use std::fmt;

/// How a prediction should treat a key the history has never seen.
///
/// Configured once per history via
/// [`HistoryBuilder::unknown_keys`](crate::HistoryBuilder::unknown_keys).
/// Neither known consumer wants this to vary between queries — one predicts
/// thousands of candidate matchups in a loop, the other's headline feature is
/// predicting a competitor nobody has faced — so it is a property of how you
/// intend to use the model rather than an argument on five call sites.
///
/// # There is deliberately no `Skip`
///
/// Dropping an unknown member is the obvious third option and it is wrong. A
/// team's performance is the *sum* of its members, so removing one removes its
/// variance too: measured on a two-member team with one unknown, skipping gives
/// a performance sigma of 2.37 where treating the member as unknown gives 6.53.
/// An unknown competitor would make the model *more* certain, which is
/// backwards. `Prior` is also the answer the model already gives for a
/// competitor it knows about but has no evidence for, so it corresponds to a
/// state the model can actually be in; skipping does not.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
#[non_exhaustive]
pub enum UnknownKeys {
    /// Reject the prediction with [`InferenceError::UnknownKey`].
    ///
    /// The default, and the right one when every key is expected to be known:
    /// a team of strangers should not silently produce a confident-looking
    /// answer.
    #[default]
    Reject,
    /// Treat an unknown competitor as one sitting at the history's configured
    /// prior.
    ///
    /// This is the honest Bayesian reading — a competitor you have never
    /// observed is exactly the prior — and it makes "predict a matchup
    /// involving someone new" a first-class question rather than something a
    /// caller fakes with a neutral constant.
    Prior,
}

/// Every way ingestion, inference or prediction can refuse to answer.
///
/// The crate reports rather than repairs. An input it cannot represent, a fit
/// that never reached its fixed point, a quadrature it cannot resolve — each
/// comes back here instead of as a clamped, skipped or truncated result that
/// would still look like a number. Several variants exist precisely because the
/// silent version was measured and found to return a plausible wrong answer.
///
/// The enum and most of its variants are `#[non_exhaustive]`: new cases and new
/// fields are additive, so match with a `_` arm and construct through the
/// library rather than by literal.
#[derive(Debug, Clone, PartialEq)]
#[non_exhaustive]
pub enum InferenceError {
    /// Expected and actual lengths of some array-shaped input differ.
    #[non_exhaustive]
    MismatchedShape {
        /// Which input disagreed, as a short label — `"ranks vs teams"`,
        /// `"weights"`, `"times"`.
        kind: &'static str,
        /// The length it had to have, taken from whatever it must line up with
        /// (usually the event's team count).
        expected: usize,
        /// The length actually supplied.
        got: usize,
    },
    /// An `Outcome` of the wrong variant was supplied for the requested inference.
    #[non_exhaustive]
    WrongOutcomeKind {
        /// The call that rejected the outcome, e.g. `"Game::ranked"`.
        context: &'static str,
        /// The [`Outcome`](crate::Outcome) variant that call needs, by name.
        expected: &'static str,
        /// The variant actually supplied, by name.
        got: &'static str,
    },
    /// A probability value is outside `[0, 1]`.
    #[non_exhaustive]
    InvalidProbability {
        /// The value supplied, as it fell outside `[0, 1]`. Today only
        /// `p_draw` reaches here.
        value: f64,
    },
    /// A scalar parameter is outside its valid range.
    #[non_exhaustive]
    InvalidParameter {
        /// The parameter, spelled as the API spells it — `"alpha"`,
        /// `"epsilon"`, `"score_sigma"`, `"drift_scale"`, `"drift variance"`.
        name: &'static str,
        /// The value supplied for it. Out of that parameter's range, or NaN,
        /// which fails every range comparison and is rejected on that basis.
        value: f64,
    },
    /// An event contains tied teams, but the draw probability is zero.
    ///
    /// A zero draw probability asserts that draws cannot occur, so a tied
    /// result has no representable likelihood. Configure a positive `p_draw`
    /// (via `HistoryBuilder::p_draw` or `GameOptions::p_draw`) to admit ties.
    #[non_exhaustive]
    TieWithoutDrawProbability {
        /// Positions in the event's team list of the first tied pair, lowest
        /// index first. Only one pair is reported — the event is rejected
        /// whole, so enumerating the rest would add nothing.
        teams: (usize, usize),
    },
    /// The convergence sweep hit `max_iter` with the step still above
    /// `epsilon`.
    ///
    /// A fit that stops short is wrong by a little, which is the worst
    /// available failure: every posterior is finite, the ordering looks sensible,
    /// and nothing in the numbers says they were still moving. Reported rather
    /// than returned as a flag on an `Ok`, because a flag has to be checked
    /// and `let _ = h.converge()` is the natural way not to.
    ///
    /// Either the history needs more iterations — raise `max_iter` — or it is
    /// oscillating rather than converging, in which case `alpha < 1.0` damps
    /// the within-game EP loop. [`History::converge_partial`](crate::History::converge_partial)
    /// returns the short fit instead when that is genuinely what is wanted.
    #[non_exhaustive]
    NotConverged {
        /// Full forward+backward sweeps run before the loop gave up.
        iterations: usize,
        /// How far the last sweep still moved the fit, as
        /// `(largest change in a mean, largest change in a standard
        /// deviation)` over every competitor posterior it touched — the same
        /// quantity as
        /// [`ConvergenceReport::final_step`](crate::ConvergenceReport).
        final_step: (f64, f64),
        /// The threshold both components of `final_step` had to reach.
        epsilon: f64,
    },
    /// Inference produced a non-finite value (NaN or infinity).
    ///
    /// Indicates numerical breakdown; the resulting skills are meaningless
    /// and must not be treated as a converged estimate.
    #[non_exhaustive]
    NonFiniteResult {
        /// Where the breakdown was caught — `"History::converge"` for a sweep,
        /// or a phrase naming the prediction that read an unusable skill.
        context: &'static str,
        /// The offending pair, at least one component of which is NaN or
        /// infinite. From `converge` it is the sweep's step; from a prediction
        /// it is the skill's own `(mu, sigma)`.
        step: (f64, f64),
    },
    /// One batch declared two different values for the same competitor's
    /// configuration.
    ///
    /// `prior` and `drift_scale` configure a competitor, not an event, so a
    /// batch that sets one of them twice with different values has no
    /// well-defined meaning: events within a batch are not ordered, so
    /// "last one wins" would make the result depend on iteration order.
    /// Declaring the same value repeatedly is fine and is the expected shape
    /// when a competitor's configuration is a property of the domain.
    #[non_exhaustive]
    ConflictingCompetitorConfig {
        /// The competitor's interned [`Index`](crate::Index) as a raw `usize`,
        /// not the user key — the batch is already flattened to indices by the
        /// time the conflict is detectable.
        competitor: usize,
        /// Which piece of configuration was declared twice: `"prior"` or
        /// `"drift_scale"`.
        field: &'static str,
    },
    /// A prediction referenced a key the history has no skill for.
    ///
    /// Reported rather than skipped: dropping unknown keys turns a team of
    /// strangers into a confident-looking probability about nobody.
    ///
    /// `key` is the offending key's `Debug` rendering. It is carried because
    /// the indices alone are not actionable: a caller that logs
    /// `UnknownKey { team: 0, member: 0 }` learns nothing about *which* of its
    /// keys the history has not seen, and the natural handling — fall back to a
    /// neutral value — turns the whole thing into a plausible constant.
    #[non_exhaustive]
    UnknownKey {
        /// Position of the offending team in the supplied matchup. `0` on the
        /// queries that take a flat list of keys rather than teams, where
        /// there is only one list to index into.
        team: usize,
        /// Position of the offending key within that team, or within the flat
        /// key list.
        member: usize,
        /// The key's `Debug` rendering, captured because `K` is only required
        /// to be `Debug` — see the variant docs for why the indices alone are
        /// not enough.
        key: String,
    },
    /// `History::register` was called for a competitor that already exists.
    ///
    /// Registration states a competitor's configuration before anything has
    /// been observed about them, so a competitor that already exists has
    /// already been configured — by an earlier `register`, or by an event that
    /// created them. Silently overwriting would reintroduce exactly the
    /// order-dependence registration exists to remove.
    ///
    /// To change an existing competitor's configuration, supply it on an event
    /// through `Member`; that refits the whole history.
    #[non_exhaustive]
    AlreadyRegistered {
        /// The already-known competitor's key, in its `Debug` rendering.
        key: String,
    },
    /// A prediction was given a team with no members.
    #[non_exhaustive]
    EmptyTeam {
        /// Position of the memberless team in the supplied list.
        team: usize,
    },
    /// The prediction grid cannot resolve the narrowest feature in the matchup.
    ///
    /// `predict_outcome` and `predict_ranking` integrate every team's density
    /// on one shared grid, whose resolution is set by the narrowest sigma (or a
    /// narrower draw margin). When the widest and narrowest are far enough
    /// apart, resolving the narrow one across the wide one's support needs more
    /// nodes than the grid is allowed to hold.
    ///
    /// Reported rather than clamped. Clamping is what this replaced, and it
    /// returned probabilities greater than one — measured, a `P` of 2.79 and a
    /// `Prediction::total()` of 5.41 — because the trapezoid rule stops
    /// resolving a density once the step exceeds roughly 1.7 of its sigma.
    ///
    /// `predict_win_probabilities` answers the same matchup through adaptive
    /// quadrature and is accurate here; use it when only the per-team win
    /// probabilities are needed.
    #[non_exhaustive]
    GridTooCoarse {
        /// Nodes required to resolve the narrowest feature.
        needed: usize,
        /// Nodes the grid may hold.
        max: usize,
    },
    /// A joint posterior was requested where one cannot be formed exactly.
    #[non_exhaustive]
    JointUnavailable {
        /// Why no exact joint exists here: the history has no events, it holds
        /// ranked events whose EP factors are not retained past convergence, or
        /// the assembled precision matrix is not positive-definite.
        reason: &'static str,
    },
    /// Fewer than two teams were supplied to a prediction.
    #[non_exhaustive]
    NotEnoughTeams {
        /// How many teams the prediction was actually given. Two is the
        /// minimum: there is nothing to compare against with fewer.
        got: usize,
    },
    /// The full outcome distribution was requested for too many teams.
    ///
    /// Each realisation sorts into exactly one (order, tie-pattern) event, so
    /// the space holds `n! * 2^(n-1)` members — 1_920 at five teams, 23_040 at
    /// six, 322_560 at seven. Past `max` this stops being something to
    /// enumerate on a caller's behalf; ask for individual rankings with
    /// `predict_ranking`, or for `predict_win_probabilities`, both of which
    /// stay cheap at any team count.
    #[non_exhaustive]
    TooManyTeams {
        /// How many teams the outcome distribution was asked for.
        got: usize,
        /// The largest team count that will be enumerated,
        /// [`MAX_PREDICTED_TEAMS`](crate::MAX_PREDICTED_TEAMS).
        max: usize,
    },
}

impl fmt::Display for InferenceError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::MismatchedShape {
                kind,
                expected,
                got,
            } => {
                write!(f, "{kind}: expected length {expected}, got {got}")
            }
            Self::WrongOutcomeKind {
                context,
                expected,
                got,
            } => {
                write!(f, "{context}: expected {expected}, got {got}")
            }
            Self::InvalidProbability { value } => {
                write!(f, "probability must be in [0, 1]; got {value}")
            }
            Self::TieWithoutDrawProbability { teams } => {
                write!(
                    f,
                    "teams {} and {} are tied, but p_draw is 0.0; set a positive draw probability to admit ties",
                    teams.0, teams.1
                )
            }
            Self::NotConverged {
                iterations,
                final_step,
                epsilon,
            } => {
                write!(
                    f,
                    "did not converge in {iterations} iterations: final step {final_step:?} \
                     is still above epsilon {epsilon}; raise max_iter, or damp with \
                     alpha < 1.0 if it is oscillating"
                )
            }
            Self::NonFiniteResult { context, step } => {
                write!(
                    f,
                    "{context}: inference produced a non-finite result (step = {step:?})"
                )
            }
            Self::InvalidParameter { name, value } => {
                write!(f, "{name} is invalid: {value}")
            }
            Self::ConflictingCompetitorConfig { competitor, field } => {
                write!(
                    f,
                    "competitor {competitor}: this batch sets {field} to two different values"
                )
            }
            Self::UnknownKey { team, member, key } => {
                write!(
                    f,
                    "team {team}, member {member}: no skill recorded for key {key} \
                     (every key must already be known to the history; pre-filter \
                     with `lookup` or `current_skill` if that is not guaranteed)"
                )
            }
            Self::AlreadyRegistered { key } => {
                write!(
                    f,
                    "competitor {key} is already registered; registration states \
                     configuration before anything is observed, so re-registering \
                     would silently overwrite it"
                )
            }
            Self::EmptyTeam { team } => {
                write!(f, "team {team} has no members")
            }
            Self::GridTooCoarse { needed, max } => {
                write!(
                    f,
                    "the prediction grid needs {needed} nodes to resolve the narrowest \
                     team's density across the widest team's support, but may hold only \
                     {max}; the sigmas in this matchup are too far apart to integrate on \
                     one grid. Use predict_win_probabilities, which is accurate here"
                )
            }
            Self::JointUnavailable { reason } => {
                write!(f, "no exact joint posterior is available: {reason}")
            }
            Self::NotEnoughTeams { got } => {
                write!(f, "prediction needs at least 2 teams, got {got}")
            }
            Self::TooManyTeams { got, max } => {
                write!(
                    f,
                    "the outcome distribution over {got} teams is too large to enumerate (limit {max}); \
                     use predict_ranking or predict_win_probabilities instead"
                )
            }
        }
    }
}

impl std::error::Error for InferenceError {}