Read a single line string from a file

ReadLine.swift

// Swift
//
// Intent: Read a single line string from a file, with some memory efficiency.
// This could be improved by use of an (unsafe) memory buffer
//
// By Andrew Kingdom
// MIT license
// 
class ReadLineTool {

    static func readLine(from fileHandle: FileHandle) throws -> String? {
        let bufferSize = 4096
        let newline = Data.Element(0x0A)
        
        // Gets the position of the file pointer within the file.
        let offset: UInt64 = try fileHandle.offset()
        
        // Create a Data object from the file handle.
        var data = Data()

        // Read data from the file handle until we encounter a newline character.
        while let chunk = try fileHandle.read(upToCount: bufferSize) {
            data.append(chunk)

            if let newlineIndex = data.firstIndex(of: newline) {
                // We found a newline character. Return the line up to the newline character.
                guard let line = String(data: data[0..<newlineIndex], encoding: .utf8) else {
                    // The data is not encoded in UTF-8.
                    return nil
                }
                let lineSize = line.lengthOfBytes(using: .utf8)
                // Reset the file position index to one character after the found newline.
                try fileHandle.seek(toOffset: offset + UInt64(lineSize + 1))
                return line
            }
        }
        // If we reach the end of the file without finding a newline character, return the remaining data as the line.
        if data.count > 0 {
            guard let line = String(data: data, encoding: .utf8) else {
                // The data is not encoded in UTF-8.
                return nil
            }
            return line
        }

        // If we reach this point, there is no more data to read from the file.
        return nil
    }
}