earthsci-ast 0.6.0

//! Build-time error type shared by the simulate pipeline and the non-gated
//! `faq` / `join` resolution passes.
//!
//! [`CompileError`] lives in its own module rather than inside `simulate`
//! because `simulate` (and `simulate_array`) are `#[cfg(not(target_arch =
//! "wasm32"))]` — they pull in the native ODE backend. The `faq` and
//! `join` modules surface build-time resolution failures as `CompileError`
//! but are compiled on every target, including `wasm32-unknown-unknown`.
//! Keeping the type here lets those modules compile for WASM while `simulate`
//! re-exports it unchanged for the native API (`crate::simulate::CompileError`).

use crate::flatten::FlattenError;
use thiserror::Error;

/// Errors raised when building a compiled model from a flattened system.
#[derive(Error, Debug)]
pub enum CompileError {
    /// A model construct this evaluator cannot run: a continuous or discrete
    /// event, an implicit equation (an equation whose LHS is an expression
    /// rather than an unknown, `D(unknown)` or `ic(unknown)`), or a Wiener-noise
    /// parameter. esm-spec §9.6.6
    /// `unsupported_construct`.
    ///
    /// Refused at build rather than skipped: a model run without its event, or
    /// without its residual solved, reports a wrong answer, and nothing in that
    /// answer says a construct was dropped.
    #[error(
        "{code}: {construct} {detail} is not supported by the {evaluator}; \
         refusing the build rather than running the model without it (esm-spec §9.6.6)",
        code = crate::diagnostic::codes::UNSUPPORTED_CONSTRUCT
    )]
    UnsupportedConstruct {
        /// Which construct: [`CONTINUOUS_EVENT`], [`DISCRETE_EVENT`],
        /// [`IMPLICIT_EQUATION`] or [`WIENER_NOISE`].
        construct: &'static str,
        /// Which evaluator refused it, e.g. `"Rust array evaluator"`.
        evaluator: &'static str,
        /// The offending instance: an event's name, or an equation's LHS.
        detail: String,
    },

    /// One unknown carries BOTH a derivative equation and a bare-LHS one
    /// (esm-spec §4.9.4 `equation_count_mismatch`).
    ///
    /// §4.9.4 counts an equation against the unknowns "whichever form its LHS
    /// takes", so `D(x) ~ f` beside `x ~ g` is two equations binding one
    /// unknown: the document is unbalanced, and `validate` reports exactly
    /// that. This crate used to tie-break in favour of the derivative, which
    /// integrates a system free of a constraint the file declares and reports
    /// the trajectory as the answer. The build is the other place that same
    /// document arrives, so it refuses under the same code.
    #[error(
        "{code}: unknown '{name}' is defined twice — by `{differential}` and by \
         `{algebraic}`. esm-spec §4.9.4 counts an equation whichever form its LHS \
         takes, so this system has one more equation than it has unknowns to bind; \
         keeping the derivative and dropping the constraint would run a model the \
         document does not describe. Remove one of the two definitions.",
        code = crate::diagnostic::codes::EQUATION_COUNT_MISMATCH
    )]
    DoublyDefinedUnknown {
        /// The unknown both equations define, spelled as the system spells it.
        name: String,
        /// The derivative equation, rendered `lhs ~ rhs`.
        differential: String,
        /// The competing bare-LHS equation, rendered `lhs ~ rhs`.
        algebraic: String,
    },

    /// The chosen compiler cannot run one of this document's rules
    /// (esm-spec §9.6.6 `compiler_refused_rule`, esm-libraries-spec §2.5.10).
    ///
    /// Raised at CONSTRUCTION, naming the compiler, the rule — an equation or
    /// an observed, component-qualified — the cadence tier the rule would have
    /// run at, and the DEEPEST decline reason reached while trying to lower
    /// it. A refusal, never a fallback: the compiler does not run that rule on
    /// a slower path, because a caller who asked for a compiled program and
    /// got a tree walk has no way to find that out.
    #[error(
        "{code}: compiler '{compiler}' cannot run the {tier}-cadence {kind} '{rule}': {reason}. \
         `native` is strict (esm-libraries-spec §2.5.10) — it refuses rather than demoting the \
         rule to the per-cell oracle; build with `compiler: Some(Compiler::Interpreter)` to run \
         this document on the reference evaluator",
        code = crate::diagnostic::codes::COMPILER_REFUSED_RULE
    )]
    CompilerRefusedRule {
        /// The vocabulary spelling of the compiler that refused.
        compiler: &'static str,
        /// `"observed"` or `"state derivative"`.
        kind: &'static str,
        /// The rule, component-qualified.
        rule: String,
        /// The cadence tier the rule would have run at: `"const"`,
        /// `"discrete"` or `"continuous"`.
        tier: &'static str,
        /// The deepest decline reason reached while trying to lower it.
        reason: String,
    },

    /// The flattened system contains a feature the v1 simulator does not support
    /// (e.g. a join over data-derived columns).
    #[error("Unsupported feature '{feature}': {message}")]
    UnsupportedFeatureError {
        /// Feature name (e.g. `"value-equality join over data-derived columns"`).
        feature: String,
        /// Why this is rejected and what to do about it.
        message: String,
    },

    /// The flattened system still carries a spatial independent variable — a
    /// spatial operator that was never discretized into a `faq` stencil.
    /// Discretized PDEs fold their spatial axis into array dimensions, leaving
    /// `independent_variables == ["t"]`, and simulate fine.
    #[error(
        "Unsupported dimensionality {independent_variables:?}: the simulator integrates systems whose only independent variable is time (independent_variables == [\"t\"]). A remaining spatial independent variable means a spatial operator was not discretized — apply the discretization template (an `expression_templates` `match` rewrite) that lowers it to a `faq` stencil, then simulate. Discretized PDEs run natively in this backend."
    )]
    UnsupportedDimensionalityError {
        /// The actual independent variables found.
        independent_variables: Vec<String>,
    },

    /// The interpreter could not build a callable representation of the
    /// flattened equations.
    #[error("Interpreter build failed: {details}")]
    InterpreterBuildError {
        /// Human-readable failure description.
        details: String,
    },

    /// A rewrite-target operator (a spatial / right-hand-side `D`, or the
    /// optional sugar ops `grad` / `div` / `laplacian` / ...) survived the
    /// lowering fixpoint into an evaluation position (esm-spec §4.2 / §9.6.8).
    /// Such ops carry NO evaluator: a discretization `match` rewrite rule MUST
    /// lower them to a `faq` / `makearray` stencil before evaluation.
    /// The gate fires here — before evaluation, not at load — with the uniform
    /// `unlowered_operator` code that supersedes the former per-binding
    /// `UnreachableSpatialOperatorError` / `UnsupportedDimensionality` errors.
    /// Silently producing zeros (the previous behavior) would mask a broken
    /// pipeline. This format ships no discretization rules — they live in
    /// EarthSciDiscretizations.
    #[error(
        "unlowered_operator: rewrite-target operator '{op}' reached evaluation without being \
         lowered to a stencil by a rewrite rule (esm-spec §4.2 / §9.6.8). Discretization rules \
         live in EarthSciDiscretizations, not this format."
    )]
    UnloweredOperatorError {
        /// The offending operator name (e.g. `"grad"` or `"D"`).
        op: String,
    },

    /// An evaluable-core operator (esm-spec §4.2) applied to the wrong number
    /// of arguments — e.g. `atan2` with one argument, or `min` with fewer than
    /// two. The schema cannot express per-operator arity (`args` is
    /// `minItems: 0`), so this is the gate that keeps a malformed-but-
    /// schema-valid node out of the evaluators, where it would otherwise either
    /// panic on an out-of-bounds index or — worse — be silently assigned two
    /// different values by the per-cell oracle and the vectorized overlay.
    /// See [`crate::op_registry`].
    #[error(
        "invalid_operator_arity: operator '{op}' takes {expected} argument(s), got {got} \
         (esm-spec §4.2)"
    )]
    InvalidOperatorArity {
        /// The offending operator name (e.g. `"atan2"`).
        op: String,
        /// How many arguments the node actually carried.
        got: usize,
        /// What the spec says the operator takes (e.g. `"exactly 2"`).
        expected: String,
    },

    /// A `makearray` node whose `regions` are malformed: either the regions
    /// disagree on rank (a ragged `regions` list), or a bound pair is inverted
    /// beyond the legal empty spelling. Per esm-spec §4.3.2 a pair with
    /// `stop == start - 1` is the canonical **empty** region and is legal;
    /// anything further inverted (`stop < start - 1`) is rejected with
    /// `makearray_region_inverted`.
    #[error("makearray_region_inverted: {reason} (esm-spec §4.3.2)")]
    MakearrayRegionInvalid {
        /// What is wrong with the regions.
        reason: String,
    },

    /// A `broadcast` node whose `fn` field is absent or does not name a scalar
    /// operator (esm-spec §4.3.4). `broadcast` is the one op whose OPERATOR is
    /// data — the node says `broadcast`, the arithmetic is named by a sibling
    /// string — and nothing used to check that string, so a typo'd or missing
    /// `fn` was silently evaluated as `+` (issue #101).
    #[error("invalid_broadcast_fn: {reason}")]
    InvalidBroadcastFn {
        /// Why the `fn` is rejected.
        reason: String,
    },

    /// An evaluable-core operator (esm-spec §4.2) that this interpreter has no
    /// evaluation rule for reached the evaluator.
    ///
    /// These ops are legal in an AST but are *eliminated before* the per-cell
    /// evaluator ever runs: the build-time query ops (`skolem`, `rank`,
    /// `distinct`, `argmin`, `argmax`) are resolved by
    /// [`crate::value_invention`], and the form/lowering ops (`ic`, `true`,
    /// `enum`, `table_lookup`, `apply_expression_template`) are consumed by
    /// their respective lowering passes. One arriving at the evaluator means the
    /// pipeline is broken.
    ///
    /// It is reported rather than evaluated to a NaN sentinel: a silent NaN is
    /// indistinguishable from a legitimate numerical result and would propagate
    /// into the solution, whereas the pipeline stage that should have eliminated
    /// the op is the actual defect.
    ///
    /// Raised by the array runtime's build gate and by the single-expression
    /// [`crate::expression::evaluate`], whose scalar bindings additionally give
    /// the array / tensor and geometry ops no value. The invariant is the same
    /// either way: an unevaluable op is a diagnostic, never a number.
    #[error(
        "unevaluable_operator: operator '{op}' is an evaluable-core op with no evaluation rule \
         in the interpreter this model was built for — it must be eliminated by an earlier \
         pipeline stage (value invention, or a lowering pass), or the document built for a \
         runtime that evaluates it, before evaluation (esm-spec §4.2)"
    )]
    UnevaluableOperatorError {
        /// The offending operator name (e.g. `"skolem"`).
        op: String,
    },

    /// An expression ranges over a `kind: "derived"` index set whose
    /// value-invention producer could not be materialized at build (esm-spec
    /// §4.2 / §9.6.6). Refused rather than contracted as an empty range, which
    /// folds to the additive identity and reads as a plausible 0.
    #[error("{code}: {reason}")]
    ValueInventionRefused {
        /// `derived_index_set_unmaterialized`, or `relational_node_in_continuous`
        /// when the producer reads live state.
        code: &'static str,
        /// The index set, its producer, and why the producer could not run.
        reason: String,
    },

    /// A `table_lookup` node that cannot be lowered to its esm-spec §9.5.3
    /// `interp.linear` / `interp.bilinear` / `index` form — an unknown table,
    /// an axis-key set that does not match the table's declared axes, an
    /// out-of-range `output`, or a table whose `interpolation` and axis count
    /// disagree.
    ///
    /// Raised by [`crate::lower_table_lookup`] during the build rather than by
    /// the evaluator: `unevaluable_operator` would name `table_lookup` without
    /// saying which of the §9.5.5 conditions the document actually trips.
    #[error("{code}: {reason}")]
    TableLookupLowering {
        /// The esm-spec §9.5.5 diagnostic code.
        code: &'static str,
        /// What is wrong with this `table_lookup` (or the table it names).
        reason: String,
    },

    /// `domain.element_type` names a precision this evaluator does not have.
    ///
    /// Reported rather than defaulted to binary64: a document that asks for a
    /// precision and silently gets a different one is the same class of defect
    /// as an `element_type` that is accepted and then ignored.
    #[error(
        "unsupported_element_type: domain.element_type '{element_type}' is not a precision this \
         evaluator implements — use \"Float64\" (the default) or \"Float32\" (esm-spec §11.3)"
    )]
    UnsupportedElementType {
        /// The offending `element_type` spelling.
        element_type: String,
    },

    /// A construct that cannot be evaluated in binary32 appears in a document
    /// declaring `domain.element_type: "Float32"`.
    ///
    /// The alternative — evaluating just that construct in binary64 — is a
    /// silent fallback: the document asked for one precision and part of the
    /// answer would be computed in another, with nothing in the result to say
    /// which part. So it is an error naming the construct.
    #[error(
        "float32_unsupported: {construct} cannot be evaluated under \
         `domain.element_type: \"Float32\"` — {reason}. Remove the construct, or evaluate the \
         document in Float64 (esm-spec §11.3)"
    )]
    Float32Unsupported {
        /// The offending construct, named (e.g. ``operator `intersect_polygon` ``).
        construct: String,
        /// Why binary32 cannot carry it.
        reason: String,
    },

    /// A declared working precision reaches a construct this evaluator cannot
    /// carry it through (esm-spec §11.3.1, §4.7 "Working precision crosses a
    /// mount").
    ///
    /// Refused rather than evaluated in another precision: the declaration
    /// would otherwise be accepted and ignored.
    #[error("element_type_uncarried: {construct} — {reason} (esm-spec §11.3.1)")]
    ElementTypeUncarried {
        /// The construct whose declared precision cannot be carried.
        construct: String,
        /// Why this evaluator cannot carry it.
        reason: String,
    },

    /// One operator's operands carry DIFFERENT declared element types
    /// (`ModelVariable.element_type`, esm-spec §11.3.1).
    ///
    /// Refused rather than resolved by a usual-arithmetic-conversions rule,
    /// because every resolution is a silent answer to a question only the
    /// author can settle. Widening to the wider operand makes a quantity
    /// declared binary32 come out binary64 — the declaration ignored, with
    /// nothing in the result to say so. Narrowing to the narrower destroys the
    /// binary64 operand, which is the very corruption per-variable element
    /// types exist to prevent (a ten-digit key rounded to binary32 collides
    /// with its neighbours). So the mix is an error naming both variables, and
    /// the author states the intent by computing the mixed step into a variable
    /// whose own `element_type` says which precision it is in.
    #[error(
        "mixed_element_type: operator '{op}' mixes operands of different element types — \
         {lhs_name} is {lhs_type} and {rhs_name} is {rhs_type}. One operator cannot be both; \
         neither widening nor narrowing is silently applied. Compute one side into a variable \
         whose own `element_type` states which precision that step lands in, then combine the \
         declared variables (esm-spec §11.3.1)"
    )]
    MixedElementType {
        /// The operator whose operands disagree (or `"equation"` for an
        /// equation whose right-hand side disagrees with its left-hand side).
        op: String,
        /// A variable supplying the first element type.
        lhs_name: String,
        /// That variable's element type.
        lhs_type: String,
        /// A variable supplying the conflicting element type.
        rhs_name: String,
        /// That variable's element type.
        rhs_type: String,
    },

    /// A BARE array-level expression whose operand is declared over an index
    /// set the result does not have (esm-spec §4.3.4).
    ///
    /// Operands of an array-level expression align by index-set NAME: an
    /// operand declared over a SUBSET of the result's index sets broadcasts
    /// along the ones it is missing. An operand carrying an index set the
    /// result does not carry has no axis to align to and no defensible value to
    /// take, so it is rejected rather than reinterpreted positionally — the
    /// positional flatten this replaces produced plausible, non-`NaN`,
    /// zero-padded garbage (issue #100).
    ///
    /// The shapes are fully known statically, so `validate()` reports the same
    /// defect as an `array_shape_mismatch` structural error before any build is
    /// attempted; this is the build-time backstop for a model compiled without
    /// being validated first.
    #[error(
        "array_shape_mismatch: operand '{operand}' of the array-level expression for '{result}' \
         is declared over index set '{axis}', which '{result}' (shape {result_axes:?}) does not \
         have — an operand's index sets must be a subset of the result's (esm-spec §4.3.4)"
    )]
    UnalignableArrayShape {
        /// The offending operand's variable name.
        operand: String,
        /// The index set the operand carries and the result does not.
        axis: String,
        /// The name of the array-level expression's result.
        result: String,
        /// The result's declared index sets, in declaration order.
        result_axes: Vec<String>,
    },

    /// A dependency cycle among the model's OBSERVED unknowns (esm-spec
    /// §4.9.6): each observed on the cycle is defined by an equation whose RHS
    /// names the next, so no evaluation order satisfies every definition.
    ///
    /// The equations decide this on their own, so `validate()` reports the same
    /// defect as an `observed_cycle` structural error before any build is
    /// attempted; this is the build-time backstop for a model compiled without
    /// being validated first — and it is the reason this variant exists at all.
    /// The ordering sweep used to *tolerate* an unorderable rule set, appending
    /// the stuck rules in declaration order so "the build still proceeds"; what
    /// actually proceeded was a materialization pass that read an observed with
    /// no value yet and reported `E_TREEWALK_UNBOUND_NAME` against whichever
    /// name it reached first — routinely an observed that is declared, defined
    /// and referenced perfectly well (issue #181).
    #[error(
        "observed_cycle: dependency cycle among observed variables: {}. Each is defined in \
         terms of the next, so no evaluation order satisfies every definition (esm-spec §4.9.6). \
         `esm validate` reports this cycle before any build.",
        cycle.join(" -> ")
    )]
    ObservedCycle {
        /// The observeds on the cycle, in traversal order, with the entry node
        /// repeated to close it (`["a", "b", "a"]`).
        cycle: Vec<String>,
    },

    /// A shaped parameter reached construction with no value: no `default`, no
    /// caller array, and — for one the document refreshes from a data source
    /// or a registered handler — no provider data either. esm-spec §10.10 makes
    /// "a parameter with neither a default nor a supplied value" an error when
    /// a problem is built. `detail` names what the document says feeds the
    /// parameter and how a caller supplies it.
    #[error("E_TREEWALK_MISSING_DATA: {detail}")]
    MissingData {
        /// The parameter, as the compiled model names it.
        parameter: String,
        /// The whole diagnostic after the code.
        detail: String,
    },

    /// An unknown that needs a starting value reached construction with none:
    /// no `default`, no initial condition or `ic` equation, and no caller `u0`
    /// (esm-spec §11.4).
    #[error("E_TREEWALK_MISSING_INITIAL_VALUE: {detail}")]
    MissingInitialValue {
        /// The first state slot with no starting value.
        slot: String,
        /// The whole diagnostic after the code.
        detail: String,
    },

    /// The convenience constructors flattened the input first; that step
    /// failed.
    #[error("Flatten failed: {0}")]
    Flatten(#[from] FlattenError),
}

impl CompileError {
    /// Shorthand constructor for [`CompileError::InterpreterBuildError`], the
    /// catch-all build failure, whose `{ details: format!(…) }` spelling is
    /// otherwise repeated at every raise site.
    pub fn build_err(details: impl Into<String>) -> Self {
        CompileError::InterpreterBuildError {
            details: details.into(),
        }
    }
}

/// [`CompileError::UnsupportedConstruct`]'s `construct` for a
/// `continuous_events` entry.
pub const CONTINUOUS_EVENT: &str = "continuous event";
/// [`CompileError::UnsupportedConstruct`]'s `construct` for a `discrete_events`
/// entry.
pub const DISCRETE_EVENT: &str = "discrete event";
/// [`CompileError::UnsupportedConstruct`]'s `construct` for an equation whose
/// LHS is an expression ([`crate::classification::LhsForm::Expression`]).
pub const IMPLICIT_EQUATION: &str = "implicit equation";
/// [`CompileError::UnsupportedConstruct`]'s `construct` for a parameter whose
/// `update.kind` is `"wiener"`: it makes the document an SDE, which no Rust
/// evaluator integrates.
pub const WIENER_NOISE: &str = "Wiener noise";
/// The array runtime (`crate::simulate_array`), as a refusal names it.
pub const ARRAY_EVALUATOR: &str = "Rust array evaluator";

/// The refusal of an event by `evaluator`: `construct` is [`CONTINUOUS_EVENT`]
/// or [`DISCRETE_EVENT`], and the message names the event.
pub fn event_refusal(
    construct: &'static str,
    evaluator: &'static str,
    name: Option<&str>,
) -> CompileError {
    CompileError::UnsupportedConstruct {
        construct,
        evaluator,
        detail: name.map_or_else(|| "(unnamed)".to_string(), |n| format!("'{n}'")),
    }
}

/// The refusal of a continuous event by `evaluator`, naming the event.
pub fn continuous_event_refusal(evaluator: &'static str, name: Option<&str>) -> CompileError {
    event_refusal(CONTINUOUS_EVENT, evaluator, name)
}

/// The refusal of a discrete event by `evaluator`, naming the event.
pub fn discrete_event_refusal(evaluator: &'static str, name: Option<&str>) -> CompileError {
    event_refusal(DISCRETE_EVENT, evaluator, name)
}

/// The first event `model` or any of its inline subsystems declares, a
/// continuous one before a discrete one: its construct ([`CONTINUOUS_EVENT`] or
/// [`DISCRETE_EVENT`]) and its name (`None` for an unnamed one). The subsystems
/// are searched in their raw JSON: mounting keeps only their variables and
/// equations, and `flatten` does not lift a subsystem's events into the
/// flattened system, so no check on the flattened event lists can see one.
pub(crate) fn first_event(model: &crate::types::Model) -> Option<(&'static str, Option<String>)> {
    if let Some(event) = model.continuous_events.as_ref().and_then(|e| e.first()) {
        return Some((CONTINUOUS_EVENT, event.name.clone()));
    }
    if let Some(event) = model.discrete_events.as_ref().and_then(|e| e.first()) {
        return Some((DISCRETE_EVENT, event.name.clone()));
    }
    model
        .subsystems
        .as_ref()
        .and_then(|subs| subs.values().find_map(first_event_in_json))
}

/// [`first_event`] over every model and reaction system in `file`, subsystems
/// included.
pub(crate) fn first_event_in_file(
    file: &crate::types::EsmFile,
) -> Option<(&'static str, Option<String>)> {
    let in_models = file
        .models
        .as_ref()
        .and_then(|models| models.values().find_map(first_event));
    in_models.or_else(|| {
        file.reaction_systems.as_ref().and_then(|systems| {
            systems.values().find_map(|rs| {
                if let Some(event) = rs.continuous_events.as_ref().and_then(|e| e.first()) {
                    return Some((CONTINUOUS_EVENT, event.name.clone()));
                }
                if let Some(event) = rs.discrete_events.as_ref().and_then(|e| e.first()) {
                    return Some((DISCRETE_EVENT, event.name.clone()));
                }
                rs.subsystems
                    .as_ref()
                    .and_then(|subs| subs.values().find_map(first_event_in_json))
            })
        })
    })
}

fn first_event_in_json(value: &serde_json::Value) -> Option<(&'static str, Option<String>)> {
    for (key, construct) in [
        ("continuous_events", CONTINUOUS_EVENT),
        ("discrete_events", DISCRETE_EVENT),
    ] {
        if let Some(event) = value
            .get(key)
            .and_then(|v| v.as_array())
            .and_then(|events| events.first())
        {
            let name = event.get("name").and_then(|n| n.as_str());
            return Some((construct, name.map(str::to_string)));
        }
    }
    value
        .get("subsystems")
        .and_then(|s| s.as_object())
        .and_then(|subs| subs.values().find_map(first_event_in_json))
}

/// The first equation that constrains its operands only implicitly, including a
/// time derivative of an expression (`D(a + b) ~ 3`), which credits no state. An
/// `ic` LHS is an initial condition, and a spatial-derivative LHS is a rewrite
/// target the `unlowered_operator` gate reports; neither is counted.
pub fn first_implicit_equation(
    equations: &[crate::types::Equation],
) -> Option<&crate::types::Equation> {
    use crate::classification::{LhsFormRef, lhs_form_ref};
    use crate::types::Expr;
    equations.iter().find(|eq| {
        let structural = matches!(
            &eq.lhs,
            Expr::Operator(n) if matches!(n.op.as_str(), "ic" | "grad" | "div" | "laplacian")
                || (n.op == "D" && n.wrt.as_deref().is_some_and(|w| w != "t"))
        );
        !structural && matches!(lhs_form_ref(&eq.lhs), LhsFormRef::Expression)
    })
}

/// The first unknown carrying BOTH a derivative equation and a bare-LHS one,
/// with the two competing equations (esm-spec §4.9.4).
///
/// The search is over EQUATION SHAPES, not over any state/observed split: the
/// derivative side accepts every spelling [`crate::classification::lhs_form`]
/// unwraps to a [`LhsForm::Derivative`](crate::classification::LhsForm), while
/// the competing definition must be a BARE variable LHS. An indexed LHS
/// (`u[i] ~ …`) is deliberately not counted here — it writes one cell of `u`
/// rather than redefining the whole unknown — which keeps this to the shape
/// Python's `_assert_no_doubly_defined_state` refuses, so the two bindings
/// refuse the same documents.
///
/// Returns the FIRST such unknown in equation order, so the message is stable
/// for a document with several.
pub fn first_doubly_defined_unknown(
    equations: &[crate::types::Equation],
) -> Option<(String, &crate::types::Equation, &crate::types::Equation)> {
    use crate::classification::{LhsFormRef, lhs_form_ref};
    use crate::types::Expr;
    use std::collections::HashMap;

    let mut bare: HashMap<&str, &crate::types::Equation> = HashMap::new();
    for eq in equations {
        if let Expr::Variable(name) = &eq.lhs {
            bare.entry(name.as_str()).or_insert(eq);
        }
    }
    if bare.is_empty() {
        return None;
    }
    let mut differential: HashMap<&str, &crate::types::Equation> = HashMap::new();
    for eq in equations {
        if let LhsFormRef::Derivative(name) = lhs_form_ref(&eq.lhs) {
            differential.entry(name).or_insert(eq);
        }
    }
    equations.iter().find_map(|eq| {
        let LhsFormRef::Derivative(name) = lhs_form_ref(&eq.lhs) else {
            return None;
        };
        let diff = *differential.get(name)?;
        let alg = *bare.get(name)?;
        Some((name.to_string(), diff, alg))
    })
}

/// The refusal of the doubly-defined unknown [`first_doubly_defined_unknown`]
/// found, naming the unknown and both equations.
pub fn doubly_defined_unknown_refusal(
    name: &str,
    differential: &crate::types::Equation,
    algebraic: &crate::types::Equation,
) -> CompileError {
    let render = |eq: &crate::types::Equation| {
        format!(
            "{} ~ {}",
            crate::to_ascii(&eq.lhs),
            crate::to_ascii(&eq.rhs)
        )
    };
    CompileError::DoublyDefinedUnknown {
        name: name.to_string(),
        differential: render(differential),
        algebraic: render(algebraic),
    }
}

/// The refusal of an implicit equation by `evaluator`, naming the equation.
pub fn implicit_equation_refusal(
    evaluator: &'static str,
    equation: &crate::types::Equation,
) -> CompileError {
    CompileError::UnsupportedConstruct {
        construct: IMPLICIT_EQUATION,
        evaluator,
        detail: format!(
            "`{} ~ {}`",
            crate::to_ascii(&equation.lhs),
            crate::to_ascii(&equation.rhs)
        ),
    }
}

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

    #[test]
    fn unsupported_construct_carries_its_registered_code() {
        let msg = discrete_event_refusal(ARRAY_EVALUATOR, Some("double")).to_string();
        assert!(
            msg.starts_with(&format!(
                "{}:",
                crate::diagnostic::codes::UNSUPPORTED_CONSTRUCT
            )),
            "{msg}"
        );
        assert!(msg.contains("discrete event 'double'"), "{msg}");
        assert!(msg.contains(ARRAY_EVALUATOR), "{msg}");
    }
}