//! Profile PIN: hashing, and the lockout policy that decides what a guess costs.
//!
//! The policy half is deliberately pure — it takes the stored counter state and
//! the current time, and returns the decision plus the next state. That is what
//! makes "five wrong guesses then a lockout that survives a restart" testable
//! without a database, a clock, or a running app.
//!
//! What this is *not*: at-rest protection. The PIN gates switching to a profile;
//! it does not encrypt that profile's access token, so anyone holding the
//! database and the keyring has every token regardless. That trade is deliberate
//! and its reasoning lives in DR-268 — a wrapped token would leave a locked
//! profile unable to resume its own downloads or drain its own sync queue until
//! somebody walked past and typed the code.
//!
//! TRACES: UR-083 | DR-268

use argon2::Argon2;
use chrono::{DateTime, Duration, Utc};
use password_hash::{rand_core::OsRng, PasswordHash, PasswordHasher, PasswordVerifier, SaltString};

/// Wrong guesses allowed before the first lockout.
pub const MAX_ATTEMPTS: u32 = 5;

/// How long the first lockout lasts. Each subsequent failure doubles it.
const BASE_LOCKOUT_SECS: i64 = 60;

/// Ceiling on the doubling, so a forgotten PIN never bricks the tile — the
/// password route is always there, and a lockout measured in hours would push
/// people towards not setting a PIN at all.
const MAX_LOCKOUT_SECS: i64 = 15 * 60;

/// Persisted counter state for one profile's PIN.
///
/// TRACES: UR-083 | DR-268
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct PinState {
    pub failed_count: u32,
    pub locked_until: Option<DateTime<Utc>>,
}

impl PinState {
    pub fn fresh() -> Self {
        Self {
            failed_count: 0,
            locked_until: None,
        }
    }
}

/// What the caller should do with an attempt.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum PinDecision {
    Accept,
    Reject { attempts_remaining: u32 },
    Locked { until: DateTime<Utc> },
}

/// Decide an attempt and produce the state to persist.
///
/// `pin_matches` is the result of the hash comparison; passing it in rather than
/// doing the comparison here is what keeps this function pure and cheap to test
/// across the whole attempt/lockout space.
///
/// A locked profile is refused *without consulting the hash*, so a caller cannot
/// burn through a lockout by guessing quickly.
///
/// TRACES: UR-083 | DR-268
pub fn evaluate(
    state: &PinState,
    now: DateTime<Utc>,
    pin_matches: bool,
) -> (PinDecision, PinState) {
    if let Some(until) = state.locked_until {
        if now < until {
            return (PinDecision::Locked { until }, state.clone());
        }
    }

    if pin_matches {
        return (PinDecision::Accept, PinState::fresh());
    }

    let failed_count = state.failed_count.saturating_add(1);

    if failed_count >= MAX_ATTEMPTS {
        let over = i64::from(failed_count - MAX_ATTEMPTS);
        let secs = BASE_LOCKOUT_SECS
            .saturating_mul(1i64.checked_shl(over.min(16) as u32).unwrap_or(i64::MAX))
            .min(MAX_LOCKOUT_SECS);
        let until = now + Duration::seconds(secs);
        (
            PinDecision::Locked { until },
            PinState {
                failed_count,
                locked_until: Some(until),
            },
        )
    } else {
        (
            PinDecision::Reject {
                attempts_remaining: MAX_ATTEMPTS - failed_count,
            },
            PinState {
                failed_count,
                locked_until: None,
            },
        )
    }
}

/// A PIN must be 4–8 digits. Rejecting non-digits here rather than in the pad
/// keeps the rule where the rule is enforced.
///
/// TRACES: UR-083 | DR-268
pub fn validate_pin(pin: &str) -> Result<(), String> {
    if pin.len() < 4 || pin.len() > 8 {
        return Err("PIN must be between 4 and 8 digits".to_string());
    }
    if !pin.chars().all(|c| c.is_ascii_digit()) {
        return Err("PIN must contain only digits".to_string());
    }
    Ok(())
}

/// Hash a PIN for storage. Returns a PHC string with the salt embedded.
///
/// TRACES: UR-083 | DR-268
pub fn hash_pin(pin: &str) -> Result<String, String> {
    let salt = SaltString::generate(&mut OsRng);
    Argon2::default()
        .hash_password(pin.as_bytes(), &salt)
        .map(|h| h.to_string())
        .map_err(|e| format!("Failed to hash PIN: {}", e))
}

/// Compare a candidate PIN against a stored PHC string.
///
/// A malformed stored hash verifies as `false` rather than erroring: a corrupt
/// row should send the user down the password route, not wedge the picker.
///
/// TRACES: UR-083 | DR-268
pub fn verify_pin(pin: &str, stored: &str) -> bool {
    match PasswordHash::new(stored) {
        Ok(parsed) => Argon2::default()
            .verify_password(pin.as_bytes(), &parsed)
            .is_ok(),
        Err(e) => {
            log::warn!("[Profiles] Stored PIN hash is unreadable: {}", e);
            false
        }
    }
}

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

    fn t0() -> DateTime<Utc> {
        DateTime::parse_from_rfc3339("2026-01-01T00:00:00Z")
            .unwrap()
            .with_timezone(&Utc)
    }

    /// UT: a correct PIN is accepted and clears any accumulated failures.
    #[test]
    fn correct_pin_accepts_and_resets() {
        let state = PinState {
            failed_count: 3,
            locked_until: None,
        };
        let (decision, next) = evaluate(&state, t0(), true);
        assert_eq!(decision, PinDecision::Accept);
        assert_eq!(next, PinState::fresh());
    }

    /// UT: wrong guesses count down, and the count is what gets persisted —
    /// this is the half that must survive an app restart.
    #[test]
    fn wrong_pin_counts_down() {
        let mut state = PinState::fresh();
        for expected in (1..MAX_ATTEMPTS).rev() {
            let (decision, next) = evaluate(&state, t0(), false);
            assert_eq!(
                decision,
                PinDecision::Reject {
                    attempts_remaining: expected
                }
            );
            state = next;
        }
        assert_eq!(state.failed_count, MAX_ATTEMPTS - 1);
    }

    /// UT: the attempt that exhausts the allowance locks out rather than
    /// reporting zero attempts remaining.
    #[test]
    fn exhausting_attempts_locks_out() {
        let state = PinState {
            failed_count: MAX_ATTEMPTS - 1,
            locked_until: None,
        };
        let (decision, next) = evaluate(&state, t0(), false);
        match decision {
            PinDecision::Locked { until } => {
                assert_eq!(until, t0() + Duration::seconds(BASE_LOCKOUT_SECS));
            }
            other => panic!("expected lockout, got {:?}", other),
        }
        assert_eq!(next.locked_until, Some(t0() + Duration::seconds(60)));
    }

    /// UT: a locked profile is refused without the hash being consulted — the
    /// correct PIN does not shortcut an active lockout.
    #[test]
    fn lockout_refuses_even_a_correct_pin() {
        let until = t0() + Duration::seconds(60);
        let state = PinState {
            failed_count: MAX_ATTEMPTS,
            locked_until: Some(until),
        };
        let (decision, next) = evaluate(&state, t0(), true);
        assert_eq!(decision, PinDecision::Locked { until });
        assert_eq!(next, state, "a refused attempt must not extend the lockout");
    }

    /// UT: once the window passes the profile accepts again.
    #[test]
    fn lockout_expires() {
        let until = t0() + Duration::seconds(60);
        let state = PinState {
            failed_count: MAX_ATTEMPTS,
            locked_until: Some(until),
        };
        let (decision, next) = evaluate(&state, until + Duration::seconds(1), true);
        assert_eq!(decision, PinDecision::Accept);
        assert_eq!(next, PinState::fresh());
    }

    /// UT: repeated lockouts escalate, but stop at the ceiling so a forgotten
    /// PIN never becomes an hours-long wait.
    #[test]
    fn lockout_escalates_to_a_ceiling() {
        let mut seen = Vec::new();
        for failed in MAX_ATTEMPTS - 1..MAX_ATTEMPTS + 12 {
            let state = PinState {
                failed_count: failed,
                locked_until: None,
            };
            if let (PinDecision::Locked { until }, _) = evaluate(&state, t0(), false) {
                seen.push((until - t0()).num_seconds());
            }
        }
        assert_eq!(seen[0], BASE_LOCKOUT_SECS);
        assert!(seen[1] > seen[0], "second lockout should be longer");
        assert_eq!(*seen.last().unwrap(), MAX_LOCKOUT_SECS);
        assert!(seen.windows(2).all(|w| w[1] >= w[0]), "must not shrink");
    }

    /// UT: hashing round-trips, and a wrong PIN does not verify.
    #[test]
    fn hash_round_trips() {
        let hash = hash_pin("1234").unwrap();
        assert!(verify_pin("1234", &hash));
        assert!(!verify_pin("4321", &hash));
    }

    /// UT: the stored hash never contains the PIN itself.
    #[test]
    fn hash_does_not_leak_the_pin() {
        let hash = hash_pin("246813").unwrap();
        assert!(!hash.contains("246813"));
    }

    /// UT: an unreadable stored hash fails closed instead of erroring, so a
    /// corrupt row sends the user to the password route.
    #[test]
    fn corrupt_hash_fails_closed() {
        assert!(!verify_pin("1234", "not-a-phc-string"));
    }

    /// UT: PIN shape is enforced in Rust, not in the pad.
    #[test]
    fn pin_shape_is_validated() {
        assert!(validate_pin("1234").is_ok());
        assert!(validate_pin("12345678").is_ok());
        assert!(validate_pin("123").is_err(), "too short");
        assert!(validate_pin("123456789").is_err(), "too long");
        assert!(validate_pin("12a4").is_err(), "non-digit");
    }
}