//! Internet peer discovery via pkarr relay servers.
//!
//! Derives deterministic keypair "slots" from the shared passphrase.
//! Each peer claims a slot by publishing its EndpointId as a TXT record.
//! All peers scan all slots periodically to discover each other.
//!
//! This works over the internet — no LAN, no port forwarding needed.
//! Uses n0's public pkarr relay servers (same infrastructure as iroh).

use std::collections::HashSet;
use std::time::Duration;

use anyhow::{Context, Result};
use iroh::EndpointId;
use pkarr::{Client as PkarrClient, Keypair, SignedPacket};
use simple_dns::rdata::RData;
use tokio::sync::mpsc;
use tracing::{debug, info, warn};

const NUM_SLOTS: usize = 8;
const PUBLISH_INTERVAL: Duration = Duration::from_secs(60);
const SCAN_INTERVAL: Duration = Duration::from_secs(15);
const RECORD_NAME: &str = "_can_sync";

/// Derive a deterministic pkarr keypair for a given slot index.
fn derive_slot_keypair(passphrase: &str, slot: usize) -> Keypair {
    let seed = blake3::hash(
        format!("can-sync-rendezvous:{}:{}", passphrase, slot).as_bytes(),
    );
    let seed_bytes: [u8; 32] = *seed.as_bytes();
    let secret = ed25519_dalek::SecretKey::from(seed_bytes);
    Keypair::from_secret_key(&secret)
}

/// Internet peer discovery via pkarr relay.
pub struct Rendezvous {
    slots: Vec<Keypair>,
    our_id: EndpointId,
    client: PkarrClient,
}

impl Rendezvous {
    /// Create a new Rendezvous by deriving keypairs for all slots from the passphrase.
    pub fn new(passphrase: &str, our_id: EndpointId) -> Result<Self> {
        let slots: Vec<Keypair> = (0..NUM_SLOTS)
            .map(|i| derive_slot_keypair(passphrase, i))
            .collect();

        let client = PkarrClient::builder()
            .build()
            .context("creating pkarr client")?;

        Ok(Self {
            slots,
            our_id,
            client,
        })
    }

    /// Run the rendezvous loop: claim a slot, periodically re-publish and scan.
    pub async fn run(self, tx: mpsc::Sender<EndpointId>) -> Result<()> {
        let our_id_hex = hex::encode(self.our_id.as_bytes());
        info!(
            "Rendezvous: starting internet discovery ({} slots, publish every {}s, scan every {}s)",
            NUM_SLOTS,
            PUBLISH_INTERVAL.as_secs(),
            SCAN_INTERVAL.as_secs(),
        );

        // Claim our slot (first empty, or hash-based fallback)
        let our_slot = self.claim_slot(&our_id_hex).await;
        info!("Rendezvous: claimed slot {}", our_slot);

        let mut known_peers: HashSet<EndpointId> = HashSet::new();
        let mut publish_tick = tokio::time::interval(PUBLISH_INTERVAL);
        let mut scan_tick = tokio::time::interval(SCAN_INTERVAL);

        // Do an initial scan immediately
        self.scan_all_slots(&mut known_peers, &tx).await;

        loop {
            tokio::select! {
                _ = publish_tick.tick() => {
                    if let Err(e) = self.publish_slot(our_slot, &our_id_hex).await {
                        warn!("Rendezvous: failed to re-publish slot {}: {:#}", our_slot, e);
                    }
                }
                _ = scan_tick.tick() => {
                    self.scan_all_slots(&mut known_peers, &tx).await;
                }
            }
        }
    }

    // Read every slot and report any newly discovered peer IDs.
    async fn scan_all_slots(
        &self,
        known_peers: &mut HashSet<EndpointId>,
        tx: &mpsc::Sender<EndpointId>,
    ) {
        for i in 0..NUM_SLOTS {
            match self.read_slot(i).await {
                Some(peer_id) if peer_id != self.our_id && known_peers.insert(peer_id) => {
                    info!(
                        "Rendezvous: discovered peer {} in slot {}",
                        peer_id.fmt_short(),
                        i
                    );
                    let _ = tx.send(peer_id).await;
                }
                _ => {}
            }
        }
    }

    // Pick an available slot for this peer: reuse our old slot, take an empty one,
    // or fall back to a deterministic slot based on our ID.
    async fn claim_slot(&self, our_id_hex: &str) -> usize {
        // Check if we already own a slot (from a previous run)
        for i in 0..NUM_SLOTS {
            if let Some(peer_id) = self.read_slot(i).await {
                if peer_id == self.our_id {
                    debug!("Rendezvous: already own slot {}", i);
                    return i;
                }
            }
        }

        // Claim first empty slot
        for i in 0..NUM_SLOTS {
            if self.read_slot(i).await.is_none() {
                if let Err(e) = self.publish_slot(i, our_id_hex).await {
                    warn!("Rendezvous: failed to claim slot {}: {:#}", i, e);
                    continue;
                }
                return i;
            }
        }

        // All slots occupied — use deterministic slot based on our ID
        let slot = {
            let h = blake3::hash(self.our_id.as_bytes());
            let bytes: [u8; 8] = h.as_bytes()[..8].try_into().unwrap();
            u64::from_le_bytes(bytes) as usize % NUM_SLOTS
        };
        let _ = self.publish_slot(slot, our_id_hex).await;
        slot
    }

    // Write our EndpointId into the given slot's DNS TXT record via the pkarr relay.
    async fn publish_slot(&self, slot: usize, our_id_hex: &str) -> Result<()> {
        let keypair = &self.slots[slot];
        let packet = SignedPacket::builder()
            .txt(
                RECORD_NAME.try_into().context("invalid record name")?,
                our_id_hex.try_into().context("invalid txt value")?,
                300, // 5 min TTL
            )
            .sign(keypair)
            .context("signing pkarr packet")?;

        self.client
            .publish(&packet, None)
            .await
            .context("publishing to pkarr relay")?;

        debug!("Rendezvous: published slot {}", slot);
        Ok(())
    }

    // Look up a slot's DNS TXT record and parse the EndpointId stored there, if any.
    async fn read_slot(&self, slot: usize) -> Option<EndpointId> {
        let public_key = self.slots[slot].public_key();
        let packet = self.client.resolve(&public_key).await?;

        // Use pkarr's resource_records iterator to find our TXT record
        for record in packet.resource_records(RECORD_NAME) {
            if let RData::TXT(txt) = &record.rdata {
                // Try to extract the hex-encoded EndpointId from TXT attributes
                if let Ok(txt_string) = String::try_from(txt.clone()) {
                    let hex_str = txt_string.trim();
                    if let Ok(bytes) = hex::decode(hex_str) {
                        if bytes.len() == 32 {
                            if let Ok(arr) = <[u8; 32]>::try_from(bytes.as_slice()) {
                                return EndpointId::from_bytes(&arr).ok();
                            }
                        }
                    }
                }
            }
        }

        None
    }
}