//! 256-bit node identity for Kademlia.
//!
//! NodeId is 32 bytes — the same size as an Ed25519
//! public key. For node IDs, `NodeId = public_key`
//! directly. For DHT key hashing, SHA-256 maps
//! arbitrary data into the 256-bit ID space.

use crate::sys;
use std::fmt;

/// Length of a node ID in bytes (32 = Ed25519 pubkey).
pub const ID_LEN: usize = 32;

/// Number of bits in a node ID.
pub const ID_BITS: usize = ID_LEN * 8; // 256

/// A 256-bit node identifier.
///
/// Provides XOR distance and lexicographic ordering as
/// required by Kademlia routing.
#[derive(Clone, Copy, PartialEq, Eq, Hash)]
pub struct NodeId([u8; ID_LEN]);

impl NodeId {
    /// Generate a random node ID.
    pub fn random() -> Self {
        let mut buf = [0u8; ID_LEN];
        sys::random_bytes(&mut buf);
        Self(buf)
    }

    /// Create a node ID from raw bytes.
    pub fn from_bytes(b: [u8; ID_LEN]) -> Self {
        Self(b)
    }

    /// Create a node ID by SHA-256 hashing arbitrary
    /// data.
    ///
    /// Used by `put`/`get` to map keys into the 256-bit
    /// Kademlia ID space.
    pub fn from_key(data: &[u8]) -> Self {
        use sha2::{Digest, Sha256};
        let hash = Sha256::digest(data);
        let mut buf = [0u8; ID_LEN];
        buf.copy_from_slice(&hash);
        Self(buf)
    }

    /// Return the raw bytes.
    pub fn as_bytes(&self) -> &[u8; ID_LEN] {
        &self.0
    }

    /// XOR distance between two node IDs.
    pub fn distance(&self, other: &NodeId) -> NodeId {
        let mut out = [0u8; ID_LEN];
        for (i, byte) in out.iter_mut().enumerate() {
            *byte = self.0[i] ^ other.0[i];
        }
        NodeId(out)
    }

    /// Number of leading zero bits in this ID.
    ///
    /// Used to determine the k-bucket index.
    pub fn leading_zeros(&self) -> u32 {
        for (i, &byte) in self.0.iter().enumerate() {
            if byte != 0 {
                return (i as u32) * 8 + byte.leading_zeros();
            }
        }
        (ID_LEN as u32) * 8
    }

    /// Check if all bytes are zero.
    pub fn is_zero(&self) -> bool {
        self.0.iter().all(|&b| b == 0)
    }

    /// Parse from a hexadecimal string (64 chars).
    pub fn from_hex(s: &str) -> Option<Self> {
        if s.len() != ID_LEN * 2 {
            return None;
        }
        let mut buf = [0u8; ID_LEN];
        for i in 0..ID_LEN {
            buf[i] = u8::from_str_radix(&s[i * 2..i * 2 + 2], 16).ok()?;
        }
        Some(Self(buf))
    }

    /// Format as a hexadecimal string (64 chars).
    pub fn to_hex(&self) -> String {
        self.0.iter().map(|b| format!("{b:02x}")).collect()
    }

    /// Serialize to a byte slice.
    ///
    /// # Panics
    /// Panics if `buf.len() < ID_LEN` (32).
    pub fn write_to(&self, buf: &mut [u8]) {
        debug_assert!(
            buf.len() >= ID_LEN,
            "NodeId::write_to: buf too small ({} < {ID_LEN})",
            buf.len()
        );
        buf[..ID_LEN].copy_from_slice(&self.0);
    }

    /// Deserialize from a byte slice.
    ///
    /// # Panics
    /// Panics if `buf.len() < ID_LEN` (32).
    pub fn read_from(buf: &[u8]) -> Self {
        debug_assert!(
            buf.len() >= ID_LEN,
            "NodeId::read_from: buf too small ({} < {ID_LEN})",
            buf.len()
        );
        let mut id = [0u8; ID_LEN];
        id.copy_from_slice(&buf[..ID_LEN]);
        Self(id)
    }
}

impl Ord for NodeId {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.0.cmp(&other.0)
    }
}

impl PartialOrd for NodeId {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl fmt::Debug for NodeId {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "NodeId({})", &self.to_hex()[..8])
    }
}

impl fmt::Display for NodeId {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.to_hex())
    }
}

impl AsRef<[u8]> for NodeId {
    fn as_ref(&self) -> &[u8] {
        &self.0
    }
}

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

    #[test]
    fn zero_distance() {
        let id = NodeId::from_bytes([0xAB; ID_LEN]);
        let d = id.distance(&id);
        assert!(d.is_zero());
    }

    #[test]
    fn xor_symmetric() {
        let a = NodeId::from_bytes([0x01; ID_LEN]);
        let b = NodeId::from_bytes([0xFF; ID_LEN]);
        assert_eq!(a.distance(&b), b.distance(&a));
    }

    #[test]
    fn leading_zeros_all_zero() {
        let z = NodeId::from_bytes([0; ID_LEN]);
        assert_eq!(z.leading_zeros(), 256);
    }

    #[test]
    fn leading_zeros_first_bit() {
        let mut buf = [0u8; ID_LEN];
        buf[0] = 0x80;
        let id = NodeId::from_bytes(buf);
        assert_eq!(id.leading_zeros(), 0);
    }

    #[test]
    fn leading_zeros_ninth_bit() {
        let mut buf = [0u8; ID_LEN];
        buf[1] = 0x80;
        let id = NodeId::from_bytes(buf);
        assert_eq!(id.leading_zeros(), 8);
    }

    #[test]
    fn hex_roundtrip() {
        let id = NodeId::from_bytes([
            0xDE, 0xAD, 0xBE, 0xEF, 0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD,
            0xEF, 0xFE, 0xDC, 0xBA, 0x98, 0x76, 0x54, 0x32, 0x10, 0xDE, 0xAD,
            0xBE, 0xEF, 0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF,
        ]);
        let hex = id.to_hex();
        assert_eq!(hex.len(), 64);
        assert_eq!(NodeId::from_hex(&hex), Some(id));
    }

    #[test]
    fn from_key_deterministic() {
        let a = NodeId::from_key(b"hello");
        let b = NodeId::from_key(b"hello");
        assert_eq!(a, b);
    }

    #[test]
    fn from_key_different_inputs() {
        let a = NodeId::from_key(b"hello");
        let b = NodeId::from_key(b"world");
        assert_ne!(a, b);
    }

    #[test]
    fn ordering_is_lexicographic() {
        let a = NodeId::from_bytes([0x00; ID_LEN]);
        let b = NodeId::from_bytes([0xFF; ID_LEN]);
        assert!(a < b);
    }

    #[test]
    fn write_read_roundtrip() {
        let id = NodeId::from_bytes([0x42; ID_LEN]);
        let mut buf = [0u8; ID_LEN];
        id.write_to(&mut buf);
        let id2 = NodeId::read_from(&buf);
        assert_eq!(id, id2);
    }
}