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package spire
package random

trait Exponential[@sp(Float, Double) A] extends Any {
  /**
   * Return an `A` that has an exponential distribution from 0, with
   * the given `rate` parameter.
   */
  def apply(rate: A): Dist[A]
}

object Exponential extends ExponentialInstances {
  @inline final def apply[@sp(Float, Double) A](implicit e: Exponential[A]): Exponential[A] = e

  def apply[@sp(Float, Double) A](rate: A)(implicit e: Exponential[A]): Dist[A] = e(rate)
}

trait ExponentialInstances {

  implicit val float: Exponential[Float] =
    new Exponential[Float] {
      def apply(rate: Float): Dist[Float] =
        new DistFromGen(g => (Ziggurat.rexp(g) / rate).toFloat)
    }

  implicit val double: Exponential[Double] =
    new Exponential[Double] {
      def apply(rate: Double): Dist[Double] =
        new DistFromGen(g => Ziggurat.rexp(g) / rate)
    }
}