package main

import (
	"encoding/json"
	"fmt"
	"math"
	"os"
	"path"
	"runtime"
	"sort"
	"strconv"
	"strings"
	"time"

	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/handler"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/notifications"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/resolver"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/logs"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/version"
	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/netmap"
	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/pool"
	"github.com/spf13/pflag"
	"github.com/spf13/viper"
	"go.uber.org/zap"
	"go.uber.org/zap/zapcore"
)

const (
	defaultRebalanceInterval  = 60 * time.Second
	defaultHealthcheckTimeout = 15 * time.Second
	defaultConnectTimeout     = 10 * time.Second
	defaultStreamTimeout      = 10 * time.Second
	defaultShutdownTimeout    = 15 * time.Second

	defaultPoolErrorThreshold uint32 = 100
	defaultPlacementPolicy           = "REP 3"

	defaultMaxClientsCount    = 100
	defaultMaxClientsDeadline = time.Second * 30

	defaultSoftMemoryLimit = math.MaxInt64
)

var defaultCopiesNumbers = []uint32{0}

const ( // Settings.
	// Logger.
	cfgLoggerLevel = "logger.level"

	// Wallet.
	cfgWalletPath       = "wallet.path"
	cfgWalletAddress    = "wallet.address"
	cfgWalletPassphrase = "wallet.passphrase"
	cmdWallet           = "wallet"
	cmdAddress          = "address"

	// Server.
	cfgServer      = "server"
	cfgTLSEnabled  = "tls.enabled"
	cfgTLSKeyFile  = "tls.key_file"
	cfgTLSCertFile = "tls.cert_file"

	// Pool config.
	cfgConnectTimeout     = "connect_timeout"
	cfgStreamTimeout      = "stream_timeout"
	cfgHealthcheckTimeout = "healthcheck_timeout"
	cfgRebalanceInterval  = "rebalance_interval"
	cfgPoolErrorThreshold = "pool_error_threshold"

	// Caching.
	cfgObjectsCacheLifetime       = "cache.objects.lifetime"
	cfgObjectsCacheSize           = "cache.objects.size"
	cfgListObjectsCacheLifetime   = "cache.list.lifetime"
	cfgListObjectsCacheSize       = "cache.list.size"
	cfgBucketsCacheLifetime       = "cache.buckets.lifetime"
	cfgBucketsCacheSize           = "cache.buckets.size"
	cfgNamesCacheLifetime         = "cache.names.lifetime"
	cfgNamesCacheSize             = "cache.names.size"
	cfgSystemCacheLifetime        = "cache.system.lifetime"
	cfgSystemCacheSize            = "cache.system.size"
	cfgAccessBoxCacheLifetime     = "cache.accessbox.lifetime"
	cfgAccessBoxCacheSize         = "cache.accessbox.size"
	cfgAccessControlCacheLifetime = "cache.accesscontrol.lifetime"
	cfgAccessControlCacheSize     = "cache.accesscontrol.size"

	// NATS.
	cfgEnableNATS             = "nats.enabled"
	cfgNATSEndpoint           = "nats.endpoint"
	cfgNATSTimeout            = "nats.timeout"
	cfgNATSTLSCertFile        = "nats.cert_file"
	cfgNATSAuthPrivateKeyFile = "nats.key_file"
	cfgNATSRootCAFiles        = "nats.root_ca"

	// Policy.
	cfgPolicyDefault       = "placement_policy.default"
	cfgPolicyRegionMapFile = "placement_policy.region_mapping"
	cfgCopiesNumbers       = "placement_policy.copies_numbers"

	// CORS.
	cfgDefaultMaxAge = "cors.default_max_age"

	// MaxClients.
	cfgMaxClientsCount    = "max_clients_count"
	cfgMaxClientsDeadline = "max_clients_deadline"

	// Metrics / Profiler / Web.
	cfgPrometheusEnabled = "prometheus.enabled"
	cfgPrometheusAddress = "prometheus.address"
	cfgPProfEnabled      = "pprof.enabled"
	cfgPProfAddress      = "pprof.address"

	// Tracing.
	cfgTracingEnabled  = "tracing.enabled"
	cfgTracingExporter = "tracing.exporter"
	cfgTracingEndpoint = "tracing.endpoint"

	cfgListenDomains = "listen_domains"

	// Peers.
	cfgPeers = "peers"

	// NeoGo.
	cfgRPCEndpoint = "rpc_endpoint"

	// Resolving.
	cfgResolveOrder = "resolve_order"

	// Application.
	cfgApplicationBuildTime = "app.build_time"

	// Kludge.
	cfgKludgeUseDefaultXMLNSForCompleteMultipartUpload = "kludge.use_default_xmlns_for_complete_multipart"
	cfgKludgeCompleteMultipartUploadKeepalive          = "kludge.complete_multipart_keepalive"
	cfgKludgeBypassContentEncodingCheckInChunks        = "kludge.bypass_content_encoding_check_in_chunks"

	// Command line args.
	cmdHelp      = "help"
	cmdVersion   = "version"
	cmdConfig    = "config"
	cmdConfigDir = "config-dir"
	cmdPProf     = "pprof"
	cmdMetrics   = "metrics"

	cmdListenAddress = "listen_address"

	// Configuration of parameters of requests to FrostFS.
	// Number of the object copies to consider PUT to FrostFS successful.
	cfgSetCopiesNumber = "frostfs.set_copies_number"
	// Enabling client side object preparing for PUT operations.
	cfgClientCut = "frostfs.client_cut"
	// Sets max attempt to make successful tree request.
	cfgTreePoolMaxAttempts = "frostfs.tree_pool_max_attempts"

	// List of allowed AccessKeyID prefixes.
	cfgAllowedAccessKeyIDPrefixes = "allowed_access_key_id_prefixes"

	// Bucket resolving options.
	cfgResolveBucketAllow = "resolve_bucket.allow"
	cfgResolveBucketDeny  = "resolve_bucket.deny"

	// Runtime.
	cfgSoftMemoryLimit = "runtime.soft_memory_limit"

	// Enable return MD5 checksum in ETag.
	cfgMD5Enabled = "features.md5.enabled"

	// envPrefix is an environment variables prefix used for configuration.
	envPrefix = "S3_GW"
)

var ignore = map[string]struct{}{
	cfgApplicationBuildTime: {},

	cfgPeers: {},

	cmdHelp:    {},
	cmdVersion: {},
}

func fetchConnectTimeout(cfg *viper.Viper) time.Duration {
	connTimeout := cfg.GetDuration(cfgConnectTimeout)
	if connTimeout <= 0 {
		connTimeout = defaultConnectTimeout
	}

	return connTimeout
}

func fetchStreamTimeout(cfg *viper.Viper) time.Duration {
	streamTimeout := cfg.GetDuration(cfgStreamTimeout)
	if streamTimeout <= 0 {
		streamTimeout = defaultStreamTimeout
	}

	return streamTimeout
}

func fetchHealthCheckTimeout(cfg *viper.Viper) time.Duration {
	healthCheckTimeout := cfg.GetDuration(cfgHealthcheckTimeout)
	if healthCheckTimeout <= 0 {
		healthCheckTimeout = defaultHealthcheckTimeout
	}

	return healthCheckTimeout
}

func fetchRebalanceInterval(cfg *viper.Viper) time.Duration {
	rebalanceInterval := cfg.GetDuration(cfgRebalanceInterval)
	if rebalanceInterval <= 0 {
		rebalanceInterval = defaultRebalanceInterval
	}

	return rebalanceInterval
}

func fetchErrorThreshold(cfg *viper.Viper) uint32 {
	errorThreshold := cfg.GetUint32(cfgPoolErrorThreshold)
	if errorThreshold <= 0 {
		errorThreshold = defaultPoolErrorThreshold
	}

	return errorThreshold
}

func fetchMaxClientsCount(cfg *viper.Viper) int {
	maxClientsCount := cfg.GetInt(cfgMaxClientsCount)
	if maxClientsCount <= 0 {
		maxClientsCount = defaultMaxClientsCount
	}

	return maxClientsCount
}

func fetchMaxClientsDeadline(cfg *viper.Viper) time.Duration {
	maxClientsDeadline := cfg.GetDuration(cfgMaxClientsDeadline)
	if maxClientsDeadline <= 0 {
		maxClientsDeadline = defaultMaxClientsDeadline
	}

	return maxClientsDeadline
}

func fetchSoftMemoryLimit(cfg *viper.Viper) int64 {
	softMemoryLimit := cfg.GetSizeInBytes(cfgSoftMemoryLimit)
	if softMemoryLimit <= 0 {
		softMemoryLimit = defaultSoftMemoryLimit
	}

	return int64(softMemoryLimit)
}

func fetchDefaultPolicy(l *zap.Logger, cfg *viper.Viper) netmap.PlacementPolicy {
	var policy netmap.PlacementPolicy

	if cfg.IsSet(cfgPolicyDefault) {
		policyStr := cfg.GetString(cfgPolicyDefault)
		if err := policy.DecodeString(policyStr); err != nil {
			l.Warn(logs.FailedToParseDefaultLocationConstraint,
				zap.String("policy", policyStr), zap.String("default", defaultPlacementPolicy), zap.Error(err))
		} else {
			return policy
		}
	}

	if err := policy.DecodeString(defaultPlacementPolicy); err != nil {
		l.Fatal(logs.FailedToParseDefaultDefaultLocationConstraint, zap.String("policy", defaultPlacementPolicy))
	}

	return policy
}

func fetchNATSTimeout(cfg *viper.Viper, l *zap.Logger) time.Duration {
	timeout := cfg.GetDuration(cfgNATSTimeout)
	if timeout <= 0 {
		l.Error(logs.InvalidLifetimeUsingDefaultValue,
			zap.String("parameter", cfgNATSTimeout),
			zap.Duration("value in config", timeout),
			zap.Duration("default", notifications.DefaultTimeout))
		timeout = notifications.DefaultTimeout
	}

	return timeout
}

func fetchCacheLifetime(v *viper.Viper, l *zap.Logger, cfgEntry string, defaultValue time.Duration) time.Duration {
	if v.IsSet(cfgEntry) {
		lifetime := v.GetDuration(cfgEntry)
		if lifetime <= 0 {
			l.Error(logs.InvalidLifetimeUsingDefaultValue,
				zap.String("parameter", cfgEntry),
				zap.Duration("value in config", lifetime),
				zap.Duration("default", defaultValue))
		} else {
			return lifetime
		}
	}

	return defaultValue
}

func fetchCacheSize(v *viper.Viper, l *zap.Logger, cfgEntry string, defaultValue int) int {
	if v.IsSet(cfgEntry) {
		size := v.GetInt(cfgEntry)
		if size <= 0 {
			l.Error(logs.InvalidCacheSizeUsingDefaultValue,
				zap.String("parameter", cfgEntry),
				zap.Int("value in config", size),
				zap.Int("default", defaultValue))
		} else {
			return size
		}
	}

	return defaultValue
}

func fetchDefaultMaxAge(cfg *viper.Viper, l *zap.Logger) int {
	defaultMaxAge := handler.DefaultMaxAge

	if cfg.IsSet(cfgDefaultMaxAge) {
		defaultMaxAge = cfg.GetInt(cfgDefaultMaxAge)

		if defaultMaxAge <= 0 && defaultMaxAge != -1 {
			l.Fatal(logs.InvalidDefaultMaxAge,
				zap.String("parameter", cfgDefaultMaxAge),
				zap.String("value in config", strconv.Itoa(defaultMaxAge)))
		}
	}

	return defaultMaxAge
}

func fetchRegionMappingPolicies(l *zap.Logger, cfg *viper.Viper) map[string]netmap.PlacementPolicy {
	filepath := cfg.GetString(cfgPolicyRegionMapFile)
	regionPolicyMap, err := readRegionMap(filepath)
	if err != nil {
		l.Warn(logs.FailedToReadRegionMapFilePolicies, zap.String("file", filepath), zap.Error(err))
		return make(map[string]netmap.PlacementPolicy)
	}

	regionMap := make(map[string]netmap.PlacementPolicy, len(regionPolicyMap))
	for region, policy := range regionPolicyMap {
		if region == api.DefaultLocationConstraint {
			l.Warn(logs.DefaultLocationConstraintCantBeOverriden, zap.String("policy", policy))
			continue
		}

		var pp netmap.PlacementPolicy
		if err = pp.DecodeString(policy); err == nil {
			regionMap[region] = pp
			continue
		}

		if err = pp.UnmarshalJSON([]byte(policy)); err == nil {
			regionMap[region] = pp
			continue
		}

		l.Warn(logs.FailedToParseLocationConstraint, zap.String("region", region), zap.String("policy", policy))
	}

	return regionMap
}

func readRegionMap(filePath string) (map[string]string, error) {
	regionMap := make(map[string]string)

	if filePath == "" {
		return regionMap, nil
	}

	data, err := os.ReadFile(filePath)
	if err != nil {
		return nil, fmt.Errorf("coudln't read file '%s'", filePath)
	}

	if err = json.Unmarshal(data, &regionMap); err != nil {
		return nil, fmt.Errorf("unmarshal policies: %w", err)
	}

	return regionMap, nil
}

func fetchDefaultCopiesNumbers(l *zap.Logger, v *viper.Viper) []uint32 {
	unparsed := v.GetStringSlice(cfgSetCopiesNumber)
	result := make([]uint32, len(unparsed))
	for i := range unparsed {
		parsedValue, err := strconv.ParseUint(unparsed[i], 10, 32)
		if err != nil {
			l.Warn(logs.FailedToParseDefaultCopiesNumbers,
				zap.Strings("copies numbers", unparsed), zap.Uint32s("default", defaultCopiesNumbers), zap.Error(err))
			return defaultCopiesNumbers
		}
		result[i] = uint32(parsedValue)
	}

	return result
}

func fetchCopiesNumbers(l *zap.Logger, v *viper.Viper) map[string][]uint32 {
	copiesNums := make(map[string][]uint32)
	for i := 0; ; i++ {
		key := cfgCopiesNumbers + "." + strconv.Itoa(i) + "."
		constraint := v.GetString(key + "location_constraint")
		vector := v.GetStringSlice(key + "vector")

		if constraint == "" || len(vector) == 0 {
			break
		}

		vector32 := make([]uint32, len(vector))
		for j := range vector {
			parsedValue, err := strconv.ParseUint(vector[j], 10, 32)
			if err != nil {
				l.Warn(logs.FailedToParseCopiesNumbers, zap.String("location", constraint),
					zap.Strings("copies numbers", vector), zap.Error(err))
				continue
			}
			vector32[j] = uint32(parsedValue)
		}

		copiesNums[constraint] = vector32
		l.Info(logs.ConstraintAdded, zap.String("location", constraint), zap.Strings("copies numbers", vector))
	}
	return copiesNums
}

func fetchPeers(l *zap.Logger, v *viper.Viper) []pool.NodeParam {
	var nodes []pool.NodeParam
	for i := 0; ; i++ {
		key := cfgPeers + "." + strconv.Itoa(i) + "."
		address := v.GetString(key + "address")
		weight := v.GetFloat64(key + "weight")
		priority := v.GetInt(key + "priority")

		if address == "" {
			l.Warn(logs.SkipEmptyAddress)
			break
		}
		if weight <= 0 { // unspecified or wrong
			weight = 1
		}
		if priority <= 0 { // unspecified or wrong
			priority = 1
		}

		nodes = append(nodes, pool.NewNodeParam(priority, address, weight))

		l.Info(logs.AddedStoragePeer,
			zap.Int("priority", priority),
			zap.String("address", address),
			zap.Float64("weight", weight))
	}

	return nodes
}

func fetchServers(v *viper.Viper) []ServerInfo {
	var servers []ServerInfo

	for i := 0; ; i++ {
		key := cfgServer + "." + strconv.Itoa(i) + "."

		var serverInfo ServerInfo
		serverInfo.Address = v.GetString(key + "address")
		serverInfo.TLS.Enabled = v.GetBool(key + cfgTLSEnabled)
		serverInfo.TLS.KeyFile = v.GetString(key + cfgTLSKeyFile)
		serverInfo.TLS.CertFile = v.GetString(key + cfgTLSCertFile)

		if serverInfo.Address == "" {
			break
		}

		servers = append(servers, serverInfo)
	}

	return servers
}

func newSettings() *viper.Viper {
	v := viper.New()

	v.AutomaticEnv()
	v.SetEnvPrefix(envPrefix)
	v.SetConfigType("yaml")
	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
	v.AllowEmptyEnv(true)

	// flags setup:
	flags := pflag.NewFlagSet("commandline", pflag.ExitOnError)
	flags.SetOutput(os.Stdout)
	flags.SortFlags = false

	flags.Bool(cmdPProf, false, "enable pprof")
	flags.Bool(cmdMetrics, false, "enable prometheus metrics")

	help := flags.BoolP(cmdHelp, "h", false, "show help")
	versionFlag := flags.BoolP(cmdVersion, "v", false, "show version")

	flags.StringP(cmdWallet, "w", "", `path to the wallet`)
	flags.String(cmdAddress, "", `address of wallet account`)
	flags.StringArray(cmdConfig, nil, "config paths")
	flags.String(cmdConfigDir, "", "config dir path")

	flags.Duration(cfgHealthcheckTimeout, defaultHealthcheckTimeout, "set timeout to check node health during rebalance")
	flags.Duration(cfgConnectTimeout, defaultConnectTimeout, "set timeout to connect to FrostFS nodes")
	flags.Duration(cfgRebalanceInterval, defaultRebalanceInterval, "set rebalance interval")

	flags.Int(cfgMaxClientsCount, defaultMaxClientsCount, "set max-clients count")
	flags.Duration(cfgMaxClientsDeadline, defaultMaxClientsDeadline, "set max-clients deadline")

	flags.String(cmdListenAddress, "0.0.0.0:8080", "set the main address to listen")
	flags.String(cfgTLSCertFile, "", "TLS certificate file to use")
	flags.String(cfgTLSKeyFile, "", "TLS key file to use")

	peers := flags.StringArrayP(cfgPeers, "p", nil, "set FrostFS nodes")

	flags.StringP(cfgRPCEndpoint, "r", "", "set RPC endpoint")
	resolveMethods := flags.StringSlice(cfgResolveOrder, []string{resolver.DNSResolver}, "set bucket name resolve order")

	domains := flags.StringSliceP(cfgListenDomains, "d", nil, "set domains to be listened")

	// set defaults:

	// logger:
	v.SetDefault(cfgLoggerLevel, "debug")

	// pool:
	v.SetDefault(cfgPoolErrorThreshold, defaultPoolErrorThreshold)
	v.SetDefault(cfgStreamTimeout, defaultStreamTimeout)

	v.SetDefault(cfgPProfAddress, "localhost:8085")
	v.SetDefault(cfgPrometheusAddress, "localhost:8086")

	// kludge
	v.SetDefault(cfgKludgeUseDefaultXMLNSForCompleteMultipartUpload, false)
	v.SetDefault(cfgKludgeCompleteMultipartUploadKeepalive, 10*time.Second)
	v.SetDefault(cfgKludgeBypassContentEncodingCheckInChunks, false)

	// Bind flags
	if err := bindFlags(v, flags); err != nil {
		panic(fmt.Errorf("bind flags: %w", err))
	}

	if err := flags.Parse(os.Args); err != nil {
		panic(err)
	}

	if v.IsSet(cfgServer+".0."+cfgTLSKeyFile) && v.IsSet(cfgServer+".0."+cfgTLSCertFile) {
		v.Set(cfgServer+".0."+cfgTLSEnabled, true)
	}

	if resolveMethods != nil {
		v.SetDefault(cfgResolveOrder, *resolveMethods)
	}

	if peers != nil && len(*peers) > 0 {
		for i := range *peers {
			v.SetDefault(cfgPeers+"."+strconv.Itoa(i)+".address", (*peers)[i])
			v.SetDefault(cfgPeers+"."+strconv.Itoa(i)+".weight", 1)
			v.SetDefault(cfgPeers+"."+strconv.Itoa(i)+".priority", 1)
		}
	}

	if domains != nil && len(*domains) > 0 {
		v.SetDefault(cfgListenDomains, *domains)
	}

	switch {
	case help != nil && *help:
		fmt.Printf("FrostFS S3 gateway %s\n", version.Version)
		flags.PrintDefaults()

		fmt.Println()
		fmt.Println("Default environments:")
		fmt.Println()
		keys := v.AllKeys()
		sort.Strings(keys)

		for i := range keys {
			if _, ok := ignore[keys[i]]; ok {
				continue
			}

			defaultValue := v.GetString(keys[i])
			if len(defaultValue) == 0 {
				continue
			}

			k := strings.Replace(keys[i], ".", "_", -1)
			fmt.Printf("%s_%s = %s\n", envPrefix, strings.ToUpper(k), defaultValue)
		}

		fmt.Println()
		fmt.Println("Peers preset:")
		fmt.Println()

		fmt.Printf("%s_%s_[N]_ADDRESS = string\n", envPrefix, strings.ToUpper(cfgPeers))
		fmt.Printf("%s_%s_[N]_WEIGHT = 0..1 (float)\n", envPrefix, strings.ToUpper(cfgPeers))

		os.Exit(0)
	case versionFlag != nil && *versionFlag:
		fmt.Printf("FrostFS S3 Gateway\nVersion: %s\nGoVersion: %s\n", version.Version, runtime.Version())
		os.Exit(0)
	}

	if err := readInConfig(v); err != nil {
		panic(err)
	}

	return v
}

func bindFlags(v *viper.Viper, flags *pflag.FlagSet) error {
	if err := v.BindPFlag(cfgPProfEnabled, flags.Lookup(cmdPProf)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgPrometheusEnabled, flags.Lookup(cmdMetrics)); err != nil {
		return err
	}
	if err := v.BindPFlag(cmdConfig, flags.Lookup(cmdConfig)); err != nil {
		return err
	}
	if err := v.BindPFlag(cmdConfigDir, flags.Lookup(cmdConfigDir)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgWalletPath, flags.Lookup(cmdWallet)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgWalletAddress, flags.Lookup(cmdAddress)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgHealthcheckTimeout, flags.Lookup(cfgHealthcheckTimeout)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgConnectTimeout, flags.Lookup(cfgConnectTimeout)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgRebalanceInterval, flags.Lookup(cfgRebalanceInterval)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgMaxClientsCount, flags.Lookup(cfgMaxClientsCount)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgMaxClientsDeadline, flags.Lookup(cfgMaxClientsDeadline)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgRPCEndpoint, flags.Lookup(cfgRPCEndpoint)); err != nil {
		return err
	}

	if err := v.BindPFlag(cfgServer+".0.address", flags.Lookup(cmdListenAddress)); err != nil {
		return err
	}
	if err := v.BindPFlag(cfgServer+".0."+cfgTLSKeyFile, flags.Lookup(cfgTLSKeyFile)); err != nil {
		return err
	}
	return v.BindPFlag(cfgServer+".0."+cfgTLSCertFile, flags.Lookup(cfgTLSCertFile))
}

func readInConfig(v *viper.Viper) error {
	if v.IsSet(cmdConfig) {
		if err := readConfig(v); err != nil {
			return err
		}
	}

	if v.IsSet(cmdConfigDir) {
		if err := readConfigDir(v); err != nil {
			return err
		}
	}

	return nil
}

func readConfigDir(v *viper.Viper) error {
	cfgSubConfigDir := v.GetString(cmdConfigDir)
	entries, err := os.ReadDir(cfgSubConfigDir)
	if err != nil {
		return err
	}

	for _, entry := range entries {
		if entry.IsDir() {
			continue
		}
		ext := path.Ext(entry.Name())
		if ext != ".yaml" && ext != ".yml" {
			continue
		}

		if err = mergeConfig(v, path.Join(cfgSubConfigDir, entry.Name())); err != nil {
			return err
		}
	}

	return nil
}

func readConfig(v *viper.Viper) error {
	for _, fileName := range v.GetStringSlice(cmdConfig) {
		if err := mergeConfig(v, fileName); err != nil {
			return err
		}
	}
	return nil
}

func mergeConfig(v *viper.Viper, fileName string) error {
	cfgFile, err := os.Open(fileName)
	if err != nil {
		return err
	}

	defer func() {
		if errClose := cfgFile.Close(); errClose != nil {
			panic(errClose)
		}
	}()

	return v.MergeConfig(cfgFile)
}

// newLogger constructs a Logger instance for the current application.
// Panics on failure.
//
// Logger contains a logger is built from zap's production logging configuration with:
//   - parameterized level (debug by default)
//   - console encoding
//   - ISO8601 time encoding
//
// and atomic log level to dynamically change it.
//
// Logger records a stack trace for all messages at or above fatal level.
//
// See also zapcore.Level, zap.NewProductionConfig, zap.AddStacktrace.
func newLogger(v *viper.Viper) *Logger {
	lvl, err := getLogLevel(v)
	if err != nil {
		panic(err)
	}

	c := zap.NewProductionConfig()
	c.Level = zap.NewAtomicLevelAt(lvl)
	c.Encoding = "console"
	c.EncoderConfig.EncodeTime = zapcore.ISO8601TimeEncoder

	l, err := c.Build(
		zap.AddStacktrace(zap.NewAtomicLevelAt(zap.FatalLevel)),
	)
	if err != nil {
		panic(fmt.Sprintf("build zap logger instance: %v", err))
	}

	return &Logger{
		logger: l,
		lvl:    c.Level,
	}
}

func getLogLevel(v *viper.Viper) (zapcore.Level, error) {
	var lvl zapcore.Level
	lvlStr := v.GetString(cfgLoggerLevel)
	err := lvl.UnmarshalText([]byte(lvlStr))
	if err != nil {
		return lvl, fmt.Errorf("incorrect logger level configuration %s (%v), "+
			"value should be one of %v", lvlStr, err, [...]zapcore.Level{
			zapcore.DebugLevel,
			zapcore.InfoLevel,
			zapcore.WarnLevel,
			zapcore.ErrorLevel,
			zapcore.DPanicLevel,
			zapcore.PanicLevel,
			zapcore.FatalLevel,
		})
	}
	return lvl, nil
}