// Computes the sum (d_sum) and count (d_count) 
//    for each of the k clusters labeled in d_centroids.
// n: number of data points
// d: number of dimensions
// k: number of clusters
// Uses shared memory of 3*k*d
static __global__ void sum_and_count(
    const float *d_data,
    const float *d_centroids,
    float *d_sum,
    int *d_count,
    int n,
    int d,
    int k
);

// Updates each centroid using d_sum and d_count
//    where the index is d * centroid number (out of k).
// d: number of dimensions
// k: number of clusters
static __global__ void update_centroids(
    float *d_centroids,
    const float *d_sum,
    const int *d_count,
    int d,
    int k
);