delvewright_dsl/

gate.rs

//! **The one gate.**
//!
//! A *gate* is the campaign's answer to "may this happen yet?". Eight object
//! classes ask it — an objective, an effect, an environment trigger, a trap, a
//! dialogue option, a cast placement, a shop offer and a lethal volume. The
//! first six once each carried its own copy of the same two fields,
//! `requires_flags` and `forbids_flags`, with nothing in the type system saying
//! they were one thing.
//!
//! That arrangement is the defect CLAUDE.md names first. When spec-0031 needed a
//! numeric comparison ("this door opens at 500", "this line is withheld below
//! 200", "this lever does nothing while the car is moving"), the shape of the
//! code offered exactly one obvious place to put it: the verb that asked. The
//! second consumer would then have had no surface, and the fix would have looked
//! like a second bespoke field. **Generality is decided at the FIRST site.**
//!
//! So `requires_state` was added to all twenty-five declaration sites at once,
//! and this module is what makes that a property rather than a coincidence:
//!
//! * [`Gate`] is the gate as one value — the three fields together, borrowed.
//!   Every consumer answers `gate()`, and every proof that reasons about gating
//!   is written against it rather than against two of three fields.
//! * [`GateConsumer`] is the **closed set** of object classes that carry a gate.
//!   `ALL` is the enumeration; adding a variant is a rustc error at every match.
//! * [`for_each_gate`] visits every gate in a campaign, in one fixed order, and
//!   returns a [`GateBinding`] ledger stating how many gates it found per
//!   consumer — because a proof over "every gate" that bound to nothing is
//!   vacuous, not a pass (CLAUDE.md).
//!
//! What this module cannot do is stop a twenty-sixth *declaration site* being
//! added tomorrow with only the flag pair on it. Nothing in Rust's type system
//! can: the fields are ordinary fields on ordinary structs, and serde's
//! `flatten` — the one construct that would have made a shared struct
//! literal — is a compile error in combination with `deny_unknown_fields`, which
//! every stage struct carries and which is what turns an author's typo into
//! `DW0100` instead of silence. That half of the obligation is
//! `crates/dsl/tests/gate_consumers.rs`, which enumerates the consumers **from
//! the generated JSON Schema** — i.e. from the types — and fails when any schema
//! object declares `requires_flags` without `requires_state`.
//!
//! Determinism (ADR-0006): iteration is over slices and `BTreeMap` keys, in a
//! fixed order that is part of this module's contract.

use crate::envelope::Campaign;
use crate::ids::FlagId;
use crate::stages::StateCompare;

/// A gate, as one value: everything that decides whether the thing carrying it
/// may happen.
///
/// Borrowed rather than owned, so `gate()` is free on every consumer and no
/// consumer has to store a second copy of its own fields.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Gate<'a> {
    /// Flags that must all be set (DSL v0.3/v0.4).
    pub requires_flags: &'a [FlagId],
    /// Flags whose being set suppresses this (DSL v0.6).
    pub forbids_flags: &'a [FlagId],
    /// Numeric comparisons that must all hold (DSL v0.10, spec-0031).
    pub requires_state: &'a [StateCompare],
}

impl<'a> Gate<'a> {
    /// Build a gate from its three fields. The one constructor, so a consumer
    /// that forgets a field is a rustc error rather than a silently narrower
    /// gate.
    pub fn of(
        requires_flags: &'a [FlagId],
        forbids_flags: &'a [FlagId],
        requires_state: &'a [StateCompare],
    ) -> Self {
        Gate {
            requires_flags,
            forbids_flags,
            requires_state,
        }
    }

    /// The always-open gate: no flags, no comparison.
    pub const OPEN: Gate<'static> = Gate {
        requires_flags: &[],
        forbids_flags: &[],
        requires_state: &[],
    };

    /// True if this gate constrains nothing — the thing carrying it is
    /// unconditional, and emission writes it verbatim with no `execute` wrapper.
    pub fn is_empty(&self) -> bool {
        self.requires_flags.is_empty()
            && self.forbids_flags.is_empty()
            && self.requires_state.is_empty()
    }

    /// How many terms this gate has, across all three axes. The number a binding
    /// ledger reports.
    pub fn terms(&self) -> usize {
        self.requires_flags.len() + self.forbids_flags.len() + self.requires_state.len()
    }
}

// ---------------------------------------------------------------------------
// The six consumers, in one place
// ---------------------------------------------------------------------------
//
// Every object class that carries a gate answers `gate()`, and all six answers
// are written HERE rather than beside their own type. That is deliberate: the
// list of gate consumers is one fact, and a fact spread over six files is a fact
// nobody can read. A seventh consumer whose author forgets to add its `gate()`
// here is a consumer no proof written against `Gate` can see — which is exactly
// the shape `crates/dsl/tests/gate_consumers.rs` fails on.

impl crate::stages::Objective {
    /// This objective's whole gate, as one value (DSL v0.10).
    pub fn gate(&self) -> Gate<'_> {
        Gate::of(
            self.requires_flags(),
            self.forbids_flags(),
            self.requires_state(),
        )
    }
}

impl crate::stages::QuestEffect {
    /// This effect's whole gate, as one value (DSL v0.10).
    pub fn gate(&self) -> Gate<'_> {
        Gate::of(
            self.requires_flags(),
            self.forbids_flags(),
            self.requires_state(),
        )
    }
}

impl crate::stages::EnvTrigger {
    /// This trigger's whole gate, as one value (DSL v0.10).
    pub fn gate(&self) -> Gate<'_> {
        Gate::of(
            &self.requires_flags,
            &self.forbids_flags,
            &self.requires_state,
        )
    }
}

impl crate::stages::Trap {
    /// This trap's whole gate, as one value (DSL v0.10).
    pub fn gate(&self) -> Gate<'_> {
        Gate::of(
            &self.requires_flags,
            &self.forbids_flags,
            &self.requires_state,
        )
    }
}

impl crate::stages::DialogueOption {
    /// This option's whole gate, as one value (DSL v0.10).
    pub fn gate(&self) -> Gate<'_> {
        Gate::of(
            &self.requires_flags,
            &self.forbids_flags,
            &self.requires_state,
        )
    }
}

impl crate::stages::CastPlacement {
    /// This placement's whole gate, as one value (DSL v0.10).
    pub fn gate(&self) -> Gate<'_> {
        Gate::of(
            &self.requires_flags,
            &self.forbids_flags,
            &self.requires_state,
        )
    }
}

impl crate::stages::ShopOffer {
    /// This offer's whole gate, as one value (DSL v0.10, spec-0032) — a **price
    /// is a gate**, so a shop declares no comparison surface of its own.
    ///
    /// Defined beside the other six rather than on the type, for the reason the
    /// section header gives: the list of gate consumers is one fact.
    pub fn gate_view(&self) -> Gate<'_> {
        Gate::of(
            &self.requires_flags,
            &self.forbids_flags,
            &self.requires_state,
        )
    }
}

impl crate::stages::Loop {
    /// This loop's whole gate, as one value (spec-0086): the loop **holds** while
    /// it is open and stands down while it is shut.
    pub fn gate(&self) -> Gate<'_> {
        Gate::of(
            &self.requires_flags,
            &self.forbids_flags,
            &self.requires_state,
        )
    }
}

impl crate::stages::LethalVolume {
    /// This volume's whole gate, as one value (spec-0088) — the [`Guard`]
    /// under `when`, or the always-open gate when it declares none.
    ///
    /// [`Guard`]: crate::stages::Guard
    pub fn gate(&self) -> Gate<'_> {
        match &self.when {
            Some(g) => Gate::of(&g.requires_flags, &g.forbids_flags, &g.requires_state),
            None => Gate::OPEN,
        }
    }
}

/// The object classes that carry a gate. **A closed set.**
///
/// `ALL` is the enumeration; [`GateConsumer::label`] and every consumer that
/// matches on one is exhaustive, so an eighth class is a compile error at every
/// site where the answer would have to change.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
pub enum GateConsumer {
    /// A stage-5 `quests[].objectives[]` — the gate decides whether the
    /// objective activates.
    Objective,
    /// Any `QuestEffect`, at any of the five effect roots, top-level or nested —
    /// the gate decides whether the effect's commands run.
    Effect,
    /// A stage-5 `triggers[]` — the gate decides whether the trigger can fire.
    Trigger,
    /// A stage-5 `traps[]` — the gate decides whether the trap is armed.
    Trap,
    /// A stage-6 `dialogues[].nodes[].options[]` — the gate decides whether the
    /// option is shown, and whether a direct `/trigger` on it does anything.
    DialogueOption,
    /// A stage-5 `quests[].cast[]` placement — the gate decides which branch's
    /// scene describes the world.
    CastPlacement,
    /// A stage-5 `shops[].offers[]` (DSL v0.10, spec-0032) — the gate decides
    /// whether the button is shown, and whether a direct `/trigger` on it does
    /// anything. **This is where a price lives**: a shop declares no comparison
    /// surface of its own, because "may this happen yet?" already has an owner.
    ShopOffer,
    /// A stage-5 `loops[]` (spec-0086) — the gate decides whether the loop
    /// holds. Read against the party on the tick, before any body is selected.
    Loop,
    /// A stage-5 `lethal_volumes[]` entry (spec-0088) — the gate decides
    /// whether the volume kills. A volume's liveness is a fact about the place,
    /// so its gate is a party predicate: the tick reads it on `#party`.
    LethalVolume,
}

impl GateConsumer {
    /// Every consumer class, in enumeration order (= visit order in
    /// [`for_each_gate`]).
    pub const ALL: [GateConsumer; 9] = [
        GateConsumer::Objective,
        GateConsumer::Effect,
        GateConsumer::Trigger,
        GateConsumer::Trap,
        GateConsumer::DialogueOption,
        GateConsumer::CastPlacement,
        GateConsumer::ShopOffer,
        GateConsumer::Loop,
        GateConsumer::LethalVolume,
    ];

    /// How many consumer classes there are.
    pub const COUNT: usize = Self::ALL.len();

    /// A short, stable label for a binding ledger or a diagnostic.
    pub fn label(self) -> &'static str {
        match self {
            GateConsumer::Objective => "objective",
            GateConsumer::Effect => "effect",
            GateConsumer::Trigger => "trigger",
            GateConsumer::Trap => "trap",
            GateConsumer::DialogueOption => "dialogue option",
            GateConsumer::CastPlacement => "cast placement",
            GateConsumer::ShopOffer => "shop offer",
            GateConsumer::Loop => "loop",
            GateConsumer::LethalVolume => "lethal volume",
        }
    }

    /// Whether emission evaluates this consumer's gate **against an acting
    /// player** (`@s`) rather than against the party holder (`#party`) — or
    /// `None` when the class alone cannot say.
    ///
    /// This is a statement about the emitter, not a preference: a dialogue
    /// option's availability is computed per player into `dw.dmask` and its
    /// `/trigger` handler runs `as @s`; a cast placement selects a scene into a
    /// per-player `dw.cast`. Three of the others are party predicates by
    /// construction — an objective's activation guard is read on the tick
    /// ("whoever finishes the last objective completes the quest for everyone"),
    /// and a trigger's and a trap's *arming* gates flip one global sentinel.
    ///
    /// **`Effect` answers `None`, and that is the whole point of the return
    /// type.** An effect's gate is evaluated wherever its bundle is run, and
    /// which that is belongs to the **root**, not to the effect: four roots have
    /// an acting player and three do not
    /// ([`EffectRootKind::runs_with_acting_player`](crate::EffectRootKind::runs_with_acting_player)),
    /// and on top of that the `sequence` / `on_arrive` seams inside a bundle drop
    /// the actor mid-walk. An earlier version of this method answered `true` for
    /// `Effect`, which is right for `on_objective_complete` and wrong for a
    /// trigger's effects, a trap's payload and a shortcut's `on_unlock` — three
    /// of the seven roots, silently. `Option` makes that wrong answer
    /// unrepresentable: a caller must handle the deferral.
    ///
    /// It is what makes a `player`-scoped datum's readability decidable
    /// (`DW0503`) from the closed consumer set rather than from a list somebody
    /// maintains — an eighth consumer class must answer this to compile.
    pub fn evaluates_per_player(self) -> Option<bool> {
        match self {
            // A shop offer's gate is computed into `dw.dmask` per player and its
            // `/trigger` handler runs `as @s`, exactly as a dialogue option's does
            // — which is what makes a `player`-scoped purse a legal price.
            GateConsumer::DialogueOption
            | GateConsumer::CastPlacement
            | GateConsumer::ShopOffer => Some(true),
            GateConsumer::Objective
            | GateConsumer::Trigger
            | GateConsumer::Trap
            | GateConsumer::Loop
            | GateConsumer::LethalVolume => Some(false),
            // Ask the root (and then the seams inside the bundle).
            GateConsumer::Effect => None,
        }
    }

    /// The stage document this consumer lives in.
    pub fn stage(self) -> &'static str {
        match self {
            GateConsumer::Objective
            | GateConsumer::Trigger
            | GateConsumer::Trap
            | GateConsumer::CastPlacement
            | GateConsumer::ShopOffer
            | GateConsumer::Loop
            | GateConsumer::LethalVolume => "quests",
            // An effect root hangs off the quests stage four times out of five and
            // off dialogue once; the site's own path says which.
            GateConsumer::Effect => "quests",
            GateConsumer::DialogueOption => "dialogue",
        }
    }
}

/// One gate the walk found: which consumer class it belongs to, and where.
pub struct GateSite {
    /// The object class carrying the gate.
    pub consumer: GateConsumer,
    /// JSON pointer, within its stage document, to the object that declares the
    /// gate's fields — so `<path>/requires_flags/0` is where an author would look.
    /// For every consumer but the effect that is the consumer object itself; an
    /// effect declares its gate in one `when`, and the pointer names it.
    pub path: String,
}

/// What a walk over the campaign's gates actually examined.
///
/// CLAUDE.md: *a green gate that binds to nothing is vacuous, not a pass.* A
/// proof over "every gate" is only as good as the consumer classes it reached
/// and the gates it found there; neither number is visible from the proof's own
/// output, so it is reported here.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct GateBinding {
    /// How many of [`GateConsumer::COUNT`] classes the walk enumerated. Always
    /// `COUNT` for a walk that ran; asserted by [`for_each_gate`].
    pub consumers_enumerated: usize,
    /// Per class: how many gate-carrying objects this campaign actually has.
    pub sites: [(GateConsumer, usize); GateConsumer::COUNT],
    /// How many of those objects carry a **non-empty** gate.
    pub gated: usize,
    /// Total gate terms across every axis and every site.
    pub terms: usize,
}

impl GateBinding {
    /// A one-line, deterministic rendering for a report or a `--json` field.
    pub fn summary(&self) -> String {
        let per: Vec<String> = self
            .sites
            .iter()
            .map(|(k, n)| format!("{}={n}", k.label()))
            .collect();
        format!(
            "consumers {}/{}, sites {}, gated {}, terms {} [{}]",
            self.consumers_enumerated,
            GateConsumer::COUNT,
            self.sites.iter().map(|(_, n)| n).sum::<usize>(),
            self.gated,
            self.terms,
            per.join(", ")
        )
    }
}

/// Visit **every gate in the campaign**, in one fixed deterministic order, as
/// `f(&site, gate)`.
///
/// Order: every objective (quest order, objective order); every effect (via
/// [`crate::stages::for_each_campaign_effect`], which inherits the single effect-root
/// enumeration and descends nesting); every trigger; every trap; every dialogue
/// option; every cast placement; every shop offer; every lethal volume.
///
/// Returns the [`GateBinding`] ledger.
///
/// # Panics
///
/// If the walk failed to enumerate all [`GateConsumer::COUNT`] classes.
/// Unreachable by construction and asserted anyway, for the same reason
/// [`crate::effects::for_each_effect_root`] asserts its own: a walk that quietly
/// stops visiting a class just answers a narrower question and stays green over
/// every campaign that does not use it.
pub fn for_each_gate(c: &Campaign, f: &mut dyn FnMut(&GateSite, Gate<'_>)) -> GateBinding {
    let mut sites = [
        (GateConsumer::Objective, 0usize),
        (GateConsumer::Effect, 0usize),
        (GateConsumer::Trigger, 0usize),
        (GateConsumer::Trap, 0usize),
        (GateConsumer::DialogueOption, 0usize),
        (GateConsumer::CastPlacement, 0usize),
        (GateConsumer::ShopOffer, 0usize),
        (GateConsumer::Loop, 0usize),
        (GateConsumer::LethalVolume, 0usize),
    ];
    debug_assert_eq!(
        sites.map(|(k, _)| k),
        GateConsumer::ALL,
        "the binding ledger's slots are GateConsumer::ALL, in order"
    );
    let mut enumerated = [false; GateConsumer::COUNT];
    let mut gated = 0usize;
    let mut terms = 0usize;
    fn slot_of(k: GateConsumer) -> usize {
        GateConsumer::ALL
            .iter()
            .position(|x| *x == k)
            .expect("every consumer is a member of GateConsumer::ALL")
    }

    let mut visit = |consumer: GateConsumer,
                     path: String,
                     gate: Gate<'_>,
                     sites: &mut [(GateConsumer, usize); GateConsumer::COUNT],
                     gated: &mut usize,
                     terms: &mut usize| {
        sites[slot_of(consumer)].1 += 1;
        if !gate.is_empty() {
            *gated += 1;
        }
        *terms += gate.terms();
        f(&GateSite { consumer, path }, gate);
    };

    // C1 objectives.
    enumerated[slot_of(GateConsumer::Objective)] = true;
    for (qi, q) in c.quests.content.quests.iter().enumerate() {
        for (oi, o) in q.objectives.iter().enumerate() {
            visit(
                GateConsumer::Objective,
                format!("/content/quests/{qi}/objectives/{oi}"),
                o.gate(),
                &mut sites,
                &mut gated,
                &mut terms,
            );
        }
    }
    // C2 effects — every root, top-level and nested, from the single enumeration.
    enumerated[slot_of(GateConsumer::Effect)] = true;
    crate::stages::for_each_campaign_effect(c, &mut |path, _site, eff| {
        visit(
            GateConsumer::Effect,
            format!("{path}/when"),
            eff.gate(),
            &mut sites,
            &mut gated,
            &mut terms,
        );
    });
    // C3 triggers.
    enumerated[slot_of(GateConsumer::Trigger)] = true;
    for (ti, t) in c.quests.content.triggers.iter().enumerate() {
        visit(
            GateConsumer::Trigger,
            format!("/content/triggers/{ti}"),
            t.gate(),
            &mut sites,
            &mut gated,
            &mut terms,
        );
    }
    // C4 traps.
    enumerated[slot_of(GateConsumer::Trap)] = true;
    for (pi, p) in c.quests.content.traps.iter().enumerate() {
        visit(
            GateConsumer::Trap,
            format!("/content/traps/{pi}"),
            p.gate(),
            &mut sites,
            &mut gated,
            &mut terms,
        );
    }
    // C5 dialogue options.
    enumerated[slot_of(GateConsumer::DialogueOption)] = true;
    for (di, tree) in c.dialogue.content.dialogues.iter().enumerate() {
        for (ni, node) in tree.nodes.iter().enumerate() {
            for (oi, opt) in node.options.iter().enumerate() {
                visit(
                    GateConsumer::DialogueOption,
                    format!("/content/dialogues/{di}/nodes/{ni}/options/{oi}"),
                    opt.gate(),
                    &mut sites,
                    &mut gated,
                    &mut terms,
                );
            }
        }
    }
    // C6 cast placements.
    enumerated[slot_of(GateConsumer::CastPlacement)] = true;
    for (qi, q) in c.quests.content.quests.iter().enumerate() {
        for (npc, entry) in &q.cast {
            for (pi, p) in entry.placements().iter().enumerate() {
                visit(
                    GateConsumer::CastPlacement,
                    format!("/content/quests/{qi}/cast/{}/{pi}", npc.as_str()),
                    p.gate(),
                    &mut sites,
                    &mut gated,
                    &mut terms,
                );
            }
        }
    }

    // C7 shop offers (DSL v0.10, spec-0032). A price is a gate term, so every
    // offer is visited here and nowhere else.
    enumerated[slot_of(GateConsumer::ShopOffer)] = true;
    for (si, shop) in c.quests.content.shops.iter().enumerate() {
        for (oi, off) in shop.offers.iter().enumerate() {
            visit(
                GateConsumer::ShopOffer,
                format!("/content/shops/{si}/offers/{oi}"),
                off.gate_view(),
                &mut sites,
                &mut gated,
                &mut terms,
            );
        }
    }

    // C8 loops (spec-0086). The gate is the release.
    enumerated[slot_of(GateConsumer::Loop)] = true;
    for (li, l) in c.quests.content.loops.iter().enumerate() {
        visit(
            GateConsumer::Loop,
            format!("/content/loops/{li}"),
            l.gate(),
            &mut sites,
            &mut gated,
            &mut terms,
        );
    }

    // C9 lethal volumes (spec-0088), after every shop offer, in declaration
    // order. The pointer names the `when` object, as an effect's does.
    enumerated[slot_of(GateConsumer::LethalVolume)] = true;
    for (vi, v) in c.quests.content.lethal_volumes.iter().enumerate() {
        visit(
            GateConsumer::LethalVolume,
            format!("/content/lethal_volumes/{vi}/when"),
            v.gate(),
            &mut sites,
            &mut gated,
            &mut terms,
        );
    }

    let missed: Vec<&str> = GateConsumer::ALL
        .iter()
        .zip(enumerated)
        .filter(|(_, seen)| !*seen)
        .map(|(k, _)| k.label())
        .collect();
    assert!(
        missed.is_empty(),
        "for_each_gate enumerated {} of {} gate consumers — missing: {}. A consumer that stops \
         being enumerated has no other symptom.",
        GateConsumer::COUNT - missed.len(),
        GateConsumer::COUNT,
        missed.join(", ")
    );

    GateBinding {
        consumers_enumerated: GateConsumer::COUNT,
        sites,
        gated,
        terms,
    }
}

// ---------------------------------------------------------------------------
// Satisfiability: the set of values that opens a gate, as a value
// ---------------------------------------------------------------------------

/// The set of integer values of ONE datum that a conjunction of comparison
/// terms leaves open — an interval with holes, or a pinned point.
///
/// This is the arithmetic shared by every question of the form "can this gate
/// ever open, and at what value?": [`Gate::contradiction`] asks it per gate,
/// and the compiler's cast-ladder solver asks it per clause while also
/// *refusing* sibling terms ([`DatumSet::forbid`]). It lives here, on the gate,
/// because a gate is the object class the question is about — a copy beside
/// each asking verb is how two answers to one question start disagreeing.
///
/// Determinism (ADR-0006): [`DatumSet::pick`] returns one canonical member, a
/// pure function of the constraint set.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct DatumSet {
    /// Lower bound (inclusive), `None` = unbounded below.
    lo: Option<i32>,
    /// Upper bound (inclusive), `None` = unbounded above.
    hi: Option<i32>,
    /// Excluded points.
    holes: std::collections::BTreeSet<i32>,
    /// `Some(v)`: the set is at most `{v}` (an `equals` term).
    pin: Option<i32>,
    /// Two constraints that no integer can satisfy at once.
    contra: bool,
}

impl Default for DatumSet {
    fn default() -> Self {
        Self::all()
    }
}

impl DatumSet {
    /// Every integer: the set before any term constrains it.
    pub fn all() -> Self {
        DatumSet {
            lo: None,
            hi: None,
            holes: std::collections::BTreeSet::new(),
            pin: None,
            contra: false,
        }
    }

    /// Intersect with the values that SATISFY `op value`.
    pub fn require(&mut self, op: crate::stages::CompareOp, value: i32) {
        use crate::stages::CompareOp::*;
        match op {
            Equals => match self.pin {
                Some(p) if p != value => self.contra = true,
                _ => self.pin = Some(value),
            },
            NotEquals => {
                self.holes.insert(value);
            }
            AtLeast => self.lo = Some(self.lo.map_or(value, |l| l.max(value))),
            AtMost => self.hi = Some(self.hi.map_or(value, |h| h.min(value))),
        }
    }

    /// Intersect with the values that VIOLATE `op value` — the negation of
    /// [`DatumSet::require`], spelled once so the two can never disagree about
    /// what a term means.
    pub fn forbid(&mut self, op: crate::stages::CompareOp, value: i32) {
        use crate::stages::CompareOp::*;
        match op {
            Equals => self.require(NotEquals, value),
            NotEquals => self.require(Equals, value),
            // ¬(x ≥ v) ⇔ x ≤ v−1; at i32::MIN nothing violates it.
            AtLeast => match value.checked_sub(1) {
                Some(v) => self.require(AtMost, v),
                None => self.contra = true,
            },
            AtMost => match value.checked_add(1) {
                Some(v) => self.require(AtLeast, v),
                None => self.contra = true,
            },
        }
    }

    /// **The smallest value in the set**, or `None` when it is unbounded below
    /// or empty.
    ///
    /// The question a price asks: *what is the least balance at which this gate
    /// opens?* It is the same arithmetic [`Self::pick`] does — an interval with
    /// holes, stepped past — asked from the bottom, so the purchase rule
    /// (`DW0901`) and the satisfiability verdict cannot disagree about what a
    /// term means. `equals` pins, and a pin is its own floor.
    pub fn min(&self) -> Option<i32> {
        let lo = self.pin.or(self.lo)?;
        let steps = self.holes.len() as i64 + 1;
        (lo as i64..lo as i64 + steps)
            .filter_map(|v| i32::try_from(v).ok())
            .find(|v| self.contains(*v))
    }

    /// **The largest value in the set**, or `None` when it is unbounded above or
    /// empty — the mirror of [`Self::min`], which is what a refusal arm's
    /// ceiling is read from.
    pub fn max(&self) -> Option<i32> {
        let hi = self.pin.or(self.hi)?;
        let steps = self.holes.len() as i64 + 1;
        ((hi as i64 - steps + 1)..=hi as i64)
            .rev()
            .filter_map(|v| i32::try_from(v).ok())
            .find(|v| self.contains(*v))
    }

    /// Whether `v` satisfies every term this set carries.
    fn contains(&self, v: i32) -> bool {
        !self.contra
            && self.pin.is_none_or(|p| p == v)
            && self.lo.is_none_or(|l| v >= l)
            && self.hi.is_none_or(|h| v <= h)
            && !self.holes.contains(&v)
    }

    /// A deterministic member of the set, or `None` when the set is empty —
    /// which is the satisfiability verdict.
    ///
    /// Complete without enumeration games: an interval-with-holes is nonempty
    /// iff a member exists within `holes.len() + 1` steps of a bound (or of 0
    /// when unbounded both ways), because each step is only ever excluded by a
    /// distinct hole.
    pub fn pick(&self) -> Option<i32> {
        if self.contra {
            return None;
        }
        let ok = |v: i32| {
            self.lo.is_none_or(|l| v >= l)
                && self.hi.is_none_or(|h| v <= h)
                && !self.holes.contains(&v)
        };
        if let Some(p) = self.pin {
            return ok(p).then_some(p);
        }
        let steps = self.holes.len() as i64 + 1;
        let from = match (self.lo, self.hi) {
            (Some(l), _) => l as i64,
            (None, Some(h)) => (h as i64) - steps + 1,
            (None, None) => 0,
        };
        (from..from + steps)
            .filter_map(|v| i32::try_from(v).ok())
            .find(|v| ok(*v))
    }
}

/// Why a gate can never open (see [`Gate::contradiction`]).
#[derive(Clone, Debug, PartialEq, Eq)]
pub enum GateContradiction {
    /// The same flag is both required and forbidden.
    Flag(String),
    /// No integer value of this datum satisfies every `requires_state` term
    /// that reads it.
    Datum(String),
}

impl Gate<'_> {
    /// The first reason this gate can NEVER open, or `None` for a satisfiable
    /// gate.
    ///
    /// A gate is a conjunction, so it is unsatisfiable exactly when one flag is
    /// on both lists, or one datum's terms intersect to the empty set. Terms on
    /// distinct flags/datums are independent and cannot contradict each other.
    pub fn contradiction(&self) -> Option<GateContradiction> {
        for f in self.requires_flags {
            if self.forbids_flags.iter().any(|g| g == f) {
                return Some(GateContradiction::Flag(f.as_str().to_string()));
            }
        }
        let mut per: std::collections::BTreeMap<&str, DatumSet> = std::collections::BTreeMap::new();
        for t in self.requires_state {
            per.entry(t.state.as_str())
                .or_default()
                .require(t.op, t.value);
        }
        for (state, set) in per {
            if set.pick().is_none() {
                return Some(GateContradiction::Datum(state.to_string()));
            }
        }
        None
    }
}

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

    #[test]
    fn open_gate_is_empty() {
        assert!(Gate::OPEN.is_empty());
        assert_eq!(Gate::OPEN.terms(), 0);
    }

    /// Every consumer names a stage and a label, and the stages are exactly the
    /// two stage documents gates live in.
    #[test]
    fn every_consumer_names_its_stage() {
        for k in GateConsumer::ALL {
            assert!(matches!(k.stage(), "quests" | "dialogue"), "{k:?}");
            assert!(!k.label().is_empty(), "{k:?}");
        }
    }

    use crate::stages::CompareOp::*;

    #[test]
    fn datum_set_picks_within_bounds_and_around_holes() {
        let mut s = DatumSet::all();
        assert_eq!(s.pick(), Some(0), "the unconstrained canonical member is 0");
        s.require(AtLeast, 3);
        s.require(AtMost, 5);
        assert_eq!(s.pick(), Some(3), "the lower boundary is canonical");
        s.require(NotEquals, 3);
        assert_eq!(s.pick(), Some(4), "a hole at the boundary steps past it");
        s.require(NotEquals, 4);
        s.require(NotEquals, 5);
        assert_eq!(s.pick(), None, "holes covering the interval empty it");
    }

    #[test]
    fn datum_set_pins_and_pin_conflicts() {
        let mut s = DatumSet::all();
        s.require(Equals, 7);
        assert_eq!(s.pick(), Some(7));
        s.require(NotEquals, 7);
        assert_eq!(s.pick(), None, "a hole at the pin empties the set");
        let mut t = DatumSet::all();
        t.require(Equals, 7);
        t.require(Equals, 8);
        assert_eq!(t.pick(), None, "two different pins contradict");
    }

    #[test]
    fn forbid_is_the_exact_negation() {
        // Violating `at-least 5` means at most 4; violating that too is empty.
        let mut s = DatumSet::all();
        s.forbid(AtLeast, 5);
        assert_eq!(s.pick(), Some(4), "the violating boundary is canonical");
        s.require(AtLeast, 5);
        assert_eq!(s.pick(), None);
        // Nothing violates `at-least i32::MIN` / `at-most i32::MAX`.
        let mut lo = DatumSet::all();
        lo.forbid(AtLeast, i32::MIN);
        assert_eq!(lo.pick(), None);
        let mut hi = DatumSet::all();
        hi.forbid(AtMost, i32::MAX);
        assert_eq!(hi.pick(), None);
        // Violating `not-equals v` pins v.
        let mut ne = DatumSet::all();
        ne.forbid(NotEquals, 9);
        assert_eq!(ne.pick(), Some(9));
    }

    /// The floor and the ceiling of a set — what a price reads. A pin is its own
    /// floor and its own ceiling; a hole at a bound steps past it; an unbounded
    /// side answers `None` rather than a number nothing stated.
    #[test]
    fn a_datum_set_states_its_floor_and_its_ceiling() {
        let mut s = DatumSet::all();
        assert_eq!((s.min(), s.max()), (None, None), "nothing bounds it");
        s.require(AtLeast, 15);
        assert_eq!((s.min(), s.max()), (Some(15), None));
        s.require(AtMost, 20);
        assert_eq!((s.min(), s.max()), (Some(15), Some(20)));
        s.require(NotEquals, 15);
        s.require(NotEquals, 20);
        assert_eq!((s.min(), s.max()), (Some(16), Some(19)));
        let mut pinned = DatumSet::all();
        pinned.require(Equals, 7);
        assert_eq!((pinned.min(), pinned.max()), (Some(7), Some(7)));
        let mut empty = DatumSet::all();
        empty.require(AtLeast, 5);
        empty.require(AtMost, 4);
        assert_eq!(
            (empty.min(), empty.max()),
            (None, None),
            "an empty set has neither"
        );
    }

    #[test]
    fn an_upper_bounded_set_picks_below_its_holes() {
        let mut s = DatumSet::all();
        s.require(AtMost, 10);
        s.require(NotEquals, 10);
        s.require(NotEquals, 9);
        assert_eq!(s.pick(), Some(8), "walks down from the upper bound");
    }

    #[test]
    fn gate_contradiction_answers_per_axis() {
        use crate::ids::FlagId;
        use crate::stages::StateCompare;
        let f: Vec<FlagId> = vec![FlagId("flag/paid".to_string())];
        let g = Gate::of(&f, &f, &[]);
        assert_eq!(
            g.contradiction(),
            Some(GateContradiction::Flag("flag/paid".to_string()))
        );
        let terms = [
            StateCompare {
                state: crate::ids::StateId("state/toll".to_string()),
                op: AtLeast,
                value: 5,
            },
            StateCompare {
                state: crate::ids::StateId("state/toll".to_string()),
                op: AtMost,
                value: 3,
            },
        ];
        let g = Gate::of(&[], &[], &terms);
        assert_eq!(
            g.contradiction(),
            Some(GateContradiction::Datum("state/toll".to_string()))
        );
        assert_eq!(Gate::OPEN.contradiction(), None);
    }
}