// Gateway pairing mode — first-connect authentication.
//
// On startup the gateway generates a one-time pairing code printed to the
// terminal. The first client must present this code via `X-Pairing-Code`
// header on a `POST /pair` request. The server responds with a bearer token
// that must be sent on all subsequent requests via `Authorization: Bearer <token>`.
//
// Already-paired tokens are persisted in config so restarts don't require
// re-pairing.

use parking_lot::Mutex;
use sha2::{Digest, Sha256};
use std::collections::HashSet;
use std::time::Instant;

/// Maximum failed pairing attempts before lockout.
const MAX_PAIR_ATTEMPTS: u32 = 5;
/// Lockout duration after too many failed pairing attempts.
const PAIR_LOCKOUT_SECS: u64 = 300; // 5 minutes

/// Manages pairing state for the gateway.
///
/// Bearer tokens are stored as SHA-256 hashes to prevent plaintext exposure
/// in config files. When a new token is generated, the plaintext is returned
/// to the client once, and only the hash is retained.
#[derive(Debug)]
pub struct PairingGuard {
    /// Whether pairing is required at all.
    require_pairing: bool,
    /// One-time pairing code (generated on startup, consumed on first pair).
    pairing_code: Mutex<Option<String>>,
    /// Set of SHA-256 hashed bearer tokens (persisted across restarts).
    paired_tokens: Mutex<HashSet<String>>,
    /// Brute-force protection: failed attempt counter + lockout time.
    failed_attempts: Mutex<(u32, Option<Instant>)>,
}

impl PairingGuard {
    /// Create a new pairing guard.
    ///
    /// If `require_pairing` is true and no tokens exist yet, a fresh
    /// pairing code is generated and returned via `pairing_code()`.
    ///
    /// Existing tokens are accepted in both forms:
    /// - Plaintext (`zc_...`): hashed on load for backward compatibility
    /// - Already hashed (64-char hex): stored as-is
    pub fn new(require_pairing: bool, existing_tokens: &[String]) -> Self {
        let tokens: HashSet<String> = existing_tokens
            .iter()
            .map(|t| {
                if is_token_hash(t) {
                    t.clone()
                } else {
                    hash_token(t)
                }
            })
            .collect();
        let code = if require_pairing && tokens.is_empty() {
            Some(generate_code())
        } else {
            None
        };
        Self {
            require_pairing,
            pairing_code: Mutex::new(code),
            paired_tokens: Mutex::new(tokens),
            failed_attempts: Mutex::new((0, None)),
        }
    }

    /// The one-time pairing code (only set when no tokens exist yet).
    pub fn pairing_code(&self) -> Option<String> {
        self.pairing_code.lock().clone()
    }

    /// Whether pairing is required at all.
    pub fn require_pairing(&self) -> bool {
        self.require_pairing
    }

    /// Attempt to pair with the given code. Returns a bearer token on success.
    /// Returns `Err(lockout_seconds)` if locked out due to brute force.
    pub fn try_pair(&self, code: &str) -> Result<Option<String>, u64> {
        // Check brute force lockout
        {
            let attempts = self.failed_attempts.lock();
            if let (count, Some(locked_at)) = &*attempts {
                if *count >= MAX_PAIR_ATTEMPTS {
                    let elapsed = locked_at.elapsed().as_secs();
                    if elapsed < PAIR_LOCKOUT_SECS {
                        return Err(PAIR_LOCKOUT_SECS - elapsed);
                    }
                }
            }
        }

        {
            let mut pairing_code = self.pairing_code.lock();
            if let Some(ref expected) = *pairing_code {
                if constant_time_eq(code.trim(), expected.trim()) {
                    // Reset failed attempts on success
                    {
                        let mut attempts = self.failed_attempts.lock();
                        *attempts = (0, None);
                    }
                    let token = generate_token();
                    let mut tokens = self.paired_tokens.lock();
                    tokens.insert(hash_token(&token));

                    // Consume the pairing code so it cannot be reused
                    *pairing_code = None;

                    return Ok(Some(token));
                }
            }
        }

        // Increment failed attempts
        {
            let mut attempts = self.failed_attempts.lock();
            attempts.0 += 1;
            if attempts.0 >= MAX_PAIR_ATTEMPTS {
                attempts.1 = Some(Instant::now());
            }
        }

        Ok(None)
    }

    /// Check if a bearer token is valid (compares against stored hashes).
    pub fn is_authenticated(&self, token: &str) -> bool {
        if !self.require_pairing {
            return true;
        }
        let hashed = hash_token(token);
        let tokens = self.paired_tokens.lock();
        tokens.contains(&hashed)
    }

    /// Returns true if the gateway is already paired (has at least one token).
    pub fn is_paired(&self) -> bool {
        let tokens = self.paired_tokens.lock();
        !tokens.is_empty()
    }

    /// Get all paired token hashes (for persisting to config).
    pub fn tokens(&self) -> Vec<String> {
        let tokens = self.paired_tokens.lock();
        tokens.iter().cloned().collect()
    }
}

/// Generate a 6-digit numeric pairing code using cryptographically secure randomness.
fn generate_code() -> String {
    // UUID v4 uses getrandom (backed by /dev/urandom on Linux, BCryptGenRandom
    // on Windows) — a CSPRNG. We extract 4 bytes from it for a uniform random
    // number in [0, 1_000_000).
    //
    // Rejection sampling eliminates modulo bias: values above the largest
    // multiple of 1_000_000 that fits in u32 are discarded and re-drawn.
    // The rejection probability is ~0.02%, so this loop almost always exits
    // on the first iteration.
    const UPPER_BOUND: u32 = 1_000_000;
    const REJECT_THRESHOLD: u32 = (u32::MAX / UPPER_BOUND) * UPPER_BOUND;

    loop {
        let uuid = uuid::Uuid::new_v4();
        let bytes = uuid.as_bytes();
        let raw = u32::from_le_bytes([bytes[0], bytes[1], bytes[2], bytes[3]]);

        if raw < REJECT_THRESHOLD {
            return format!("{:06}", raw % UPPER_BOUND);
        }
    }
}

/// Generate a cryptographically-adequate bearer token with 256-bit entropy.
///
/// Uses `rand::thread_rng()` which is backed by the OS CSPRNG
/// (/dev/urandom on Linux, BCryptGenRandom on Windows, SecRandomCopyBytes
/// on macOS). The 32 random bytes (256 bits) are hex-encoded for a
/// 64-character token, providing 256 bits of entropy.
fn generate_token() -> String {
    use rand::RngCore;
    let mut bytes = [0u8; 32];
    rand::thread_rng().fill_bytes(&mut bytes);
    format!("zc_{}", hex::encode(&bytes))
}

/// SHA-256 hash a bearer token for storage. Returns lowercase hex.
fn hash_token(token: &str) -> String {
    format!("{:x}", Sha256::digest(token.as_bytes()))
}

/// Check if a stored value looks like a SHA-256 hash (64 hex chars)
/// rather than a plaintext token.
fn is_token_hash(value: &str) -> bool {
    value.len() == 64 && value.chars().all(|c| c.is_ascii_hexdigit())
}

/// Constant-time string comparison to prevent timing attacks.
///
/// Does not short-circuit on length mismatch — always iterates over the
/// longer input to avoid leaking length information via timing.
pub fn constant_time_eq(a: &str, b: &str) -> bool {
    let a = a.as_bytes();
    let b = b.as_bytes();

    // Track length mismatch as a usize (non-zero = different lengths)
    let len_diff = a.len() ^ b.len();

    // XOR each byte, padding the shorter input with zeros.
    // Iterates over max(a.len(), b.len()) to avoid timing differences.
    let max_len = a.len().max(b.len());
    let mut byte_diff = 0u8;
    for i in 0..max_len {
        let x = *a.get(i).unwrap_or(&0);
        let y = *b.get(i).unwrap_or(&0);
        byte_diff |= x ^ y;
    }
    (len_diff == 0) & (byte_diff == 0)
}

/// Check if a host string represents a non-localhost bind address.
pub fn is_public_bind(host: &str) -> bool {
    !matches!(
        host,
        "127.0.0.1" | "localhost" | "::1" | "[::1]" | "0:0:0:0:0:0:0:1"
    )
}

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

    // ── PairingGuard ─────────────────────────────────────────

    #[test]
    fn new_guard_generates_code_when_no_tokens() {
        let guard = PairingGuard::new(true, &[]);
        assert!(guard.pairing_code().is_some());
        assert!(!guard.is_paired());
    }

    #[test]
    fn new_guard_no_code_when_tokens_exist() {
        let guard = PairingGuard::new(true, &["zc_existing".into()]);
        assert!(guard.pairing_code().is_none());
        assert!(guard.is_paired());
    }

    #[test]
    fn new_guard_no_code_when_pairing_disabled() {
        let guard = PairingGuard::new(false, &[]);
        assert!(guard.pairing_code().is_none());
    }

    #[test]
    fn try_pair_correct_code() {
        let guard = PairingGuard::new(true, &[]);
        let code = guard.pairing_code().unwrap().to_string();
        let token = guard.try_pair(&code).unwrap();
        assert!(token.is_some());
        assert!(token.unwrap().starts_with("zc_"));
        assert!(guard.is_paired());
    }

    #[test]
    fn try_pair_wrong_code() {
        let guard = PairingGuard::new(true, &[]);
        let result = guard.try_pair("000000").unwrap();
        // Might succeed if code happens to be 000000, but extremely unlikely
        // Just check it returns Ok(None) normally
        let _ = result;
    }

    #[test]
    fn try_pair_empty_code() {
        let guard = PairingGuard::new(true, &[]);
        assert!(guard.try_pair("").unwrap().is_none());
    }

    #[test]
    fn is_authenticated_with_valid_token() {
        // Pass plaintext token — PairingGuard hashes it on load
        let guard = PairingGuard::new(true, &["zc_valid".into()]);
        assert!(guard.is_authenticated("zc_valid"));
    }

    #[test]
    fn is_authenticated_with_prehashed_token() {
        // Pass an already-hashed token (64 hex chars)
        let hashed = hash_token("zc_valid");
        let guard = PairingGuard::new(true, &[hashed]);
        assert!(guard.is_authenticated("zc_valid"));
    }

    #[test]
    fn is_authenticated_with_invalid_token() {
        let guard = PairingGuard::new(true, &["zc_valid".into()]);
        assert!(!guard.is_authenticated("zc_invalid"));
    }

    #[test]
    fn is_authenticated_when_pairing_disabled() {
        let guard = PairingGuard::new(false, &[]);
        assert!(guard.is_authenticated("anything"));
        assert!(guard.is_authenticated(""));
    }

    #[test]
    fn tokens_returns_hashes() {
        let guard = PairingGuard::new(true, &["zc_a".into(), "zc_b".into()]);
        let tokens = guard.tokens();
        assert_eq!(tokens.len(), 2);
        // Tokens should be stored as 64-char hex hashes, not plaintext
        for t in &tokens {
            assert_eq!(t.len(), 64, "Token should be a SHA-256 hash");
            assert!(t.chars().all(|c| c.is_ascii_hexdigit()));
            assert!(!t.starts_with("zc_"), "Token should not be plaintext");
        }
    }

    #[test]
    fn pair_then_authenticate() {
        let guard = PairingGuard::new(true, &[]);
        let code = guard.pairing_code().unwrap().to_string();
        let token = guard.try_pair(&code).unwrap().unwrap();
        assert!(guard.is_authenticated(&token));
        assert!(!guard.is_authenticated("wrong"));
    }

    // ── Token hashing ────────────────────────────────────────

    #[test]
    fn hash_token_produces_64_hex_chars() {
        let hash = hash_token("zc_test_token");
        assert_eq!(hash.len(), 64);
        assert!(hash.chars().all(|c| c.is_ascii_hexdigit()));
    }

    #[test]
    fn hash_token_is_deterministic() {
        assert_eq!(hash_token("zc_abc"), hash_token("zc_abc"));
    }

    #[test]
    fn hash_token_differs_for_different_inputs() {
        assert_ne!(hash_token("zc_a"), hash_token("zc_b"));
    }

    #[test]
    fn is_token_hash_detects_hash_vs_plaintext() {
        assert!(is_token_hash(&hash_token("zc_test")));
        assert!(!is_token_hash("zc_test_token"));
        assert!(!is_token_hash("too_short"));
        assert!(!is_token_hash(""));
    }

    // ── is_public_bind ───────────────────────────────────────

    #[test]
    fn localhost_variants_not_public() {
        assert!(!is_public_bind("127.0.0.1"));
        assert!(!is_public_bind("localhost"));
        assert!(!is_public_bind("::1"));
        assert!(!is_public_bind("[::1]"));
    }

    #[test]
    fn zero_zero_is_public() {
        assert!(is_public_bind("0.0.0.0"));
    }

    #[test]
    fn real_ip_is_public() {
        assert!(is_public_bind("192.168.1.100"));
        assert!(is_public_bind("10.0.0.1"));
    }

    // ── constant_time_eq ─────────────────────────────────────

    #[test]
    fn constant_time_eq_same() {
        assert!(constant_time_eq("abc", "abc"));
        assert!(constant_time_eq("", ""));
    }

    #[test]
    fn constant_time_eq_different() {
        assert!(!constant_time_eq("abc", "abd"));
        assert!(!constant_time_eq("abc", "ab"));
        assert!(!constant_time_eq("a", ""));
    }

    // ── generate helpers ─────────────────────────────────────

    #[test]
    fn generate_code_is_6_digits() {
        let code = generate_code();
        assert_eq!(code.len(), 6);
        assert!(code.chars().all(|c| c.is_ascii_digit()));
    }

    #[test]
    fn generate_code_is_not_deterministic() {
        // Two codes should differ with overwhelming probability. We try
        // multiple pairs so a single 1-in-10^6 collision doesn't cause
        // a flaky CI failure. All 10 pairs colliding is ~1-in-10^60.
        for _ in 0..10 {
            if generate_code() != generate_code() {
                return; // Pass: found a non-matching pair.
            }
        }
        panic!("Generated 10 pairs of codes and all were collisions — CSPRNG failure");
    }

    #[test]
    fn generate_token_has_prefix() {
        let token = generate_token();
        assert!(token.starts_with("zc_"));
        assert!(token.len() > 10);
    }

    // ── Brute force protection ───────────────────────────────

    #[test]
    fn brute_force_lockout_after_max_attempts() {
        let guard = PairingGuard::new(true, &[]);
        // Exhaust all attempts with wrong codes
        for i in 0..MAX_PAIR_ATTEMPTS {
            let result = guard.try_pair(&format!("wrong_{i}"));
            assert!(result.is_ok(), "Attempt {i} should not be locked out yet");
        }
        // Next attempt should be locked out
        let result = guard.try_pair("another_wrong");
        assert!(
            result.is_err(),
            "Should be locked out after {MAX_PAIR_ATTEMPTS} attempts"
        );
        let lockout_secs = result.unwrap_err();
        assert!(lockout_secs > 0, "Lockout should have remaining seconds");
        assert!(
            lockout_secs <= PAIR_LOCKOUT_SECS,
            "Lockout should not exceed max"
        );
    }

    #[test]
    fn correct_code_resets_failed_attempts() {
        let guard = PairingGuard::new(true, &[]);
        let code = guard.pairing_code().unwrap().to_string();
        // Fail a few times
        for _ in 0..3 {
            let _ = guard.try_pair("wrong");
        }
        // Correct code should still work (under MAX_PAIR_ATTEMPTS)
        let result = guard.try_pair(&code).unwrap();
        assert!(result.is_some(), "Correct code should work before lockout");
    }

    #[test]
    fn lockout_returns_remaining_seconds() {
        let guard = PairingGuard::new(true, &[]);
        for _ in 0..MAX_PAIR_ATTEMPTS {
            let _ = guard.try_pair("wrong");
        }
        let err = guard.try_pair("wrong").unwrap_err();
        // Should be close to PAIR_LOCKOUT_SECS (within a second)
        assert!(
            err >= PAIR_LOCKOUT_SECS - 1,
            "Remaining lockout should be ~{PAIR_LOCKOUT_SECS}s, got {err}s"
        );
    }
}
