use crate::matrix::Matrix;
use crate::matrix::{FloatMatrix, SeriesOps};
use rand::rng; // Changed from rand::thread_rng
use rand::seq::SliceRandom;

pub struct KMeans {
    pub centroids: Matrix<f64>, // (k, n_features)
}

impl KMeans {
    /// Fit with k clusters.
    pub fn fit(x: &Matrix<f64>, k: usize, max_iter: usize, tol: f64) -> (Self, Vec<usize>) {
        let m = x.rows();
        let n = x.cols();
        assert!(k <= m, "k must be ≤ number of samples");

        // ----- initialise centroids -----
        let mut centroids = Matrix::zeros(k, n);
        if k == 1 {
            // For k=1, initialize the centroid to the mean of the data
            for j in 0..n {
                centroids[(0, j)] = x.column(j).iter().sum::<f64>() / m as f64;
            }
        } else {
            // For k > 1, pick k distinct rows at random
            let mut rng = rng(); // Changed from thread_rng()
            let mut indices: Vec<usize> = (0..m).collect();
            indices.shuffle(&mut rng);
            for (c, &i) in indices[..k].iter().enumerate() {
                for j in 0..n {
                    centroids[(c, j)] = x[(i, j)];
                }
            }
        }

        let mut labels = vec![0usize; m];
        let mut old_centroids = centroids.clone(); // Store initial centroids for first iteration's convergence check

        for _iter in 0..max_iter {
            // Renamed loop variable to _iter for clarity
            // ----- assignment step -----
            let mut changed = false;
            for i in 0..m {
                let sample_row = x.row(i);
                let sample_matrix = FloatMatrix::from_rows_vec(sample_row, 1, n);

                let mut best = 0usize;
                let mut best_dist_sq = f64::MAX;

                for c in 0..k {
                    let centroid_row = old_centroids.row(c); // Use old_centroids for distance calculation
                    let centroid_matrix = FloatMatrix::from_rows_vec(centroid_row, 1, n);

                    let diff = &sample_matrix - &centroid_matrix;
                    let sq_diff = &diff * &diff;
                    let dist_sq = sq_diff.sum_horizontal()[0];

                    if dist_sq < best_dist_sq {
                        best_dist_sq = dist_sq;
                        best = c;
                    }
                }
                if labels[i] != best {
                    labels[i] = best;
                    changed = true;
                }
            }

            // ----- update step -----
            let mut counts = vec![0usize; k];
            let mut new_centroids = Matrix::zeros(k, n); // New centroids for this iteration
            for i in 0..m {
                let c = labels[i];
                counts[c] += 1;
                for j in 0..n {
                    new_centroids[(c, j)] += x[(i, j)];
                }
            }
            for c in 0..k {
                if counts[c] > 0 {
                    for j in 0..n {
                        new_centroids[(c, j)] /= counts[c] as f64;
                    }
                }
            }

            // ----- convergence test -----
            if !changed {
                break; // assignments stable
            }
            if tol > 0.0 {
                // optional centroid-shift tolerance
                let diff = &new_centroids - &old_centroids; // Calculate difference between new and old centroids
                let sq_diff = &diff * &diff;
                let shift = sq_diff.data().iter().sum::<f64>().sqrt(); // Sum all squared differences

                if shift < tol {
                    break;
                }
            }
            old_centroids = new_centroids; // Update old_centroids for next iteration
        }
        (
            Self {
                centroids: old_centroids,
            },
            labels,
        ) // Return the final centroids
    }

    /// Predict nearest centroid for each sample.
    pub fn predict(&self, x: &Matrix<f64>) -> Vec<usize> {
        let m = x.rows();
        let k = self.centroids.rows();
        let n = x.cols();

        if m == 0 {
            // Handle empty input matrix
            return Vec::new();
        }

        let mut labels = vec![0usize; m];
        for i in 0..m {
            let sample_row = x.row(i);
            let sample_matrix = FloatMatrix::from_rows_vec(sample_row, 1, n);

            let mut best = 0usize;
            let mut best_dist_sq = f64::MAX;
            for c in 0..k {
                let centroid_row = self.centroids.row(c);
                let centroid_matrix = FloatMatrix::from_rows_vec(centroid_row, 1, n);

                let diff = &sample_matrix - &centroid_matrix;
                let sq_diff = &diff * &diff;
                let dist_sq = sq_diff.sum_horizontal()[0];

                if dist_sq < best_dist_sq {
                    best_dist_sq = dist_sq;
                    best = c;
                }
            }
            labels[i] = best;
        }
        labels
    }
}

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

    fn create_test_data() -> (FloatMatrix, usize) {
        // Simple 2D data for testing K-Means
        // Cluster 1: (1,1), (1.5,1.5)
        // Cluster 2: (5,8), (8,8), (6,7)
        let data = vec![
            1.0, 1.0, // Sample 0
            1.5, 1.5, // Sample 1
            5.0, 8.0, // Sample 2
            8.0, 8.0, // Sample 3
            6.0, 7.0, // Sample 4
        ];
        let x = FloatMatrix::from_rows_vec(data, 5, 2);
        let k = 2;
        (x, k)
    }

    // Helper for single cluster test with exact mean
    fn create_simple_integer_data() -> FloatMatrix {
        // Data points: (1,1), (2,2), (3,3)
        FloatMatrix::from_rows_vec(vec![1.0, 1.0, 2.0, 2.0, 3.0, 3.0], 3, 2)
    }

    #[test]
    fn test_k_means_fit_predict_basic() {
        let (x, k) = create_test_data();
        let max_iter = 100;
        let tol = 1e-6;

        let (kmeans_model, labels) = KMeans::fit(&x, k, max_iter, tol);

        // Assertions for fit
        assert_eq!(kmeans_model.centroids.rows(), k);
        assert_eq!(kmeans_model.centroids.cols(), x.cols());
        assert_eq!(labels.len(), x.rows());

        // Check if labels are within expected range (0 to k-1)
        for &label in &labels {
            assert!(label < k);
        }

        // Predict with the same data
        let predicted_labels = kmeans_model.predict(&x);

        // The exact labels might vary due to random initialization,
        // but the clustering should be consistent.
        // We expect two clusters. Let's check if samples 0,1 are in one cluster
        // and samples 2,3,4 are in another.
        let cluster_0_members = vec![labels[0], labels[1]];
        let cluster_1_members = vec![labels[2], labels[3], labels[4]];

        // All members of cluster 0 should have the same label
        assert_eq!(cluster_0_members[0], cluster_0_members[1]);
        // All members of cluster 1 should have the same label
        assert_eq!(cluster_1_members[0], cluster_1_members[1]);
        assert_eq!(cluster_1_members[0], cluster_1_members[2]);
        // The two clusters should have different labels
        assert_ne!(cluster_0_members[0], cluster_1_members[0]);

        // Check predicted labels are consistent with fitted labels
        assert_eq!(labels, predicted_labels);

        // Test with a new sample
        let new_sample_data = vec![1.2, 1.3]; // Should be close to cluster 0
        let new_sample = FloatMatrix::from_rows_vec(new_sample_data, 1, 2);
        let new_sample_label = kmeans_model.predict(&new_sample)[0];
        assert_eq!(new_sample_label, cluster_0_members[0]);

        let new_sample_data_2 = vec![7.0, 7.5]; // Should be close to cluster 1
        let new_sample_2 = FloatMatrix::from_rows_vec(new_sample_data_2, 1, 2);
        let new_sample_label_2 = kmeans_model.predict(&new_sample_2)[0];
        assert_eq!(new_sample_label_2, cluster_1_members[0]);
    }

    #[test]
    fn test_k_means_fit_k_equals_m() {
        // Test case where k (number of clusters) equals m (number of samples)
        let (x, _) = create_test_data(); // 5 samples
        let k = 5; // 5 clusters
        let max_iter = 10;
        let tol = 1e-6;

        let (kmeans_model, labels) = KMeans::fit(&x, k, max_iter, tol);

        assert_eq!(kmeans_model.centroids.rows(), k);
        assert_eq!(labels.len(), x.rows());

        // Each sample should be its own cluster, so labels should be unique
        let mut sorted_labels = labels.clone();
        sorted_labels.sort_unstable();
        assert_eq!(sorted_labels, vec![0, 1, 2, 3, 4]);
    }

    #[test]
    #[should_panic(expected = "k must be ≤ number of samples")]
    fn test_k_means_fit_k_greater_than_m() {
        let (x, _) = create_test_data(); // 5 samples
        let k = 6; // k > m
        let max_iter = 10;
        let tol = 1e-6;

        let (_kmeans_model, _labels) = KMeans::fit(&x, k, max_iter, tol);
    }

    #[test]
    fn test_k_means_fit_single_cluster() {
        // Test with k=1
        let x = create_simple_integer_data(); // Use integer data
        let k = 1;
        let max_iter = 100;
        let tol = 1e-6; // Reset tolerance

        let (kmeans_model, labels) = KMeans::fit(&x, k, max_iter, tol);

        assert_eq!(kmeans_model.centroids.rows(), 1);
        assert_eq!(labels.len(), x.rows());

        // All labels should be 0
        assert!(labels.iter().all(|&l| l == 0));

        // Centroid should be the mean of all data points
        let expected_centroid_x = x.column(0).iter().sum::<f64>() / x.rows() as f64;
        let expected_centroid_y = x.column(1).iter().sum::<f64>() / x.rows() as f64;

        // Relax the assertion tolerance to match the algorithm's convergence tolerance
        assert!((kmeans_model.centroids[(0, 0)] - expected_centroid_x).abs() < 1e-6);
        assert!((kmeans_model.centroids[(0, 1)] - expected_centroid_y).abs() < 1e-6);
    }

    #[test]
    fn test_k_means_predict_empty_matrix() {
        let (x, k) = create_test_data();
        let max_iter = 10;
        let tol = 1e-6;
        let (kmeans_model, _labels) = KMeans::fit(&x, k, max_iter, tol);

        // Create a 0x0 matrix. This is allowed by Matrix constructor.
        let empty_x = FloatMatrix::from_rows_vec(vec![], 0, 0);
        let predicted_labels = kmeans_model.predict(&empty_x);
        assert!(predicted_labels.is_empty());
    }

    #[test]
    fn test_k_means_predict_single_sample() {
        let (x, k) = create_test_data();
        let max_iter = 10;
        let tol = 1e-6;
        let (kmeans_model, _labels) = KMeans::fit(&x, k, max_iter, tol);

        let single_sample = FloatMatrix::from_rows_vec(vec![1.1, 1.2], 1, 2);
        let predicted_label = kmeans_model.predict(&single_sample);
        assert_eq!(predicted_label.len(), 1);
        assert!(predicted_label[0] < k);
    }
}