use smallvec::SmallVec;

use crate::{
    InferenceError, Outcome,
    drift::Drift,
    event::{Event, Member, Team},
    history::History,
    observer::Observer,
    time::Time,
};

pub struct EventBuilder<'h, T, D, O, K>
where
    T: Time,
    D: Drift<T>,
    O: Observer<T>,
    K: Eq + std::hash::Hash + Clone,
{
    history: &'h mut History<T, D, O, K>,
    event: Event<T, K>,
    current_team_idx: Option<usize>,
    /// First validation failure seen while building, surfaced by `commit`.
    ///
    /// The setters return `Self` so the chain stays fluent; they cannot return
    /// a `Result` without breaking that. Recording the failure and reporting it
    /// at `commit` keeps the check enforced in release, where the previous
    /// `debug_assert!` was compiled out and a mismatched event was ingested
    /// silently.
    error: Option<InferenceError>,
}

impl<'h, T, D, O, K> EventBuilder<'h, T, D, O, K>
where
    T: Time,
    D: Drift<T>,
    O: Observer<T>,
    K: Eq + std::hash::Hash + Clone,
{
    pub(crate) fn new(history: &'h mut History<T, D, O, K>, time: T) -> Self {
        Self {
            history,
            event: Event {
                time,
                teams: SmallVec::new(),
                outcome: Outcome::Ranked(SmallVec::new()),
            },
            current_team_idx: None,
            error: None,
        }
    }

    /// Add a team by its member keys (weight 1.0 each, no prior overrides).
    ///
    /// Use [`EventBuilder::members`] to set `prior` or `drift_scale`.
    pub fn team<I: IntoIterator<Item = K>>(mut self, keys: I) -> Self {
        let members: SmallVec<[Member<K>; 4]> = keys.into_iter().map(Member::new).collect();
        self.event.teams.push(Team { members });
        self.current_team_idx = Some(self.event.teams.len() - 1);
        self
    }

    /// Add a team from fully-specified [`Member`] values.
    ///
    /// [`EventBuilder::team`] is the common case and builds members with
    /// `Member::new`, which leaves `prior` and `drift_scale` unset. This is the
    /// escape hatch for when they matter:
    ///
    /// ```
    /// # use trueskill_tt::{Gaussian, History, Member};
    /// # let mut h = History::builder().build();
    /// h.event(0)
    ///     .team(["player"])
    ///     .members([Member::new("layout_7")
    ///         .with_drift_scale(0.0)
    ///         .with_prior(Gaussian::from_ms(0.0, 1.0))])
    ///     .ranking([0, 1])
    ///     .commit()?;
    /// # Ok::<(), trueskill_tt::InferenceError>(())
    /// ```
    ///
    /// One method rather than a `priors` and a `drift_scales` setter beside
    /// `weights`: those would have to grow a parallel array — and a parallel
    /// length check — every time `Member` gains a field, and each one would be
    /// a new way to get the lengths wrong. `Member`'s own builder already
    /// expresses all of it.
    ///
    /// `prior` and `drift_scale` are competitor configuration rather than
    /// per-event values; see [`Member`] for what that means for a key the
    /// history already knows.
    pub fn members<I: IntoIterator<Item = Member<K>>>(mut self, members: I) -> Self {
        self.event.teams.push(Team::with_members(members));
        self.current_team_idx = Some(self.event.teams.len() - 1);
        self
    }

    /// Set per-member weights for the most recently added team.
    ///
    /// A length mismatch is recorded and returned by [`EventBuilder::commit`]
    /// as `InferenceError::MismatchedShape`, in both debug and release. The
    /// weights are not applied in that case, so a partially-weighted team
    /// cannot reach the history.
    ///
    /// # Panics
    ///
    /// Panics if called before any `.team(...)`.
    pub fn weights<I: IntoIterator<Item = f64>>(mut self, weights: I) -> Self {
        let idx = self
            .current_team_idx
            .expect(".weights(...) called before any .team(...)");

        let ws: Vec<f64> = weights.into_iter().collect();
        let team = &mut self.event.teams[idx];

        if ws.len() != team.members.len() {
            self.error.get_or_insert(InferenceError::MismatchedShape {
                kind: "weights",
                expected: team.members.len(),
                got: ws.len(),
            });

            return self;
        }

        for (m, w) in team.members.iter_mut().zip(ws) {
            m.weight = w;
        }

        self
    }

    /// Set explicit ranks per team (length must equal number of teams).
    pub fn ranking<I: IntoIterator<Item = u32>>(mut self, ranks: I) -> Self {
        self.event.outcome = Outcome::ranking(ranks);
        self
    }

    /// Set explicit per-team continuous scores; higher = better.
    pub fn scores<I: IntoIterator<Item = f64>>(mut self, scores: I) -> Self {
        self.event.outcome = crate::Outcome::scores(scores);
        self
    }

    /// Set explicit per-team continuous scores with a per-event noise override.
    ///
    /// `sigma` overrides `HistoryBuilder::score_sigma` for this event only.
    /// Must be `> 0.0`. Constructing the outcome with a non-positive or NaN
    /// sigma is allowed; the value is rejected with
    /// `InferenceError::InvalidParameter` when the event is ingested, so
    /// callers get an error from `commit` rather than a panic.
    pub fn scores_with_sigma<I: IntoIterator<Item = f64>>(mut self, scores: I, sigma: f64) -> Self {
        self.event.outcome = crate::Outcome::scores_with_sigma(scores, sigma);
        self
    }

    /// Mark team `winner_idx` as winner; others tied for last.
    pub fn winner(mut self, winner_idx: u32) -> Self {
        self.event.outcome = Outcome::winner(winner_idx, self.event.teams.len() as u32);
        self
    }

    /// All teams tied.
    pub fn draw(mut self) -> Self {
        self.event.outcome = Outcome::draw(self.event.teams.len() as u32);
        self
    }

    /// Commit the event to the history.
    ///
    /// # Errors
    ///
    /// Returns the first validation failure recorded while building — see
    /// [`EventBuilder::weights`] — otherwise forwards to
    /// [`History::add_events`] and returns its errors.
    pub fn commit(self) -> Result<(), InferenceError> {
        if let Some(error) = self.error {
            return Err(error);
        }

        self.history.add_events(std::iter::once(self.event))
    }
}