gam_problem/

basis_error.rs

//! Leaf error type for basis construction.
//!
//! `BasisError` lives in the neutral `gam-problem` crate (not `gam-terms`) so
//! that downstream consumers — families, design assembly, the terms cluster —
//! can resolve it without dragging in a `gam-terms` dependency cycle.

use gam_linalg::faer_ndarray::FaerLinalgError;
use thiserror::Error;

/// A comprehensive error type for all operations within the basis module.
#[derive(Error, Debug)]
pub enum BasisError {
    #[error("Spline degree must be at least 1, but was {0}.")]
    InvalidDegree(usize),

    #[error(
        "Spline degree {degree} is too low for derivative order {derivative_order}; need degree >= {minimum_degree}."
    )]
    InsufficientDegreeForDerivative {
        degree: usize,
        derivative_order: usize,
        minimum_degree: usize,
    },

    #[error("Data range is invalid: start ({0}) must be less than or equal to end ({1}).")]
    InvalidRange(f64, f64),

    #[error(
        "Data range has zero width (min equals max), which collapses the B-spline knot domain; requested {0} internal knots."
    )]
    DegenerateRange(usize),

    #[error(
        "Penalty order ({order}) must be positive and less than the number of basis functions ({num_basis})."
    )]
    InvalidPenaltyOrder { order: usize, num_basis: usize },

    #[error(
        "Insufficient knots for degree {degree} spline: need at least {required} knots but only {provided} were provided."
    )]
    InsufficientKnotsForDegree {
        degree: usize,
        required: usize,
        provided: usize,
    },

    #[error(
        "Cannot apply sum-to-zero constraint: requires at least 2 basis functions, but only {found} were provided."
    )]
    InsufficientColumnsForConstraint { found: usize },

    #[error(
        "Constraint matrix must have the same number of rows as the basis: basis has {basisrows}, constraint has {constraintrows}."
    )]
    ConstraintMatrixRowMismatch {
        basisrows: usize,
        constraintrows: usize,
    },

    #[error(
        "Weights dimension mismatch: expected {expected} weights to match basis matrix rows, but got {found}."
    )]
    WeightsDimensionMismatch { expected: usize, found: usize },

    #[error("QR decomposition failed while applying constraints: {0}")]
    LinalgError(#[from] FaerLinalgError),

    #[error(
        "Failed to identify a constraint nullspace basis at {site}: \
         coefficient dim {coeff_dim}, cross-rank {cross_rank}, \
         constraint Frobenius {cross_frobenius:.3e}, \
         constrained Gram spectrum {gram_spectrum}. \
         The smooth basis collapses onto the parametric block — typical causes: \
         (a) the smooth's evaluated kernel underflows after projecting out the \
         polynomial nullspace, leaving only floating-point noise (Duchon hybrid \
         in moderate-to-high d with length_scale near pairwise center distances); \
         (b) the parametric block already spans the smooth's column space \
         (over-restrictive identifiability constraint); \
         (c) the smooth has effective rank ≤ parametric-block size on this data."
    )]
    ConstraintNullspaceCollapsed {
        site: &'static str,
        cross_rank: usize,
        coeff_dim: usize,
        cross_frobenius: f64,
        /// Pre-formatted constrained-Gram spectrum summary. The structural
        /// early-return sites bail at the cross-rank check before the Gram is
        /// ever eigendecomposed, so they report `not computed` rather than a
        /// misleading NaN; only the spectral-rank-deficiency site fills in real
        /// max/min eigenvalues and tolerance.
        gram_spectrum: String,
    },

    #[error(
        "Knot vector is degenerate: all Greville abscissae are equal, so linear constraint cannot be applied."
    )]
    DegenerateKnots,

    #[error(
        "The provided knot vector is invalid: {0}. It must be non-decreasing and contain only finite values."
    )]
    InvalidKnotVector(String),

    #[error("Failed to build sparse basis matrix: {0}")]
    SparseCreation(String),

    #[error("Dimension mismatch: {0}")]
    DimensionMismatch(String),

    #[error(
        "Indefinite penalty matrix in {context}: minimum eigenvalue {min_eigenvalue:.3e} is below tolerance {tolerance:.3e}. {guidance}"
    )]
    IndefinitePenalty {
        context: String,
        min_eigenvalue: f64,
        tolerance: f64,
        guidance: String,
    },

    #[error("Invalid input: {0}")]
    InvalidInput(String),

    #[error(
        "Radial basis derivative is undefined at center collision (r = 0) for {kernel} \
         with dim = {dim}, m = {m}: {message}. The first/second derivative of the \
         underlying φ(r) does not have a finite limit as r → 0+, so the design-row \
         gradient and Hessian have no well-defined value at coincident points."
    )]
    DegenerateAtCollision {
        kernel: &'static str,
        dim: usize,
        m: f64,
        message: &'static str,
    },

    /// A Duchon spline is not smooth enough for an operator a fit path applies
    /// to its radial kernel: the kernel's spectral order `2(p+s)` must exceed
    /// the covariate dimension by `margin` for that operator to be finite at
    /// the origin.
    #[error(
        "{}",
        duchon_smoothness_message(operator, *margin, *spectral_order, *dimension, *nullspace_order, *power, *minimum_power)
    )]
    DuchonSmoothnessInsufficient {
        /// The operator that needs the smoothness ("pointwise kernel values",
        /// "D2 collocation", "collision derivative phi^(4)", ...).
        operator: String,
        /// The required excess of `2(p+s)` over the dimension.
        margin: usize,
        /// The realised spectral order `2(p+s)`.
        spectral_order: f64,
        dimension: usize,
        /// The Duchon nullspace order `p`.
        nullspace_order: usize,
        /// The Duchon power `s`.
        power: f64,
        /// The smallest integer power that would admit the operator.
        minimum_power: usize,
    },

    #[error("{0}")]
    Other(String),
}

fn duchon_smoothness_message(
    operator: &str,
    margin: usize,
    spectral_order: f64,
    dimension: usize,
    nullspace_order: usize,
    power: f64,
    minimum_power: usize,
) -> String {
    let bound = if margin == 0 {
        "dimension".to_string()
    } else {
        format!("dimension+{margin}")
    };
    format!(
        "Duchon {operator}: 2*(p+s) > {bound} is required; got 2*(p+s)={spectral_order}, \
         dimension={dimension}, p={nullspace_order}, s={power}. The operator is finite only \
         for a smoother spline: raise power to >= {minimum_power} (or reduce the joint \
         smooth's dimension)."
    )
}

impl BasisError {
    /// The typed Duchon smoothness refusal for `operator`, computing the
    /// smallest admitting power from the same inequality that refused.
    #[must_use]
    pub fn duchon_smoothness_insufficient(
        operator: impl Into<String>,
        margin: usize,
        dimension: usize,
        nullspace_order: usize,
        power: f64,
    ) -> Self {
        // `2(p+s) > dimension + margin` ⇔ `s > (dimension + margin)/2 − p`; the
        // smallest integer power satisfying it is `⌊(dimension + margin)/2⌋ + 1 − p`.
        let minimum_power = ((dimension + margin) / 2 + 1).saturating_sub(nullspace_order);
        Self::DuchonSmoothnessInsufficient {
            operator: operator.into(),
            margin,
            spectral_order: 2.0 * (nullspace_order as f64 + power),
            dimension,
            nullspace_order,
            power,
            minimum_power,
        }
    }

    /// The remediation a user can act on, when the failure has one. This is
    /// the single source of the advice every front end prints beside the
    /// error; it is keyed on the variant, never on the rendered text (#2470).
    #[must_use]
    pub fn advice(&self) -> Option<String> {
        match self {
            Self::DuchonSmoothnessInsufficient { minimum_power, .. } => Some(format!(
                "Raise the Duchon smooth's `power=...` to at least {minimum_power}, or reduce \
                 the joint smooth's dimension."
            )),
            Self::InvalidDegree(_)
            | Self::InsufficientDegreeForDerivative { .. }
            | Self::InvalidRange(..)
            | Self::DegenerateRange(_)
            | Self::InvalidPenaltyOrder { .. }
            | Self::InsufficientKnotsForDegree { .. }
            | Self::InsufficientColumnsForConstraint { .. }
            | Self::ConstraintMatrixRowMismatch { .. }
            | Self::WeightsDimensionMismatch { .. }
            | Self::LinalgError(_)
            | Self::ConstraintNullspaceCollapsed { .. }
            | Self::DegenerateKnots
            | Self::InvalidKnotVector(_)
            | Self::SparseCreation(_)
            | Self::DimensionMismatch(_)
            | Self::IndefinitePenalty { .. }
            | Self::InvalidInput(_)
            | Self::DegenerateAtCollision { .. }
            | Self::Other(_) => None,
        }
    }
}

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

    #[test]
    fn duchon_smoothness_refusal_names_the_smallest_admitting_power() {
        // 2(p+s) > dim + margin with p = 1, dim = 16, margin = 2 needs s ≥ 9.
        let err = BasisError::duchon_smoothness_insufficient("D2 collocation", 2, 16, 1, 8.0);
        let BasisError::DuchonSmoothnessInsufficient { minimum_power, spectral_order, .. } = &err
        else {
            panic!("expected the typed Duchon refusal, got {err:?}");
        };
        assert_eq!(*minimum_power, 9);
        assert_eq!(*spectral_order, 18.0);
        let text = err.to_string();
        assert!(text.contains("2*(p+s) > dimension+2"), "{text}");
        assert!(text.contains("raise power to >= 9"), "{text}");
        let advice = err.advice().expect("advice");
        assert!(advice.contains("at least 9"), "{advice}");
        // A refusal with no remediation carries none.
        assert!(BasisError::DegenerateKnots.advice().is_none());
    }

    #[test]
    fn a_zero_margin_refusal_states_the_bare_dimension_bound() {
        let err = BasisError::duchon_smoothness_insufficient("pointwise kernel values", 0, 4, 2, 0.5);
        let text = err.to_string();
        assert!(text.contains("2*(p+s) > dimension is required;"), "{text}");
        assert!(text.contains("2*(p+s)=5"), "{text}");
    }
}

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

    #[test]
    fn invalid_degree_mentions_degree_in_message() {
        let err = BasisError::InvalidDegree(0);
        let msg = err.to_string();
        assert!(msg.contains("0"), "expected degree in message, got: {msg}");
        assert!(msg.to_lowercase().contains("degree"));
    }

    #[test]
    fn invalid_range_mentions_start_and_end() {
        let err = BasisError::InvalidRange(2.5, 1.0);
        let msg = err.to_string();
        assert!(
            msg.contains("2.5") || msg.contains("start"),
            "message: {msg}"
        );
    }

    #[test]
    fn degenerate_range_mentions_zero_width() {
        let err = BasisError::DegenerateRange(4);
        let msg = err.to_string().to_lowercase();
        assert!(msg.contains("zero"), "message: {msg}");
    }

    #[test]
    fn invalid_penalty_order_mentions_order_and_num_basis() {
        let err = BasisError::InvalidPenaltyOrder {
            order: 5,
            num_basis: 3,
        };
        let msg = err.to_string();
        assert!(msg.contains("5") && msg.contains("3"), "message: {msg}");
    }

    #[test]
    fn insufficient_knots_mentions_degree() {
        let err = BasisError::InsufficientKnotsForDegree {
            degree: 3,
            required: 10,
            provided: 5,
        };
        let msg = err.to_string();
        assert!(
            msg.contains("3") && msg.contains("10") && msg.contains("5"),
            "message: {msg}"
        );
    }

    #[test]
    fn invalid_knot_vector_includes_reason() {
        let err = BasisError::InvalidKnotVector("decreasing knots".to_string());
        let msg = err.to_string();
        assert!(msg.contains("decreasing knots"), "message: {msg}");
    }

    #[test]
    fn invalid_input_passthrough() {
        let err = BasisError::InvalidInput("bad value".to_string());
        assert!(err.to_string().contains("bad value"));
    }

    #[test]
    fn other_passthrough() {
        let err = BasisError::Other("catch-all".to_string());
        assert_eq!(err.to_string(), "catch-all");
    }
}