src/main/java/au/id/zancanaro/Generators.java


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package au.id.zancanaro;

import java.util.Iterator;
import java.util.function.Predicate;

public class Generators {
    public static <T> Generator<T> suchThat(Generator<T> gen, Predicate<T> pred) {
        return (random, size) -> {
            RoseTree<T> result = gen.generate(random, size);
            if (pred.test(result.getValue())) {
                return result.filter(pred);
            } else {
                return suchThat(gen, pred).generate(random, size);
            }
        };
    }

    public static Generator<Integer> integer() {
        return (random, size) -> {
            int value = random.nextInt(size);
            return new RoseTree<>(value, intShrinkingIterable(value));
        };
    }

    private static Iterable<RoseTree<Integer>> intShrinkingIterable(final int value) {
        return () -> new Iterator<RoseTree<Integer>>() {
            int curr = value;

            @Override
            public boolean hasNext() {
                return curr > 0;
            }

            @Override
            public RoseTree<Integer> next() {
                int prevCurr = curr;
                curr = curr / 2;
                return new RoseTree<>(value - prevCurr, intShrinkingIterable(value - prevCurr));
            }
        };
    }
}