import Foundation

/// Bound before decoding: AsyncBytes.lines can allocate an unbounded line.
/// Package access so ScriptedTransport frames streams exactly as NativeHTTP does.
package struct NativeStreamLines {
  private var buffer = Data()
  private var eventBytes = 0
  private var afterCR = false
  private var firstLine = true
  let lineLimit: Int
  let eventLimit: Int

  package init(lineLimit: Int = 256 * 1024, eventLimit: Int = 1024 * 1024) {
    self.lineLimit = lineLimit
    self.eventLimit = eventLimit
  }

  package mutating func append(_ byte: UInt8) throws -> String? {
    if afterCR {
      afterCR = false
      if byte == 10 { return nil }
    }
    if byte == 10 || byte == 13 {
      afterCR = byte == 13
      guard var line = String(data: buffer, encoding: .utf8) else {
        throw NativeClientError.message("The live connection returned invalid text.")
      }
      if firstLine {
        firstLine = false
        if line.hasPrefix("\u{FEFF}") { line.removeFirst() }
      }
      buffer.removeAll(keepingCapacity: true)
      if line.isEmpty { eventBytes = 0 }
      return line
    }
    guard buffer.count < lineLimit, eventBytes < eventLimit else {
      throw NativeClientError.message("The live update is too large. Reconnect to refresh it.")
    }
    buffer.append(byte)
    eventBytes += 1
    return nil
  }
}