package bjc.data;

import java.util.*;
import java.util.function.*;

/**
 * Represents a pair where only one side has a value.
 *
 * @author ben
 *
 * @param <LeftType>
 *                    The type that could be on the left.
 *
 * @param <RightType>
 *                    The type that could be on the right.
 *
 */
public class Either<LeftType, RightType> {
	/**
	 * Create a new either with the left value occupied.
	 *
	 * @param <LeftType>
	 *                    The type of the left value.
	 *
	 * @param <RightType>
	 *                    The type of the empty right value.
	 *
	 * @param left
	 *                    The value to put on the left.
	 *
	 * @return An either with the left side occupied.
	 */
	public static <LeftType, RightType> Either<LeftType, RightType>
			left(final LeftType left) {
		return new Either<>(left, null);
	}

	/**
	 * Create a new either with the right value occupied.
	 *
	 * @param <LeftType>
	 *                    The type of the empty left value.
	 *
	 * @param <RightType>
	 *                    The type of the right value.
	 *
	 * @param right
	 *                    The value to put on the right.
	 *
	 * @return An either with the right side occupied.
	 */
	public static <LeftType, RightType> Either<LeftType, RightType>
			right(final RightType right) {
		return new Either<>(null, right);
	}

	/* The left value of the either. */
	private LeftType leftVal;
	/* The right value of the either. */
	private RightType rightVal;
	/* Whether the left value is the one filled out. */
	private boolean isLeft;

	/* Create a new either with specifed values. */
	private Either(final LeftType left, final RightType right) {
		if (left == null) {
			rightVal = right;
		} else {
			leftVal = left;

			isLeft = true;
		}
	}

	/**
	 * Perform a mapping over this either.
	 * 
	 * @param <NewLeft> The new left type.
	 * @param <NewRight> The new right type.
	 * 
	 * @param leftFunc The function to apply if this is a left either.
	 * @param rightFunc The function to apply if this is a right either.
	 * 
	 * @return A new either, containing a value transformed by the appropriate function.
	 */
	public <NewLeft, NewRight> Either<NewLeft, NewRight> map(
			Function<LeftType, NewLeft> leftFunc,
			Function<RightType, NewRight> rightFunc)
	{
		if (isLeft) return left(leftFunc.apply(leftVal));
		else        return right(rightFunc.apply(rightVal));
	}
	
	/**
	 * Extract the value from this Either.
	 * 
	 * @param <Common> The common type to extract.
	 * 
	 * @param leftHandler The function to handle left-values.
	 * @param rightHandler The function to handle right-values.
	 * 
	 * @return The result of applying the proper function.
	 */
	public <Common> Common extract(
			Function<LeftType, Common> leftHandler,
			Function<RightType, Common> rightHandler)
	{
		if (isLeft) return leftHandler.apply(leftVal);
		else        return rightHandler.apply(rightVal);
	}
	
	/**
	 * Perform an action on this either.
	 * 
	 * @param leftHandler The handler of left values.
	 * @param rightHandler The handler of right values.
	 */
	public void pick(
			Consumer<LeftType> leftHandler, Consumer<RightType> rightHandler)
	{
		if (isLeft) leftHandler.accept(leftVal);
		else        rightHandler.accept(rightVal);
	}

	/**
	 * Check if this either is left-aligned (has the left value filled,
	 * not the right value).
	 * 
	 * @return Whether this either is left-aligned.
	 */
	public boolean isLeft() {
		return isLeft;
	}

	/**
	 * Get the left value of this either if there is one.
	 * 
	 * @return An optional containing the left value, if there is one.
	 */
	public Optional<LeftType> getLeft() {
		return Optional.ofNullable(leftVal);
	}
	
	/**
	 * Get the left value of this either, or get a {@link NoSuchElementException}
	 * if there isn't one.
	 * 
	 * @return The left value of this either.
	 * 
	 * @throws NoSuchElementException If this either doesn't have a left value.
	 */
	public LeftType forceLeft() {
		if (isLeft)
		{
			return leftVal;
		} else
		{
			throw new NoSuchElementException("Either has no left value, is right value");
		}
	}

	/**
	 * Get the right value of this either if there is one.
	 * 
	 * @return An optional containing the right value, if there is one.
	 */
	public Optional<RightType> getRight() {
		return Optional.ofNullable(rightVal);
	}
	
	/**
	 * Get the right value of this either, or get a {@link NoSuchElementException}
	 * if there isn't one.
	 * 
	 * @return The right value of this either.
	 * 
	 * @throws NoSuchElementException If this either doesn't have a right value.
	 */
	public RightType forceRight() {
		if (isLeft)
		{
			throw new NoSuchElementException("Either has no right value, has left value");
		} else
		{
			return rightVal;
		}
	}
	
	// Misc. overrides
	
	@Override
	public int hashCode() {
		return Objects.hash(isLeft, leftVal, rightVal);
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)                  return true;
		if (obj == null)                  return false;
		if (getClass() != obj.getClass()) return false;
		
		Either<?, ?> other = (Either<?, ?>) obj;
		
		return isLeft == other.isLeft
				&& Objects.equals(leftVal, other.leftVal)
				&& Objects.equals(rightVal, other.rightVal);
	}

	@Override
	public String toString() {
		return String.format("Either [leftVal='%s', rightVal='%s', isLeft=%s]", leftVal,
				rightVal, isLeft);
	}
}