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Service Provider: A TestEngine that runs specs on JUnit Jupiter
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/**
 * MIT License
 *
 * Copyright (c) 2014–2022 Kyle Krull
 *
 * 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 info.javaspec.engine;

import info.javaspec.api.BehaviorDeclaration;
import info.javaspec.api.JavaSpec;
import info.javaspec.api.SpecClass;
import info.javaspec.api.Verification;
import java.lang.reflect.Constructor;
import java.util.Optional;
import java.util.Stack;
import org.junit.platform.engine.TestDescriptor;
import org.junit.platform.engine.UniqueId;

//Runs JavaSpec declarations in a SpecClass, converting them to TestDescriptors Jupiter can use.
final class SpecClassDeclaration implements JavaSpec {
	private final Class selectedClass;
	private final Stack containersInScope;

	public SpecClassDeclaration(Class selectedClass) {
		this.selectedClass = selectedClass;
		this.containersInScope = new Stack<>();
	}

	// Optional because the Jupiter-selected class often is not a SpecClass
	public Optional run(UniqueId engineId) {
		return Optional.of(this.selectedClass)
			.filter(SpecClass.class::isAssignableFrom)
			.map(this::instantiate)
			.map(SpecClass.class::cast)
			.map(declaringInstance -> this.discover(engineId, declaringInstance));
	}

	private ContextDescriptor discover(UniqueId engineId, SpecClass declaringInstance) {
		enterScope(ContextDescriptor.forDeclaringClass(engineId, declaringInstance.getClass()));
		declaringInstance.declareSpecs(this);
		return exitScope();
	}

	private Object instantiate(Class specClass) {
		try {
			Constructor constructor = specClass.getDeclaredConstructor();
			return constructor.newInstance();
		} catch (Exception e) {
			throw new RuntimeException("Failed to instantiate spec class", e);
		}
	}

	/* Context discovery */

	@Override
	public void describe(Class aClass, BehaviorDeclaration declaration) {
		pushChildContainer(
			declaration,
			parent -> ContextDescriptor.describe(parent.getUniqueId(), aClass.getSimpleName())
		);
	}

	@Override
	public void describe(String what, BehaviorDeclaration declaration) {
		pushChildContainer(
			declaration,
			parent -> ContextDescriptor.describe(parent.getUniqueId(), what)
		);
	}

	@Override
	public void given(String what, BehaviorDeclaration declaration) {
		pushChildContainer(
			declaration,
			parent -> ContextDescriptor.given(parent.getUniqueId(), what)
		);
	}

	private void pushChildContainer(BehaviorDeclaration block, ContainerDescriptorFactory factory) {
		ContextDescriptor current = currentContainer();
		ContextDescriptor child = factory.makeChildContainer(current);
		current.addChild(child);

		enterScope(child);
		block.declare();
		exitScope();
	}

	// Makes a TestDescriptor for a container, as a child of the given parent
	@FunctionalInterface
	private interface ContainerDescriptorFactory {
		ContextDescriptor makeChildContainer(ContextDescriptor parent);
	}

	/* Spec discovery */

	@Override
	public void it(String behavior, Verification verification) {
		addToCurrentContainer(
			container -> SpecDescriptor.of(
				container.getUniqueId(),
				behavior,
				verification
			)
		);
	}

	@Override
	public void pending(String futureBehavior) {
		addToCurrentContainer(
			container -> SkippedSpecDescriptor.pending(
				container.getUniqueId(),
				futureBehavior
			)
		);
	}

	@Override
	public void skip(String intendedBehavior, Verification brokenVerification) {
		addToCurrentContainer(
			container -> SkippedSpecDescriptor.disabled(
				container.getUniqueId(),
				intendedBehavior
			)
		);
	}

	private void addToCurrentContainer(TestDescriptorFactory factory) {
		ContextDescriptor container = currentContainer();
		TestDescriptor specDescriptor = factory.makeTestDescriptor(container);
		container.addChild(specDescriptor);
	}

	// Makes a TestDescriptor for a spec, as a child of the given container
	@FunctionalInterface
	private interface TestDescriptorFactory {
		TestDescriptor makeTestDescriptor(ContextDescriptor parent);
	}

	/* Declaration scope */

	private ContextDescriptor currentContainer() {
		return this.containersInScope.peek();
	}

	private void enterScope(ContextDescriptor container) {
		this.containersInScope.push(container);
	}

	private ContextDescriptor exitScope() {
		return this.containersInScope.pop();
	}
}