//! Append-only usage observations and the derived footer projection.
//!
//! Decision 0002 requires every usage value to carry its provenance, and the
//! product footer must answer five questions without inventing precision:
//! which profile is active, how much of the window is consumed, when the window
//! resets, what the burn rate is, and how trustworthy those numbers are.
//!
//! This module stores facts, derives views from them, and reports what it
//! cannot know. Nothing here reads a clock: callers supply timestamps so the
//! projection is deterministic and testable. All arithmetic is integer
//! arithmetic; fractions are carried as permille so a rendered percentage never
//! drifts from the stored fact.
//!
//! ```
//! use lumbridge_core::{
//!     AccountProfile, FooterUsage, UsageLedger, UsageObservation, UsageProvenance, UsageUnit,
//!     UsageWindow,
//! };
//!
//! let profile = AccountProfile::new("codex-plus", "Codex", "ChatGPT", "gpt-5.2", "work")?;
//! let window = UsageWindow::new(0, 18_000_000)?;
//! let mut ledger = UsageLedger::new();
//!
//! for (at_ms, consumed) in [(0, 0), (3_600_000, 620_000)] {
//!     ledger.record(
//!         UsageObservation::counted(
//!             profile.id().clone(),
//!             UsageUnit::Tokens,
//!             consumed,
//!             UsageProvenance::ProviderReported,
//!             at_ms,
//!         )?
//!         .with_limit(1_000_000)?
//!         .with_window(window)?,
//!     )?;
//! }
//!
//! let projection = ledger.project(profile.id(), 3_600_000);
//! assert_eq!(projection.consumed_permille(), Some(620));
//! assert_eq!(projection.burn_per_hour(), Some(620_000));
//! let footer = FooterUsage::new(&profile, &projection);
//! assert_eq!(footer.window(), "62.0% used");
//! # Ok::<(), lumbridge_core::UsageError>(())
//! ```

use std::collections::{BTreeMap, VecDeque};
use std::fmt;

use thiserror::Error;

const MAX_IDENTITY_BYTES: usize = 256;
const DEFAULT_RETENTION_PER_PROFILE: usize = 64;
const MIN_RETENTION_PER_PROFILE: usize = 2;
const DEFAULT_STALENESS_BUDGET_MS: u64 = 300_000;
const MIN_BURN_SAMPLE_MS: u64 = 1_000;
const MILLIS_PER_HOUR: u128 = 3_600_000;
const MILLIS_PER_MINUTE: u64 = 60_000;
const MILLIS_PER_SECOND: u64 = 1_000;
const MINUTES_PER_HOUR: u64 = 60;
const HOURS_PER_DAY: u64 = 24;
const PERMILLE_SCALE: u128 = 1_000;
const MAX_PERMILLE: u64 = 1_000;

/// Failures produced while recording or shaping usage facts.
#[derive(Clone, Debug, Eq, Error, PartialEq)]
pub enum UsageError {
    #[error("invalid {0}")]
    InvalidIdentifier(&'static str),
    #[error("a usage window must reset after it starts")]
    EmptyWindow,
    #[error("a usage limit must be greater than zero")]
    ZeroLimit,
    #[error("observation at {observed_at_ms} ms precedes the newest observation at {newest_ms} ms")]
    OutOfOrderObservation { observed_at_ms: u64, newest_ms: u64 },
    #[error("observation at {observed_at_ms} ms falls outside its declared window")]
    ObservationOutsideWindow { observed_at_ms: u64 },
    #[error("retention must keep at least two observations per profile")]
    RetentionTooSmall,
    #[error("an unavailable observation cannot carry a counted value")]
    UnavailableWithValue,
}

/// Where a usage number came from.
///
/// The ordering is deliberate: variants are declared from most to least
/// authoritative so [`UsageProvenance::weakest`] can take the maximum.
#[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
pub enum UsageProvenance {
    /// The provider's own account or quota API reported this value.
    ProviderReported,
    /// The upstream harness reported it through a documented surface.
    HarnessReported,
    /// Lumbridge counted it locally from traffic it owns.
    LocallyMeasured,
    /// Lumbridge derived it from other observations.
    Estimated,
    /// No usable value exists. This is a fact, not a zero.
    Unavailable,
}

impl UsageProvenance {
    /// The label kept in detail views and exports.
    #[must_use]
    pub const fn label(self) -> &'static str {
        match self {
            Self::ProviderReported => "provider-reported",
            Self::HarnessReported => "harness-reported",
            Self::LocallyMeasured => "locally measured",
            Self::Estimated => "estimated",
            Self::Unavailable => "unavailable",
        }
    }

    /// The short label a compact surface such as the footer may substitute.
    #[must_use]
    pub const fn compact_label(self) -> &'static str {
        match self {
            Self::ProviderReported => "provider",
            Self::HarnessReported => "harness",
            Self::LocallyMeasured => "local",
            Self::Estimated => "est",
            Self::Unavailable => "none",
        }
    }

    /// Whether an external party stated this value rather than Lumbridge.
    #[must_use]
    pub const fn is_reported(self) -> bool {
        matches!(self, Self::ProviderReported | Self::HarnessReported)
    }

    #[must_use]
    pub const fn is_available(self) -> bool {
        !matches!(self, Self::Unavailable)
    }

    #[must_use]
    pub const fn confidence(self) -> UsageConfidence {
        match self {
            Self::ProviderReported => UsageConfidence::Exact,
            Self::HarnessReported | Self::LocallyMeasured => UsageConfidence::Approximate,
            Self::Estimated => UsageConfidence::Projected,
            Self::Unavailable => UsageConfidence::Unknown,
        }
    }

    /// The less authoritative of two provenances.
    ///
    /// A derived value is only as trustworthy as its weakest input.
    #[must_use]
    pub fn weakest(self, other: Self) -> Self {
        self.max(other)
    }
}

/// How much a displayed number can be trusted.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum UsageConfidence {
    Exact,
    Approximate,
    Projected,
    Unknown,
}

impl UsageConfidence {
    #[must_use]
    pub const fn label(self) -> &'static str {
        match self {
            Self::Exact => "exact",
            Self::Approximate => "approximate",
            Self::Projected => "projected",
            Self::Unknown => "unknown",
        }
    }
}

/// What a usage number counts.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum UsageUnit {
    Tokens,
    Requests,
    /// A window fraction the provider reports directly, in permille.
    WindowPermille,
}

impl UsageUnit {
    #[must_use]
    pub const fn label(self) -> &'static str {
        match self {
            Self::Tokens => "tokens",
            Self::Requests => "requests",
            Self::WindowPermille => "window",
        }
    }

    /// Renders an amount in this unit without inflating its precision.
    #[must_use]
    pub fn format_amount(self, amount: u64) -> String {
        match self {
            Self::Tokens => format_count(amount, "tokens"),
            Self::Requests => format_count(amount, "requests"),
            Self::WindowPermille => format_permille(amount),
        }
    }
}

fn format_count(amount: u64, suffix: &str) -> String {
    if amount >= 1_000_000 {
        format!(
            "{}.{}M {suffix}",
            amount / 1_000_000,
            (amount % 1_000_000) / 100_000
        )
    } else if amount >= 1_000 {
        format!("{}.{}k {suffix}", amount / 1_000, (amount % 1_000) / 100)
    } else {
        format!("{amount} {suffix}")
    }
}

fn format_permille(permille: u64) -> String {
    format!("{}.{}%", permille / 10, permille % 10)
}

/// Renders a duration for a compact surface. Never rounds up to a false zero.
#[must_use]
pub fn format_duration_ms(duration_ms: u64) -> String {
    if duration_ms < MILLIS_PER_MINUTE {
        return format!("{}s", duration_ms / MILLIS_PER_SECOND);
    }
    let minutes = duration_ms / MILLIS_PER_MINUTE;
    if minutes < MINUTES_PER_HOUR {
        return format!("{minutes}m");
    }
    let hours = minutes / MINUTES_PER_HOUR;
    if hours < HOURS_PER_DAY {
        return format!("{hours}h {}m", minutes % MINUTES_PER_HOUR);
    }
    // Weekly and monthly quota windows are common, and "151h 30m" is not a
    // duration anyone reads at a glance.
    format!("{}d {}h", hours / HOURS_PER_DAY, hours % HOURS_PER_DAY)
}

fn validate_identity(value: &str, label: &'static str) -> Result<(), UsageError> {
    if value.trim().is_empty()
        || value.len() > MAX_IDENTITY_BYTES
        || value.chars().any(char::is_control)
    {
        return Err(UsageError::InvalidIdentifier(label));
    }
    Ok(())
}

/// A stable identifier for one harness/provider/account combination.
#[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct AccountProfileId(String);

impl AccountProfileId {
    /// Creates a stable, non-empty account profile identifier.
    ///
    /// # Errors
    ///
    /// Returns [`UsageError::InvalidIdentifier`] for an empty, oversized, or
    /// control-bearing identifier.
    pub fn new(value: impl Into<String>) -> Result<Self, UsageError> {
        let value = value.into();
        validate_identity(&value, "account profile ID")?;
        Ok(Self(value))
    }

    #[must_use]
    pub fn as_str(&self) -> &str {
        &self.0
    }
}

impl fmt::Display for AccountProfileId {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        formatter.write_str(&self.0)
    }
}

/// The active harness, provider, model, and account label shown in the footer.
///
/// These are display names only. Credentials stay in the operating-system
/// credential store and are never part of this type.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct AccountProfile {
    id: AccountProfileId,
    harness: String,
    provider: String,
    model: String,
    account: String,
}

impl AccountProfile {
    /// Creates a footer-ready account profile.
    ///
    /// # Errors
    ///
    /// Returns [`UsageError::InvalidIdentifier`] when any field is empty,
    /// oversized, or contains control characters.
    pub fn new(
        id: impl Into<String>,
        harness: impl Into<String>,
        provider: impl Into<String>,
        model: impl Into<String>,
        account: impl Into<String>,
    ) -> Result<Self, UsageError> {
        let harness = harness.into();
        let provider = provider.into();
        let model = model.into();
        let account = account.into();
        validate_identity(&harness, "harness name")?;
        validate_identity(&provider, "provider name")?;
        validate_identity(&model, "model name")?;
        validate_identity(&account, "account label")?;
        Ok(Self {
            id: AccountProfileId::new(id)?,
            harness,
            provider,
            model,
            account,
        })
    }

    #[must_use]
    pub const fn id(&self) -> &AccountProfileId {
        &self.id
    }

    #[must_use]
    pub fn harness(&self) -> &str {
        &self.harness
    }

    #[must_use]
    pub fn provider(&self) -> &str {
        &self.provider
    }

    #[must_use]
    pub fn model(&self) -> &str {
        &self.model
    }

    #[must_use]
    pub fn account(&self) -> &str {
        &self.account
    }

    /// The footer's first answer: which harness, provider, and model is active.
    #[must_use]
    pub fn identity_line(&self) -> String {
        format!("{} · {} · {}", self.harness, self.provider, self.model)
    }
}

/// A provider-declared usage window.
///
/// The start is optional because providers report these fields independently:
/// a reset time can arrive without a window length. Knowing when a window ends
/// is the useful half — it bounds the exhaustion forecast and identifies which
/// window an observation belongs to — so discarding it for want of a start
/// would throw away a fact the provider actually stated.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct UsageWindow {
    started_at_ms: Option<u64>,
    resets_at_ms: u64,
}

impl UsageWindow {
    /// Creates a window that resets strictly after it starts.
    ///
    /// # Errors
    ///
    /// Returns [`UsageError::EmptyWindow`] when the reset is not after the start.
    pub const fn new(started_at_ms: u64, resets_at_ms: u64) -> Result<Self, UsageError> {
        if resets_at_ms <= started_at_ms {
            return Err(UsageError::EmptyWindow);
        }
        Ok(Self {
            started_at_ms: Some(started_at_ms),
            resets_at_ms,
        })
    }

    /// Creates a window whose reset is known and whose start is not.
    ///
    /// Two of these compare equal when they reset at the same moment, which is
    /// what lets a burn rate be scoped to one window without inventing a start.
    #[must_use]
    pub const fn until(resets_at_ms: u64) -> Self {
        Self {
            started_at_ms: None,
            resets_at_ms,
        }
    }

    /// When the window opened, if the provider said.
    #[must_use]
    pub const fn started_at_ms(self) -> Option<u64> {
        self.started_at_ms
    }

    #[must_use]
    pub const fn resets_at_ms(self) -> u64 {
        self.resets_at_ms
    }

    #[must_use]
    pub const fn contains(self, at_ms: u64) -> bool {
        if at_ms >= self.resets_at_ms {
            return false;
        }
        match self.started_at_ms {
            Some(started_at_ms) => at_ms >= started_at_ms,
            None => true,
        }
    }

    #[must_use]
    pub const fn has_expired(self, now_ms: u64) -> bool {
        now_ms >= self.resets_at_ms
    }

    #[must_use]
    pub const fn remaining_ms(self, now_ms: u64) -> u64 {
        self.resets_at_ms.saturating_sub(now_ms)
    }
}

/// One recorded usage fact. Observations are never edited, only appended.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct UsageObservation {
    profile: AccountProfileId,
    unit: Option<UsageUnit>,
    consumed: u64,
    limit: Option<u64>,
    window: Option<UsageWindow>,
    provenance: UsageProvenance,
    observed_at_ms: u64,
}

impl UsageObservation {
    /// Records a counted usage value with its provenance.
    ///
    /// # Errors
    ///
    /// Returns [`UsageError::UnavailableWithValue`] when the caller labels a
    /// counted value as unavailable. Use [`UsageObservation::unavailable`] to
    /// record a genuine gap.
    pub fn counted(
        profile: AccountProfileId,
        unit: UsageUnit,
        consumed: u64,
        provenance: UsageProvenance,
        observed_at_ms: u64,
    ) -> Result<Self, UsageError> {
        if !provenance.is_available() {
            return Err(UsageError::UnavailableWithValue);
        }
        Ok(Self {
            profile,
            unit: Some(unit),
            consumed,
            limit: None,
            window: None,
            provenance,
            observed_at_ms,
        })
    }

    /// Records that no usage value could be obtained at this moment.
    ///
    /// This is deliberately a first-class observation. Recording it stops the
    /// projection from presenting an older reading as current.
    #[must_use]
    pub const fn unavailable(profile: AccountProfileId, observed_at_ms: u64) -> Self {
        Self {
            profile,
            unit: None,
            consumed: 0,
            limit: None,
            window: None,
            provenance: UsageProvenance::Unavailable,
            observed_at_ms,
        }
    }

    /// Attaches the quota ceiling this observation was measured against.
    ///
    /// # Errors
    ///
    /// Returns [`UsageError::ZeroLimit`] for a zero limit and
    /// [`UsageError::UnavailableWithValue`] for an unavailable observation.
    pub fn with_limit(mut self, limit: u64) -> Result<Self, UsageError> {
        if !self.provenance.is_available() {
            return Err(UsageError::UnavailableWithValue);
        }
        if limit == 0 {
            return Err(UsageError::ZeroLimit);
        }
        self.limit = Some(limit);
        Ok(self)
    }

    /// Attaches the provider window this observation belongs to.
    ///
    /// # Errors
    ///
    /// Returns [`UsageError::ObservationOutsideWindow`] when the observation
    /// time is not inside the window, and [`UsageError::UnavailableWithValue`]
    /// for an unavailable observation.
    pub fn with_window(mut self, window: UsageWindow) -> Result<Self, UsageError> {
        if !self.provenance.is_available() {
            return Err(UsageError::UnavailableWithValue);
        }
        if !window.contains(self.observed_at_ms) {
            return Err(UsageError::ObservationOutsideWindow {
                observed_at_ms: self.observed_at_ms,
            });
        }
        self.window = Some(window);
        Ok(self)
    }

    #[must_use]
    pub const fn profile(&self) -> &AccountProfileId {
        &self.profile
    }

    #[must_use]
    pub const fn unit(&self) -> Option<UsageUnit> {
        self.unit
    }

    #[must_use]
    pub const fn consumed(&self) -> u64 {
        self.consumed
    }

    #[must_use]
    pub const fn limit(&self) -> Option<u64> {
        self.limit
    }

    #[must_use]
    pub const fn window(&self) -> Option<UsageWindow> {
        self.window
    }

    #[must_use]
    pub const fn provenance(&self) -> UsageProvenance {
        self.provenance
    }

    #[must_use]
    pub const fn observed_at_ms(&self) -> u64 {
        self.observed_at_ms
    }

    /// Units left before the ceiling, when a ceiling is known.
    #[must_use]
    pub fn remaining(&self) -> Option<u64> {
        self.limit.map(|limit| limit.saturating_sub(self.consumed))
    }
}

/// A bounded, append-only observation stream per account profile.
#[derive(Clone, Debug)]
pub struct UsageLedger {
    retention_per_profile: usize,
    staleness_budget_ms: u64,
    observations: BTreeMap<AccountProfileId, VecDeque<UsageObservation>>,
}

impl Default for UsageLedger {
    fn default() -> Self {
        Self::new()
    }
}

impl UsageLedger {
    #[must_use]
    pub fn new() -> Self {
        Self {
            retention_per_profile: DEFAULT_RETENTION_PER_PROFILE,
            staleness_budget_ms: DEFAULT_STALENESS_BUDGET_MS,
            observations: BTreeMap::new(),
        }
    }

    /// Creates a ledger that retains a bounded history per profile.
    ///
    /// # Errors
    ///
    /// Returns [`UsageError::RetentionTooSmall`] below two observations, which
    /// is the minimum a burn rate can be derived from.
    pub fn with_retention(retention_per_profile: usize) -> Result<Self, UsageError> {
        if retention_per_profile < MIN_RETENTION_PER_PROFILE {
            return Err(UsageError::RetentionTooSmall);
        }
        Ok(Self {
            retention_per_profile,
            ..Self::new()
        })
    }

    /// Sets how old the newest observation may be before it reads as stale.
    #[must_use]
    pub const fn with_staleness_budget_ms(mut self, staleness_budget_ms: u64) -> Self {
        self.staleness_budget_ms = staleness_budget_ms;
        self
    }

    /// Appends an observation to its profile's history.
    ///
    /// # Errors
    ///
    /// Returns [`UsageError::OutOfOrderObservation`] when the observation is
    /// older than the newest one already recorded for that profile. The stream
    /// is append-only, so time must not move backwards.
    pub fn record(&mut self, observation: UsageObservation) -> Result<(), UsageError> {
        let history = self
            .observations
            .entry(observation.profile.clone())
            .or_default();
        if let Some(newest) = history.back()
            && observation.observed_at_ms < newest.observed_at_ms
        {
            return Err(UsageError::OutOfOrderObservation {
                observed_at_ms: observation.observed_at_ms,
                newest_ms: newest.observed_at_ms,
            });
        }
        history.push_back(observation);
        while history.len() > self.retention_per_profile {
            history.pop_front();
        }
        Ok(())
    }

    #[must_use]
    pub fn latest(&self, profile: &AccountProfileId) -> Option<&UsageObservation> {
        self.observations.get(profile).and_then(VecDeque::back)
    }

    #[must_use]
    pub fn observation_count(&self, profile: &AccountProfileId) -> usize {
        self.observations.get(profile).map_or(0, VecDeque::len)
    }

    pub fn profiles(&self) -> impl Iterator<Item = &AccountProfileId> {
        self.observations.keys()
    }

    /// Derives the current view of a profile's usage.
    ///
    /// Reported facts keep their own provenance. Anything this method computes
    /// is labelled [`UsageProvenance::Estimated`], even when every input was
    /// provider-reported: a derived rate is not a reported fact.
    #[must_use]
    pub fn project(&self, profile: &AccountProfileId, now_ms: u64) -> UsageProjection {
        let Some(history) = self.observations.get(profile) else {
            return UsageProjection::unavailable(None);
        };
        let Some(latest) = history.back() else {
            return UsageProjection::unavailable(None);
        };
        if !latest.provenance.is_available() {
            return UsageProjection::unavailable(Some(latest.observed_at_ms));
        }
        if let Some(window) = latest.window
            && window.has_expired(now_ms)
        {
            return UsageProjection::window_rolled(latest.observed_at_ms);
        }

        let consumed_permille = latest
            .limit
            .map(|limit| permille_of(latest.consumed, limit))
            .or_else(|| {
                matches!(latest.unit, Some(UsageUnit::WindowPermille))
                    .then_some(latest.consumed.min(MAX_PERMILLE))
            });
        let burn_per_hour = burn_per_hour(history, latest);
        let resets_in_ms = latest.window.map(|window| window.remaining_ms(now_ms));
        let exhaustion_in_ms = burn_per_hour
            .and_then(|rate| exhaustion_in_ms(latest.remaining()?, rate, resets_in_ms));

        UsageProjection {
            unit: latest.unit,
            consumed: Some(latest.consumed),
            limit: latest.limit,
            consumed_permille,
            resets_in_ms,
            burn_per_hour,
            exhaustion_in_ms,
            provenance: latest.provenance,
            stale: now_ms.saturating_sub(latest.observed_at_ms) > self.staleness_budget_ms,
            window_rolled: false,
            observed_at_ms: Some(latest.observed_at_ms),
        }
    }
}

fn permille_of(consumed: u64, limit: u64) -> u64 {
    if limit == 0 {
        return 0;
    }
    let permille = u128::from(consumed) * PERMILLE_SCALE / u128::from(limit);
    u64::try_from(permille)
        .unwrap_or(MAX_PERMILLE)
        .min(MAX_PERMILLE)
}

/// Averages consumption across the newest observation's window.
///
/// Returns `None` unless two observations share a window and a unit, are far
/// enough apart to divide by, and did not move backwards. A single reading
/// yields no rate at all rather than a rate of zero.
fn burn_per_hour(history: &VecDeque<UsageObservation>, latest: &UsageObservation) -> Option<u64> {
    let baseline = history.iter().find(|candidate| {
        candidate.window == latest.window
            && candidate.unit == latest.unit
            && candidate.provenance.is_available()
            && candidate.observed_at_ms < latest.observed_at_ms
    })?;
    if latest.consumed < baseline.consumed {
        return None;
    }
    let elapsed_ms = latest.observed_at_ms - baseline.observed_at_ms;
    if elapsed_ms < MIN_BURN_SAMPLE_MS {
        return None;
    }
    let consumed_delta = u128::from(latest.consumed - baseline.consumed);
    let rate = consumed_delta * MILLIS_PER_HOUR / u128::from(elapsed_ms);
    u64::try_from(rate).ok()
}

/// Projects when a ceiling is reached, unless the window resets first.
fn exhaustion_in_ms(remaining: u64, burn_per_hour: u64, resets_in_ms: Option<u64>) -> Option<u64> {
    if burn_per_hour == 0 {
        return None;
    }
    let millis = u128::from(remaining) * MILLIS_PER_HOUR / u128::from(burn_per_hour);
    let millis = u64::try_from(millis).ok()?;
    match resets_in_ms {
        Some(resets_in_ms) if millis >= resets_in_ms => None,
        _ => Some(millis),
    }
}

/// The derived view a footer or detail panel renders.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct UsageProjection {
    unit: Option<UsageUnit>,
    consumed: Option<u64>,
    limit: Option<u64>,
    consumed_permille: Option<u64>,
    resets_in_ms: Option<u64>,
    burn_per_hour: Option<u64>,
    exhaustion_in_ms: Option<u64>,
    provenance: UsageProvenance,
    stale: bool,
    window_rolled: bool,
    observed_at_ms: Option<u64>,
}

impl UsageProjection {
    const fn unavailable(observed_at_ms: Option<u64>) -> Self {
        Self {
            unit: None,
            consumed: None,
            limit: None,
            consumed_permille: None,
            resets_in_ms: None,
            burn_per_hour: None,
            exhaustion_in_ms: None,
            provenance: UsageProvenance::Unavailable,
            stale: false,
            window_rolled: false,
            observed_at_ms,
        }
    }

    const fn window_rolled(observed_at_ms: u64) -> Self {
        Self {
            stale: true,
            window_rolled: true,
            ..Self::unavailable(Some(observed_at_ms))
        }
    }

    #[must_use]
    pub const fn unit(self) -> Option<UsageUnit> {
        self.unit
    }

    #[must_use]
    pub const fn consumed(self) -> Option<u64> {
        self.consumed
    }

    #[must_use]
    pub const fn limit(self) -> Option<u64> {
        self.limit
    }

    /// Share of the window consumed, in permille, when a ceiling is known.
    #[must_use]
    pub const fn consumed_permille(self) -> Option<u64> {
        self.consumed_permille
    }

    #[must_use]
    pub const fn resets_in_ms(self) -> Option<u64> {
        self.resets_in_ms
    }

    /// Units per hour averaged over the current window.
    #[must_use]
    pub const fn burn_per_hour(self) -> Option<u64> {
        self.burn_per_hour
    }

    /// Time until the ceiling is reached, absent unless it precedes the reset.
    #[must_use]
    pub const fn exhaustion_in_ms(self) -> Option<u64> {
        self.exhaustion_in_ms
    }

    /// The provenance of the reported facts in this projection.
    #[must_use]
    pub const fn provenance(self) -> UsageProvenance {
        self.provenance
    }

    /// The provenance of the burn rate, which is always derived.
    #[must_use]
    pub const fn burn_provenance(self) -> UsageProvenance {
        match self.burn_per_hour {
            Some(_) => UsageProvenance::Estimated,
            None => UsageProvenance::Unavailable,
        }
    }

    /// The trust classification a detail view must keep.
    #[must_use]
    pub const fn confidence(self) -> UsageConfidence {
        if self.stale {
            return UsageConfidence::Unknown;
        }
        self.provenance.confidence()
    }

    /// Whether the newest observation is older than the staleness budget.
    #[must_use]
    pub const fn is_stale(self) -> bool {
        self.stale
    }

    /// Whether the last known window ended before this projection was taken.
    #[must_use]
    pub const fn window_has_rolled(self) -> bool {
        self.window_rolled
    }

    #[must_use]
    pub const fn observed_at_ms(self) -> Option<u64> {
        self.observed_at_ms
    }

    #[must_use]
    pub const fn is_available(self) -> bool {
        self.provenance.is_available()
    }
}

/// Renders the five footer answers for one profile.
///
/// Every method returns an explicit unavailable phrase instead of a placeholder
/// number, so a missing fact reads as missing rather than as zero.
#[derive(Clone, Copy, Debug)]
pub struct FooterUsage<'a> {
    profile: &'a AccountProfile,
    projection: &'a UsageProjection,
}

impl<'a> FooterUsage<'a> {
    #[must_use]
    pub const fn new(profile: &'a AccountProfile, projection: &'a UsageProjection) -> Self {
        Self {
            profile,
            projection,
        }
    }

    /// Which harness, provider, model, and account profile is active.
    #[must_use]
    pub fn identity(&self) -> String {
        format!(
            "{} · {}",
            self.profile.identity_line(),
            self.profile.account()
        )
    }

    /// How much of the current usage window has been consumed.
    #[must_use]
    pub fn window(&self) -> String {
        match (
            self.projection.consumed_permille(),
            self.projection.consumed(),
            self.projection.unit(),
        ) {
            (Some(permille), _, _) => format!("{} used", format_permille(permille)),
            (None, Some(consumed), Some(unit)) => {
                format!(
                    "{} used · no ceiling reported",
                    unit.format_amount(consumed)
                )
            }
            _ if self.projection.window_has_rolled() => "window rolled over".to_owned(),
            _ => "usage unavailable".to_owned(),
        }
    }

    /// When the window resets.
    #[must_use]
    pub fn reset(&self) -> String {
        self.projection.resets_in_ms().map_or_else(
            || "reset time unavailable".to_owned(),
            |remaining| format!("resets in {}", format_duration_ms(remaining)),
        )
    }

    /// The recent burn rate and, when meaningful, the projected exhaustion.
    #[must_use]
    pub fn burn(&self) -> String {
        let Some(rate) = self.projection.burn_per_hour() else {
            return "burn rate unavailable".to_owned();
        };
        let unit = self.projection.unit().unwrap_or(UsageUnit::Tokens);
        let rate = format!("~{}/hr est", unit.format_amount(rate));
        self.projection
            .exhaustion_in_ms()
            .map_or(rate.clone(), |exhaustion| {
                format!("{rate} · empty in {}", format_duration_ms(exhaustion))
            })
    }

    /// How trustworthy the numbers above are.
    #[must_use]
    pub fn trust(&self) -> String {
        let provenance = self.projection.provenance();
        if !provenance.is_available() {
            return if self.projection.window_has_rolled() {
                "awaiting a new window reading".to_owned()
            } else {
                "no usage source".to_owned()
            };
        }
        let base = format!(
            "{} · {}",
            provenance.compact_label(),
            self.projection.confidence().label()
        );
        if self.projection.is_stale() {
            return format!("{base} · stale");
        }
        base
    }
}

impl fmt::Display for FooterUsage<'_> {
    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            formatter,
            "{} · {} · {} · {} · {}",
            self.identity(),
            self.window(),
            self.reset(),
            self.burn(),
            self.trust()
        )
    }
}

#[cfg(test)]
mod tests {
    use super::{
        AccountProfile, AccountProfileId, FooterUsage, UsageConfidence, UsageError, UsageLedger,
        UsageObservation, UsageProvenance, UsageUnit, UsageWindow, format_duration_ms,
    };

    const HOUR_MS: u64 = 3_600_000;

    fn profile() -> AccountProfile {
        AccountProfile::new("codex-plus", "Codex", "ChatGPT", "gpt-5.2", "work")
            .expect("the fixture profile is valid")
    }

    fn window() -> UsageWindow {
        UsageWindow::new(0, 5 * HOUR_MS).expect("the fixture window is valid")
    }

    fn observation(consumed: u64, at_ms: u64, provenance: UsageProvenance) -> UsageObservation {
        UsageObservation::counted(
            AccountProfileId::new("codex-plus").expect("valid ID"),
            UsageUnit::Tokens,
            consumed,
            provenance,
            at_ms,
        )
        .expect("a counted observation is available")
        .with_limit(1_000_000)
        .expect("a positive limit is valid")
        .with_window(window())
        .expect("the observation is inside the window")
    }

    fn ledger_with(samples: &[(u64, u64)]) -> UsageLedger {
        let mut ledger = UsageLedger::new();
        for &(consumed, at_ms) in samples {
            ledger
                .record(observation(
                    consumed,
                    at_ms,
                    UsageProvenance::ProviderReported,
                ))
                .expect("fixture observations are ordered");
        }
        ledger
    }

    #[test]
    fn provenance_labels_and_confidence_are_stable() {
        assert_eq!(
            UsageProvenance::ProviderReported.label(),
            "provider-reported"
        );
        assert_eq!(UsageProvenance::Estimated.compact_label(), "est");
        assert!(UsageProvenance::HarnessReported.is_reported());
        assert!(!UsageProvenance::LocallyMeasured.is_reported());
        assert_eq!(
            UsageProvenance::ProviderReported.confidence(),
            UsageConfidence::Exact
        );
        assert_eq!(
            UsageProvenance::Unavailable.confidence(),
            UsageConfidence::Unknown
        );
    }

    #[test]
    fn the_weakest_provenance_wins() {
        assert_eq!(
            UsageProvenance::ProviderReported.weakest(UsageProvenance::Estimated),
            UsageProvenance::Estimated
        );
        assert_eq!(
            UsageProvenance::Estimated.weakest(UsageProvenance::Unavailable),
            UsageProvenance::Unavailable
        );
    }

    #[test]
    fn a_counted_value_cannot_be_labelled_unavailable() {
        let error = UsageObservation::counted(
            AccountProfileId::new("codex-plus").expect("valid ID"),
            UsageUnit::Tokens,
            10,
            UsageProvenance::Unavailable,
            0,
        )
        .expect_err("unavailable counted values are rejected");
        assert_eq!(error, UsageError::UnavailableWithValue);
    }

    #[test]
    fn windows_and_limits_are_validated() {
        assert_eq!(UsageWindow::new(10, 10), Err(UsageError::EmptyWindow));
        let observation = UsageObservation::counted(
            AccountProfileId::new("codex-plus").expect("valid ID"),
            UsageUnit::Tokens,
            10,
            UsageProvenance::ProviderReported,
            9 * HOUR_MS,
        )
        .expect("a counted observation is available");
        assert_eq!(
            observation.clone().with_limit(0),
            Err(UsageError::ZeroLimit)
        );
        assert!(matches!(
            observation.with_window(window()),
            Err(UsageError::ObservationOutsideWindow { .. })
        ));
    }

    #[test]
    fn the_ledger_refuses_to_move_backwards() {
        let mut ledger = ledger_with(&[(0, 0), (100, HOUR_MS)]);
        let error = ledger
            .record(observation(
                50,
                HOUR_MS / 2,
                UsageProvenance::ProviderReported,
            ))
            .expect_err("an older observation is rejected");
        assert!(matches!(
            error,
            UsageError::OutOfOrderObservation { newest_ms, .. } if newest_ms == HOUR_MS
        ));
    }

    #[test]
    fn retention_is_bounded_and_keeps_the_newest_samples() {
        let mut ledger = UsageLedger::with_retention(2).expect("two samples are allowed");
        for step in 0..5 {
            ledger
                .record(observation(
                    step * 1_000,
                    step * HOUR_MS / 4,
                    UsageProvenance::ProviderReported,
                ))
                .expect("ordered observations are accepted");
        }
        let id = AccountProfileId::new("codex-plus").expect("valid ID");
        assert_eq!(ledger.observation_count(&id), 2);
        assert_eq!(
            ledger.latest(&id).map(UsageObservation::consumed),
            Some(4_000)
        );
        assert!(matches!(
            UsageLedger::with_retention(1),
            Err(UsageError::RetentionTooSmall)
        ));
    }

    #[test]
    fn a_single_observation_yields_no_burn_rate() {
        let ledger = ledger_with(&[(620_000, 0)]);
        let projection = ledger.project(profile().id(), 0);
        assert_eq!(projection.consumed_permille(), Some(620));
        assert_eq!(projection.burn_per_hour(), None, "a rate needs two samples");
        assert_eq!(projection.exhaustion_in_ms(), None);
    }

    #[test]
    fn burn_and_exhaustion_are_derived_from_the_window() {
        let ledger = ledger_with(&[(0, 0), (300_000, HOUR_MS)]);
        let projection = ledger.project(profile().id(), HOUR_MS);
        assert_eq!(projection.burn_per_hour(), Some(300_000));
        assert_eq!(projection.consumed_permille(), Some(300));
        assert_eq!(projection.resets_in_ms(), Some(4 * HOUR_MS));
        assert_eq!(
            projection.exhaustion_in_ms(),
            Some(2 * HOUR_MS + 20 * 60_000),
            "700k remaining at 300k/hr is reached before the four-hour reset"
        );
        assert_eq!(projection.burn_provenance(), UsageProvenance::Estimated);
        assert_eq!(projection.provenance(), UsageProvenance::ProviderReported);
    }

    #[test]
    fn exhaustion_is_withheld_when_the_window_resets_first() {
        let ledger = ledger_with(&[(0, 0), (10_000, HOUR_MS)]);
        let projection = ledger.project(profile().id(), HOUR_MS);
        assert_eq!(projection.burn_per_hour(), Some(10_000));
        assert_eq!(
            projection.exhaustion_in_ms(),
            None,
            "the window resets long before the ceiling is reached"
        );
    }

    #[test]
    fn an_unavailable_observation_invalidates_older_facts() {
        let mut ledger = ledger_with(&[(0, 0), (200_000, HOUR_MS)]);
        ledger
            .record(UsageObservation::unavailable(
                AccountProfileId::new("codex-plus").expect("valid ID"),
                2 * HOUR_MS,
            ))
            .expect("a gap is recordable");
        let projection = ledger.project(profile().id(), 2 * HOUR_MS);
        assert!(!projection.is_available());
        assert_eq!(projection.consumed_permille(), None);
        assert_eq!(projection.burn_per_hour(), None);
        assert_eq!(projection.confidence(), UsageConfidence::Unknown);
    }

    #[test]
    fn an_expired_window_is_not_presented_as_current() {
        let ledger = ledger_with(&[(0, 0), (200_000, HOUR_MS)]);
        let projection = ledger.project(profile().id(), 6 * HOUR_MS);
        assert!(projection.window_has_rolled());
        assert!(projection.is_stale());
        assert_eq!(projection.consumed_permille(), None);
        assert_eq!(projection.burn_per_hour(), None);
    }

    #[test]
    fn an_unknown_profile_projects_as_unavailable() {
        let ledger = UsageLedger::new();
        let projection = ledger.project(profile().id(), 0);
        assert!(!projection.is_available());
        assert_eq!(projection.observed_at_ms(), None);
    }

    #[test]
    fn staleness_demotes_confidence_without_erasing_the_fact() {
        let ledger = UsageLedger::new().with_staleness_budget_ms(1_000);
        let mut ledger = ledger;
        ledger
            .record(observation(200_000, 0, UsageProvenance::ProviderReported))
            .expect("ordered");
        ledger
            .record(observation(
                400_000,
                HOUR_MS,
                UsageProvenance::ProviderReported,
            ))
            .expect("ordered");
        let projection = ledger.project(profile().id(), HOUR_MS + 5_000);
        assert!(projection.is_stale());
        assert_eq!(projection.consumed_permille(), Some(400));
        assert_eq!(projection.confidence(), UsageConfidence::Unknown);
    }

    #[test]
    fn the_footer_answers_five_questions() {
        let ledger = ledger_with(&[(0, 0), (620_000, HOUR_MS)]);
        let projection = ledger.project(profile().id(), HOUR_MS);
        let profile = profile();
        let footer = FooterUsage::new(&profile, &projection);
        assert_eq!(footer.identity(), "Codex · ChatGPT · gpt-5.2 · work");
        assert_eq!(footer.window(), "62.0% used");
        assert_eq!(footer.reset(), "resets in 4h 0m");
        assert!(footer.burn().starts_with("~620.0k tokens/hr est"));
        assert_eq!(footer.trust(), "provider · exact");
    }

    #[test]
    fn the_footer_says_unavailable_instead_of_zero() {
        let ledger = UsageLedger::new();
        let projection = ledger.project(profile().id(), 0);
        let profile = profile();
        let footer = FooterUsage::new(&profile, &projection);
        assert_eq!(footer.window(), "usage unavailable");
        assert_eq!(footer.reset(), "reset time unavailable");
        assert_eq!(footer.burn(), "burn rate unavailable");
        assert_eq!(footer.trust(), "no usage source");
        assert!(!footer.to_string().contains('%'));
    }

    #[test]
    fn a_locally_measured_count_without_a_ceiling_says_so() {
        let mut ledger = UsageLedger::new();
        ledger
            .record(
                UsageObservation::counted(
                    AccountProfileId::new("codex-plus").expect("valid ID"),
                    UsageUnit::Requests,
                    42,
                    UsageProvenance::LocallyMeasured,
                    0,
                )
                .expect("available"),
            )
            .expect("ordered");
        let projection = ledger.project(profile().id(), 0);
        let profile = profile();
        let footer = FooterUsage::new(&profile, &projection);
        assert_eq!(footer.window(), "42 requests used · no ceiling reported");
        assert_eq!(footer.trust(), "local · approximate");
    }

    #[test]
    fn durations_render_without_false_precision() {
        assert_eq!(format_duration_ms(45_000), "45s");
        assert_eq!(format_duration_ms(14 * 60_000), "14m");
        assert_eq!(format_duration_ms(2 * HOUR_MS + 14 * 60_000), "2h 14m");
        // A weekly quota window is a real provider shape, not a hypothetical.
        assert_eq!(format_duration_ms(10_080 * 60_000), "7d 0h");
        assert_eq!(format_duration_ms(151 * HOUR_MS + 30 * 60_000), "6d 7h");
    }

    #[test]
    fn a_window_can_know_its_reset_without_knowing_its_start() {
        let window = UsageWindow::until(5 * HOUR_MS);
        assert_eq!(window.started_at_ms(), None);
        assert_eq!(window.resets_at_ms(), 5 * HOUR_MS);
        assert!(window.contains(0), "an unknown start bounds nothing below");
        assert!(window.contains(5 * HOUR_MS - 1));
        assert!(
            !window.contains(5 * HOUR_MS),
            "the reset still bounds it above"
        );
        assert!(window.has_expired(5 * HOUR_MS));
        assert_eq!(window.remaining_ms(HOUR_MS), 4 * HOUR_MS);
    }

    #[test]
    fn windows_with_the_same_reset_share_an_identity() {
        assert_eq!(UsageWindow::until(HOUR_MS), UsageWindow::until(HOUR_MS));
        assert_ne!(UsageWindow::until(HOUR_MS), UsageWindow::until(2 * HOUR_MS));
        assert_ne!(
            UsageWindow::until(HOUR_MS),
            UsageWindow::new(0, HOUR_MS).expect("a bounded window is valid"),
            "a known start is a different fact from an unknown one"
        );
    }

    #[test]
    fn a_start_only_window_still_scopes_a_burn_rate_to_one_period() {
        // The provider reported a reset but no window length, then the quota
        // rolled. A baseline must not be drawn across the two periods.
        let id = AccountProfileId::new("codex-plus").expect("valid ID");
        let mut ledger = UsageLedger::new();
        for (consumed, at_ms, resets_at_ms) in [
            (800, 0, 5 * HOUR_MS),
            (50, 5 * HOUR_MS + 1_000, 10 * HOUR_MS),
            (850, 6 * HOUR_MS, 10 * HOUR_MS),
        ] {
            ledger
                .record(
                    UsageObservation::counted(
                        id.clone(),
                        UsageUnit::WindowPermille,
                        consumed,
                        UsageProvenance::ProviderReported,
                        at_ms,
                    )
                    .expect("available")
                    .with_limit(1_000)
                    .expect("a positive limit")
                    .with_window(UsageWindow::until(resets_at_ms))
                    .expect("the observation precedes its reset"),
                )
                .expect("ordered");
        }
        let projection = ledger.project(&id, 6 * HOUR_MS);
        assert_eq!(projection.resets_in_ms(), Some(4 * HOUR_MS));
        assert_eq!(
            projection.burn_per_hour(),
            Some(800),
            "the baseline must be the reading from this window, not the last one"
        );
        assert_eq!(
            projection.exhaustion_in_ms(),
            Some(675_000),
            "150 permille left at 800 per hour is about eleven minutes"
        );
    }

    #[test]
    fn identities_reject_empty_and_control_values() {
        assert!(matches!(
            AccountProfile::new("id", "", "p", "m", "a"),
            Err(UsageError::InvalidIdentifier("harness name"))
        ));
        assert!(matches!(
            AccountProfileId::new("bad\nid"),
            Err(UsageError::InvalidIdentifier("account profile ID"))
        ));
    }
}