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/**
 * The MIT License
 * Copyright (c) 2014-2016 Ilkka Seppälä
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package com.iluwatar.monad;

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
import java.util.function.Function;
import java.util.function.Predicate;

/**
 * Class representing Monad design pattern. Monad is a way of chaining operations on the
 * given object together step by step. In Validator each step results in either success or
 * failure indicator, giving a way of receiving each of them easily and finally getting
 * validated object or list of exceptions.
 *
 * @param <T> Placeholder for an object.
 */
public class Validator < T > {
    /**
     * Object that is validated
     */
    private final T t;

    /**
     * List of exception thrown during validation.
     */
    private final List < Throwable > exceptions = new ArrayList < > ();

    /**
     * Creates a monadic value of given object.
     * @param t object to be validated
     */
    private Validator(T t) {
        this.t = t;
    }

    /**
     * Creates validator against given object
     *
     * @param t   object to be validated
     * @param <T> object's type
     * @return new instance of a validator
     */
    public static < T > Validator < T > of (T t) {
        return new Validator < > (Objects.requireNonNull(t));
    }

    /**
     * @param validation one argument boolean-valued function that
     *                   represents one step of validation. Adds exception to main validation exception
     *                   list when single step validation ends with failure.
     * @param message    error message when object is invalid
     * @return this
     */
    public Validator < T > validate(Predicate < T > validation, String message) {
        if (!validation.test(t)) {
            exceptions.add(new IllegalStateException(message));
        }
        return this;
    }

    /**
     * Extension for the {@link Validator#validate(Function, Predicate, String)} method,
     * dedicated for objects, that need to be projected before requested validation.
     *
     * @param projection function that gets an objects, and returns projection representing
     *                   element to be validated.
     * @param validation see {@link Validator#validate(Function, Predicate, String)}
     * @param message    see {@link Validator#validate(Function, Predicate, String)}
     * @param <U>        see {@link Validator#validate(Function, Predicate, String)}
     * @return this
     */
    public < U > Validator < T > validate(Function < T, U > projection, Predicate < U > validation,
        String message) {
        return validate(projection.andThen(validation::test)::apply, message);
    }

    /**
     * Receives validated object or throws exception when invalid.
     *
     * @return object that was validated
     * @throws IllegalStateException when any validation step results with failure
     */
    public T get() throws IllegalStateException {
        if (exceptions.isEmpty()) {
            return t;
        }
        IllegalStateException e = new IllegalStateException();
        exceptions.forEach(e::addSuppressed);
        throw e;
    }
}