
                    
                    let exclDateStrings = lineWithoutPrefix.components(separatedBy: ",")
                    
                    for exclDate in exclDateStrings {
                        let dateFormatter = DateFormatter()
                        if(lineWithoutPrefix.contains("Z")){
                            dateFormatter.dateFormat = "yyyyMMdd'T'HHmmss'Z'"
                        } else {
                            dateFormatter.dateFormat = "yyyyMMdd'T'HHmmss"
                        }
                        let date = dateFormatter.date(from: exclDate)!
                        evt.excludedDates.append(date)
                    }
                }
            }
            
            events.append(evt)
        }
        
        return events
    }
    
    // Save the given iCalEvent objects to CoreData
    // Updates the events if they already exist and deletes old (/invalid) ones
    private class func saveToCoreData(eventObjects: [iCalEvent]) -> Bool{
        // Get known UIDs
        let existingEvents: [RaPlaEvent] = RaPlaEvent.getAll()
        var existingEventsDict: [String:RaPlaEvent] = [:]
        for event in existingEvents {
            existingEventsDict[event.uid!] = event
        }
        // List for new UIDs
        var newEventUIDs: [String] = []
        
        for event in eventObjects {
            // If the event already exists locally, update it. Otherwise, create a new record
            if(event.isRecurring){
                // Create as many events as we need
                // If we e.g. need 12 events, we create 0...11
                for iteration in 0..<event.recCount {
                    // Calculate start- and enddate
                    // Calculate offset
                    let offsetType: Calendar.Component
                    let offsetAmount: Int
                    
                    switch event.frequency {
                    case "DAILY":
                        offsetType = Calendar.Component.day
                        offsetAmount = 1
                        break
                    case "WEEKLY":
                        offsetType = Calendar.Component.day
                        offsetAmount = 7
                        break
                    case "MONTHLY":
                        offsetType = Calendar.Component.month
                        offsetAmount = 1
                        break
                    case "YEARLY":
                        offsetType = Calendar.Component.year
                        offsetAmount = 1
                        break
                    default:
                        offsetType = Calendar.Component.day
                        offsetAmount = 0
                        print("Found unknown frequency: " + event.frequency)
                    }
                    
                    let startDate = Calendar.current.date(byAdding: offsetType, value: (offsetAmount * iteration), to: event.startDate)!
                    let endDate = Calendar.current.date(byAdding: offsetType, value: (offsetAmount * iteration), to: event.endDate)!
                    
                    // Check if this recurrence should be excluded
                    if(event.excludedDates.contains(startDate)){
                        continue
                    }
                    
                    // Generate UID
                    // Appending iteration to distinguish between recurring events
                    let newUID = event.uid + "---" + String(iteration)
                    
                    // Create or update existing CoreData object
                    let evt: RaPlaEvent
                    if existingEventsDict.keys.contains(newUID) {
                        evt = existingEventsDict[newUID]!
                    } else {
                        evt = RaPlaEvent(context: PersistenceController.shared.context)
                        
                        // Set default values for new object
                        evt.isHidden = false
                    }
                    
                    // Populate fields
                    // (offsetAmount * iteration) because for the 1st event, we dont want to add an offset, and
                    // for every event after that we want to add e.g. 1 week, 2 weeks, 3 weeks etc.
                    evt.startDate = startDate
                    evt.endDate = endDate
                    evt.summary = event.summary
                    evt.descr = event.description
                    evt.location = event.location
                    evt.category = event.category
                    evt.uid = newUID
                    for lecturer in event.lecturers {
                        let lect = Lecturer(context: PersistenceController.shared.context)
                        lect.name = lecturer.name
                        lect.email = lecturer.email
                        lect.event = evt
                    }
                    
                    // Add UID to new UIDs list
                    newEventUIDs.append(newUID)
                }
            } else {
                // Generate UID
                let newUID = event.uid + "---0"  // Appending ---0 to distinguish between recurring events
                
                // Create or update existing CoreData object
                let evt: RaPlaEvent
                if existingEventsDict.keys.contains(newUID) {
                    evt = existingEventsDict[newUID]!
                } else {
                    evt = RaPlaEvent(context: PersistenceController.shared.context)
                    
                    // Set default values for new object
                    evt.isHidden = false
                }
                
                // Populate fields
                evt.startDate = event.startDate
                evt.endDate = event.endDate
                evt.summary = event.summary
                evt.descr = event.description
                evt.location = event.location
                evt.category = event.category
                evt.uid = newUID
                for lecturer in event.lecturers {
                    let lect = Lecturer(context: PersistenceController.shared.context)
                    lect.name = lecturer.name
                    lect.email = lecturer.email
                    lect.event = evt
                }
                
                // Add UID to new UIDs list
                newEventUIDs.append(newUID)
            }
        }
        
        // Now we also have to delete locally stored events that have been deleted from RaPla
        for localUid in existingEventsDict.keys {
            if(!newEventUIDs.contains(localUid)){
                // Locally stored event does not exist in RaPla anymore, delete it
                let evt = existingEventsDict[localUid]
                PersistenceController.shared.context.delete(evt!)
            }
        }
        
        PersistenceController.shared.save()
        
        return true
    }
    
    private class func removeBlankLines(lines: [String]) -> [String] {
        var newLines = lines
        
        // Remove all blank lines that somehow are generated by the .components function
        for line in newLines {
            if(line.isEmpty){
                newLines.remove(at: newLines.firstIndex(of: line)!)
            }
        }
        
        return newLines
    }
}