First version of DBSCAN in Kotlin

dbscanV1.kt

import kotlin.math.*


// First, we need to define, what a 'distance' is
interface Distance<T> {
    fun T.distance(to: T): Double
}

// Now let's define few concrete data classes that will be used to store the data
data class Location(val lat: Double, val lon: Double)
data class Point(val x: Double, val y: Double)


// It is possible to define different distance functions for different data classes
// It is also possible to define different distance functions for the same data class
// For example, we could define full blown 'geodesic' distance for locations
// I tried to reproduce something like 'Typeclass' pattern which I would use in Scala/Haskell
val euclideanDistance = object : Distance<Point> {
    override fun Point.distance(to: Point): Double {
        return sqrt((x - to.x).pow(2) + (y - to.y).pow(2))
    }
}

val haversineDistance = object : Distance<Location> {
    override fun Location.distance(to: Location): Double {
        val R = 6371000.0
        val lat1 = Math.toRadians(lat)
        val lat2 = Math.toRadians(to.lat)
        val lon1 = Math.toRadians(lon)
        val lon2 = Math.toRadians(to.lon)

        return 2 * R * asin(sqrt(((lat1 - lat2) / 2).pow(2) + cos(lat1) * cos(lat2) * sin((lon1 - lon2) / 2).pow(2)))
    }
}


// We will only require a 'cluster' to have an id for now
// And let's say we use integers as identifiers
data class Cluster(val id: Int)

// Now we can try to define what types can be 'clustered'
// 'We know how to compute a distance between 2 objects of a type' => 'We can cluster a list of such objects'
interface ClusteringAlgorithm<T, D> {
    fun<D: Distance<T>> fit_transform(xs: List<T>, dist: D): List<Cluster>
}


// I first implemented a minimalistic version of a Matrix to hold pairwise distances,
// but I didn't really use everything from here after all.
// On a bright side, it's easy to add a method to ClusteringAlgorithm that would get
// an arbitrary pairwise distance matrix and skip its own distance calculations
// as it's done in sklearn
interface DummyMatrix<T> {
    fun get(i: Int, j: Int): T
    fun bitMask(f: (item: T) -> Boolean): DummyMatrix<Boolean>
}

open class DummySquareMatrix<T>(val entries: List<T>, val dim: Int): DummyMatrix<T> {
    override fun get(i: Int, j: Int): T {
        return entries[i * dim + j]
    }

    override fun bitMask(f: (item: T) -> Boolean): DummyMatrix<Boolean> {
        return DummySquareMatrix(entries.map(f), dim)
    }
}

class PairwiseDistanceMatrix private constructor(entries: List<Double>, dim: Int): DummySquareMatrix<Double>(entries, dim) {
    companion object {
        operator fun <T, D: Distance<T>> invoke(locs: List<T>, dist: D): PairwiseDistanceMatrix {
            val n = locs.size
            val distances = mutableListOf<Double>()
            for (i in 0 until n) {
                for (j in 0 until n) {
                    distances.add(dist.run { locs[i].distance(locs[j]) })
                }
            }
            return PairwiseDistanceMatrix(distances, n)
        }
    }
}

// Simplified implementation of DBSCAN
// It doesn't take into account edge points
class DBSCAN(val eps: Double, val minPts: Int): ClusteringAlgorithm<Location, Distance<Location>> {
    override fun <D : Distance<Location>> fit_transform(xs: List<Location>, dist: D): List<Cluster> {
        val result = MutableList(xs.size) {Cluster(-1)}
        var c = 0
        val distances = PairwiseDistanceMatrix(xs, dist)
        var neighbors: MutableList<Int>
        for (i in xs.indices) {
            if (result[i].id != -1) {
                continue
            }
            neighbors = mutableListOf()
            for (j in xs.indices) {
                if (distances.get(i, j) < eps) {
                    neighbors.add(j)
                }
            }
            if (neighbors.size < minPts) {
                continue
            }
            c += 1
            for (j in neighbors) {
                if(result[j].id == -1) {
                    result[j] = Cluster(c)
                }
            }
        }
        return result
    }
}


fun main() {
    val a = Point(1.0, 1.0)
    val b = Point(0.0, 0.0)
    val d = euclideanDistance.run { a.distance(b) }
    println(d)

    val loc1 = Location(-34.83333, -58.5166646)
    val loc2 = Location(49.0083899664, 2.53844117956)

    val d2 = haversineDistance.run{ loc1.distance(loc2) } / 1000
    val d3 = haversineDistance.run{ loc2.distance(loc1) } / 1000
    val d4 = haversineDistance.run{ loc1.distance(loc1) } / 1000
    println(d2)
    println(d3)
    println(d4)

    val dataset = listOf<Location>(
        // Groenplaats
        Location(51.219227, 4.401766),
        Location(51.218729, 4.401970),
        Location(51.219162, 4.401155),

        // Sentiance
        Location(51.196732, 4.407988),
        Location(51.196743, 4.408003),
        Location(51.196677, 4.408226),

        // Stadspark
        Location(51.211933, 4.413849),
        Location(51.212721, 4.414198),
        Location(51.212113, 4.414257),
    )

    val dbscan = DBSCAN(150.0, 2)
    val dbscanClusters = dbscan.fit_transform(dataset, haversineDistance)
    println(dbscanClusters.map { it.id })
}