ReadOperation

ReadOperation.swift

//
//  Created by Oliver Epper on 16.02.21.
//
//

import Combine
import Foundation

final class ReadOperation: Operation {
    enum ReadOperationError: Error {
        case cannotOpenFile(URL)
    }

    private let url: URL
    private let linesToRead: Int?
    private let subject = PassthroughSubject<String, ReadOperationError>()

    var linePublisher: AnyPublisher<String, ReadOperationError> {
        subject.eraseToAnyPublisher()
    }

    init(url: URL, lines linesToRead: Int? = nil) {
        self.url = url
        self.linesToRead = linesToRead
        super.init()
    }

    override func main() {
        guard !isCancelled else {
            return
        }

        guard let filePointer: UnsafeMutablePointer<FILE> = fopen(url.path, "r") else {
            subject.send(completion: .failure(.cannotOpenFile(url)))
            return
        }

        var buffer: UnsafeMutablePointer<CChar>?
        var lineCap: Int = 0
        var bytes = 0
        var readLines = 0

        defer {
            free(buffer)
            fclose(filePointer)
        }

        bytes = getline(&buffer, &lineCap, filePointer)

        while bytes > 0, !isCancelled {
            if readLines == linesToRead {
                break
            }
            let line = String(cString: buffer!).trimmingCharacters(in: .newlines)
            subject.send(line)
            bytes = getline(&buffer, &lineCap, filePointer)
            readLines += 1
        }

        subject.send(completion: .finished)
    }
}