Generate random integers that follow a normal (Gaussian) distribution

Prng.kt

/**
 * Copyright 2020 John Hunt
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated 
 * documentation files (the "Software"), to deal in the Software without restriction, including without limitation the 
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to 
 * permit persons to whom the Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all copies or substantial portions of 
 * the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE 
 * WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
 * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR 
 * OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

import java.util.Random
import kotlin.math.ceil

/**
 * Pseudo-random number generator
 */
object Prng {
    private val random: Random = Random()

    /**
     * Generates a pseudo-random integer in the given range. Values roughly fit a normal (Gaussian) distribution
     * centered around the midpoint of the range.
     */
    fun randomInRange(lowerBound: Int, upperBound: Int, rounds: Int = 0): Int {
        require(lowerBound >= 0)
        require(upperBound >= lowerBound)

        val stdDevUpDown = random.nextGaussian()
        return when {
            stdDevUpDown >= 3.0 -> { // top 0.13% of all values
                // workaround for tail probability higher than it should be
                if (rounds > 0) upperBound else randomInRange(lowerBound, upperBound, rounds + 1)
            }
            stdDevUpDown <= -3.0 -> { // bottom 0.13% of all values
                // workaround for tail probability higher than it should be
                if (rounds > 0) lowerBound else randomInRange(lowerBound, upperBound, rounds + 1)
            }
            else -> {
                val mid = ((lowerBound - 1 + upperBound).toDouble()) / 2.0
                val stdDev = (upperBound.toDouble() - mid) / 3.0
                val calc = mid + (stdDevUpDown * stdDev)
                ceil(calc).toInt()
            }
        }
    }
}