net.jqwik.engine.support.ChooseRandomlyByFrequency Maven / Gradle / Ivy
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package net.jqwik.engine.support;
import java.util.*;
import java.util.function.*;
import net.jqwik.api.*;
import org.jspecify.annotations.*;
public class ChooseRandomlyByFrequency implements Function {
private int[] upperBounds;
private int size = 0;
private List valuesToChooseFrom;
public ChooseRandomlyByFrequency(List> frequencies) {
calculateUpperBorders(frequencies);
if (size <= 0) {
throw new JqwikException(String.format(
"%s does not contain any positive frequencies.",
JqwikStringSupport.displayString(frequencies)
));
}
}
protected List possibleValues() {
return valuesToChooseFrom;
}
private void calculateUpperBorders(List> frequencies) {
List values = new ArrayList<>(frequencies.size());
// Zero-frequency elements are possible, so we start with a list, and transform to array later
List upperBounds = new ArrayList<>(frequencies.size());
for (Tuple.Tuple2 tuple : frequencies) {
int frequency = tuple.get1();
if (frequency <= 0)
continue;
size = Math.addExact(size, frequency);
T value = tuple.get2();
values.add(value);
upperBounds.add(size);
}
valuesToChooseFrom = values;
this.upperBounds = upperBounds.stream().mapToInt(i -> i).toArray();
}
private T choose(int index) {
int i = Arrays.binarySearch(upperBounds, index);
if (i < 0) {
// Exact value not found => convert "negative" insertion point to the actual index
i = -(i + 1);
} else {
// Exact value is found, use the next bucket
// For instance, if weights are {2,3}, the upperBounds are {2,5},
// and the input indices could be 0..4.
// "index" {0, 1} should be mapped to the first element
// "index" {2, 3, 4} should be mapped to the second
// If (input) index==2, binarySearch returns 0, so we must advance i by one
// This advance is never triggered for the very last element since random.nextInt
// excludes upper bound.
i++;
}
return valuesToChooseFrom.get(i);
}
public T apply(Random random) {
return choose(random.nextInt(size));
}
}