rustyml 0.12.0

//! Integration tests for [`rustyml::machine_learning::LinearRegression`]
//!
//! Every expected value is derived from the problem design or a closed-form analytic result,
//! never recorded from model output

use approx::assert_abs_diff_eq;
use ndarray::{Array1, Array2, array};
use rustyml::error::Error;
use rustyml::machine_learning::{LinearRegression, RegularizationType};

use crate::common::assert_allclose;

// Constructor validation

/// learning_rate = 0.0 is not positive -> InvalidParameter
#[test]
fn constructor_zero_learning_rate_is_invalid() {
    let result = LinearRegression::new(true, 0.0, 100, 1e-6);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// learning_rate < 0 -> InvalidParameter
#[test]
fn constructor_negative_learning_rate_is_invalid() {
    let result = LinearRegression::new(true, -0.01, 100, 1e-6);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// learning_rate = NaN -> InvalidParameter
#[test]
fn constructor_nan_learning_rate_is_invalid() {
    let result = LinearRegression::new(true, f64::NAN, 100, 1e-6);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// learning_rate = +inf -> InvalidParameter
#[test]
fn constructor_inf_learning_rate_is_invalid() {
    let result = LinearRegression::new(true, f64::INFINITY, 100, 1e-6);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// max_iterations = 0 -> InvalidParameter
#[test]
fn constructor_zero_max_iter_is_invalid() {
    let result = LinearRegression::new(true, 0.01, 0, 1e-6);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// tolerance = 0.0 is not positive -> InvalidParameter
#[test]
fn constructor_zero_tolerance_is_invalid() {
    let result = LinearRegression::new(true, 0.01, 100, 0.0);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// tolerance < 0 -> InvalidParameter
#[test]
fn constructor_negative_tolerance_is_invalid() {
    let result = LinearRegression::new(true, 0.01, 100, -1e-6);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// tolerance = NaN -> InvalidParameter
#[test]
fn constructor_nan_tolerance_is_invalid() {
    let result = LinearRegression::new(true, 0.01, 100, f64::NAN);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// tolerance = +inf -> InvalidParameter
#[test]
fn constructor_inf_tolerance_is_invalid() {
    let result = LinearRegression::new(true, 0.01, 100, f64::INFINITY);
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// Negative L2 alpha -> InvalidParameter
#[test]
fn constructor_negative_l2_alpha_is_invalid() {
    let result = LinearRegression::new(true, 0.01, 100, 1e-6)
        .unwrap()
        .with_regularization(RegularizationType::L2(-0.1));
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// Negative L1 alpha -> InvalidParameter
#[test]
fn constructor_negative_l1_alpha_is_invalid() {
    let result = LinearRegression::new(true, 0.01, 100, 1e-6)
        .unwrap()
        .with_regularization(RegularizationType::L1(-0.5));
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// NaN L2 alpha -> InvalidParameter
#[test]
fn constructor_nan_l2_alpha_is_invalid() {
    let result = LinearRegression::new(true, 0.01, 100, 1e-6)
        .unwrap()
        .with_regularization(RegularizationType::L2(f64::NAN));
    assert!(
        matches!(result, Err(Error::InvalidParameter { .. })),
        "expected InvalidParameter, got {:?}",
        result
    );
}

/// Valid constructor with all legal parameters -> Ok
#[test]
fn constructor_valid_parameters_succeeds() {
    let result = LinearRegression::new(true, 0.01, 1000, 1e-6);
    assert!(result.is_ok(), "expected Ok, got {:?}", result);
}

/// Getters on a freshly constructed model return the supplied values
#[test]
fn constructor_getters_round_trip() {
    let model = LinearRegression::new(false, 0.05, 500, 1e-4).unwrap();
    assert!(!model.get_fit_intercept());
    assert_abs_diff_eq!(model.get_learning_rate(), 0.05, epsilon = 1e-15);
    assert_eq!(model.get_max_iterations(), 500);
    assert_abs_diff_eq!(model.get_tolerance(), 1e-4, epsilon = 1e-20);
    assert!(model.get_coefficients().is_none());
    assert!(model.get_intercept().is_none());
    assert!(model.get_actual_iterations().is_none());
}

// NotFitted errors before fit

/// predict() on an unfitted model -> NotFitted
#[test]
fn predict_before_fit_returns_not_fitted() {
    let model = LinearRegression::new(true, 0.01, 100, 1e-6).unwrap();
    let x = array![[1.0, 2.0]];
    let result = model.predict(&x);
    assert!(
        matches!(result, Err(Error::NotFitted(_))),
        "expected NotFitted, got {:?}",
        result
    );
}

// fit() input-validation errors

/// fit() with empty X -> EmptyInput
#[test]
fn fit_empty_x_returns_empty_input() {
    let mut model = LinearRegression::new(true, 0.01, 100, 1e-6).unwrap();
    let x: Array2<f64> = Array2::zeros((0, 2));
    let y: Array1<f64> = Array1::zeros(0);
    let result = model.fit(&x, &y);
    assert!(
        matches!(result, Err(Error::EmptyInput(_))),
        "expected EmptyInput, got {:?}",
        result
    );
}

/// fit() with NaN in X -> NonFinite
#[test]
fn fit_nan_in_x_returns_non_finite() {
    let mut model = LinearRegression::new(true, 0.01, 100, 1e-6).unwrap();
    let x = array![[1.0, f64::NAN], [2.0, 3.0]];
    let y = array![1.0, 2.0];
    let result = model.fit(&x, &y);
    assert!(
        matches!(result, Err(Error::NonFinite(_))),
        "expected NonFinite, got {:?}",
        result
    );
}

/// fit() with Inf in X -> NonFinite
#[test]
fn fit_inf_in_x_returns_non_finite() {
    let mut model = LinearRegression::new(true, 0.01, 100, 1e-6).unwrap();
    let x = array![[1.0, f64::INFINITY], [2.0, 3.0]];
    let y = array![1.0, 2.0];
    let result = model.fit(&x, &y);
    assert!(
        matches!(result, Err(Error::NonFinite(_))),
        "expected NonFinite, got {:?}",
        result
    );
}

/// fit() with mismatched y length -> DimensionMismatch
#[test]
fn fit_y_length_mismatch_returns_dimension_mismatch() {
    let mut model = LinearRegression::new(true, 0.01, 100, 1e-6).unwrap();
    // 3 rows in x but 2 elements in y
    let x = array![[1.0], [2.0], [3.0]];
    let y = array![1.0, 2.0];
    let result = model.fit(&x, &y);
    assert!(
        matches!(result, Err(Error::DimensionMismatch { .. })),
        "expected DimensionMismatch, got {:?}",
        result
    );
}

// predict() input-validation errors (after fit)

/// predict() with empty matrix -> EmptyInput
#[test]
fn predict_empty_matrix_returns_empty_input() {
    let mut model = LinearRegression::new(true, 0.01, 5000, 1e-8).unwrap();
    // train on y = 2x + 1
    let x_train = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y_train = array![3.0, 5.0, 7.0, 9.0, 11.0];
    model.fit(&x_train, &y_train).unwrap();

    let x_empty: Array2<f64> = Array2::zeros((0, 1));
    let result = model.predict(&x_empty);
    assert!(
        matches!(result, Err(Error::EmptyInput(_))),
        "expected EmptyInput, got {:?}",
        result
    );
}

/// predict() with wrong number of columns -> DimensionMismatch
#[test]
fn predict_wrong_feature_count_returns_dimension_mismatch() {
    let mut model = LinearRegression::new(true, 0.01, 5000, 1e-8).unwrap();
    // trained on 1 feature
    let x_train = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y_train = array![3.0, 5.0, 7.0, 9.0, 11.0];
    model.fit(&x_train, &y_train).unwrap();

    // predict with 2 features, mismatched
    let x_wrong = array![[1.0, 2.0]];
    let result = model.predict(&x_wrong);
    assert!(
        matches!(result, Err(Error::DimensionMismatch { .. })),
        "expected DimensionMismatch, got {:?}",
        result
    );
}

/// predict() with NaN in X -> NonFinite
#[test]
fn predict_nan_in_x_returns_non_finite() {
    let mut model = LinearRegression::new(true, 0.01, 5000, 1e-8).unwrap();
    let x_train = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y_train = array![3.0, 5.0, 7.0, 9.0, 11.0];
    model.fit(&x_train, &y_train).unwrap();

    let x_nan = array![[f64::NAN]];
    let result = model.predict(&x_nan);
    assert!(
        matches!(result, Err(Error::NonFinite(_))),
        "expected NonFinite, got {:?}",
        result
    );
}

/// predict() with +Inf in X -> NonFinite
#[test]
fn predict_inf_in_x_returns_non_finite() {
    let mut model = LinearRegression::new(true, 0.01, 5000, 1e-8).unwrap();
    let x_train = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y_train = array![3.0, 5.0, 7.0, 9.0, 11.0];
    model.fit(&x_train, &y_train).unwrap();

    let x_inf = array![[f64::INFINITY]];
    let result = model.predict(&x_inf);
    assert!(
        matches!(result, Err(Error::NonFinite(_))),
        "expected NonFinite, got {:?}",
        result
    );
}

// Correctness: univariate y = 2x + 1
// OLS on x=[1..5], y=[3,5,7,9,11] gives slope 2.0, intercept 1.0

/// After fit, coefficient ~= 2.0 and intercept ~= 1.0 (tight tolerance)
#[test]
fn univariate_y_equals_2x_plus_1_coefficient_and_intercept() {
    // small learning rate and many iterations so gradient descent converges
    let mut model = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];
    model.fit(&x, &y).unwrap();

    // iterative solver reaches the OLS solution (coef 2, intercept 1) to ~1e-3, so assert
    // within iterative-solver tolerance rather than exact-OLS precision
    let coeff = model.get_coefficients().unwrap();
    assert_abs_diff_eq!(coeff[0], 2.0, epsilon = 3e-3);

    let intercept = model.get_intercept().unwrap();
    assert_abs_diff_eq!(intercept, 1.0, epsilon = 3e-3);
}

/// predict on x=6 -> 13.0; predict on x=0 -> 1.0
#[test]
fn univariate_y_equals_2x_plus_1_predictions() {
    let mut model = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];
    model.fit(&x, &y).unwrap();

    // predict x=6 -> 13.0, predict x=0 -> 1.0
    let preds = model.predict(&array![[6.0], [0.0]]).unwrap();
    let expected = array![13.0, 1.0];
    assert_allclose(&preds, &expected, 1e-3);
}

/// After fit, n_iter is set (model ran at least 1 iteration)
#[test]
fn fit_sets_n_iter() {
    let mut model = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];
    model.fit(&x, &y).unwrap();

    let n_iter = model.get_actual_iterations();
    assert!(n_iter.is_some(), "n_iter should be set after fit");
    assert!(n_iter.unwrap() >= 1, "n_iter must be at least 1");
}

// Correctness: multivariate y = 2*x1 + 3*x2 + 1
// Coefficients converge to [2.0, 3.0], intercept to 1.0

/// Multivariate coefficients and intercept converge to known values
#[test]
fn multivariate_y_equals_2x1_plus_3x2_plus_1_coefficients() {
    let mut model = LinearRegression::new(true, 0.01, 20_000, 1e-10).unwrap();
    // 6 training points spanning the feature space
    let x = Array2::from_shape_vec(
        (6, 2),
        vec![
            1.0, 1.0, // y = 2+3+1 = 6
            2.0, 1.0, // y = 4+3+1 = 8
            1.0, 2.0, // y = 2+6+1 = 9
            3.0, 2.0, // y = 6+6+1 = 13
            2.0, 3.0, // y = 4+9+1 = 14
            4.0, 1.0, // y = 8+3+1 = 12
        ],
    )
    .unwrap();
    let y = array![6.0, 8.0, 9.0, 13.0, 14.0, 12.0];
    model.fit(&x, &y).unwrap();

    let coeff = model.get_coefficients().unwrap();
    assert_abs_diff_eq!(coeff[0], 2.0, epsilon = 3e-3);
    assert_abs_diff_eq!(coeff[1], 3.0, epsilon = 3e-3);

    let intercept = model.get_intercept().unwrap();
    assert_abs_diff_eq!(intercept, 1.0, epsilon = 3e-3);
}

/// Multivariate predictions match closed-form y = 2*x1 + 3*x2 + 1
#[test]
fn multivariate_predictions_match_closed_form() {
    let mut model = LinearRegression::new(true, 0.01, 20_000, 1e-10).unwrap();
    let x = Array2::from_shape_vec(
        (6, 2),
        vec![1.0, 1.0, 2.0, 1.0, 1.0, 2.0, 3.0, 2.0, 2.0, 3.0, 4.0, 1.0],
    )
    .unwrap();
    let y = array![6.0, 8.0, 9.0, 13.0, 14.0, 12.0];
    model.fit(&x, &y).unwrap();

    // predict at (1,1) -> 6.0, (2,3) -> 14.0
    let x_new = array![[1.0, 1.0], [2.0, 3.0]];
    let preds = model.predict(&x_new).unwrap();
    let expected = array![6.0, 14.0];
    assert_allclose(&preds, &expected, 5e-3);
}

// fit_intercept = false
// Data y = 2x through the origin: OLS slope 2.0, stored intercept 0.0 by contract

/// With fit_intercept=false the stored intercept is exactly 0.0
#[test]
fn no_intercept_stored_intercept_is_zero() {
    let mut model = LinearRegression::new(false, 0.01, 10_000, 1e-10).unwrap();
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    // y = 2x, passes through the origin
    let y = array![2.0, 4.0, 6.0, 8.0, 10.0];
    model.fit(&x, &y).unwrap();

    let intercept = model.get_intercept().unwrap();
    assert_abs_diff_eq!(intercept, 0.0, epsilon = 1e-15);
}

/// With fit_intercept=false the coefficient converges to slope ~= 2.0
#[test]
fn no_intercept_coefficient_converges_to_slope() {
    let mut model = LinearRegression::new(false, 0.01, 10_000, 1e-10).unwrap();
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![2.0, 4.0, 6.0, 8.0, 10.0];
    model.fit(&x, &y).unwrap();

    let coeff = model.get_coefficients().unwrap();
    assert_abs_diff_eq!(coeff[0], 2.0, epsilon = 1e-4);
}

/// With fit_intercept=false get_fit_intercept() returns false
#[test]
fn no_intercept_getter_returns_false() {
    let model = LinearRegression::new(false, 0.01, 1000, 1e-6).unwrap();
    assert!(!model.get_fit_intercept());
}

// Analytic OLS sanity: y = 3x + 2
// OLS on x=[1..5], y=[5,8,11,14,17] gives slope 3.0, intercept 2.0

/// OLS converges to slope=3.0, intercept=2.0 on y=3x+2
#[test]
fn ols_sanity_y_equals_3x_plus_2_parameters() {
    let mut model = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![5.0, 8.0, 11.0, 14.0, 17.0];
    model.fit(&x, &y).unwrap();

    let coeff = model.get_coefficients().unwrap();
    // analytic slope = 3.0 (gradient descent converges to ~1e-3)
    assert_abs_diff_eq!(coeff[0], 3.0, epsilon = 3e-3);

    // analytic intercept = 2.0
    let intercept = model.get_intercept().unwrap();
    assert_abs_diff_eq!(intercept, 2.0, epsilon = 3e-3);
}

/// OLS prediction at x=6 -> 20.0, x=10 -> 32.0 on y=3x+2
#[test]
fn ols_sanity_y_equals_3x_plus_2_predictions() {
    let mut model = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![5.0, 8.0, 11.0, 14.0, 17.0];
    model.fit(&x, &y).unwrap();

    let preds = model.predict(&array![[6.0], [10.0]]).unwrap();
    let expected = array![20.0, 32.0];
    assert_allclose(&preds, &expected, 3e-3);
}

// fit_predict matches separate fit + predict

/// fit_predict() returns the same predictions as fit() + predict() on training data
#[test]
fn fit_predict_matches_fit_then_predict() {
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];

    let mut model_a = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    let preds_a = model_a.fit_predict(&x, &y).unwrap();

    let mut model_b = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    model_b.fit(&x, &y).unwrap();
    let preds_b = model_b.predict(&x).unwrap();

    assert_allclose(&preds_a, &preds_b, 1e-12);
}

/// fit_predict() predictions match known true values from y=2x+1
#[test]
fn fit_predict_values_match_known_true_values() {
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];

    let mut model = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    let preds = model.fit_predict(&x, &y).unwrap();

    // Trained on exactly this data, so predictions on it closely match y = 2x + 1
    let expected = array![3.0, 5.0, 7.0, 9.0, 11.0];
    assert_allclose(&preds, &expected, 5e-3);
}

// Regularization: L2 shrinks the coefficient
// Ridge alpha > 0 makes |w_ridge| strictly smaller than |w_ols|; only that inequality is asserted

/// L2 regularization shrinks the L2-norm of coefficients below the unregularized value
#[test]
fn l2_regularization_shrinks_coefficient_norm() {
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];

    let mut unregularized = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    unregularized.fit(&x, &y).unwrap();
    let coeff_unreg = unregularized.get_coefficients().unwrap()[0].abs();

    let mut ridge = LinearRegression::new(true, 0.01, 10_000, 1e-10)
        .unwrap()
        .with_regularization(RegularizationType::L2(5.0))
        .unwrap();
    ridge.fit(&x, &y).unwrap();
    let coeff_ridge = ridge.get_coefficients().unwrap()[0].abs();

    assert!(
        coeff_ridge < coeff_unreg,
        "Ridge coefficient {coeff_ridge} should be smaller than unregularized {coeff_unreg}"
    );
}

// Regularization: L1 shrinks the coefficient
// Lasso alpha drives the coefficient toward zero, so |w_lasso| < |w_ols|

/// L1 regularization shrinks the coefficient below the unregularized value
#[test]
fn l1_regularization_shrinks_coefficient() {
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];

    let mut unregularized = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    unregularized.fit(&x, &y).unwrap();
    let coeff_unreg = unregularized.get_coefficients().unwrap()[0].abs();

    let mut lasso = LinearRegression::new(true, 0.01, 10_000, 1e-10)
        .unwrap()
        .with_regularization(RegularizationType::L1(5.0))
        .unwrap();
    lasso.fit(&x, &y).unwrap();
    let coeff_lasso = lasso.get_coefficients().unwrap()[0].abs();

    assert!(
        coeff_lasso < coeff_unreg,
        "Lasso coefficient {coeff_lasso} should be smaller than unregularized {coeff_unreg}"
    );
}

/// With fit_intercept=true and moderate L2 alpha the intercept stays close to 1.0
#[test]
fn l2_regularization_intercept_within_reasonable_range() {
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];

    let mut ridge = LinearRegression::new(true, 0.01, 10_000, 1e-10)
        .unwrap()
        .with_regularization(RegularizationType::L2(0.1))
        .unwrap();
    ridge.fit(&x, &y).unwrap();

    // With small alpha=0.1 the intercept stays close to 1.0 (within +/- 0.5)
    let intercept = ridge.get_intercept().unwrap();
    assert!(
        (intercept - 1.0).abs() < 0.5,
        "Intercept {intercept} deviates too far from 1.0 under weak L2 regularization"
    );
}

// Determinism: same data -> identical results
// Gradient descent has no internal randomness, so identical runs are bit-identical

/// Two identical LinearRegression models trained on the same data produce identical predictions
#[test]
fn determinism_same_data_identical_predictions() {
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];
    let x_test = array![[6.0], [7.0], [8.0]];

    let mut model_a = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    model_a.fit(&x, &y).unwrap();
    let preds_a = model_a.predict(&x_test).unwrap();

    let mut model_b = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    model_b.fit(&x, &y).unwrap();
    let preds_b = model_b.predict(&x_test).unwrap();

    // Gradient descent is deterministic, so results must be bit-identical
    assert_allclose(&preds_a, &preds_b, 0.0);
}

// Save / load round-trip

/// save_to_path + load_from_path round-trip yields identical predictions
#[test]
fn save_load_round_trip_identical_predictions() {
    let x_train = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y_train = array![3.0, 5.0, 7.0, 9.0, 11.0];
    let x_test = array![[6.0], [7.0], [0.5]];

    let mut model = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    model.fit(&x_train, &y_train).unwrap();
    let preds_before = model.predict(&x_test).unwrap();

    // Tmp path unique to this test to avoid collisions
    let path = "/tmp/rustyml_linear_regression_test_round_trip.json";
    model.save_to_path(path).unwrap();

    let loaded = LinearRegression::load_from_path(path).unwrap();
    let preds_after = loaded.predict(&x_test).unwrap();

    assert_allclose(&preds_before, &preds_after, 0.0);

    let _ = std::fs::remove_file(path);
}

/// After load, getter values match those of the original model
#[test]
fn save_load_preserves_model_state() {
    let x_train = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y_train = array![3.0, 5.0, 7.0, 9.0, 11.0];

    let mut model = LinearRegression::new(false, 0.005, 8_000, 1e-9).unwrap();
    model.fit(&x_train, &y_train).unwrap();

    let path = "/tmp/rustyml_linear_regression_test_state.json";
    model.save_to_path(path).unwrap();
    let loaded = LinearRegression::load_from_path(path).unwrap();

    assert_eq!(loaded.get_fit_intercept(), model.get_fit_intercept());
    assert_abs_diff_eq!(
        loaded.get_learning_rate(),
        model.get_learning_rate(),
        epsilon = 1e-15
    );
    assert_eq!(loaded.get_max_iterations(), model.get_max_iterations());

    // Coefficients must survive the round-trip exactly
    let orig_coeff = model.get_coefficients().unwrap();
    let load_coeff = loaded.get_coefficients().unwrap();
    assert_allclose(orig_coeff, load_coeff, 0.0);

    let _ = std::fs::remove_file(path);
}

// Default constructor

/// Default model has fit_intercept=true, learning_rate=0.01, max_iter=1000, tol=1e-5
#[test]
fn default_constructor_has_expected_hyperparameters() {
    let model = LinearRegression::default();
    assert!(model.get_fit_intercept());
    assert_abs_diff_eq!(model.get_learning_rate(), 0.01, epsilon = 1e-15);
    assert_eq!(model.get_max_iterations(), 1000);
    assert_abs_diff_eq!(model.get_tolerance(), 1e-5, epsilon = 1e-20);
    assert!(model.get_coefficients().is_none());
    assert!(model.get_intercept().is_none());
    assert!(model.get_actual_iterations().is_none());
}

/// Default model can be fit and predict without error
#[test]
fn default_constructor_can_fit_and_predict() {
    let mut model = LinearRegression::default();
    let x = array![[1.0], [2.0], [3.0]];
    let y = array![3.0, 5.0, 7.0];
    model.fit(&x, &y).unwrap();
    let preds = model.predict(&array![[4.0]]).unwrap();
    // y = 2x + 1, predict(4) ~= 9.0
    assert_abs_diff_eq!(preds[0], 9.0, epsilon = 5e-2);
}

// Clone

/// Clone of a fitted model makes identical predictions
#[test]
fn clone_of_fitted_model_makes_identical_predictions() {
    let x_train = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y_train = array![3.0, 5.0, 7.0, 9.0, 11.0];
    let x_test = array![[6.0], [0.0]];

    let mut model = LinearRegression::new(true, 0.01, 10_000, 1e-10).unwrap();
    model.fit(&x_train, &y_train).unwrap();
    let preds_orig = model.predict(&x_test).unwrap();

    let cloned = model.clone();
    let preds_clone = cloned.predict(&x_test).unwrap();

    assert_allclose(&preds_orig, &preds_clone, 0.0);
}

// In-loop NonFinite divergence guards
// A huge learning_rate overshoots to +/- inf inside the loop, tripping the in-loop finiteness guard

/// fit() on finite, valid data but with a huge learning_rate diverges to +/- inf inside the
/// gradient-descent loop -> Error::NonFinite (in-loop guard, not the up-front X check)
#[test]
fn fit_huge_learning_rate_diverges_returns_non_finite() {
    // learning_rate = 1e8 is positive and finite, so the constructor accepts it
    let mut model = LinearRegression::new(true, 1e8, 1000, 1e-10).unwrap();

    // clean, finite data: y = 2x + 1 on x = [1..5]
    let x = array![[1.0], [2.0], [3.0], [4.0], [5.0]];
    let y = array![3.0, 5.0, 7.0, 9.0, 11.0];

    let result = model.fit(&x, &y);
    assert!(
        matches!(result, Err(Error::NonFinite(_))),
        "expected NonFinite from in-loop divergence guard, got {:?}",
        result
    );
}

// L1 regularization parallel branch (n_features >= 200)
// At 200 features L1 uses rayon paths; column 0 carries the signal (y=3*x0), the rest is noise

/// L1 regularization with >=200 features hits the parallel L1 branch; the one informative
/// feature (column 0, y = 3*x0) ends up with the dominant coefficient over the noise columns
#[test]
fn l1_regularization_parallel_branch_recovers_informative_feature() {
    let n_samples = 12usize;
    let n_features = 200usize; // exactly the parallel threshold (>= 200 triggers the branch)

    // column 0 = centered signal, columns 1.. = tiny noise
    let x = Array2::from_shape_fn((n_samples, n_features), |(i, j)| {
        if j == 0 {
            // centered, varying signal in [-5.5, 5.5]
            (i as f64) - 5.5
        } else {
            // bounded zig-zag noise in {-0.03, ..., 0.03}, uncorrelated with y
            0.01 * (((i * 31 + j * 17) % 7) as f64 - 3.0)
        }
    });

    // y depends only on column 0: y = 3 * x0 (no intercept needed)
    let y = Array1::from_shape_fn(n_samples, |i| 3.0 * ((i as f64) - 5.5));

    // weak L1 so the dominant coefficient is shrunk only slightly; fit_intercept = false
    let mut model = LinearRegression::new(false, 0.01, 20_000, 1e-12)
        .unwrap()
        .with_regularization(RegularizationType::L1(1e-3))
        .unwrap();
    model
        .fit(&x, &y)
        .expect("fit with >=200 features and L1 should succeed");

    let coeffs = model.get_coefficients().unwrap();
    assert_eq!(
        coeffs.len(),
        n_features,
        "coefficient vector length must equal feature count"
    );

    let c0 = coeffs[0];
    // (a) the informative coefficient is substantial and positive (true slope is 3.0)
    assert!(
        c0 > 1.0,
        "informative coefficient[0] = {c0} should be a large positive value (true slope 3.0)"
    );

    // (b) every noise coefficient stays small; the 0.5 bound sits above what the bounded
    // (|x_j| <= 0.03) noise columns can earn yet far below coefficient[0]
    let max_other = coeffs
        .iter()
        .skip(1)
        .fold(0.0_f64, |acc, &w| acc.max(w.abs()));
    assert!(
        max_other < 0.5,
        "uninformative coefficients should stay small; largest |other| = {max_other}"
    );

    // (c) coefficient[0] strictly dominates every other |coefficient|
    assert!(
        c0.abs() > max_other,
        "|coefficient[0]| = {} should dominate the largest other |coefficient| = {}",
        c0.abs(),
        max_other
    );
}