use std::collections::VecDeque;
use std::str;

extern crate custom_error;
use custom_error::custom_error;

//  Defines how many bytes is used to encode "AMOUNT" field in the response from ue-server about
//  amount of bytes it received since the last update
const UE_RECEIVED_FIELD_SIZE: usize = 4;
//  Defines how many bytes is used to encode "LENGTH" field, describing length of
//  next JSON message from ue-server
const UE_LENGTH_FIELD_SIZE: usize = 4;
//  Value indicating that next byte sequence from ue-server reports amount of bytes received by
//  that server so far. Value itself is arbitrary.

const HEAD_UE_RECEIVED: u8 = 85;
//  Value indicating that next byte sequence from ue-server contains JSON message.
//  Value itself is arbitrary.
const HEAD_UE_MESSAGE: u8 = 42;
//  Maximum allowed size of JSON message sent from ue-server.
const MAX_UE_MESSAGE_LENGTH: usize = 25 * 1024 * 1024;

custom_error! { pub ReadingStreamError
    InvalidHead{input: u8}          = "Invalid byte used as a HEAD: {input}",
    MessageTooLong{length: usize}   = "Message to receive is too long: {length}",
    InvalidUnicode                  = "Invalid utf-8 was received",
    BrokenStream                    = "Used stream is broken"
}

enum ReadingState {
    Head,
    ReceivedBytes,
    Length,
    Payload,
}

pub struct MessageReader {
    is_broken: bool,
    reading_state: ReadingState,
    read_bytes: usize,
    current_message_length: usize,
    current_message: Vec<u8>,
    read_messages: VecDeque<String>,
    next_received_bytes: u32,
    received_bytes: u64,
}

///     For converting byte stream that is expected from the ue-server into actual messages.
/// Expected format is a sequence of either:
///     1.  [HEAD_UE_RECEIVED: marker byte | 1 byte]
///         [AMOUNT: amount of bytes received by ue-server since last update | 4 bytes: u32 BE]
///     2.  [HEAD_UE_MESSAGE: marker byte | 1 byte]
///         [LENGTH: length of the JSON message in utf8 encoding | 4 bytes: u32 BE]
///         [PAYLOAD: utf8-encoded string | `LENGTH` bytes]
///     On any invalid input enters into a failure state (can be checked by `is_broken()`) and
/// never recovers from it.
///     Use either `push_byte()` or `push()` to input byte stream from ue-server and `pop()` to
/// retrieve resulting messages.
impl MessageReader {
    pub fn new() -> MessageReader {
        MessageReader {
            is_broken: false,
            reading_state: ReadingState::Head,
            read_bytes: 0,
            current_message_length: 0,
            current_message: Vec::new(),
            read_messages: VecDeque::new(),
            next_received_bytes: 0,
            received_bytes: 0,
        }
    }

    pub fn push_byte(&mut self, input: u8) -> Result<(), ReadingStreamError> {
        if self.is_broken {
            return Err(ReadingStreamError::BrokenStream);
        }
        match &self.reading_state {
            ReadingState::Head => {
                if input == HEAD_UE_RECEIVED {
                    self.reading_state = ReadingState::ReceivedBytes;
                } else if input == HEAD_UE_MESSAGE {
                    self.reading_state = ReadingState::Length;
                } else {
                    self.is_broken = true;
                    return Err(ReadingStreamError::InvalidHead { input });
                }
            }
            ReadingState::ReceivedBytes => {
                self.next_received_bytes = self.next_received_bytes << 8;
                self.next_received_bytes += input as u32;
                self.read_bytes += 1;
                if self.read_bytes >= UE_RECEIVED_FIELD_SIZE {
                    self.received_bytes += self.next_received_bytes as u64;
                    self.next_received_bytes = 0;
                    self.read_bytes = 0;
                    self.reading_state = ReadingState::Head;
                }
            }
            ReadingState::Length => {
                self.current_message_length = self.current_message_length << 8;
                self.current_message_length += input as usize;
                self.read_bytes += 1;
                if self.read_bytes >= UE_LENGTH_FIELD_SIZE {
                    self.read_bytes = 0;
                    self.reading_state = ReadingState::Payload;
                    if self.current_message_length > MAX_UE_MESSAGE_LENGTH {
                        self.is_broken = true;
                        return Err(ReadingStreamError::MessageTooLong {
                            length: self.current_message_length,
                        });
                    }
                    self.current_message = Vec::with_capacity(self.current_message_length);
                }
            }
            ReadingState::Payload => {
                self.current_message.push(input);
                self.read_bytes += 1 as usize;
                if self.read_bytes >= self.current_message_length {
                    match str::from_utf8(&self.current_message) {
                        Ok(next_message) => self.read_messages.push_front(next_message.to_owned()),
                        _ => {
                            self.is_broken = true;
                            return Err(ReadingStreamError::InvalidUnicode);
                        }
                    };
                    self.current_message.clear();
                    self.current_message_length = 0;
                    self.read_bytes = 0;
                    self.reading_state = ReadingState::Head;
                }
            }
        }
        Ok(())
    }

    pub fn push(&mut self, input: &[u8]) -> Result<(), ReadingStreamError> {
        for &byte in input {
            self.push_byte(byte)?;
        }
        Ok(())
    }

    pub fn pop(&mut self) -> Option<String> {
        self.read_messages.pop_back()
    }

    pub fn received_bytes(&self) -> u64 {
        self.received_bytes
    }

    pub fn is_broken(&self) -> bool {
        self.is_broken
    }
}

#[test]
fn message_push_byte() {
    let mut reader = MessageReader::new();
    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(13).unwrap();
    reader.push_byte(0x48).unwrap(); // H
    reader.push_byte(0x65).unwrap(); // e
    reader.push_byte(0x6c).unwrap(); // l
    reader.push_byte(0x6c).unwrap(); // l
    reader.push_byte(0x6f).unwrap(); // o
    reader.push_byte(0x2c).unwrap(); // ,
    reader.push_byte(0x20).unwrap(); // <space>
    reader.push_byte(0x77).unwrap(); // w
    reader.push_byte(0x6f).unwrap(); // o
    reader.push_byte(0x72).unwrap(); // r
    reader.push_byte(0x6c).unwrap(); // l
    reader.push_byte(0x64).unwrap(); // d
    reader.push_byte(0x21).unwrap(); // <exclamation mark>
    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(3).unwrap();
    reader.push_byte(0x59).unwrap(); // Y
    reader.push_byte(0x6f).unwrap(); // o
    reader.push_byte(0x21).unwrap(); // <exclamation mark>
    assert_eq!(reader.pop().unwrap(), "Hello, world!");
    assert_eq!(reader.pop().unwrap(), "Yo!");
    assert_eq!(reader.pop(), None);
}

#[test]
fn received_push_byte() {
    let mut reader = MessageReader::new();
    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(243).unwrap();
    assert_eq!(reader.received_bytes(), 243);
    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
    reader.push_byte(65).unwrap();
    reader.push_byte(25).unwrap();
    reader.push_byte(178).unwrap();
    reader.push_byte(4).unwrap();
    assert_eq!(reader.received_bytes(), 1092203255);
    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
    reader.push_byte(231).unwrap();
    reader.push_byte(34).unwrap();
    reader.push_byte(154).unwrap();
    assert_eq!(reader.received_bytes(), 1092203255);
}

#[test]
fn mixed_push_byte() {
    let mut reader = MessageReader::new();
    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(243).unwrap();
    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(3).unwrap();
    reader.push_byte(0x59).unwrap(); // Y
    reader.push_byte(0x6f).unwrap(); // o
    reader.push_byte(0x21).unwrap(); // <exclamation mark>
    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
    reader.push_byte(65).unwrap();
    reader.push_byte(25).unwrap();
    reader.push_byte(178).unwrap();
    reader.push_byte(4).unwrap();
    assert_eq!(reader.received_bytes(), 1092203255);
    assert_eq!(reader.pop().unwrap(), "Yo!");
    assert_eq!(reader.pop(), None);
}

#[test]
fn pushing_many_bytes_at_once() {
    let mut reader = MessageReader::new();
    reader
        .push(&[
            HEAD_UE_RECEIVED,
            0,
            0,
            0,
            243,
            HEAD_UE_MESSAGE,
            0,
            0,
            0,
            3,
            0x59, // Y
            0x6f, // o
            0x21, // <exclamation mark>
            HEAD_UE_RECEIVED,
            65,
            25,
            178,
            4,
        ])
        .unwrap();
    assert_eq!(reader.received_bytes(), 1092203255);
    assert_eq!(reader.pop().unwrap(), "Yo!");
    assert_eq!(reader.pop(), None);
}

#[test]
fn generates_error_invalid_head() {
    let mut reader = MessageReader::new();
    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(243).unwrap();
    assert!(!reader.is_broken());
    reader
        .push_byte(25)
        .expect_err("Testing failing on incorrect HEAD");
    assert!(reader.is_broken());
}

#[test]
fn generates_error_message_too_long() {
    let mut reader = MessageReader::new();
    let huge_length = MAX_UE_MESSAGE_LENGTH + 1;
    let bytes = (huge_length as u32).to_be_bytes();

    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
    reader.push_byte(bytes[0]).unwrap();
    reader.push_byte(bytes[1]).unwrap();
    reader.push_byte(bytes[2]).unwrap();
    assert!(!reader.is_broken());
    reader
        .push_byte(bytes[3])
        .expect_err("Testing failing on exceeding allowed message length");
    assert!(reader.is_broken());
}

#[test]
fn generates_error_invalid_unicode() {
    let mut reader = MessageReader::new();
    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(0).unwrap();
    reader.push_byte(2).unwrap();
    reader.push_byte(0b11010011).unwrap(); // start of 2-byte sequence
    assert!(!reader.is_broken());
    //  Bytes inside multi-byte code point have to have `1` for their high bit
    reader
        .push_byte(0b01010011)
        .expect_err("Testing failing on incorrect unicode");
    assert!(reader.is_broken());
}