/*
 * Copyright 2012-2023 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
 *
 *      https://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.actuate.metrics;

import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.TreeSet;
import java.util.function.BiFunction;

import io.micrometer.core.instrument.Meter;
import io.micrometer.core.instrument.MeterRegistry;
import io.micrometer.core.instrument.Statistic;
import io.micrometer.core.instrument.Tag;
import io.micrometer.core.instrument.composite.CompositeMeterRegistry;

import org.springframework.boot.actuate.endpoint.InvalidEndpointRequestException;
import org.springframework.boot.actuate.endpoint.OperationResponseBody;
import org.springframework.boot.actuate.endpoint.annotation.Endpoint;
import org.springframework.boot.actuate.endpoint.annotation.ReadOperation;
import org.springframework.boot.actuate.endpoint.annotation.Selector;
import org.springframework.lang.Nullable;

/**
 * An {@link Endpoint @Endpoint} for exposing the metrics held by a {@link MeterRegistry}.
 *
 * @author Jon Schneider
 * @author Phillip Webb
 * @since 2.0.0
 */
@Endpoint(id = "metrics")
public class MetricsEndpoint {

	private final MeterRegistry registry;

	public MetricsEndpoint(MeterRegistry registry) {
		this.registry = registry;
	}

	@ReadOperation
	public MetricNamesDescriptor listNames() {
		Set<String> names = new TreeSet<>();
		collectNames(names, this.registry);
		return new MetricNamesDescriptor(names);
	}

	private void collectNames(Set<String> names, MeterRegistry registry) {
		if (registry instanceof CompositeMeterRegistry compositeMeterRegistry) {
			compositeMeterRegistry.getRegistries().forEach((member) -> collectNames(names, member));
		}
		else {
			registry.getMeters().stream().map(this::getName).forEach(names::add);
		}
	}

	private String getName(Meter meter) {
		return meter.getId().getName();
	}

	@ReadOperation
	public MetricDescriptor metric(@Selector String requiredMetricName, @Nullable List<String> tag) {
		List<Tag> tags = parseTags(tag);
		Collection<Meter> meters = findFirstMatchingMeters(this.registry, requiredMetricName, tags);
		if (meters.isEmpty()) {
			return null;
		}
		Map<Statistic, Double> samples = getSamples(meters);
		Map<String, Set<String>> availableTags = getAvailableTags(meters);
		tags.forEach((t) -> availableTags.remove(t.getKey()));
		Meter.Id meterId = meters.iterator().next().getId();
		return new MetricDescriptor(requiredMetricName, meterId.getDescription(), meterId.getBaseUnit(),
				asList(samples, Sample::new), asList(availableTags, AvailableTag::new));
	}

	private List<Tag> parseTags(List<String> tags) {
		return (tags != null) ? tags.stream().map(this::parseTag).toList() : Collections.emptyList();
	}

	private Tag parseTag(String tag) {
		String[] parts = tag.split(":", 2);
		if (parts.length != 2) {
			throw new InvalidEndpointRequestException(
					"Each tag parameter must be in the form 'key:value' but was: " + tag,
					"Each tag parameter must be in the form 'key:value'");
		}
		return Tag.of(parts[0], parts[1]);
	}

	private Collection<Meter> findFirstMatchingMeters(MeterRegistry registry, String name, Iterable<Tag> tags) {
		if (registry instanceof CompositeMeterRegistry compositeMeterRegistry) {
			return findFirstMatchingMeters(compositeMeterRegistry, name, tags);
		}
		return registry.find(name).tags(tags).meters();
	}

	private Collection<Meter> findFirstMatchingMeters(CompositeMeterRegistry composite, String name,
			Iterable<Tag> tags) {
		return composite.getRegistries()
			.stream()
			.map((registry) -> findFirstMatchingMeters(registry, name, tags))
			.filter((matching) -> !matching.isEmpty())
			.findFirst()
			.orElse(Collections.emptyList());
	}

	private Map<Statistic, Double> getSamples(Collection<Meter> meters) {
		Map<Statistic, Double> samples = new LinkedHashMap<>();
		meters.forEach((meter) -> mergeMeasurements(samples, meter));
		return samples;
	}

	private void mergeMeasurements(Map<Statistic, Double> samples, Meter meter) {
		meter.measure()
			.forEach((measurement) -> samples.merge(measurement.getStatistic(), measurement.getValue(),
					mergeFunction(measurement.getStatistic())));
	}

	private BiFunction<Double, Double, Double> mergeFunction(Statistic statistic) {
		return Statistic.MAX.equals(statistic) ? Double::max : Double::sum;
	}

	private Map<String, Set<String>> getAvailableTags(Collection<Meter> meters) {
		Map<String, Set<String>> availableTags = new HashMap<>();
		meters.forEach((meter) -> mergeAvailableTags(availableTags, meter));
		return availableTags;
	}

	private void mergeAvailableTags(Map<String, Set<String>> availableTags, Meter meter) {
		meter.getId().getTags().forEach((tag) -> {
			Set<String> value = Collections.singleton(tag.getValue());
			availableTags.merge(tag.getKey(), value, this::merge);
		});
	}

	private <T> Set<T> merge(Set<T> set1, Set<T> set2) {
		Set<T> result = new HashSet<>(set1.size() + set2.size());
		result.addAll(set1);
		result.addAll(set2);
		return result;
	}

	private <K, V, T> List<T> asList(Map<K, V> map, BiFunction<K, V, T> mapper) {
		return map.entrySet().stream().map((entry) -> mapper.apply(entry.getKey(), entry.getValue())).toList();
	}

	/**
	 * Description of metric names.
	 */
	public static final class MetricNamesDescriptor implements OperationResponseBody {

		private final Set<String> names;

		MetricNamesDescriptor(Set<String> names) {
			this.names = names;
		}

		public Set<String> getNames() {
			return this.names;
		}

	}

	/**
	 * Description of a metric.
	 */
	public static final class MetricDescriptor implements OperationResponseBody {

		private final String name;

		private final String description;

		private final String baseUnit;

		private final List<Sample> measurements;

		private final List<AvailableTag> availableTags;

		MetricDescriptor(String name, String description, String baseUnit, List<Sample> measurements,
				List<AvailableTag> availableTags) {
			this.name = name;
			this.description = description;
			this.baseUnit = baseUnit;
			this.measurements = measurements;
			this.availableTags = availableTags;
		}

		public String getName() {
			return this.name;
		}

		public String getDescription() {
			return this.description;
		}

		public String getBaseUnit() {
			return this.baseUnit;
		}

		public List<Sample> getMeasurements() {
			return this.measurements;
		}

		public List<AvailableTag> getAvailableTags() {
			return this.availableTags;
		}

	}

	/**
	 * A set of tags for further dimensional drill-down and their potential values.
	 */
	public static final class AvailableTag {

		private final String tag;

		private final Set<String> values;

		AvailableTag(String tag, Set<String> values) {
			this.tag = tag;
			this.values = values;
		}

		public String getTag() {
			return this.tag;
		}

		public Set<String> getValues() {
			return this.values;
		}

	}

	/**
	 * A measurement sample combining a {@link Statistic statistic} and a value.
	 */
	public static final class Sample {

		private final Statistic statistic;

		private final Double value;

		Sample(Statistic statistic, Double value) {
			this.statistic = statistic;
			this.value = value;
		}

		public Statistic getStatistic() {
			return this.statistic;
		}

		public Double getValue() {
			return this.value;
		}

		@Override
		public String toString() {
			return "MeasurementSample{statistic=" + this.statistic + ", value=" + this.value + '}';
		}

	}

}
