//! Datagram transport with automatic fragmentation.
//!
//! Messages larger
//! than `MAX_DGRAM_PAYLOAD` (896 bytes) are split into
//! fragments and reassembled at the destination.
//!
//! Routing is automatic: if the local node is behind
//! symmetric NAT, datagrams are sent through the proxy.

use std::collections::HashMap;
use std::time::{Duration, Instant};

use crate::id::NodeId;

/// Maximum payload per datagram fragment (bytes).
///
/// Ensures each fragment
/// fits in a single UDP packet with protocol overhead.
pub const MAX_DGRAM_PAYLOAD: usize = 896;

/// Timeout for incomplete fragment reassembly.
pub const REASSEMBLY_TIMEOUT: Duration = Duration::from_secs(10);

/// A queued datagram waiting for address resolution.
#[derive(Debug, Clone)]
pub struct QueuedDgram {
    pub data: Vec<u8>,
    pub src: NodeId,
    pub queued_at: Instant,
}

// ── Fragmentation ───────────────────────────────────

/// Fragment header: 4 bytes.
///
/// - `total` (u16 BE): total number of fragments.
/// - `index` (u16 BE): this fragment's index (0-based).
const FRAG_HEADER_SIZE: usize = 4;

/// Split a message into fragments, each with a 4-byte
/// header and up to `MAX_DGRAM_PAYLOAD` bytes of data.
pub fn fragment(data: &[u8]) -> Vec<Vec<u8>> {
    if data.is_empty() {
        return vec![make_fragment(1, 0, &[])];
    }

    let chunk_size = MAX_DGRAM_PAYLOAD;
    let total = data.len().div_ceil(chunk_size);
    let total = total as u16;

    data.chunks(chunk_size)
        .enumerate()
        .map(|(i, chunk)| make_fragment(total, i as u16, chunk))
        .collect()
}

fn make_fragment(total: u16, index: u16, data: &[u8]) -> Vec<u8> {
    let mut buf = Vec::with_capacity(FRAG_HEADER_SIZE + data.len());
    buf.extend_from_slice(&total.to_be_bytes());
    buf.extend_from_slice(&index.to_be_bytes());
    buf.extend_from_slice(data);
    buf
}

/// Parse a fragment header.
///
/// Returns `(total_fragments, fragment_index, payload)`.
pub fn parse_fragment(buf: &[u8]) -> Option<(u16, u16, &[u8])> {
    if buf.len() < FRAG_HEADER_SIZE {
        return None;
    }
    let total = u16::from_be_bytes([buf[0], buf[1]]);
    let index = u16::from_be_bytes([buf[2], buf[3]]);
    Some((total, index, &buf[FRAG_HEADER_SIZE..]))
}

// ── Reassembly ──────────────────────────────────────

/// State for reassembling fragments from a single sender.
#[derive(Debug)]
struct ReassemblyState {
    total: u16,
    fragments: HashMap<u16, Vec<u8>>,
    started_at: Instant,
}

/// Fragment reassembler.
///
/// Tracks incoming fragments per sender and produces
/// complete messages when all fragments arrive.
pub struct Reassembler {
    pending: HashMap<NodeId, ReassemblyState>,
}

impl Reassembler {
    pub fn new() -> Self {
        Self {
            pending: HashMap::new(),
        }
    }

    /// Feed a fragment. Returns the complete message if
    /// all fragments have arrived.
    pub fn feed(
        &mut self,
        sender: NodeId,
        total: u16,
        index: u16,
        data: Vec<u8>,
    ) -> Option<Vec<u8>> {
        // S2-8: cap fragments to prevent memory bomb
        const MAX_FRAGMENTS: u16 = 10;
        if total == 0 {
            log::debug!("Dgram: dropping fragment with total=0");
            return None;
        }
        if total > MAX_FRAGMENTS {
            log::debug!(
                "Dgram: dropping fragment with total={total} > {MAX_FRAGMENTS}"
            );
            return None;
        }

        // Single fragment → no reassembly needed
        if total == 1 && index == 0 {
            self.pending.remove(&sender);
            return Some(data);
        }

        let state =
            self.pending
                .entry(sender)
                .or_insert_with(|| ReassemblyState {
                    total,
                    fragments: HashMap::new(),
                    started_at: Instant::now(),
                });

        // Total mismatch → reset
        if state.total != total {
            *state = ReassemblyState {
                total,
                fragments: HashMap::new(),
                started_at: Instant::now(),
            };
        }

        if index < total {
            state.fragments.insert(index, data);
        }

        if state.fragments.len() == total as usize {
            // All fragments received → reassemble
            let mut result = Vec::new();
            for i in 0..total {
                if let Some(frag) = state.fragments.get(&i) {
                    result.extend_from_slice(frag);
                } else {
                    // Should not happen, but guard
                    self.pending.remove(&sender);
                    return None;
                }
            }
            self.pending.remove(&sender);
            Some(result)
        } else {
            None
        }
    }

    /// Remove incomplete reassembly state older than the
    /// timeout.
    pub fn expire(&mut self) {
        self.pending
            .retain(|_, state| state.started_at.elapsed() < REASSEMBLY_TIMEOUT);
    }

    /// Number of pending incomplete messages.
    pub fn pending_count(&self) -> usize {
        self.pending.len()
    }
}

impl Default for Reassembler {
    fn default() -> Self {
        Self::new()
    }
}

// ── Send queue ──────────────────────────────────────

/// Queue of datagrams waiting for address resolution.
pub struct SendQueue {
    queues: HashMap<NodeId, Vec<QueuedDgram>>,
}

impl SendQueue {
    pub fn new() -> Self {
        Self {
            queues: HashMap::new(),
        }
    }

    /// Enqueue a datagram for a destination.
    pub fn push(&mut self, dst: NodeId, data: Vec<u8>, src: NodeId) {
        self.queues.entry(dst).or_default().push(QueuedDgram {
            data,
            src,
            queued_at: Instant::now(),
        });
    }

    /// Drain the queue for a destination.
    pub fn drain(&mut self, dst: &NodeId) -> Vec<QueuedDgram> {
        self.queues.remove(dst).unwrap_or_default()
    }

    /// Check if there's a pending queue for a destination.
    pub fn has_pending(&self, dst: &NodeId) -> bool {
        self.queues.contains_key(dst)
    }

    /// Remove stale queued messages (>10s).
    pub fn expire(&mut self) {
        self.queues.retain(|_, q| {
            q.retain(|d| d.queued_at.elapsed() < REASSEMBLY_TIMEOUT);
            !q.is_empty()
        });
    }
}

impl Default for SendQueue {
    fn default() -> Self {
        Self::new()
    }
}

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

    // ── Fragmentation tests ─────────────────────────

    #[test]
    fn small_message_single_fragment() {
        let frags = fragment(b"hello");
        assert_eq!(frags.len(), 1);
        let (total, idx, data) = parse_fragment(&frags[0]).unwrap();
        assert_eq!(total, 1);
        assert_eq!(idx, 0);
        assert_eq!(data, b"hello");
    }

    #[test]
    fn large_message_multiple_fragments() {
        let msg = vec![0xAB; MAX_DGRAM_PAYLOAD * 3 + 100];
        let frags = fragment(&msg);
        assert_eq!(frags.len(), 4);

        for (i, frag) in frags.iter().enumerate() {
            let (total, idx, _) = parse_fragment(frag).unwrap();
            assert_eq!(total, 4);
            assert_eq!(idx, i as u16);
        }
    }

    #[test]
    fn empty_message() {
        let frags = fragment(b"");
        assert_eq!(frags.len(), 1);
        let (total, idx, data) = parse_fragment(&frags[0]).unwrap();
        assert_eq!(total, 1);
        assert_eq!(idx, 0);
        assert!(data.is_empty());
    }

    #[test]
    fn fragment_roundtrip() {
        let msg = vec![0x42; MAX_DGRAM_PAYLOAD * 2 + 50];
        let frags = fragment(&msg);

        let mut reassembled = Vec::new();
        for frag in &frags {
            let (_, _, data) = parse_fragment(frag).unwrap();
            reassembled.extend_from_slice(data);
        }
        assert_eq!(reassembled, msg);
    }

    // ── Reassembler tests ───────────────────────────

    #[test]
    fn reassemble_single() {
        let mut r = Reassembler::new();
        let sender = NodeId::from_bytes([0x01; 32]);
        let result = r.feed(sender, 1, 0, b"hello".to_vec());
        assert_eq!(result.unwrap(), b"hello");
        assert_eq!(r.pending_count(), 0);
    }

    #[test]
    fn reassemble_multi() {
        let mut r = Reassembler::new();
        let sender = NodeId::from_bytes([0x01; 32]);

        assert!(r.feed(sender, 3, 0, b"aaa".to_vec()).is_none());
        assert!(r.feed(sender, 3, 2, b"ccc".to_vec()).is_none());
        let result = r.feed(sender, 3, 1, b"bbb".to_vec());

        assert_eq!(result.unwrap(), b"aaabbbccc");
        assert_eq!(r.pending_count(), 0);
    }

    #[test]
    fn reassemble_out_of_order() {
        let mut r = Reassembler::new();
        let sender = NodeId::from_bytes([0x01; 32]);

        // Fragments arrive in reverse order
        assert!(r.feed(sender, 2, 1, b"world".to_vec()).is_none());
        let result = r.feed(sender, 2, 0, b"hello".to_vec());
        assert_eq!(result.unwrap(), b"helloworld");
    }

    // ── SendQueue tests ─────────────────────────────

    #[test]
    fn send_queue_push_drain() {
        let mut q = SendQueue::new();
        let dst = NodeId::from_bytes([0x01; 32]);
        let src = NodeId::from_bytes([0x02; 32]);

        q.push(dst, b"msg1".to_vec(), src);
        q.push(dst, b"msg2".to_vec(), src);

        assert!(q.has_pending(&dst));
        let msgs = q.drain(&dst);
        assert_eq!(msgs.len(), 2);
        assert!(!q.has_pending(&dst));
    }

    #[test]
    fn parse_truncated() {
        assert!(parse_fragment(&[0, 1]).is_none());
    }
}