Add a counter to a URL

IncrementCounter.swift

//
//  IncrementCounter.swift
//
//  Created by Emory Dunn on 2019-04-01.
//

import Foundation

extension URL {
    
    /// Appends or increments a counter on a file name
    ///
    /// The last integer in a file name is used as a counter, such that `Some File 1`
    /// would be incremented to `Some File 2`. If there is no counter, `Some File`, a
    /// counter will be added starting with `1`: `Some File 1`.
    ///
    /// If a counter is found but can't be converted to an integer the file
    /// will be treated as if it had no counter.
    ///
    /// - Parameters:
    ///   - count: The initial counter to use if none exists
    ///   - increment: How much to increment the counter by
    ///   - format: A format string for the counter. Defaults to `"%01d"`
    ///   - delimiter: The string separating the file name and the counter. Defaults to `" "` (a single space)
    /// - Returns: A new URL with a counter
    mutating func incrementCounter(starting count: Int = 1, by increment: Int = 1, format: String = "%01d", delimiter: String = " ") {
        self = self.incrementingCounter(starting: count, by: increment, format: format, delimiter: delimiter)
    }
    
    /// Returns a URL constructed by appending or incrementing a counter on a file name
    ///
    /// The last integer in a file name is used as a counter, such that `Some File 1`
    /// would be incremented to `Some File 2`. If there is no counter, `Some File`, a
    /// counter will be added starting with `1`: `Some File 1`.
    ///
    /// If a counter is found but can't be converted to an integer the file
    /// will be treated as if it had no counter.
    ///
    /// - Parameters:
    ///   - count: The initial counter to use if none exists
    ///   - increment: How much to increment the counter by
    ///   - format: A format string for the counter. Defaults to `"%01d"`
    ///   - delimiter: The string separating the file name and the counter. Defaults to `" "` (a single space)
    /// - Returns: A new URL with a counter
    func incrementingCounter(starting count: Int = 1, by increment: Int = 1, format: String = "%01d", delimiter: String = " ") -> URL {
        
        // Break apart the URL
        let ext = self.pathExtension
        var fileName = self.deletingPathExtension().lastPathComponent
        
        let tempDest = self.deletingLastPathComponent()
        var counter = count
        
        // Find any suffix digits
        fileCounter: if let digitRange = fileName.range(of: "\(delimiter)\\d+$", options: .regularExpression) {
            
            let delimiterSet = CharacterSet(charactersIn: delimiter)
            
            // Extract the digits
            let subString = String(fileName[digitRange]).trimmingCharacters(in: delimiterSet)
            guard let fileCounter = Int(subString) else { break fileCounter}
            
            // Increment the counter
            counter = fileCounter + increment
            
            // Remove the existing counter
            fileName.removeSubrange(digitRange)
            
        }
        
        // Append the counter with a space
        let formattedCounter = String(format: format, counter)
        let newName = fileName.appending("\(delimiter)\(formattedCounter)")
        return tempDest.appendingPathComponent(newName).appendingPathExtension(ext)
        
    }
}