/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the "Elastic License
 * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
 * Public License v 1"; you may not use this file except in compliance with, at
 * your election, the "Elastic License 2.0", the "GNU Affero General Public
 * License v3.0 only", or the "Server Side Public License, v 1".
 */

package org.elasticsearch.search.fetch;

import org.elasticsearch.index.mapper.IgnoredSourceFieldMapper;
import org.elasticsearch.index.mapper.IgnoredSourceFieldMapper.IgnoredSourceFormat;

import java.util.Collection;
import java.util.HashSet;
import java.util.Set;
import java.util.function.Function;

/**
 * Defines which stored fields need to be loaded during a fetch
 * @param requiresSource        should source be loaded
 * @param requiredStoredFields  a set of stored fields to load
 */
public record StoredFieldsSpec(
    boolean requiresSource,
    boolean requiresMetadata,
    Set<String> requiredStoredFields,
    IgnoredSourceFormat ignoredSourceFormat,
    Set<String> sourcePaths
) {
    public StoredFieldsSpec(boolean requiresSource, boolean requiresMetadata, Set<String> requiredStoredFields) {
        this(requiresSource, requiresMetadata, requiredStoredFields, IgnoredSourceFormat.NO_IGNORED_SOURCE, Set.of());
    }

    public boolean noRequirements() {
        return requiresSource == false && requiresMetadata == false && requiredStoredFields.isEmpty() && sourcePaths.isEmpty();
    }

    public boolean onlyRequiresSourcePaths() {
        return requiresSource && requiresMetadata == false && requiredStoredFields.isEmpty() && sourcePaths.isEmpty() == false;
    }

    /**
     * Use when no stored fields are required
     */
    public static final StoredFieldsSpec NO_REQUIREMENTS = new StoredFieldsSpec(false, false, Set.of());

    /**
     * Use when the source should be loaded but no other stored fields are required
     */
    public static final StoredFieldsSpec NEEDS_SOURCE = new StoredFieldsSpec(true, false, Set.of());

    /**
     * @return a stored field spec that requires source only for the specified source paths.
     *         This is more efficient than using {@link #NEEDS_SOURCE}.
     */
    public static StoredFieldsSpec withSourcePaths(IgnoredSourceFormat ignoredSourceFormat, Set<String> sourcePaths) {
        return new StoredFieldsSpec(true, false, Set.of(), ignoredSourceFormat, sourcePaths);
    }

    /**
     * Combine these stored field requirements with those from another StoredFieldsSpec
     */
    public StoredFieldsSpec merge(StoredFieldsSpec other) {
        if (this == other) {
            return this;
        }
        Set<String> mergedFields;
        if (other.requiredStoredFields.isEmpty()) {
            /*
             * In the very very common case that we don't need new stored fields
             * let's not clone the existing array.
             */
            mergedFields = this.requiredStoredFields;
        } else {
            mergedFields = new HashSet<>(this.requiredStoredFields);
            mergedFields.addAll(other.requiredStoredFields);
        }
        Set<String> mergedSourcePaths = mergeSourcePaths(other);
        IgnoredSourceFormat mergedFormat;
        if (this.ignoredSourceFormat == IgnoredSourceFormat.NO_IGNORED_SOURCE) {
            mergedFormat = other.ignoredSourceFormat;
        } else if (other.ignoredSourceFormat == IgnoredSourceFormat.NO_IGNORED_SOURCE) {
            mergedFormat = this.ignoredSourceFormat;
        } else if (this.ignoredSourceFormat != other.ignoredSourceFormat) {
            throw new IllegalStateException(
                "failed to merge IgnoredFieldsSpec with differing formats ["
                    + this.ignoredSourceFormat.name()
                    + ","
                    + other.ignoredSourceFormat.name()
                    + "]"
            );
        } else {
            mergedFormat = this.ignoredSourceFormat;
        }

        return new StoredFieldsSpec(
            this.requiresSource || other.requiresSource,
            this.requiresMetadata || other.requiresMetadata,
            mergedFields,
            mergedFormat,
            mergedSourcePaths
        );
    }

    /**
     * Returns the unique source paths that should be loaded from source. Other source paths may be filtered out.
     * If an empty set is returned, then all source paths need to be loaded.
     */
    private Set<String> mergeSourcePaths(StoredFieldsSpec other) {
        Set<String> mergedSourcePaths;
        if (this.sourcePaths.isEmpty() == false && other.sourcePaths.isEmpty() == false) {
            mergedSourcePaths = new HashSet<>(this.sourcePaths);
            mergedSourcePaths.addAll(other.sourcePaths);
        } else if (this.sourcePaths.isEmpty() == false) {
            if (other.requiresSource) {
                mergedSourcePaths = Set.of();
            } else {
                mergedSourcePaths = this.sourcePaths;
            }
        } else if (other.sourcePaths.isEmpty() == false) {
            if (this.requiresSource) {
                mergedSourcePaths = Set.of();
            } else {
                mergedSourcePaths = other.sourcePaths;
            }
        } else {
            mergedSourcePaths = Set.of();
        }
        return mergedSourcePaths;
    }

    public Set<String> requiredStoredFields() {
        if (sourcePaths.isEmpty() || ignoredSourceFormat == IgnoredSourceFormat.NO_IGNORED_SOURCE) {
            return requiredStoredFields;
        }
        if (requiredStoredFields.isEmpty()) {
            return Set.of(IgnoredSourceFieldMapper.NAME);
        }
        Set<String> mergedFields = new HashSet<>(requiredStoredFields);
        mergedFields.add(IgnoredSourceFieldMapper.NAME);
        return mergedFields;
    }

    public static <T> StoredFieldsSpec build(Collection<T> sources, Function<T, StoredFieldsSpec> converter) {
        StoredFieldsSpec storedFieldsSpec = StoredFieldsSpec.NO_REQUIREMENTS;
        for (T source : sources) {
            storedFieldsSpec = storedFieldsSpec.merge(converter.apply(source));
        }
        return storedFieldsSpec;
    }
}