/* 
 * 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.data;

import java.util.function.Function;
import java.util.function.Supplier;
import java.util.function.UnaryOperator;

import bjc.data.internals.BoundLazy;
import bjc.funcdata.FunctionalList;
import bjc.funcdata.ListEx;

/**
 * A holder that holds a means to create a value, but doesn't actually compute
 * the value until it's needed.
 *
 * @author ben
 *
 * @param <ContainedType>
 *                        The type of the value being held.
 */
public class Lazy<ContainedType> implements Holder<ContainedType> {
	/* The supplier of the type. */
	private Supplier<ContainedType> valueSupplier;
	/* The actual type value. */
	private ContainedType heldValue;
	/* Whether the value has been created. */
	private boolean valueMaterialized;

	/* The list of pending actions on the value. */
	private ListEx<UnaryOperator<ContainedType>> actions = new FunctionalList<>();

	/**
	 * Create a new lazy value from the specified seed value.
	 *
	 * @param value
	 *              The seed value to use.
	 */
	public Lazy(final ContainedType value) {
		heldValue = value;

		valueMaterialized = true;
	}

	/**
	 * Create a new lazy value from the specified value source.
	 *
	 * @param supp
	 *             The source of a value to use.
	 */
	public Lazy(final Supplier<ContainedType> supp) {
		valueSupplier = new SingleSupplier<>(supp);

		valueMaterialized = false;
	}

	/* Create a new value from a supplier and a list of actions. */
	private Lazy(final Supplier<ContainedType> supp,
			final ListEx<UnaryOperator<ContainedType>> pendingActions) {
		valueSupplier = supp;

		actions = pendingActions;
	}

	@Override
	public <BoundType> Holder<BoundType>
			bind(final Function<ContainedType, Holder<BoundType>> binder) {
		final ListEx<UnaryOperator<ContainedType>> pendingActions = new FunctionalList<>();

		for (UnaryOperator<ContainedType> action : actions) {
			pendingActions.add(action);
		}
		

		final Supplier<ContainedType> supplier = () -> {
			if (valueMaterialized) return heldValue;
			else                   return valueSupplier.get();
		};

		return new BoundLazy<>(() -> new Lazy<>(supplier, pendingActions), binder);
	}

	@Override
	public <NewType> Function<ContainedType, Holder<NewType>>
			lift(final Function<ContainedType, NewType> func) {
		return val -> new Lazy<>(func.apply(val));
	}

	@Override
	public <MappedType> Holder<MappedType>
			map(final Function<ContainedType, MappedType> mapper) {
		final ListEx<UnaryOperator<ContainedType>> pendingActions = new FunctionalList<>();
		
		for (UnaryOperator<ContainedType> action : actions) {
			pendingActions.add(action);
		}

		return new Lazy<>(() -> {
			ContainedType currVal = heldValue;

			if (!valueMaterialized) {
				currVal = valueSupplier.get();
			}

			return pendingActions.reduceAux(currVal,
					UnaryOperator<ContainedType>::apply,
					value -> mapper.apply(value));
		});
	}

	@Override
	public String toString() {
		if (valueMaterialized) {
			if (actions.isEmpty()) {
				return String.format("value[v='%s']", heldValue);
			} else {
				return String.format("value[v='%s'] (has %d pending transforms)",
						heldValue, actions.getSize());
			}
		}

		if (actions.isEmpty()) {
			return"(unmaterialized)";
		} else {
			return String.format("(unmaterialized; has %d pending transforms",
					actions.getSize());
		}
	}

	@Override
	public Holder<ContainedType>
			transform(final UnaryOperator<ContainedType> transformer) {
		actions.add(transformer);

		return this;
	}

	@Override
	public <UnwrappedType> UnwrappedType
			unwrap(final Function<ContainedType, UnwrappedType> unwrapper) {
		if (!valueMaterialized) {
			heldValue = valueSupplier.get();

			valueMaterialized = true;
		}

		for (UnaryOperator<ContainedType> action : actions) {
			heldValue = action.apply(heldValue);	
		}

		actions = new FunctionalList<>();

		return unwrapper.apply(heldValue);
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;

		result = prime * result + (actions == null ? 0 : actions.hashCode());
		result = prime * result + (heldValue == null ? 0 : heldValue.hashCode());
		result = prime * result + (valueMaterialized ? 1231 : 1237);

		return result;
	}

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

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

		if (valueMaterialized != other.valueMaterialized)
			return false;

		if (valueMaterialized) {
			if (heldValue == null) {
				if (other.heldValue != null) return false;
			} else if (!heldValue.equals(other.heldValue)) {
				return false;
			}
		} else {
			return false;
		}

		if (actions == null) {
			if (other.actions != null) return false;
		} else if (actions.getSize() > 0 || other.actions.getSize() > 0) {
			return false;
		}

		return true;
	}

	/**
	 * Create a new lazy container with an already present value.
	 * 
	 * @param <ContainedType> The type of the contained value.
	 * 
	 * @param val The value for the lazy container.
	 *
	 * @return A new lazy container holding that value.
	 */
	public static <ContainedType> Lazy<ContainedType> lazy(final ContainedType val) {
		return new Lazy<>(val);
	}

	/**
	 * Create a new lazy container with a suspended value.
	 *
	 * @param <ContainedType> The type of the contained value.
	 * 
	 * @param supp The suspended value for the lazy container.
	 *
	 * @return A new lazy container that will un-suspend the value when necessary.
	 */
	public static <ContainedType> Lazy<ContainedType>
			lazy(final Supplier<ContainedType> supp) {
		return new Lazy<>(supp);
	}
}