package au.id.zancanaro.javacheck;

import java.util.List;
import java.util.Random;
import java.util.function.Function;

/**
 * Generators are a way of producing random objects and their associated shrink
 * trees in a controlled and deterministic way.
 *
 * A generator must implement one method: {@link #generate(Random, int)}. The
 * {@link RoseTree} it produces defines both the value to be returned, as well
 * as the shrink tree for that value.
 *
 * Generators form a "monad", and hence have the {@link #map(Function)} and
 * {@link #flatMap(Function)} methods for composition. The helper methods {@link
 * #pure(Object)}, {@link #tuple(Generator[])} and {@link #list(int, Generator)}
 * allow for composition at a lower level than the monadic actions..
 *
 * @param <T> The type generated by this generator.
 */
public interface Generator<T> {
    /**
     * Return a {@link RoseTree} containing a new random value of the required
     * type, as well as its associated shrink tree.
     *
     * Generators also have an abstract notion of "size". Represented by an
     * integer, generators should use this as a guide for how large an object to
     * generate. There is no specific meaning for this value, so the precise
     * interpretation will depend on the specifics of the generator.
     *
     * @param random The random source for generation purposes
     * @param size   An integer specifying how "big" a thing to produce
     * @return The {@link RoseTree} specifying the generated thing and its
     * shrink tree
     */
    RoseTree<T> generate(Random random, int size);

    /**
     * A generator which simply generates the provided value. Does not shrink.
     *
     * @param value The value to generate
     * @param <T>   The type of the generated value
     * @return A {@link Generator} which generates the provided value
     */
    static <T> Generator<T> pure(T value) {
        return (random, size) -> RoseTree.pure(value);
    }

    /**
     * A generator which generates a {@link List}, with each element taken from
     * its corresponding generator. (That is: return[i] =
     * generators[i].generate(...).)
     *
     * Shrinking for this type involves attempting to shrink each subtree in
     * turn, recursively.
     *
     * @param generators The generators to use for each term in the generated
     *                   result
     * @param <T>        The parameter type of the generated {@link List}
     * @return A {@link Generator} returning a {@link List}
     */
    @SafeVarargs
    static <T> Generator<List<T>> tuple(Generator<T>... generators) {
        return (random, size) -> {
            @SuppressWarnings("unchecked")
            RoseTree<T>[] result = (RoseTree<T>[]) new RoseTree[generators.length];
            int index = 0;
            for (Generator<T> generator : generators) {
                result[index++] = generator.generate(random, size);
            }
            return RoseTree.zip(Function.identity(), result);
        };
    }

    /**
     * A generator which generates a {@link List} of length count, with each
     * element taken from the provided generator.
     *
     * Shrinking for this type involves attempting to remove terms and shrink
     * each subtree in turn, recursively. (If the length must remain fixed then
     * the {@link RoseTree} produced by this generator should be filtered with
     * {@link RoseTree#filter(java.util.function.Predicate)}.
     *
     * @param count     The length of the list to generate
     * @param generator The generator to use for each term in the generated
     *                  result
     * @param <T>       The parameter type of the generated {@link List}
     * @return A {@link Generator} returning a {@link List}
     */
    static <T> Generator<List<T>> list(int count, Generator<T> generator) {
        return (random, size) -> {
            @SuppressWarnings("unchecked")
            RoseTree<T>[] result = (RoseTree<T>[]) new RoseTree[count];
            for (int i = 0; i < count; ++i) {
                result[i] = generator.generate(random, size);
            }
            return RoseTree.shrink(Function.identity(), result);
        };
    }

    /**
     * Transform the result of a generator by passing it through a function.
     *
     * Maps all values in the shrink of this through f, as well.
     *
     * @param f   The transformation function
     * @param <R> The result of the transformation
     * @return A new generator resulting from mapping f over this
     */
    default <R> Generator<R> map(Function<T, R> f) {
        return (random, size) -> this.generate(random, size).map(f);
    }

    /**
     * Produce a new generator relying on the value generated by this generator
     *
     * Shrinking is a bit hard to predict under {@link #flatMap}, as it will
     * first attempt to shrink this, resulting in the re-evaluation of action,
     * and hence the re-generation of the subtree.
     *
     * @param action A function to produce the new generator
     * @param <R>    The type of the returned generator
     * @return A new generator resulting from calling the provided action on the
     * result of this
     */
    default <R> Generator<R> flatMap(Function<T, Generator<R>> action) {
        return (random, size) -> RoseTree.join(this.generate(random, size).map(action).map(g -> g.generate(random, size)));
    }
}