/* 
 * esodata - data structures and other things, of varying utility
 * Copyright 2022, Ben Culkin
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *   
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
package bjc.funcdata.bst;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.function.Predicate;

import bjc.funcdata.FunctionalList;
import bjc.funcdata.ListEx;

/**
 * A binary search tree, with some mild support for functional traversal.
 *
 * @author ben
 *
 * @param <T>
 *            The data type stored in the node.
 */
public class BinarySearchTree<T> {
	/* The comparator for use in ordering items */
	private final Comparator<T> comparator;

	/* The current count of elements in the tree */
	private int elementCount;

	/* The root element of the tree */
	private TreePart<T> root;

	/**
	 * Create a new tree using the specified way to compare elements.
	 *
	 * @param cmp
	 *            The thing to use for comparing elements
	 */
	public BinarySearchTree(final Comparator<T> cmp) {
		if (cmp == null) throw new NullPointerException("Comparator must not be null");

		elementCount = 0;
		comparator = cmp;
	}

	/**
	 * Add a node to the binary search tree.
	 *
	 * @param element
	 *                The data to add to the binary search tree.
	 */
	public void addNode(final T element) {
		elementCount++;

		if (root == null) root = new BinarySearchTreeNode<>(element, null, null);
		else              root.add(element, comparator);
	}

	/**
	 * Check if an adjusted pivot falls with the bounds of a list.
	 *
	 * @param elements
	 *                        The list to get bounds from.
	 *
	 * @param pivot
	 *                        The pivot.
	 *
	 * @param pivotAdjustment
	 *                        The distance from the pivot.
	 *
	 * @return Whether the adjusted pivot is with the list.
	 */
	private boolean adjustedPivotInBounds(final ListEx<T> elements, final int pivot,
			final int pivotAdjustment) {
		return ((pivot - pivotAdjustment) >= 0)
				&& ((pivot + pivotAdjustment) < elements.getSize());
	}

	/**
	 * Balance the tree, and remove soft-deleted nodes for free.
	 *
	 * Takes O(N) time, but also O(N) space.
	 */
	public void balance() {
		final ListEx<T> elements = new FunctionalList<>();

		/* Add each element to the list in sorted order. */
		root.forEach(TreeLinearizationMethod.INORDER, element -> elements.add(element));

		/* Clear the tree. */
		root = null;

		/* Set up the pivot and adjustment for readding elements. */
		final int pivot = elements.getSize() / 2;
		int pivotAdjustment = 0;

		/* Add elements until there aren't any left. */
		while (adjustedPivotInBounds(elements, pivot, pivotAdjustment)) {
			if (root == null) {
				/* Create a new root element. */
				root = new BinarySearchTreeNode<>(elements.getByIndex(pivot), null, null);
			} else {
				/*
				 * Add the left and right elements in a balanced manner.
				 */
				root.add(elements.getByIndex(pivot + pivotAdjustment), comparator);
				root.add(elements.getByIndex(pivot - pivotAdjustment), comparator);
			}

			/* Increase the distance from the pivot. */
			pivotAdjustment++;
		}

		/* Add any trailing unbalanced elements. */
		if (pivot - pivotAdjustment >= 0) {
			root.add(elements.getByIndex(pivot - pivotAdjustment), comparator);
		} else if (pivot + pivotAdjustment < elements.getSize()) {
			root.add(elements.getByIndex(pivot + pivotAdjustment), comparator);
		}
	}

	/**
	 * Soft-delete a node from the tree.
	 *
	 * Soft-deleted nodes stay in the tree until trim()/balance() is invoked, and
	 * are not included in traversals/finds.
	 *
	 * @param element
	 *                The node to delete
	 */
	public void deleteNode(final T element) {
		elementCount--;

		root.delete(element, comparator);
	}

	/**
	 * Get the root of the tree.
	 *
	 * @return The root of the tree.
	 */
	public TreePart<T> getRoot() {
		return root;
	}

	/**
	 * Check if a node is in the tree.
	 *
	 * @param element
	 *                The node to check the presence of for the tree..
	 *
	 * @return Whether or not the node is in the tree.
	 */
	public boolean isInTree(final T element) {
		return root.contains(element, comparator);
	}

	/**
	 * Traverse the tree in a specified way until the function fails.
	 *
	 * @param linearizationMethod
	 *                            The way to linearize the tree for traversal.
	 *
	 * @param traversalPredicate
	 *                            The function to use until it fails.
	 */
	public void traverse(final TreeLinearizationMethod linearizationMethod,
			final Predicate<T> traversalPredicate) {
		if (linearizationMethod == null) {
			throw new NullPointerException("Linearization method must not be null");
		} else if (traversalPredicate == null) {
			throw new NullPointerException("Predicate must not be nulls");
		}

		root.forEach(linearizationMethod, traversalPredicate);
	}

	/** Remove all soft-deleted nodes from the tree. */
	public void trim() {
		final List<T> nodes = new ArrayList<>(elementCount);

		/*
		 * Add all non-soft deleted nodes to the tree in insertion order.
		 */
		traverse(TreeLinearizationMethod.PREORDER, node -> {
			nodes.add(node);
			
			return true;
		});

		/* Clear the tree. */
		root = null;

		/* Add the nodes to the tree in the order they were inserted. */
		nodes.forEach(node -> addNode(node));
	}

	@Override
	public String toString() {
		return String.format("BinarySearchTree [elementCount=%s, root='%s']",
				elementCount, root);
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + elementCount;
		result = prime * result + (root == null ? 0 : root.hashCode());
		return result;
	}

	@Override
	public boolean equals(final Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (!(obj instanceof BinarySearchTree<?>))
			return false;

		final BinarySearchTree<?> other = (BinarySearchTree<?>) obj;

		if (elementCount != other.elementCount)
			return false;
		if (root == null) {
			if (other.root != null)
				return false;
		} else if (!root.equals(other.root))
			return false;

		return true;
	}
}