LineByLine.swift

import Foundation

/// Adapted from https://stackoverflow.com/a/62112007
/// https://stackoverflow.com/questions/31778700/read-a-text-file-line-by-line-in-swift

extension URL {
	
	/// Note: Only supports newline as the line break character.
	
	func processLineByLine(processLine: @escaping (_ line: String) -> ()) {
		/// Make sure the file exists.
		guard FileManager.default.fileExists(atPath: self.path) else {
			preconditionFailure("file expected at \(self.absoluteString) is missing")
		}
		
		/// Open the file for reading.
		/// Note: user should be prompted the first time to allow reading from this location.
		guard let filePointer: UnsafeMutablePointer<FILE> = fopen(self.path, "r") else {
			preconditionFailure("Could not open file at \(self.absoluteString)")
		}
		
		defer {
			/// Remember to close the file when done.
			fclose(filePointer)
		}
		
		/// A pointer to a null-terminated, UTF-8 encoded sequence of bytes.
		var lineByteArrayPointer: UnsafeMutablePointer<CChar>? = nil
		
		/// The smallest multiple of 16 that will fit the byte array for this line.
		var lineCap: Int = 0
		
		/// Define nextLine as closure, because we need to use it twice.
		let nextLine = {
			/// For details regarding `getline()` have a look at
			/// https://www.man7.org/linux/man-pages/man3/getline.3.html
			return getline(&lineByteArrayPointer, &lineCap, filePointer)
		}

		defer {
			/// If *lineptr is set to NULL before the call, then getline() will
			/// allocate a buffer for storing the line.  This buffer should be
			/// freed by the user program even if getline() failed.
			lineByteArrayPointer?.deallocate()
		}
		
		/// Initial iteration.
		var bytesRead = nextLine()
		
		while (bytesRead > 0) {
			guard let linePointer = lineByteArrayPointer else {
				break
			}
			
			/// Note: this translates the sequence of bytes to a string
			/// using the UTF-8 encoding for interpreting byte sequences.
			var lineString = String.init(cString: linePointer)
			
			/// `lineString` initially includes the newline character, if one was found.
			if lineString.last?.isNewline == true {
				lineString = String(lineString.dropLast())
			}
			
			/// Process this single line of text.
			processLine(lineString)
			
			/// Update number of bytes read and the pointers for the next iteration.
			bytesRead = nextLine()
		}
	}
	
}