/*
 * Copyright 2012-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot.context.properties.source;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.function.Function;
import java.util.stream.IntStream;

import org.springframework.boot.context.properties.source.ConfigurationPropertyName.Form;
import org.springframework.core.env.PropertySource;
import org.springframework.util.StringUtils;

/**
 * {@link PropertyMapper} for system environment variables. Names are mapped by removing
 * invalid characters, converting to lower case and replacing "{@code _}" with
 * "{@code .}". For example, "{@code SERVER_PORT}" is mapped to "{@code server.port}". In
 * addition, numeric elements are mapped to indexes (e.g. "{@code HOST_0}" is mapped to
 * "{@code host[0]}").
 * <p>
 * List shortcuts (names that end with double underscore) are also supported by this
 * mapper. For example, "{@code MY_LIST__=a,b,c}" is mapped to "{@code my.list[0]=a}",
 * "{@code my.list[1]=b}", "{@code my.list[2]=c}".
 *
 * @author Phillip Webb
 * @author Madhura Bhave
 * @see PropertyMapper
 * @see SpringConfigurationPropertySource
 */
final class SystemEnvironmentPropertyMapper implements PropertyMapper {

	public static final PropertyMapper INSTANCE = new SystemEnvironmentPropertyMapper();

	private SystemEnvironmentPropertyMapper() {
	}

	@Override
	public List<PropertyMapping> map(PropertySource<?> propertySource,
			String propertySourceName) {
		ConfigurationPropertyName name = convertName(propertySourceName);
		if (name == null || name.isEmpty()) {
			return Collections.emptyList();
		}
		if (propertySourceName.endsWith("__")) {
			return expandListShortcut(propertySourceName, name,
					propertySource.getProperty(propertySourceName));
		}
		return Collections.singletonList(new PropertyMapping(propertySourceName, name));
	}

	private ConfigurationPropertyName convertName(String propertySourceName) {
		try {
			return ConfigurationPropertyName.adapt(propertySourceName, '_',
					this::processElementValue);
		}
		catch (Exception ex) {
			return null;
		}
	}

	private List<PropertyMapping> expandListShortcut(String propertySourceName,
			ConfigurationPropertyName rootName, Object value) {
		if (value == null) {
			return Collections.emptyList();
		}
		List<PropertyMapping> mappings = new ArrayList<>();
		String[] elements = StringUtils
				.commaDelimitedListToStringArray(String.valueOf(value));
		for (int i = 0; i < elements.length; i++) {
			ConfigurationPropertyName name = rootName.append("[" + i + "]");
			mappings.add(new PropertyMapping(propertySourceName, name,
					new ElementExtractor(i)));
		}
		return mappings;
	}

	@Override
	public List<PropertyMapping> map(PropertySource<?> propertySource,
			ConfigurationPropertyName configurationPropertyName) {
		String name = convertName(configurationPropertyName);
		List<PropertyMapping> result = Collections
				.singletonList(new PropertyMapping(name, configurationPropertyName));
		if (isListShortcutPossible(configurationPropertyName)) {
			result = new ArrayList<>(result);
			result.addAll(mapListShortcut(propertySource, configurationPropertyName));
		}
		return result;
	}

	private String convertName(ConfigurationPropertyName name) {
		return convertName(name, name.getNumberOfElements());
	}

	private String convertName(ConfigurationPropertyName name, int numberOfElements) {
		StringBuilder result = new StringBuilder();
		for (int i = 0; i < numberOfElements; i++) {
			result.append(result.length() == 0 ? "" : "_");
			result.append(name.getElement(i, Form.UNIFORM).toUpperCase());
		}
		return result.toString();
	}

	private boolean isListShortcutPossible(ConfigurationPropertyName name) {
		return (name.isLastElementIndexed() && isNumber(name.getLastElement(Form.UNIFORM))
				&& name.getNumberOfElements() >= 1);
	}

	private List<PropertyMapping> mapListShortcut(PropertySource<?> propertySource,
			ConfigurationPropertyName name) {
		String result = convertName(name, name.getNumberOfElements() - 1) + "__";
		if (propertySource.containsProperty(result)) {
			int index = Integer.parseInt(name.getLastElement(Form.UNIFORM));
			return Collections.singletonList(
					new PropertyMapping(result, name, new ElementExtractor(index)));
		}
		return Collections.emptyList();
	}

	private CharSequence processElementValue(CharSequence value) {
		String result = value.toString().toLowerCase();
		return (isNumber(result) ? "[" + result + "]" : result);
	}

	private static boolean isNumber(String string) {
		IntStream nonDigits = string.chars().filter((c) -> !Character.isDigit(c));
		boolean hasNonDigit = nonDigits.findFirst().isPresent();
		return !hasNonDigit;
	}

	/**
	 * Function used to extract an element from a comma list.
	 */
	private static class ElementExtractor implements Function<Object, Object> {

		private final int index;

		ElementExtractor(int index) {
			this.index = index;
		}

		@Override
		public Object apply(Object value) {
			if (value == null) {
				return null;
			}
			return StringUtils
					.commaDelimitedListToStringArray(value.toString())[this.index];
		}

	}

}
