//! Lock levels and the friction ladder (CLAUDE.md sections 1.4 and 1.5).
//!
//! This module is where "pain to turn off" stops being a slogan and becomes
//! arithmetic on a monotonic clock. The governing rule from section 1.5.1 is
//! that a level can be **raised instantly but lowered only by going through
//! that level's own disable flow** — so the state machine here is deliberately
//! asymmetric.
//!
//! # The invariant that matters
//!
//! Section 1.5.4: the longest possible disable path is capped at 24 hours, and
//! no lock may exist without an exit. Every wait in this module is bounded by a
//! constant below, and [`LockState::worst_case_wait`] is what the M1 tests
//! assert against that cap. Adding an unbounded wait here breaks the project's
//! primary safety guarantee, so the constants are `const` and the cap is
//! asserted rather than documented.

use crate::engine::{BootId, CivilTime, EngineInput, Monotonic};

/// Longest any single wait in the ladder may be. Section 1.5.4 caps the whole
/// disable path at 24 hours; this is the single largest component.
pub const MAX_WAIT_MS: u64 = 24 * 60 * 60 * 1000;

/// Longest possible wait in whole hours, for display (section 1.5.1 requires the
/// cost be shown in plain language). Declared as its own constant rather than
/// derived from [`MAX_WAIT_MS`] by division: the workspace denies
/// `cast_possible_truncation`, and a truncation here would misstate the cost the
/// user is asked to confirm.
pub const MAX_WAIT_HOURS: u32 = 24;

/// Default cooldown before the disable screen accepts input (section 1.5.2.1).
pub const DEFAULT_COOLDOWN_MINUTES: u32 = 10;
pub const DEFAULT_COOLDOWN_MS: u64 = (DEFAULT_COOLDOWN_MINUTES as u64) * 60_000;

/// Ceiling on a cooldown after escalation doubles it (section 1.5.2.4).
pub const MAX_COOLDOWN_MINUTES: u32 = 30;
pub const MAX_COOLDOWN_MS: u64 = (MAX_COOLDOWN_MINUTES as u64) * 60_000;

/// Default delay before a filed disable request takes effect (section 1.5.2.2).
pub const DEFAULT_TIME_LOCK_HOURS: u32 = 12;
pub const DEFAULT_TIME_LOCK_MS: u64 = (DEFAULT_TIME_LOCK_HOURS as u64) * 3_600_000;

/// Length of the confirmation window once the delay elapses.
pub const CONFIRMATION_WINDOW_MINUTES: u32 = 10;
pub const CONFIRMATION_WINDOW_MS: u64 = (CONFIRMATION_WINDOW_MINUTES as u64) * 60_000;

/// Length of the typing challenge at the Stubborn level (section 1.5.2.3).
pub const TYPING_CHALLENGE_CHARS: u32 = 300;

/// Escalation resets after this long with no attempts (section 1.5.2.4).
pub const ESCALATION_RESET_MS: u64 = 7 * 24 * 60 * 60 * 1000;

/// Compile-time proof that the 24-hour cap and its display form agree. If these
/// ever drift, the number shown to the user at activation would no longer be the
/// number actually enforced — which is exactly the lie section 1.5.1 forbids.
const _: () = assert!(MAX_WAIT_HOURS as u64 * 3_600_000 == MAX_WAIT_MS);

/// The three lock levels (section 1.5.1).
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
pub enum LockLevel {
    /// Schedules edit and pause freely.
    Normal,
    /// AppBlock-equivalent Strict Mode (section 1.4): no editing, pausing or
    /// uninstalling any schedule.
    Strict,
    /// Strict plus the full friction ladder.
    Stubborn,
}

impl LockLevel {
    /// The disable flow required to leave this level.
    ///
    /// Section 1.5.1 requires the cost to be shown and confirmed at activation
    /// time, so this is also what the activation screen renders as plain
    /// language — never as a number the user has to interpret.
    pub fn disable_cost(&self) -> DisableCost {
        match self {
            Self::Normal => DisableCost::Immediate,
            Self::Strict => DisableCost::Cooldown { minutes: DEFAULT_COOLDOWN_MINUTES },
            Self::Stubborn => DisableCost::TimeLockAndTyping {
                wait_hours: DEFAULT_TIME_LOCK_HOURS,
                type_chars: TYPING_CHALLENGE_CHARS,
            },
        }
    }
}

/// Human-readable cost of undoing a lock, shown at activation (section 1.5.1).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum DisableCost {
    /// The level can be left immediately.
    Immediate,
    /// A wait on the disable screen before it accepts input.
    Cooldown {
        /// How long the wait runs, in whole minutes.
        minutes: u32,
    },
    /// A filed request that stays fully blocking, plus a typing challenge.
    TimeLockAndTyping {
        /// How long blocking continues before the request can be confirmed.
        wait_hours: u32,
        /// Length of the string that must be retyped exactly.
        type_chars: u32,
    },
}

/// A disable request that has been filed but not yet granted
/// (section 1.5.2.2 — "disabling doesn't disable, it files a request").
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct PendingRequest {
    /// Monotonic deadline. Never wall clock: section 1.5.3 requires a system
    /// date change to never shorten a delay.
    pub ready_at: Monotonic,
    /// The confirmation window close. Missing it cancels the request and the
    /// user starts over.
    pub window_closes_at: Monotonic,
}

/// One recorded attempt to disable (section 1.5.2.5). Local only.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Attempt {
    /// Monotonic time of the attempt.
    pub at: Monotonic,
    /// Which boot it happened in, so attempts spanning a reboot are not
    /// miscounted as simultaneous.
    pub boot_id: BootId,
}

/// The lock subsystem's state, as a pure function of its input.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct LockState {
    /// The active lock level (section 1.5.1).
    pub level: LockLevel,
    /// True while any restriction from section 1.4 is in force: no schedule may
    /// be edited, paused or deleted.
    pub schedules_locked: bool,
    /// A request is filed and blocking stays fully active (section 1.5.2.2).
    pub pending: Option<PendingRequest>,
    /// Cooldown on the disable screen. Leaving the screen resets it.
    pub cooldown_remaining: Option<Monotonic>,
    /// Every attempt, for the block screen to display (section 1.5.2.5).
    pub attempts_today: u32,
    /// The keyholder code, if the user chose that option at activation
    /// (section 1.5.2.7). Generated once, shown once.
    pub keyholder_code: Option<String>,
}

impl LockState {
    /// Derive lock state from elapsed monotonic time. Pure: no clock read, so
    /// the caller supplies [`EngineInput::monotonic`].
    pub fn evaluate(input: &EngineInput<'_>) -> Self {
        // M1 wires this to persisted lock configuration. The timing logic that
        // matters — cooldown elapse, request windows, escalation — is written
        // here against the monotonic field so it is testable without a clock.
        let now = input.monotonic;
        let _ = (now, input.boot_id, input.now);
        Self {
            level: LockLevel::Normal,
            schedules_locked: false,
            pending: None,
            cooldown_remaining: None,
            attempts_today: 0,
            keyholder_code: None,
        }
    }

    /// Whether a schedule may currently be edited (section 1.4).
    ///
    /// Strict Mode locks **all** schedules, not only the one it was tied to —
    /// that is the AppBlock-equivalent behaviour section 1.4 specifies.
    pub fn may_edit_schedule(&self) -> bool {
        !self.schedules_locked
    }

    /// Whether a schedule may be paused (section 1.4).
    pub fn may_pause_schedule(&self) -> bool {
        !self.schedules_locked
    }

    /// Whether the uninstall block applies. Section 1.4 allows this to be
    /// configured independently of Strict Mode being on.
    pub fn blocks_uninstall(&self) -> bool {
        matches!(self.level, LockLevel::Strict | LockLevel::Stubborn)
    }

    /// The longest possible wait to fully disable, in milliseconds.
    ///
    /// Section 1.5.4 caps this at 24 hours. Every term here is a `const` above,
    /// so the bound is structural: to exceed 24 hours, someone would have to add
    /// an unbounded wait to this module, and the M1 test asserts against
    /// [`MAX_WAIT_MS`].
    pub fn worst_case_wait(&self) -> u64 {
        match self.level {
            LockLevel::Normal => 0,
            LockLevel::Strict => MAX_COOLDOWN_MS,
            LockLevel::Stubborn => MAX_WAIT_MS,
        }
    }
}

/// Escalate the cooldown, doubling it but never past the cap
/// (section 1.5.2.4).
///
/// `attempted_at` and `now` are monotonic. `last_attempt_at` is `None` for a
/// first attempt.
pub fn escalate_cooldown(previous: Option<Monotonic>, now: Monotonic) -> u64 {
    let previous_ms = previous.map_or(DEFAULT_COOLDOWN_MS, |p| now.0.saturating_sub(p.0));
    previous_ms.saturating_mul(2).min(MAX_COOLDOWN_MS)
}

/// Whether an escalation has aged out (section 1.5.2.4: resets after 7 days
/// with no attempts).
pub fn escalation_has_reset(last_attempt_at: Option<Monotonic>, now: Monotonic) -> bool {
    last_attempt_at.is_some_and(|t| now.0.saturating_sub(t.0) >= ESCALATION_RESET_MS)
}

/// Measure how far the wall clock has drifted from what real elapsed time
/// predicts.
///
/// `observed_wall` is what the OS now reports. `expected_wall` is what it should
/// report given `elapsed_monotonic` since the last sample. Returns `None` when
/// the clock agrees, and otherwise the signed skew in minutes: **positive means
/// the observed clock is behind**, i.e. the user moved the date backwards.
///
/// `elapsed_monotonic` is accepted for symmetry with the caller's sampling loop
/// but is not needed for the comparison — it is the *expected* wall value that
/// already accounts for it.
pub fn wall_clock_correction(
    observed_wall: CivilTime,
    expected_wall: CivilTime,
    elapsed_monotonic: Monotonic,
) -> Option<i64> {
    let _ = elapsed_monotonic;
    let observed = observed_wall.epoch_minutes;
    let expected = expected_wall.epoch_minutes;
    if observed == expected {
        return None;
    }
    Some(observed - expected)
}

/// Apply a detected wall-clock skew to a lock deadline.
///
/// **Always extends, never subtracts, and never past the cap.** Three rules, in
/// order of importance:
///
/// 1. *Never shortens.* Section 1.5.3. A user who moves the system date is
///    trying to shorten a wait, and an implementation that trusted the wall
///    clock would oblige them. The magnitude is taken unsigned so the direction
///    of the jump cannot matter.
/// 2. *Always extends when a jump is detected.* Section 1.5.3 again. The
///    engine cannot distinguish deliberate tampering from a machine that was
///    suspended longer than the monotonic clock accounted for, so it takes the
///    conservative branch and lengthens the lock.
/// 3. *Never past the cap.* Section 1.5.4 caps the longest possible disable
///    path at 24 hours. Without this clamp, rule 2 would let a 3-day clock
///    rollback turn a 12-hour lock into a 3-day wait — correct behaviour on the
///    anti-tamper axis and a violation on the safety axis.
///
/// Over-extending is the acceptable failure mode. A lock that ends slightly late
/// is an inconvenience; a lock that ends early defeats the entire mechanism.
/// The clamp bounds the cost of that choice.
pub fn apply_correction(deadline: Monotonic, correction_minutes: i64) -> Monotonic {
    let extension_ms = correction_minutes.unsigned_abs().saturating_mul(60_000);
    let extended = deadline.0.saturating_add(extension_ms);
    Monotonic(extended.min(MAX_WAIT_MS))
}

/// Resume a lock after a reboot, conservatively (section 1.5.3).
///
/// A new boot id means we cannot know how much of the elapsed time was real. The
/// conservative reading is that less time passed than we hoped, so the deadline
/// is pushed out rather than assumed satisfied.
pub fn resume_after_reboot(deadline: Monotonic, elapsed_before_reboot: Monotonic) -> Monotonic {
    Monotonic(deadline.0.max(elapsed_before_reboot.0))
}