package sysconf

import (
	"encoding"
	"encoding/csv"
	"errors"
	"fmt"
	"os"
	"reflect"
	"sort"
	"strconv"
	"strings"
	"time"
)

type Config struct {
	ini       *Ini
	envPrefix string
	envLookup func(string) (string, bool)
}

type ConfigFieldInfo struct {
	Section  string
	Key      string
	Field    string
	Type     string
	Default  string
	Env      string
	Split    string
	Required bool
}

type ConfigEntryInfo struct {
	Section string
	Key     string
	Path    string
	Values  []string
	NoValue bool
}

type ConfigOption func(*Config)

type ConfigSource interface {
	Load(*Config) error
}

type ConfigValidator interface {
	Validate() error
}

type ConfigError struct {
	Section string
	Key     string
	Field   string
	Reason  string
	Err     error
}

type ConfigSourceError struct {
	Path     string
	Optional bool
	Err      error
}

type FileSource struct {
	Path     string
	Optional bool
}

type MemorySource struct {
	Name string
	Data []byte
}

func (e *ConfigError) Error() string {
	if e == nil {
		return ""
	}
	location := e.Key
	if e.Section != "" {
		location = e.Section + "." + e.Key
	}
	if e.Field != "" {
		return fmt.Sprintf("sysconf config %s (%s): %s", location, e.Field, e.Reason)
	}
	return fmt.Sprintf("sysconf config %s: %s", location, e.Reason)
}

func (e *ConfigError) Unwrap() error {
	if e == nil {
		return nil
	}
	return e.Err
}

func (e *ConfigSourceError) Error() string {
	if e == nil {
		return ""
	}
	kind := "required"
	if e.Optional {
		kind = "optional"
	}
	return fmt.Sprintf("sysconf config %s source %q: %v", kind, e.Path, e.Err)
}

func (e *ConfigSourceError) Unwrap() error {
	if e == nil {
		return nil
	}
	return e.Err
}

func (s FileSource) Load(cfg *Config) error {
	if cfg == nil {
		return ErrDocumentClosed
	}
	if s.Path == "" {
		return nil
	}
	if err := cfg.LoadFile(s.Path); err != nil {
		if s.Optional && os.IsNotExist(err) {
			return nil
		}
		return &ConfigSourceError{Path: s.Path, Optional: s.Optional, Err: err}
	}
	return nil
}

func (s MemorySource) Load(cfg *Config) error {
	if cfg == nil {
		return ErrDocumentClosed
	}
	if err := cfg.LoadBytes(s.Data); err != nil {
		return &ConfigSourceError{Path: s.Name, Err: err}
	}
	return nil
}

func RequiredFile(path string) FileSource {
	return FileSource{Path: path}
}

func OptionalFile(path string) FileSource {
	return FileSource{Path: path, Optional: true}
}

func BytesSource(name string, data []byte) MemorySource {
	return MemorySource{Name: name, Data: append([]byte(nil), data...)}
}

func StringSource(name, data string) MemorySource {
	return BytesSource(name, []byte(data))
}

func DescribeConfig(src interface{}) ([]ConfigFieldInfo, error) {
	v, err := configReadableStructValue(src)
	if err != nil {
		return nil, err
	}
	fields := make([]ConfigFieldInfo, 0)
	err = walkReadableConfigFields(v, "", "", func(field configField) error {
		fields = append(fields, ConfigFieldInfo{
			Section:  field.section,
			Key:      field.key,
			Field:    field.fieldPath,
			Type:     field.structField.Type.String(),
			Default:  field.defaultValue,
			Env:      field.env,
			Split:    field.split,
			Required: field.required,
		})
		return nil
	})
	if err != nil {
		return nil, err
	}
	return fields, nil
}

func SampleConfig(src interface{}) ([]byte, error) {
	v, err := configReadableStructValue(src)
	if err != nil {
		return nil, err
	}
	cfg := NewConfig()
	err = walkReadableConfigFields(v, "", "", func(field configField) error {
		values, err := sampleValuesForField(field)
		if err != nil {
			return configFieldErrorWithErr(field, "invalid sample value: "+err.Error(), err)
		}
		cfg.setAll(field.section, field.key, values, "")
		return nil
	})
	if err != nil {
		return nil, err
	}
	return cfg.Build(), nil
}

func NewConfig(options ...ConfigOption) *Config {
	cfg := &Config{
		ini: NewIni(),
	}
	for _, option := range options {
		if option != nil {
			option(cfg)
		}
	}
	if cfg.ini == nil {
		cfg.ini = NewIni()
	}
	return cfg
}

func LoadConfig(dst interface{}, files []string, options ...ConfigOption) (*Config, error) {
	sources := make([]ConfigSource, 0, len(files))
	for _, path := range files {
		sources = append(sources, RequiredFile(path))
	}
	return LoadConfigSources(dst, sources, options...)
}

func LoadConfigSources(dst interface{}, sources []ConfigSource, options ...ConfigOption) (*Config, error) {
	cfg := NewConfig(options...)
	if err := cfg.LoadSources(sources...); err != nil {
		return nil, err
	}
	if dst != nil {
		if err := cfg.Bind(dst); err != nil {
			return nil, err
		}
	}
	return cfg, nil
}

func WithEnvPrefix(prefix string) ConfigOption {
	return func(cfg *Config) {
		cfg.envPrefix = strings.Trim(prefix, "_")
		if cfg.envLookup == nil {
			cfg.envLookup = os.LookupEnv
		}
	}
}

func WithEnvLookup(lookup func(string) (string, bool)) ConfigOption {
	return func(cfg *Config) {
		cfg.envLookup = lookup
	}
}

func WithIni(ini *Ini) ConfigOption {
	return func(cfg *Config) {
		cfg.ini = ini
	}
}

func (c *Config) Ini() *Ini {
	if c == nil {
		return nil
	}
	if c.ini == nil {
		c.ini = NewIni()
	}
	return c.ini
}

func (c *Config) Section(name string) *Section {
	if c == nil {
		return nil
	}
	return c.Ini().Section(name)
}

func (c *Config) Get(section, key string) string {
	if c == nil {
		return ""
	}
	return c.Ini().Get(section, key)
}

func (c *Config) GetAll(section, key string) []string {
	if c == nil {
		return nil
	}
	return c.Ini().GetAll(section, key)
}

func (c *Config) SectionNames() []string {
	if c == nil {
		return nil
	}
	ini := c.Ini()
	if ini == nil || ini.Document == nil {
		return nil
	}
	ini.Document.mu.RLock()
	defer ini.Document.mu.RUnlock()

	seen := make(map[string]string)
	for _, section := range ini.Document.sections {
		if section == nil {
			continue
		}
		if section.Name == "" && !sectionHasPairs(section) {
			continue
		}
		normalized := normalize(section.Name, ini.CaseSensitive)
		if _, ok := seen[normalized]; !ok {
			seen[normalized] = section.Name
		}
	}
	names := make([]string, 0, len(seen))
	for _, name := range seen {
		names = append(names, name)
	}
	sort.Strings(names)
	return names
}

func (c *Config) Keys(section string) []string {
	if c == nil {
		return nil
	}
	ini := c.Ini()
	if ini == nil || ini.Document == nil {
		return nil
	}
	ini.Document.mu.RLock()
	defer ini.Document.mu.RUnlock()

	wanted := normalize(section, ini.CaseSensitive)
	seen := make(map[string]string)
	for _, sec := range ini.Document.sections {
		if sec == nil || normalize(sec.Name, ini.CaseSensitive) != wanted {
			continue
		}
		for _, entry := range sec.Entries {
			if !isConfigEntry(entry) {
				continue
			}
			normalized := normalize(entry.Key, sec.CaseSensitive)
			if _, ok := seen[normalized]; !ok {
				seen[normalized] = entry.Key
			}
		}
	}
	keys := make([]string, 0, len(seen))
	for _, key := range seen {
		keys = append(keys, key)
	}
	sort.Strings(keys)
	return keys
}

func (c *Config) Flatten() map[string][]string {
	out := make(map[string][]string)
	for _, entry := range c.FlattenEntries() {
		if entry.NoValue || len(entry.Values) == 0 {
			out[entry.Path] = append(out[entry.Path], "")
			continue
		}
		out[entry.Path] = append(out[entry.Path], entry.Values...)
	}
	return out
}

func (c *Config) FlattenEntries() []ConfigEntryInfo {
	if c == nil {
		return nil
	}
	ini := c.Ini()
	if ini == nil || ini.Document == nil {
		return nil
	}
	ini.Document.mu.RLock()
	defer ini.Document.mu.RUnlock()

	entries := make([]ConfigEntryInfo, 0)
	for _, section := range ini.Document.sections {
		if section == nil {
			continue
		}
		for _, entry := range section.Entries {
			if !isConfigEntry(entry) {
				continue
			}
			item := ConfigEntryInfo{
				Section: section.Name,
				Key:     entry.Key,
				Path:    configPath(section.Name, entry.Key),
				Values:  append([]string(nil), entry.Values...),
				NoValue: entry.NoValue || len(entry.Values) == 0,
			}
			entries = append(entries, item)
		}
	}
	return entries
}

func (c *Config) GetStringE(section, key string) (string, error) {
	item, err := c.firstValue(section, key)
	if err != nil {
		return "", err
	}
	return item.text, nil
}

func (c *Config) GetBoolE(section, key string) (bool, error) {
	item, err := c.firstValue(section, key)
	if err != nil {
		return false, err
	}
	if item.noValue {
		return true, nil
	}
	value, err := strconv.ParseBool(item.text)
	if err != nil {
		return false, configKeyError(section, key, err.Error(), err)
	}
	return value, nil
}

func (c *Config) GetIntE(section, key string) (int, error) {
	item, err := c.firstValue(section, key)
	if err != nil {
		return 0, err
	}
	value, err := strconv.Atoi(item.text)
	if err != nil {
		return 0, configKeyError(section, key, err.Error(), err)
	}
	return value, nil
}

func (c *Config) GetInt64E(section, key string) (int64, error) {
	item, err := c.firstValue(section, key)
	if err != nil {
		return 0, err
	}
	value, err := strconv.ParseInt(item.text, 10, 64)
	if err != nil {
		return 0, configKeyError(section, key, err.Error(), err)
	}
	return value, nil
}

func (c *Config) GetUint64E(section, key string) (uint64, error) {
	item, err := c.firstValue(section, key)
	if err != nil {
		return 0, err
	}
	value, err := strconv.ParseUint(item.text, 10, 64)
	if err != nil {
		return 0, configKeyError(section, key, err.Error(), err)
	}
	return value, nil
}

func (c *Config) GetFloat64E(section, key string) (float64, error) {
	item, err := c.firstValue(section, key)
	if err != nil {
		return 0, err
	}
	value, err := strconv.ParseFloat(item.text, 64)
	if err != nil {
		return 0, configKeyError(section, key, err.Error(), err)
	}
	return value, nil
}

func (c *Config) GetDurationE(section, key string) (time.Duration, error) {
	item, err := c.firstValue(section, key)
	if err != nil {
		return 0, err
	}
	value, err := time.ParseDuration(item.text)
	if err != nil {
		return 0, configKeyError(section, key, err.Error(), err)
	}
	return value, nil
}

func (c *Config) Has(section, key string) bool {
	if c == nil {
		return false
	}
	return c.Ini().Has(section, key)
}

func (c *Config) Set(section, key, value string) {
	if c == nil {
		return
	}
	c.Ini().Set(section, key, value)
}

func (c *Config) SetAll(section, key string, values []string) {
	if c == nil {
		return
	}
	c.setAll(section, key, values, "")
}

func (c *Config) Delete(section, key string) bool {
	if c == nil {
		return false
	}
	return c.Ini().Delete(section, key)
}

func (c *Config) DeleteSection(section string) bool {
	if c == nil {
		return false
	}
	return c.Ini().DeleteSection(section)
}

func (c *Config) Build() []byte {
	if c == nil {
		return nil
	}
	return c.Ini().Build()
}

func (c *Config) Bytes() []byte {
	return c.Build()
}

func (c *Config) Save(path string) error {
	if c == nil {
		return ErrDocumentClosed
	}
	return c.Ini().Save(path)
}

func (c *Config) SaveAtomic(path string) error {
	if c == nil {
		return ErrDocumentClosed
	}
	return c.Ini().SaveAtomic(path)
}

func (c *Config) LoadBytes(data []byte) error {
	if c == nil {
		return ErrDocumentClosed
	}
	tmp := newIniLike(c.Ini())
	if err := tmp.Parse(data); err != nil {
		return err
	}
	c.merge(tmp)
	return nil
}

func newIniLike(base *Ini) *Ini {
	tmp := NewIni()
	if base == nil || base.Document == nil {
		return tmp
	}
	tmp.SectionOpen = base.SectionOpen
	tmp.SectionClose = base.SectionClose
	tmp.Assign = base.Assign
	tmp.AssignDelimiters = append([]string(nil), base.AssignDelimiters...)
	tmp.CommentHeads = append([]string(nil), base.CommentHeads...)
	tmp.AllowInline = base.AllowInline
	tmp.InlineCommentRequiresSpace = base.InlineCommentRequiresSpace
	tmp.AllowNoValue = base.AllowNoValue
	tmp.AllowMulti = base.AllowMulti
	tmp.AllowContinuation = base.AllowContinuation
	tmp.TrimSpace = base.TrimSpace
	tmp.CaseSensitive = base.CaseSensitive
	tmp.Strict = base.Strict
	return tmp
}

func (c *Config) LoadFile(path string) error {
	data, err := os.ReadFile(path)
	if err != nil {
		return err
	}
	return c.LoadBytes(data)
}

func (c *Config) LoadSource(source ConfigSource) error {
	if c == nil {
		return ErrDocumentClosed
	}
	if source == nil {
		return nil
	}
	return source.Load(c)
}

func (c *Config) LoadSources(sources ...ConfigSource) error {
	for _, source := range sources {
		if source == nil {
			continue
		}
		if err := c.LoadSource(source); err != nil {
			return err
		}
	}
	return nil
}

func (c *Config) LoadFiles(paths ...string) error {
	sources := make([]ConfigSource, 0, len(paths))
	for _, path := range paths {
		sources = append(sources, RequiredFile(path))
	}
	return c.LoadSources(sources...)
}

func (c *Config) LoadOptionalFiles(paths ...string) error {
	sources := make([]ConfigSource, 0, len(paths))
	for _, path := range paths {
		sources = append(sources, OptionalFile(path))
	}
	return c.LoadSources(sources...)
}

func (c *Config) SetDefault(section, key, value string) {
	c.SetDefaultAll(section, key, []string{value})
}

func (c *Config) SetDefaultAll(section, key string, values []string) {
	if c == nil || key == "" {
		return
	}
	ini := c.Ini()
	if ini.Has(section, key) {
		return
	}
	ini.Document.mu.Lock()
	sec := ini.Document.ensureSection(section)
	ini.Document.mu.Unlock()
	if sec != nil {
		_ = sec.SetAll(key, append([]string(nil), values...), "")
	}
}

func (c *Config) Bind(dst interface{}) error {
	if c == nil {
		return ErrDocumentClosed
	}
	v, err := configStructValue(dst)
	if err != nil {
		return err
	}
	if err := c.applyStructDefaults(v, ""); err != nil {
		return err
	}
	if err := c.applyEnv(v, ""); err != nil {
		return err
	}
	if err := c.checkRequired(v, ""); err != nil {
		return err
	}
	if err := c.bindStruct(v, ""); err != nil {
		return err
	}
	return validateConfig(dst)
}

func (c *Config) SetStruct(src interface{}) error {
	if c == nil {
		return ErrDocumentClosed
	}
	v, err := configReadableStructValue(src)
	if err != nil {
		return err
	}
	return c.setStruct(v, "")
}

type mergeValue struct {
	key     string
	values  []string
	comment string
	noValue bool
}

func (c *Config) merge(src *Ini) {
	if c == nil || src == nil || src.Document == nil {
		return
	}
	sections := make(map[string]map[string]*mergeValue)
	sectionOrder := make([]string, 0)
	keyOrder := make(map[string][]string)
	for _, section := range src.Document.Sections() {
		if section == nil {
			continue
		}
		secKey := normalize(section.Name, src.CaseSensitive)
		if _, ok := sections[secKey]; !ok {
			sections[secKey] = make(map[string]*mergeValue)
			sectionOrder = append(sectionOrder, section.Name)
		}
		for _, entry := range section.Entries {
			if entry == nil || entry.kind != linePair {
				continue
			}
			entryKey := normalize(entry.Key, section.CaseSensitive)
			mv := sections[secKey][entryKey]
			if mv == nil {
				mv = &mergeValue{}
				sections[secKey][entryKey] = mv
				keyOrder[secKey] = append(keyOrder[secKey], entry.Key)
			}
			mv.key = entry.Key
			mv.comment = entry.Comment
			mv.noValue = entry.NoValue
			if entry.NoValue {
				mv.values = nil
			} else {
				mv.values = append(mv.values, entry.Values...)
			}
		}
	}
	for _, sectionName := range sectionOrder {
		secKey := normalize(sectionName, src.CaseSensitive)
		for _, key := range keyOrder[secKey] {
			mv := sections[secKey][normalize(key, src.CaseSensitive)]
			if mv == nil {
				continue
			}
			c.setAll(sectionName, mv.key, mv.values, mv.comment)
		}
	}
}

func (c *Config) setAll(section, key string, values []string, comment string) {
	ini := c.Ini()
	ini.Document.mu.Lock()
	sec := ini.Document.ensureSection(section)
	ini.Document.mu.Unlock()
	if sec != nil {
		_ = sec.SetAll(key, append([]string(nil), values...), comment)
	}
}

func configStructValue(dst interface{}) (reflect.Value, error) {
	if dst == nil {
		return reflect.Value{}, errors.New("destination is nil")
	}
	v := reflect.ValueOf(dst)
	if v.Kind() != reflect.Ptr || v.IsNil() {
		return reflect.Value{}, errors.New("destination must be a non-nil pointer")
	}
	v = v.Elem()
	if v.Kind() != reflect.Struct {
		return reflect.Value{}, errors.New("destination must point to a struct")
	}
	return v, nil
}

func configReadableStructValue(src interface{}) (reflect.Value, error) {
	if src == nil {
		return reflect.Value{}, errors.New("source is nil")
	}
	v := reflect.ValueOf(src)
	for v.Kind() == reflect.Ptr {
		if v.IsNil() {
			return reflect.Value{}, errors.New("source must be a non-nil pointer or struct")
		}
		v = v.Elem()
	}
	if v.Kind() != reflect.Struct {
		return reflect.Value{}, errors.New("source must be struct")
	}
	return v, nil
}

func (c *Config) applyStructDefaults(v reflect.Value, inheritedSection string) error {
	return c.walkConfigFields(v, inheritedSection, func(field configField) error {
		if field.defaultValue == "" || c.has(field.section, field.key) {
			return nil
		}
		values, err := defaultValuesForField(field, field.defaultValue)
		if err != nil {
			return configFieldErrorWithErr(field, "invalid default: "+err.Error(), err)
		}
		c.setAll(field.section, field.key, values, "")
		return nil
	})
}

func (c *Config) applyEnv(v reflect.Value, inheritedSection string) error {
	if c.envLookup == nil {
		return nil
	}
	return c.walkConfigFields(v, inheritedSection, func(field configField) error {
		envName := c.envName(field)
		if envName == "" {
			return nil
		}
		value, ok := c.envLookup(envName)
		if !ok {
			return nil
		}
		values, err := defaultValuesForField(field, value)
		if err != nil {
			return configFieldErrorWithErr(field, "invalid env "+envName+": "+err.Error(), err)
		}
		c.setAll(field.section, field.key, values, "")
		return nil
	})
}

func (c *Config) checkRequired(v reflect.Value, inheritedSection string) error {
	return c.walkConfigFields(v, inheritedSection, func(field configField) error {
		if !field.required {
			return nil
		}
		items := c.values(field.section, field.key)
		if len(items) == 0 {
			return configFieldError(field, "required value is missing")
		}
		for _, item := range items {
			if item.noValue {
				if configFieldAcceptsNoValue(field.value) {
					return nil
				}
				continue
			}
			if strings.TrimSpace(item.text) != "" {
				return nil
			}
		}
		return configFieldError(field, "required value is empty")
	})
}

func (c *Config) bindStruct(v reflect.Value, inheritedSection string) error {
	return c.walkConfigFields(v, inheritedSection, func(field configField) error {
		items, err := expandConfigValues(field.value, c.values(field.section, field.key), field.split)
		if err != nil {
			return configFieldErrorWithErr(field, err.Error(), err)
		}
		if len(items) == 0 {
			return nil
		}
		if err := setConfigValueItems(field.value, items); err != nil {
			return configFieldErrorWithErr(field, err.Error(), err)
		}
		return nil
	})
}

func (c *Config) setStruct(v reflect.Value, inheritedSection string) error {
	t := v.Type()
	for idx := 0; idx < t.NumField(); idx++ {
		field := t.Field(idx)
		value := v.Field(idx)
		if !value.CanInterface() {
			continue
		}
		section := field.Tag.Get("seg")
		if section == "" {
			section = inheritedSection
		}
		key := field.Tag.Get("key")
		if nested, ok := nestedConfigValueForWrite(value, key); ok {
			if nested.IsValid() {
				if err := c.setStruct(nested, section); err != nil {
					return err
				}
			}
			continue
		}
		if key == "" {
			key = fieldNameKey(field.Name)
		}
		if key == "-" {
			continue
		}
		cfgField := configField{
			value:       value,
			structField: field,
			fieldPath:   field.Name,
			section:     section,
			key:         key,
		}
		values, err := outputValuesForField(cfgField)
		if err != nil {
			return configFieldErrorWithErr(cfgField, err.Error(), err)
		}
		if values == nil {
			continue
		}
		c.setAll(section, key, values, "")
	}
	return nil
}

type configField struct {
	value        reflect.Value
	structField  reflect.StructField
	fieldPath    string
	section      string
	key          string
	env          string
	defaultValue string
	split        string
	required     bool
}

func (c *Config) walkConfigFields(v reflect.Value, inheritedSection string, visit func(configField) error) error {
	t := v.Type()
	for idx := 0; idx < t.NumField(); idx++ {
		field := t.Field(idx)
		value := v.Field(idx)
		if !value.CanSet() {
			continue
		}
		section := field.Tag.Get("seg")
		if section == "" {
			section = inheritedSection
		}
		key := field.Tag.Get("key")
		if isNestedConfigStruct(value, key) {
			nested := value
			for nested.Kind() == reflect.Ptr {
				if nested.IsNil() {
					nested.Set(reflect.New(nested.Type().Elem()))
				}
				nested = nested.Elem()
			}
			if err := c.walkConfigFields(nested, section, visit); err != nil {
				return err
			}
			continue
		}
		if key == "" {
			key = fieldNameKey(field.Name)
		}
		if key == "-" {
			continue
		}
		cfgField := configField{
			value:        value,
			structField:  field,
			fieldPath:    field.Name,
			section:      section,
			key:          key,
			env:          field.Tag.Get("env"),
			defaultValue: field.Tag.Get("default"),
			split:        field.Tag.Get("split"),
			required:     parseTagBool(field.Tag.Get("required")),
		}
		if err := visit(cfgField); err != nil {
			return err
		}
	}
	return nil
}

func walkReadableConfigFields(v reflect.Value, inheritedSection, inheritedPath string, visit func(configField) error) error {
	t := v.Type()
	for idx := 0; idx < t.NumField(); idx++ {
		field := t.Field(idx)
		if field.PkgPath != "" {
			continue
		}
		value := v.Field(idx)
		section := field.Tag.Get("seg")
		if section == "" {
			section = inheritedSection
		}
		fieldPath := joinFieldPath(inheritedPath, field.Name)
		key := field.Tag.Get("key")
		if isNestedConfigFieldType(field.Type, key) {
			nested := readableNestedValue(value, field.Type)
			if err := walkReadableConfigFields(nested, section, fieldPath, visit); err != nil {
				return err
			}
			continue
		}
		if key == "" {
			key = fieldNameKey(field.Name)
		}
		if key == "-" {
			continue
		}
		cfgField := configField{
			value:        value,
			structField:  field,
			fieldPath:    fieldPath,
			section:      section,
			key:          key,
			env:          field.Tag.Get("env"),
			defaultValue: field.Tag.Get("default"),
			split:        field.Tag.Get("split"),
			required:     parseTagBool(field.Tag.Get("required")),
		}
		if err := visit(cfgField); err != nil {
			return err
		}
	}
	return nil
}

func isNestedConfigStruct(value reflect.Value, key string) bool {
	if key != "" {
		return false
	}
	t := value.Type()
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	if t.Kind() != reflect.Struct {
		return false
	}
	if t.PkgPath() == "time" {
		return false
	}
	return !implementsTextUnmarshaler(t)
}

func isNestedConfigFieldType(t reflect.Type, key string) bool {
	if key != "" {
		return false
	}
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	if t.Kind() != reflect.Struct {
		return false
	}
	if t.PkgPath() == "time" {
		return false
	}
	return !implementsTextUnmarshaler(t)
}

func readableNestedValue(value reflect.Value, t reflect.Type) reflect.Value {
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	for value.Kind() == reflect.Ptr {
		if value.IsNil() {
			return reflect.Zero(t)
		}
		value = value.Elem()
	}
	return value
}

func nestedConfigValueForWrite(value reflect.Value, key string) (reflect.Value, bool) {
	if !isNestedConfigStruct(value, key) {
		return reflect.Value{}, false
	}
	for value.Kind() == reflect.Ptr {
		if value.IsNil() {
			return reflect.Value{}, true
		}
		value = value.Elem()
	}
	return value, true
}

func configFieldAcceptsNoValue(value reflect.Value) bool {
	t := value.Type()
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	return t.Kind() == reflect.Bool
}

func (c *Config) has(section, key string) bool {
	return c.Ini().Has(section, key)
}

func (c *Config) value(section, key string) string {
	return c.Ini().Get(section, key)
}

type configValue struct {
	text    string
	noValue bool
}

func (c *Config) values(section, key string) []configValue {
	if c == nil {
		return nil
	}
	return configValuesFromSections(c.Ini().Sections(section), key)
}

func (c *Config) firstValue(section, key string) (configValue, error) {
	items := c.values(section, key)
	if len(items) == 0 {
		return configValue{}, configKeyError(section, key, "value is missing", ErrKeyNotFound)
	}
	return items[0], nil
}

func (c *Config) envName(field configField) string {
	if field.env == "-" {
		return ""
	}
	if field.env != "" {
		return field.env
	}
	parts := make([]string, 0, 3)
	if c.envPrefix != "" {
		parts = append(parts, c.envPrefix)
	}
	if field.section != "" {
		parts = append(parts, field.section)
	}
	parts = append(parts, field.key)
	return normalizeEnvName(strings.Join(parts, "_"))
}

func normalizeEnvName(name string) string {
	var builder strings.Builder
	builder.Grow(len(name))
	underscore := false
	for _, r := range name {
		if (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') {
			if r >= 'a' && r <= 'z' {
				r -= 'a' - 'A'
			}
			builder.WriteRune(r)
			underscore = false
			continue
		}
		if !underscore {
			builder.WriteByte('_')
			underscore = true
		}
	}
	return strings.Trim(builder.String(), "_")
}

func joinFieldPath(prefix, name string) string {
	if prefix == "" {
		return name
	}
	return prefix + "." + name
}

func fieldNameKey(name string) string {
	var builder strings.Builder
	for idx, r := range name {
		if r >= 'A' && r <= 'Z' {
			if idx > 0 {
				builder.WriteByte('_')
			}
			r += 'a' - 'A'
		}
		builder.WriteRune(r)
	}
	return builder.String()
}

func parseTagBool(value string) bool {
	switch strings.ToLower(strings.TrimSpace(value)) {
	case "1", "t", "true", "y", "yes", "required":
		return true
	default:
		return false
	}
}

func sampleValuesForField(field configField) ([]string, error) {
	if field.defaultValue != "" {
		return defaultValuesForField(field, field.defaultValue)
	}
	values, err := outputValuesForField(field)
	if err != nil {
		return nil, err
	}
	if field.required && needsRequiredSamplePlaceholder(values) {
		return requiredSampleValuesForType(field.value.Type())
	}
	if len(values) > 0 {
		return values, nil
	}
	values, err = sampleValuesForType(field.value.Type())
	if err != nil {
		return nil, err
	}
	if field.required && needsRequiredSamplePlaceholder(values) {
		return requiredSampleValuesForType(field.value.Type())
	}
	return values, nil
}

func sampleValuesForType(t reflect.Type) ([]string, error) {
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	switch t.Kind() {
	case reflect.Slice:
		return sampleScalarValue(t.Elem())
	case reflect.Array:
		if t.Len() == 0 {
			return nil, nil
		}
		values := make([]string, 0, t.Len())
		for idx := 0; idx < t.Len(); idx++ {
			items, err := sampleScalarValue(t.Elem())
			if err != nil {
				return nil, err
			}
			values = append(values, items...)
		}
		return values, nil
	case reflect.Map:
		if t.Key().Kind() != reflect.String {
			return nil, fmt.Errorf("unsupported map key kind %s", t.Key().Kind())
		}
		values, err := sampleScalarValue(t.Elem())
		if err != nil {
			return nil, err
		}
		if len(values) == 0 {
			return nil, nil
		}
		return []string{"key=" + values[0]}, nil
	default:
		return sampleScalarValue(t)
	}
}

func sampleScalarValue(t reflect.Type) ([]string, error) {
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	value := reflect.Zero(t)
	text, err := scalarToString(value)
	if err != nil {
		return nil, err
	}
	return []string{text}, nil
}

func needsRequiredSamplePlaceholder(values []string) bool {
	if len(values) == 0 {
		return true
	}
	for _, value := range values {
		if strings.TrimSpace(value) != "" {
			return false
		}
	}
	return true
}

func requiredSampleValuesForType(t reflect.Type) ([]string, error) {
	for t.Kind() == reflect.Ptr {
		t = t.Elem()
	}
	switch t.Kind() {
	case reflect.String:
		return []string{"value"}, nil
	case reflect.Slice:
		return requiredSampleValuesForType(t.Elem())
	case reflect.Array:
		if t.Len() == 0 {
			return nil, nil
		}
		values := make([]string, 0, t.Len())
		for idx := 0; idx < t.Len(); idx++ {
			items, err := requiredSampleValuesForType(t.Elem())
			if err != nil {
				return nil, err
			}
			values = append(values, items...)
		}
		return values, nil
	case reflect.Map:
		if t.Key().Kind() != reflect.String {
			return nil, fmt.Errorf("unsupported map key kind %s", t.Key().Kind())
		}
		values, err := requiredSampleValuesForType(t.Elem())
		if err != nil {
			return nil, err
		}
		if needsRequiredSamplePlaceholder(values) {
			values, err = sampleValuesForType(t.Elem())
			if err != nil {
				return nil, err
			}
		}
		if len(values) == 0 {
			return nil, nil
		}
		return []string{"key=" + values[0]}, nil
	default:
		return sampleValuesForType(t)
	}
}

func defaultValuesForField(field configField, text string) ([]string, error) {
	v := field.value
	for v.Kind() == reflect.Ptr {
		if v.IsNil() {
			v = reflect.New(v.Type().Elem()).Elem()
			break
		}
		v = v.Elem()
	}
	switch v.Kind() {
	case reflect.Slice, reflect.Array, reflect.Map:
		return splitFieldValue(text, field.split)
	}
	return []string{text}, nil
}

func splitFieldValue(text, split string) ([]string, error) {
	switch {
	case split == "":
		return []string{text}, nil
	case split == "," || strings.EqualFold(split, "csv"):
		return parseCSVList(text)
	default:
		return parseDelimitedList(text, split), nil
	}
}

func parseCSVList(text string) ([]string, error) {
	if text == "" {
		return []string{""}, nil
	}
	reader := csv.NewReader(strings.NewReader(text))
	reader.TrimLeadingSpace = true
	record, err := reader.Read()
	if err != nil {
		return nil, err
	}
	for idx := range record {
		record[idx] = strings.TrimSpace(record[idx])
	}
	return record, nil
}

func parseDelimitedList(text, split string) []string {
	if split == "" {
		return []string{text}
	}
	parts := strings.Split(text, split)
	for idx := range parts {
		parts[idx] = strings.TrimSpace(parts[idx])
	}
	return parts
}

func setConfigField(value reflect.Value, section *Section, key, split string) error {
	for value.Kind() == reflect.Ptr {
		if value.IsNil() {
			value.Set(reflect.New(value.Type().Elem()))
		}
		value = value.Elem()
	}
	items, err := expandConfigValues(value, configValuesFromSection(section, key), split)
	if err != nil {
		return err
	}
	if len(items) == 0 {
		return nil
	}
	return setConfigValueItems(value, items)
}

func setConfigValueItems(value reflect.Value, items []configValue) error {
	switch value.Kind() {
	case reflect.Slice:
		return setConfigSlice(value, items)
	case reflect.Array:
		return setConfigArray(value, items)
	case reflect.Map:
		return setConfigMap(value, items)
	default:
		return setConfigScalar(value, items[0])
	}
}

func expandConfigValues(value reflect.Value, items []configValue, split string) ([]configValue, error) {
	if split == "" {
		return items, nil
	}
	switch value.Kind() {
	case reflect.Slice, reflect.Array, reflect.Map:
	default:
		return items, nil
	}
	out := make([]configValue, 0, len(items))
	for _, item := range items {
		if item.noValue {
			out = append(out, item)
			continue
		}
		parts, err := splitFieldValue(item.text, split)
		if err != nil {
			return nil, err
		}
		for _, part := range parts {
			out = append(out, configValue{text: part})
		}
	}
	return out, nil
}

func configValuesFromSection(section *Section, key string) []configValue {
	return configValuesFromSections([]*Section{section}, key)
}

func configValuesFromSections(sections []*Section, key string) []configValue {
	values := make([]configValue, 0)
	for _, section := range sections {
		if section == nil {
			continue
		}
		entries := section.EntriesByKey(key)
		for _, entry := range entries {
			if entry == nil {
				continue
			}
			if entry.NoValue && len(entry.Values) == 0 {
				values = append(values, configValue{noValue: true})
				continue
			}
			for _, value := range entry.Values {
				values = append(values, configValue{text: value})
			}
		}
	}
	if len(values) == 0 {
		return nil
	}
	return values
}

func setConfigSlice(value reflect.Value, items []configValue) error {
	out := reflect.MakeSlice(value.Type(), 0, len(items))
	for _, item := range items {
		elem := reflect.New(value.Type().Elem()).Elem()
		if err := setConfigScalar(elem, item); err != nil {
			return err
		}
		out = reflect.Append(out, elem)
	}
	value.Set(out)
	return nil
}

func setConfigArray(value reflect.Value, items []configValue) error {
	if len(items) != value.Len() {
		return fmt.Errorf("array needs %d values, got %d", value.Len(), len(items))
	}
	for idx, item := range items {
		if err := setConfigScalar(value.Index(idx), item); err != nil {
			return err
		}
	}
	return nil
}

func setConfigMap(value reflect.Value, items []configValue) error {
	if value.Type().Key().Kind() != reflect.String {
		return fmt.Errorf("unsupported map key kind %s", value.Type().Key().Kind())
	}
	out := reflect.MakeMapWithSize(value.Type(), len(items))
	for idx, item := range items {
		key, text := parseMapItem(idx, item.text)
		elem := reflect.New(value.Type().Elem()).Elem()
		mapItem := configValue{text: text, noValue: item.noValue}
		if err := setConfigScalar(elem, mapItem); err != nil {
			return err
		}
		out.SetMapIndex(reflect.ValueOf(key), elem)
	}
	value.Set(out)
	return nil
}

func parseMapItem(idx int, text string) (string, string) {
	if at := strings.Index(text, "="); at >= 0 {
		return strings.TrimSpace(text[:at]), strings.TrimSpace(text[at+1:])
	}
	return strconv.Itoa(idx), text
}

func outputValuesForField(field configField) ([]string, error) {
	value := field.value
	for value.Kind() == reflect.Ptr {
		if value.IsNil() {
			return nil, nil
		}
		value = value.Elem()
	}
	switch value.Kind() {
	case reflect.Slice, reflect.Array:
		values := make([]string, 0, value.Len())
		for idx := 0; idx < value.Len(); idx++ {
			text, err := scalarToString(value.Index(idx))
			if err != nil {
				return nil, err
			}
			values = append(values, text)
		}
		return values, nil
	case reflect.Map:
		if value.Type().Key().Kind() != reflect.String {
			return nil, fmt.Errorf("unsupported map key kind %s", value.Type().Key().Kind())
		}
		keys := make([]string, 0, value.Len())
		for _, key := range value.MapKeys() {
			keys = append(keys, key.String())
		}
		sort.Strings(keys)
		values := make([]string, 0, len(keys))
		for _, key := range keys {
			text, err := scalarToString(value.MapIndex(reflect.ValueOf(key)))
			if err != nil {
				return nil, err
			}
			values = append(values, key+"="+text)
		}
		return values, nil
	default:
		text, err := scalarToString(value)
		if err != nil {
			return nil, err
		}
		return []string{text}, nil
	}
}

func scalarToString(value reflect.Value) (string, error) {
	for value.Kind() == reflect.Ptr {
		if value.IsNil() {
			return "", nil
		}
		value = value.Elem()
	}
	if ok, text, err := textMarshalerString(value); ok || err != nil {
		return text, err
	}
	if value.Type() == reflect.TypeOf(time.Duration(0)) {
		return time.Duration(value.Int()).String(), nil
	}
	switch value.Kind() {
	case reflect.String:
		return value.String(), nil
	case reflect.Bool:
		return strconv.FormatBool(value.Bool()), nil
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return strconv.FormatInt(value.Int(), 10), nil
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		return strconv.FormatUint(value.Uint(), 10), nil
	case reflect.Float32, reflect.Float64:
		return strconv.FormatFloat(value.Float(), 'g', -1, value.Type().Bits()), nil
	default:
		return "", fmt.Errorf("unsupported field kind %s", value.Kind())
	}
}

func textMarshalerString(value reflect.Value) (bool, string, error) {
	if value.CanInterface() {
		if marshaler, ok := value.Interface().(encoding.TextMarshaler); ok {
			data, err := marshaler.MarshalText()
			return true, string(data), err
		}
	}
	if value.CanAddr() {
		if marshaler, ok := value.Addr().Interface().(encoding.TextMarshaler); ok {
			data, err := marshaler.MarshalText()
			return true, string(data), err
		}
	}
	return false, "", nil
}

func setConfigScalar(value reflect.Value, item configValue) error {
	for value.Kind() == reflect.Ptr {
		if value.IsNil() {
			value.Set(reflect.New(value.Type().Elem()))
		}
		value = value.Elem()
	}
	if ok, err := setTextUnmarshaler(value, item.text); ok || err != nil {
		if err != nil {
			return err
		}
		return nil
	}
	if value.Type() == reflect.TypeOf(time.Duration(0)) {
		duration, err := time.ParseDuration(item.text)
		if err != nil {
			return err
		}
		value.SetInt(int64(duration))
		return nil
	}
	switch value.Kind() {
	case reflect.String:
		value.SetString(item.text)
	case reflect.Bool:
		if item.noValue {
			value.SetBool(true)
			return nil
		}
		v, err := strconv.ParseBool(item.text)
		if err != nil {
			return err
		}
		value.SetBool(v)
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		n, err := strconv.ParseInt(item.text, 10, value.Type().Bits())
		if err != nil {
			return err
		}
		value.SetInt(n)
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		n, err := strconv.ParseUint(item.text, 10, value.Type().Bits())
		if err != nil {
			return err
		}
		value.SetUint(n)
	case reflect.Float32, reflect.Float64:
		n, err := strconv.ParseFloat(item.text, value.Type().Bits())
		if err != nil {
			return err
		}
		value.SetFloat(n)
	default:
		return fmt.Errorf("unsupported field kind %s", value.Kind())
	}
	return nil
}

func setTextUnmarshaler(value reflect.Value, text string) (bool, error) {
	if value.CanAddr() {
		if unmarshaler, ok := value.Addr().Interface().(encoding.TextUnmarshaler); ok {
			return true, unmarshaler.UnmarshalText([]byte(text))
		}
	}
	if value.CanInterface() {
		if unmarshaler, ok := value.Interface().(encoding.TextUnmarshaler); ok {
			return true, unmarshaler.UnmarshalText([]byte(text))
		}
	}
	return false, nil
}

func implementsTextUnmarshaler(t reflect.Type) bool {
	textUnmarshaler := reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
	if t.Implements(textUnmarshaler) {
		return true
	}
	return reflect.PtrTo(t).Implements(textUnmarshaler)
}

func sectionHasPairs(section *Section) bool {
	if section == nil {
		return false
	}
	for _, entry := range section.Entries {
		if isConfigEntry(entry) {
			return true
		}
	}
	return false
}

func isConfigEntry(entry *Entry) bool {
	return entry != nil && entry.Key != ""
}

func configPath(section, key string) string {
	if section == "" {
		return key
	}
	return section + "." + key
}

func configFieldError(field configField, reason string) error {
	return configFieldErrorWithErr(field, reason, nil)
}

func configFieldErrorWithErr(field configField, reason string, err error) error {
	return &ConfigError{
		Section: field.section,
		Key:     field.key,
		Field:   field.structField.Name,
		Reason:  reason,
		Err:     err,
	}
}

func configKeyError(section, key, reason string, err error) error {
	return &ConfigError{
		Section: section,
		Key:     key,
		Reason:  reason,
		Err:     err,
	}
}

func validateConfig(dst interface{}) error {
	if validator, ok := dst.(ConfigValidator); ok {
		return validator.Validate()
	}
	value := reflect.ValueOf(dst)
	if value.Kind() == reflect.Ptr && !value.IsNil() {
		if validator, ok := value.Elem().Interface().(ConfigValidator); ok {
			return validator.Validate()
		}
	}
	return nil
}