// Copyright The OpenTelemetry 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 service // import "go.opentelemetry.io/collector/service"

import (
	"context"
	"fmt"

	"go.opentelemetry.io/collector/component"
	"go.opentelemetry.io/collector/confmap"
	"go.opentelemetry.io/collector/confmap/converter/expandconverter"
	"go.opentelemetry.io/collector/confmap/provider/envprovider"
	"go.opentelemetry.io/collector/confmap/provider/fileprovider"
	"go.opentelemetry.io/collector/confmap/provider/yamlprovider"
	"go.opentelemetry.io/collector/service/internal/configunmarshaler"
)

// ConfigProvider provides the service configuration.
//
// The typical usage is the following:
//
//		cfgProvider.Get(...)
//		cfgProvider.Watch() // wait for an event.
//		cfgProvider.Get(...)
//		cfgProvider.Watch() // wait for an event.
//		// repeat Get/Watch cycle until it is time to shut down the Collector process.
//		cfgProvider.Shutdown()
type ConfigProvider interface {
	// Get returns the service configuration, or error otherwise.
	//
	// Should never be called concurrently with itself, Watch or Shutdown.
	Get(ctx context.Context, factories component.Factories) (*Config, error)

	// Watch blocks until any configuration change was detected or an unrecoverable error
	// happened during monitoring the configuration changes.
	//
	// Error is nil if the configuration is changed and needs to be re-fetched. Any non-nil
	// error indicates that there was a problem with watching the config changes.
	//
	// Should never be called concurrently with itself or Get.
	Watch() <-chan error

	// Shutdown signals that the provider is no longer in use and the that should close
	// and release any resources that it may have created.
	//
	// This function must terminate the Watch channel.
	//
	// Should never be called concurrently with itself or Get.
	Shutdown(ctx context.Context) error
}

type configProvider struct {
	mapResolver *confmap.Resolver
}

// ConfigProviderSettings are the settings to configure the behavior of the ConfigProvider.
// TODO: embed confmap.ResolverSettings into this to avoid duplicates.
type ConfigProviderSettings struct {
	// Locations from where the confmap.Conf is retrieved, and merged in the given order.
	// It is required to have at least one location.
	Locations []string

	// MapProviders is a map of pairs <scheme, confmap.Provider>.
	// It is required to have at least one confmap.Provider.
	MapProviders map[string]confmap.Provider

	// MapConverters is a slice of confmap.Converter.
	MapConverters []confmap.Converter
}

func newDefaultConfigProviderSettings(locations []string) ConfigProviderSettings {
	return ConfigProviderSettings{
		Locations:     locations,
		MapProviders:  makeMapProvidersMap(fileprovider.New(), envprovider.New(), yamlprovider.New()),
		MapConverters: []confmap.Converter{expandconverter.New()},
	}
}

// NewConfigProvider returns a new ConfigProvider that provides the service configuration:
// * Initially it resolves the "configuration map":
//	 * Retrieve the confmap.Conf by merging all retrieved maps from the given `locations` in order.
// 	 * Then applies all the confmap.Converter in the given order.
// * Then unmarshalls the confmap.Conf into the service Config.
func NewConfigProvider(set ConfigProviderSettings) (ConfigProvider, error) {
	mr, err := confmap.NewResolver(confmap.ResolverSettings{URIs: set.Locations, Providers: set.MapProviders, Converters: set.MapConverters})
	if err != nil {
		return nil, err
	}

	return &configProvider{
		mapResolver: mr,
	}, nil
}

func (cm *configProvider) Get(ctx context.Context, factories component.Factories) (*Config, error) {
	retMap, err := cm.mapResolver.Resolve(ctx)
	if err != nil {
		return nil, fmt.Errorf("cannot resolve the configuration: %w", err)
	}

	var cfg *Config
	if cfg, err = configunmarshaler.New().Unmarshal(retMap, factories); err != nil {
		return nil, fmt.Errorf("cannot unmarshal the configuration: %w", err)
	}

	if err = cfg.Validate(); err != nil {
		return nil, fmt.Errorf("invalid configuration: %w", err)
	}

	return cfg, nil
}

func (cm *configProvider) Watch() <-chan error {
	return cm.mapResolver.Watch()
}

func (cm *configProvider) Shutdown(ctx context.Context) error {
	return cm.mapResolver.Shutdown(ctx)
}

func makeMapProvidersMap(providers ...confmap.Provider) map[string]confmap.Provider {
	ret := make(map[string]confmap.Provider, len(providers))
	for _, provider := range providers {
		ret[provider.Scheme()] = provider
	}
	return ret
}
