Iterates over a large string line-by-line, with configurable line endings. Uses Swift 4's Substring for efficiency.

LineParser.swift

//
//  LineParser.swift
//  Mail
//
//  Created by Adam Knight on 1/26/18.
//

import Foundation

public class LineParser: Sequence, IteratorProtocol {
	public let text: String
	private var remaining: Substring
	var currentIndex: String.Index
	var delimiter: String
	
	public init(_ text: String, delimiter: String = "\r\n") {
		self.text = text
		self.delimiter = delimiter
		
		remaining = text[text.startIndex..<text.endIndex]
		currentIndex = remaining.startIndex
	}
	
	public func next() -> Substring? {
		let line: Substring
		
		// Find the next delimiter
		if let range = remaining.range(of: delimiter, options: .literal) {
			// Get the line
			line = remaining[remaining.startIndex..<range.lowerBound]
			
			// Update the remaining text
			remaining = remaining[range.upperBound..<remaining.endIndex]
			
			// Update the current index.
			currentIndex = range.upperBound
			
			return line
		}
		
		// Are we done? Use nil as EOF.
		if remaining.isEmpty { return nil }
		
		// Return the rest of the string as the last line.
		line = remaining
		remaining = remaining[remaining.endIndex..<remaining.endIndex]
		
		return line
	}
}