#![warn(missing_docs)]

use itertools::izip;
use serde::Deserialize;
use serde::Serialize;

use crate::dht::Did;
use crate::error::Error;
use crate::error::Result;

/// MessageRelay guide message passing on rings network by relay.
///
/// All messages should be sent with `MessageRelay`.
/// By calling `relay` method in correct place, `MessageRelay` help to do things:
/// - Record the whole transport path for inspection.
/// - Get the sender of a message.
#[derive(Deserialize, Serialize, Debug, Clone, PartialEq, Eq)]
pub struct MessageRelay {
    /// A push only stack. Record routes when handling messages.
    pub path: Vec<Did>,

    /// The next node to handle the message.
    /// A message handler will pick transport by this field.
    pub next_hop: Did,

    /// The destination of the message.
    /// It may help the handler to find out `next_hop` in some situations.
    pub destination: Did,
}

impl MessageRelay {
    /// Create a new `MessageRelay`.
    pub fn new(path: Vec<Did>, next_hop: Did, destination: Did) -> Self {
        Self {
            path,
            next_hop,
            destination,
        }
    }

    /// Validate relay, then create a new `MessageRelay` that have `current` did in the end of path.
    /// The new relay will use `next_hop` as `next_hop` and `self.destination` as `destination`.
    pub fn forward(&self, current: Did, next_hop: Did) -> Result<Self> {
        self.validate(current)?;

        if self.next_hop != current {
            return Err(Error::InvalidNextHop);
        }

        let mut path = self.path.clone();
        path.push(current);

        Ok(Self {
            path,
            next_hop,
            destination: self.destination,
        })
    }

    /// Validate relay, then create a new `MessageRelay` that used to report the message.
    /// The new relay will use `self.path[self.path.len() - 1]` as `next_hop` and `self.sender()` as `destination`.
    /// In the new relay, the path will be cleared and only have `current` did.
    pub fn report(&self, current: Did) -> Result<Self> {
        self.validate(current)?;

        if self.path.is_empty() {
            return Err(Error::CannotInferNextHop);
        }

        Ok(Self {
            path: vec![current],
            next_hop: self.path[self.path.len() - 1],
            destination: self.origin_sender(),
        })
    }

    /// Sometime the sender may not know the destination of the message. They just use next_hop as destination.
    /// The next node can find a new next_hop, and may use this function to set that next_hop as destination again.
    pub fn reset_destination(&self, destination: Did) -> Self {
        let mut relay = self.clone();
        relay.destination = destination;
        relay
    }

    /// Check if path and destination is valid.
    pub fn validate(&self, current: Did) -> Result<()> {
        if self.next_hop != current {
            return Err(Error::InvalidNextHop);
        }

        // Adjacent elements in self.path cannot be equal
        if self.path.windows(2).any(|w| w[0] == w[1]) {
            return Err(Error::InvalidRelayPath);
        }

        // Prevent infinite loop
        if has_infinite_loop(&self.path) {
            tracing::error!("Infinite path detected {:?}", self.path);
            return Err(Error::InfiniteRelayPath);
        }

        Ok(())
    }

    /// Get the origin sender of current message.
    /// Should be the first element of path.
    #[deprecated(note = "please use `origin_sender` instead")]
    pub fn sender(&self) -> Did {
        *self.path.first().unwrap()
    }

    /// Get the origin sender of current message.
    /// Should be the first element of path.
    pub fn origin_sender(&self) -> Did {
        *self.path.first().unwrap()
    }
}

// Since rust cannot zip N iterators, when you change this number,
// you should also change the code of `has_infinite_loop` below.
const INFINITE_LOOP_TOLERANCE: usize = 3;

fn has_infinite_loop<T>(path: &[T]) -> bool
where T: PartialEq + std::fmt::Debug {
    if let Some(last) = path.last() {
        let indexes = path
            .iter()
            .rev()
            .enumerate()
            .filter(|(_, r)| r == &last)
            .map(|(index, _)| index)
            .take(INFINITE_LOOP_TOLERANCE)
            .collect::<Vec<_>>();

        if indexes.len() >= INFINITE_LOOP_TOLERANCE {
            let p1 = path.iter().rev().skip(indexes[0]);
            let p2 = path.iter().rev().skip(indexes[1]);
            let p3 = path.iter().rev().skip(indexes[2]);

            let lens = [
                indexes[1] - indexes[0],
                indexes[2] - indexes[1],
                path.len() - indexes[2],
            ];

            let min_len = lens.iter().min().unwrap();

            for (i, (x, y, z)) in izip!(p1, p2, p3).enumerate() {
                if !(x == y && y == z) {
                    return false;
                }

                if i == min_len - 1 {
                    break;
                }
            }

            if lens[0] == lens[1] {
                return true;
            }
        }
    }

    false
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    #[rustfmt::skip]
    fn test_has_infinite_loop() {
        assert!(!has_infinite_loop(&Vec::<u8>::new()));

        assert!(!has_infinite_loop(&[
            1, 2, 3,
        ]));

        assert!(!has_infinite_loop(&[
            1, 2, 3,
            1, 2, 3,
        ]));

        assert!(has_infinite_loop(&[
            1, 2, 3,
            1, 2, 3,
            1, 2, 3,
        ]));

        assert!(has_infinite_loop(&[
            1, 1, 2, 3,
               1, 2, 3,
               1, 2, 3,
        ]));

        assert!(!has_infinite_loop(&[
               1, 2, 3,
            1, 1, 2, 3,
               1, 2, 3,
        ]));

        assert!(has_infinite_loop(&[
            1, 2, 1, 2, 3,
                  1, 2, 3,
                  1, 2, 3,
        ]));

        assert!(has_infinite_loop(&[
            4, 5, 1, 2, 3,
                  1, 2, 3,
                  1, 2, 3,
        ]));

        assert!(!has_infinite_loop(&[
            1, 2, 3,
                  3,
            1, 2, 3,
                  3,
            1, 2, 3,
        ]));

        assert!(!has_infinite_loop(&[
                  1,
            1, 2, 3,
                  3,
            1, 2, 3,
                  3,
            1, 2, 3,
        ]));

        // TODO: try to detect this earlier.
        assert!(!has_infinite_loop(&[
                  3,
            1, 2, 3,
                  3,
            1, 2, 3,
                  3,
            1, 2, 3,
        ]));

        // TODO: try to detect this earlier.
        assert!(!has_infinite_loop(&[
            1, 2, 3,
            1, 2, 3,
                  3,
            1, 2, 3,
                  3,
            1, 2, 3,
        ]));

        // The above two cases will be detected finally.
        assert!(has_infinite_loop(&[
                  1, 2,
               3, 1, 2,
            3, 3, 1, 2,
            3, 3, 1, 2,
            3, 3, 1, 2,
        ]));

        assert!(!has_infinite_loop(&[
               2, 3,
               4, 3,
            1, 2, 3,
               4, 3,
            1, 2, 3,
               4, 3,
        ]));

        // TODO: try to detect this earlier.
        assert!(!has_infinite_loop(&[
            1, 2, 3,
               4, 3,
            1, 2, 3,
               4, 3,
            1, 2, 3,
               4, 3,
        ]));

        // The above case will be detected finally.
        assert!(has_infinite_loop(&[
               1, 2, 3, 4,
            3, 1, 2, 3, 4,
            3, 1, 2, 3, 4,
            3, 1, 2, 3, 4,
        ]));
    }
}