walkmc.WeightedDice.kt Maven / Gradle / Ivy

Go to download
package walkmc

import walkmc.collections.*
import walkmc.extensions.numbers.*

/**
 * Assigns a probabilty to each distinct [T] item, and randomly selects [T] values given those probabilities.
 *
 * In other words, this is a Probability Density Function (PDF) for discrete [T] values
 */
open class WeightedDice(probabilities: Map) {
	constructor(vararg values: Pair) : this(values.toMap())
	
	/**
	 * The sum of all probabilities.
	 */
	private val sum = probabilities.values.sum()
	
	/**
	 * The distribution of the probabilities.
	 */
	val distribution = with(probabilities) {
		var binStart = 0.0
		
		asSequence()
			.sortedBy { it.value }
			.map { it.key to OpenDoubleRange(binStart, it.value + binStart) }
			.onEach { binStart = it.second.endExclusive }
			.toMap()
	}
	
	/**
	 * Randomly selects a [T] value with probability
	 */
	fun roll() = with(randomDouble(0.0..sum)) {
		distribution
			.asIterable()
			.first { this in it.value }
			.key
	}
}

/**
 * Creates a new empty [WeightedDice].
 */
fun  weightedDiceOf() = WeightedDice()

/**
 * Creates a new [WeightedDice] with the given [elements].
 */
fun  weightedDiceOf(vararg elements: Pair) = WeightedDice(*elements)

/**
 * Creates a new [WeightedDice] by this map.
 */
fun  Map.toWeightedDice() = WeightedDice(this)