Lazily read a line at a time in Swift

SwiftLineReader.swift

import Foundation

// known good: Swift 5

typealias LineIteratorState = (
  // pointer to a C string representing a line
  linePtr:UnsafeMutablePointer<CChar>?,
  linecap:Int,
  filePtr:UnsafeMutablePointer<FILE>?
)

/// Returns a sequence which iterates through all lines of the the file at the URL.
///
/// - Parameter url: file URL of a file to read
/// - Returns: a Sequence which lazily iterates through lines of the file
///
/// - warning: the caller of this function **must** iterate through all lines of the file, since aborting iteration midway will leak memory and a file pointer
/// - precondition: the file must be UTF8-encoded (which includes, ASCII-encoded)
func lines(ofFile url:URL) -> UnfoldSequence<String,LineIteratorState>
{
  var state:LineIteratorState = (linePtr:nil,linecap:0,filePtr:nil)
  state.filePtr = fopen(fileURL.path,"r")
  func cleanup(state s:LineIteratorState) {
    if let actualLine = s.linePtr  { free(actualLine) }
    fclose(s.filePtr)
  }
  return sequence(state: state, next: { (currentState) -> String? in
    if getline(&currentState.linePtr, &currentState.linecap, currentState.filePtr) > 0,
      let theLine = currentState.linePtr  {
      return String.init(cString:theLine)
    }
    else {
      cleanup(state: currentState)
      return nil
    }
  })
}


// example of how to use:
var seq = lines(ofFile: fileURL)

for line in seq {
  print(line)
}