//! Sequence handling module

use crate::{Error, Frame, Recipient};
use identity::Identity;
use {
    std::hash::{BuildHasher, Hasher},
    twox_hash::{RandomXxHashBuilder64 as RXHash64, XxHash64},
};

/// A unique identifier to represents a sequence of frames
pub type SeqId = Identity;

/// An XxHash signature and initialisation seed
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub struct XxSignature {
    sig: u64,
    seed: u64,
}

impl XxSignature {
    fn new(data: &Vec<u8>) -> Self {
        let mut hasher = RXHash64::default().build_hasher();
        hasher.write(data);
        Self {
            sig: hasher.finish(),
            seed: hasher.seed(),
        }
    }

    fn verify(&self, data: &Vec<u8>) -> bool {
        let mut hasher = XxHash64::with_seed(self.seed);
        hasher.write(data);
        hasher.finish() == self.sig
    }
}

/// Encoded signature information related to a data sequence
///
/// When a large chunk of data is split across a `Frame` set,
/// signature hashes are used to verify data integrity, as well as
/// sequence ordering.  The "Sequence ID" itself can be used to
/// re-order frames received out of order, as well as verifying that a
/// `Frame` was transmitted without error.
///
/// Check the crate documentation for more details.
#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
pub struct SeqData {
    /// Frame number in sequence
    pub num: u32,
    /// A hash signature of the payload
    pub sig: XxSignature,
    /// Global frame sequence ID
    pub seqid: SeqId,
    /// Next sequenced Frame SIG
    pub next: Option<u64>,
}

/// Utility wrapping around `Vec<Frame>` with `SeqId` initialisation.
///
/// This type implements a builder, which is initialised with header
/// data, then filled with various sliced payloads, and then made into
/// a frame sequence, as outlined in the root netmod docs.
pub struct SeqBuilder {
    #[doc(hidden)]
    pub seqid: SeqId,
    #[doc(hidden)]
    pub sender: Identity,
    #[doc(hidden)]
    pub recp: Recipient,
    #[doc(hidden)]
    pub data: Vec<Vec<u8>>,
}

impl SeqBuilder {
    /// Initialise a Sequence builder
    pub fn new(sender: Identity, recp: Recipient, seqid: SeqId) -> Self {
        Self {
            sender,
            recp,
            seqid,
            data: vec![],
        }
    }

    /// Add a slice of payload to the sequence set
    pub fn add(mut self, data: Vec<u8>) -> Self {
        self.data.push(data);
        self
    }

    /// Consume the builder into a set of frames
    pub fn build(self) -> Vec<Frame> {
        let seqid = self.seqid;
        let sender = self.sender;
        let recipient = self.recp;
        let signed = self
            .data
            .into_iter()
            .map(|d| (XxSignature::new(&d), d))
            .collect::<Vec<_>>();

        (0..signed.len())
            .enumerate()
            .map(|(num, i)| match (signed.get(i), signed.get(i + 1)) {
                (
                    Some((ref sig, data)),
                    Some((
                        XxSignature {
                            sig: ref next,
                            seed: _,
                        },
                        _,
                    )),
                ) => (
                    SeqData {
                        num: num as u32,
                        seqid,
                        sig: *sig,
                        next: Some(*next),
                    },
                    data,
                ),
                (Some((ref sig, data)), None) => (
                    SeqData {
                        num: num as u32,
                        seqid,
                        sig: *sig,
                        next: None,
                    },
                    data,
                ),
                _ => unreachable!(),
            })
            .map(|(seq, data)| Frame {
                sender,
                recipient,
                seq,
                payload: data.to_vec(),
            })
            .collect()
    }

    /// Take a sequence of frames and turn it into a complete payload
    ///
    /// This function assumes a complete set of frame that has
    /// previously been sorted along the `seq.num` metric.
    pub fn restore(buf: &mut Vec<Frame>) -> Vec<u8> {
        // FIXME: `windows` are weird when there's less than n Items.
        // This hack just pretends that there are two.  We also
        // communicate to the fold that we should drop the last frame
        // in the set again.  A much better approach would be to use a
        // windowing iterator that doesn't need to copy and can handle
        // this situation more gracefully
        //
        // ~k
        let fake = if buf.len() == 1 {
            buf.push(buf.get(0).unwrap().clone());
            true
        } else {
            false
        };

        let wins = buf.windows(2);
        let len = wins.len();

        let r: Result<Vec<u8>, Error> = wins.enumerate().into_iter().fold(
            Ok(Vec::with_capacity(buf.len())),
            |mut res, (i, win)| {
                let last = i == (len - 1);
                let a = &win[0];
                let seqa = &a.seq;
                let b = &win[1];
                let seqb = &b.seq;

                if !seqa.sig.verify(&a.payload) {
                    res = Err(Error::DesequenceFault);
                }

                if last && !seqb.sig.verify(&b.payload) {
                    res = Err(Error::DesequenceFault);
                }

                fn append(vec: &mut Vec<u8>, other: &Vec<u8>) {
                    let mut f = other.clone();
                    vec.append(&mut f);
                }

                match (res, last) {
                    (Ok(mut vec), false) => {
                        append(&mut vec, &a.payload);
                        Ok(vec)
                    }
                    (Ok(mut vec), true) => {
                        append(&mut vec, &a.payload);

                        if !fake {
                            append(&mut vec, &b.payload);
                        }
                        Ok(vec)
                    }
                    _ => Err(Error::DesequenceFault),
                }
            },
        );

        r.expect("SeqBuilder::restore failed with invalid inputs!")
    }

    /// Read the sequence ID back from the builder
    pub fn seqid(&self) -> &SeqId {
        &self.seqid
    }

    /// Read the sender back from the builder
    pub fn sender(&self) -> Identity {
        self.sender
    }

    /// Read the recipient back from the builder
    pub fn recp(&self) -> Recipient {
        self.recp
    }

    /// Read the payload data set back from the builder
    pub fn data(&self) -> Vec<u8> {
        self.data.get(0).unwrap().clone()
    }
}

#[cfg(test)]
fn setup() -> Vec<Frame> {
    let sender = Identity::with_digest(&vec![1]);
    let recp = Identity::with_digest(&vec![2]);
    SeqBuilder::new(sender, Recipient::User(recp), Identity::random())
        .add(vec![42])
        .add(vec![13, 12])
        .add(vec![13, 37])
        .build()
}

#[test]
fn simple() {
    let seq = setup();
    assert!(seq.len() == 3);
    assert!(seq.get(0).unwrap().seq.next == Some(seq.get(1).unwrap().seq.sig.sig));
}

/// A simple test to see if the sequence numbers are ok
#[test]
fn seq_num() {
    let seq = setup();
    assert_eq!(seq[0].seq.num, 0);
    assert_eq!(seq[1].seq.num, 1);
    assert_eq!(seq[2].seq.num, 2);
}

/// Hash sequence test
#[test]
fn hash_seq() {
    let seq = setup();
    assert_eq!(seq[0].seq.next, Some(seq[1].seq.sig.sig));
    assert_eq!(seq[1].seq.next, Some(seq[2].seq.sig.sig));
    assert_eq!(seq[2].seq.next, None);
}