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use core::ops::Deref;
use core::mem::{size_of};
use core::fmt;

use crate::{Answer, Header, Questions, QueryKind, QueryClass, Name};
use crate::question::read_question;

const HEADER_SIZE: usize = size_of::<Header>();
const MAX_MESSAGE_SIZE: usize = 512 - HEADER_SIZE;

/// Helper type for constructing a buffer with the maximum UDP message size.
pub type MessageBuffer = [u8; HEADER_SIZE + MAX_MESSAGE_SIZE];

/// A DNS message.
#[repr(C)]
pub struct Message<'a> {
  buf: &'a mut [u8],
  len: usize,
}

/// Builder for [`Message`](struct.Message.html).
pub struct MessageBuilder<'a> {
  buf: &'a mut [u8],
  len: usize,
}

impl<'a> MessageBuilder<'a> {
  pub fn header(mut self, header: Header) -> Self {
    *self.header_mut() = header;

    self.len = HEADER_SIZE;

    self
  }

  fn header_mut(&mut self) -> &mut Header {
    unsafe { &mut *(self.buf[..HEADER_SIZE].as_mut_ptr() as *mut _ as *mut Header) }
  }

  pub fn build(self) -> Message<'a> {
    let Self { buf, len } = self;
    Message { buf, len }
  }
}

impl<'a> Message<'a> {
  pub const BUFFER: [u8; 512] = [0; 512];

  pub fn builder(buf: &'a mut [u8]) -> MessageBuilder<'a> {
    for b in &mut buf[..HEADER_SIZE] {
      *b = 0;
    }

    MessageBuilder { buf, len: HEADER_SIZE }
  }

  pub fn parse(buffer: &'a mut [u8]) -> Result<Message<'a>, ()> {
    if buffer.len() < HEADER_SIZE || buffer.len() > HEADER_SIZE + MAX_MESSAGE_SIZE {
      return Err(())
    }

    let len = buffer.len();
    let mut frame = Self { buf: buffer, len };

    let mut i = HEADER_SIZE;

    for _ in 0..frame.header().question_count() {
      if !read_question(&frame.buf, &mut i) {
        return Err(())
      }
    }

    frame.len = i;

    Ok(frame)
  }
}

impl fmt::Debug for Message<'_> {
  fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
    fmt.debug_struct("Message")
      .field("header", &self.header())
      .field("body", &format_args!("{:?}", &self.buf[HEADER_SIZE..]))
      .field("len", &self.len)
      .finish()
  }
}

impl Deref for Message<'_> {
  type Target = [u8];

  fn deref(&self) -> &Self::Target {
    self.as_bytes()
  }
}

impl Message<'_> {
  pub fn header(&self) -> &Header {
    unsafe { &*(self.buf[..HEADER_SIZE].as_ptr() as *const _ as *const Header) }
  }

  pub fn header_mut(&mut self) -> &mut Header {
    unsafe { &mut *(self.buf[..HEADER_SIZE].as_mut_ptr() as *mut _ as *mut Header) }
  }

  pub fn add_answer(&mut self, answer: &Answer<'_>) {
    self.add_name(&answer.name);
    self.add_kind(&answer.kind);
    self.add_class(&answer.class);
    self.add_ttl(answer.ttl);
    self.add_rdata(answer.rdata);

    let header = self.header_mut();
    unsafe { header.set_answer_count(header.answer_count() + 1) };
  }

  fn add_name(&mut self, name: &Name<'_>) {
    for label in name.labels() {
      self.extend(&[label.len() as u8]);
      self.extend(label);
    }
    self.extend(&[0]);
  }

  fn add_kind(&mut self, kind: &QueryKind) {
    self.extend(&kind.to_be_bytes());
  }

  fn add_class(&mut self, class: &QueryClass) {
    self.extend(&class.to_be_bytes());
  }

  fn add_ttl(&mut self, ttl: u32) {
    self.extend(&ttl.to_be_bytes());
  }

  fn add_rdata(&mut self, data: &[u8]) {
    self.extend(&(data.len() as u16).to_be_bytes());
    self.extend(data);
  }

  fn extend(&mut self, bytes: &[u8]) {
    self.buf[self.len..(self.len + bytes.len())].copy_from_slice(bytes);
    self.len += bytes.len();
  }

  pub fn as_bytes(&self) -> &[u8] {
    &self.buf[..self.len]
  }

  pub fn questions(&self) -> Questions {
    Questions {
      question_count: self.header().question_count() as usize,
      current_question: 0,
      buf: &self.as_bytes(),
      buf_i: HEADER_SIZE,
    }
  }
}