package au.id.zancanaro.javacheck;

import java.io.IOException;
import java.io.Writer;
import java.util.*;
import java.util.function.Function;
import java.util.function.Predicate;

@SuppressWarnings("unused")
public class ShrinkTree<T> {
    private final T value;
    private final Iterable<ShrinkTree<T>> children;

    public ShrinkTree(T value, Iterable<ShrinkTree<T>> children) {
        this.value = value;
        this.children = children;
    }

    public T getValue() {
        return value;
    }

    public Iterator<ShrinkTree<T>> getChildren() {
        return children.iterator();
    }

    public static <T> ShrinkTree<T> pure(T value) {
        return new ShrinkTree<>(value, Collections.emptyList());
    }

    public static <T> ShrinkTree<T> join(ShrinkTree<ShrinkTree<T>> tree) {
        return new ShrinkTree<>(
                tree.getValue().getValue(),
                () -> Iterators.concat(
                        Iterators.mappingIterator(ShrinkTree::join, tree.children.iterator()),
                        tree.getValue().children.iterator()));
    }

    private static <T> Iterator<ShrinkTree<T>[]> permutations(ShrinkTree<T>[] trees) {
        return Iterators.flatten(
                Iterators.rangeIterator(trees.length,
                        index -> Iterators.mappingIterator(child -> {
                            @SuppressWarnings("unchecked")
                            ShrinkTree<T>[] result = (ShrinkTree<T>[]) new ShrinkTree[trees.length];
                            for (int i = 0; i < trees.length; ++i) {
                                result[i] = (i == index ? child : trees[i]);
                            }
                            return result;
                        }, trees[index].getChildren())
                ));
    }

    private static <T> List<T> makeHeadList(ShrinkTree<T>[] trees) {
        List<T> heads = new ArrayList<>(trees.length);
        for (ShrinkTree<T> tree : trees) {
            heads.add(tree.getValue());
        }
        return heads;
    }

    public static <T, R> ShrinkTree<R> zip(Function<List<T>, R> fn, ShrinkTree<T>[] trees) {
        return new ShrinkTree<>(
                fn.apply(makeHeadList(trees)),
                () -> Iterators.mappingIterator(
                        shrinks -> ShrinkTree.zip(fn, shrinks),
                        ShrinkTree.permutations(trees)));
    }

    private static <T> Iterator<ShrinkTree<T>[]> removeEach(ShrinkTree<T>[] trees) {
        return Iterators.concat(
                Iterators.rangeIterator(trees.length, index -> {
                    @SuppressWarnings("unchecked")
                    ShrinkTree<T>[] result = (ShrinkTree<T>[]) new ShrinkTree[trees.length - 1];
                    for (int i = 0; i < trees.length - 1; ++i) {
                        result[i] = trees[(i >= index ? i + 1 : i)];
                    }
                    return result;
                }),
                permutations(trees));
    }

    public static <T, R> ShrinkTree<R> shrink(Function<List<T>, R> fn, ShrinkTree<T>[] trees) {
        return new ShrinkTree<>(
                fn.apply(makeHeadList(trees)),
                () -> Iterators.mappingIterator(
                        shrinks -> ShrinkTree.shrink(fn, shrinks),
                        ShrinkTree.removeEach(trees)));
    }

    public <R> ShrinkTree<R> map(Function<T, R> f) {
        return new ShrinkTree<>(
                f.apply(this.value),
                () -> Iterators.mappingIterator(tree -> tree.map(f), this.children.iterator()));
    }

    public <R> ShrinkTree<R> flatMap(Function<T, ShrinkTree<R>> f) {
        return ShrinkTree.join(this.map(f));
    }

    public ShrinkTree<T> filter(Predicate<T> predicate) {
        if (predicate.test(this.getValue())) {
            return new ShrinkTree<>(
                    this.getValue(),
                    () -> Iterators.mappingIterator(tree -> tree.filter(predicate),
                            Iterators.filteringIterator(
                                    tree -> predicate.test(tree.getValue()),
                                    this.getChildren())));
        } else {
            throw new IllegalArgumentException("Current value doesn't match predicate: whoops!");
        }
    }

    @SuppressWarnings("unused")
    public void print(Writer output) throws IOException {
        print(output, Object::toString);
    }

    @SuppressWarnings("unused")
    public void print(Writer output, Function<T, String> toString) throws IOException {
        output.write(toString.apply(this.getValue()));
        output.write('[');
        for (ShrinkTree<T> child : children) {
            child.print(output, toString);
        }
        output.write(']');
        output.flush();
    }
}