// Internal Helpers
type $MergeBy<T, K> = Omit<T, keyof K> & K;
type $Dictionary<T = any> = { [key: string]: T };
type $Value<Obj, Key> = Key extends keyof Obj ? Obj[Key] : never;
type $OmitArrayKeys<Arr> = Arr extends readonly any[] ? Omit<Arr, keyof any[]> : Arr;
type $PreservedValue<Value, Fallback> = [Value] extends [never] ? Fallback : Value;
type $FirstNamespace<Ns extends Namespace> = Ns extends readonly any[] ? Ns[0] : Ns;
type $IsResourcesDefined = [keyof _Resources] extends [never] ? false : true;
type $ValueIfResourcesDefined<Value, Fallback> = $IsResourcesDefined extends true
  ? Value
  : Fallback;
type $SpecialObject = object | Array<string | object>;

/**
 * This interface can be augmented by users to add types to `i18next` default TypeOptions.
 *
 * Usage:
 * ```ts
 * // i18next.d.ts
 * import 'i18next';
 * declare module 'i18next' {
 *   interface CustomTypeOptions {
 *     defaultNS: 'custom';
 *     returnNull: false;
 *     returnObjects: false;
 *     nsSeparator: ':';
 *     keySeparator: '.';
 *     jsonFormat: 'v4';
 *     allowObjectInHTMLChildren: false;
 *     resources: {
 *       custom: {
 *         foo: 'foo';
 *       };
 *     };
 *   }
 * }
 * ```
 */
export interface CustomTypeOptions {}

/**
 * This interface can be augmented by users to add types to `i18next` default PluginOptions.
 */
export interface CustomPluginOptions {}

export type TypeOptions = $MergeBy<
  {
    /**
     * Allows null values as valid translation
     */
    returnNull: false;

    /**
     * Allows objects as valid translation result
     */
    returnObjects: false;

    /**
     * Char to separate keys
     */
    keySeparator: '.';

    /**
     * Char to split namespace from key
     */
    nsSeparator: ':';

    /**
     * Char to split namespace from key
     */
    pluralSeparator: '_';

    /**
     * Default namespace used if not passed to translation function
     */
    defaultNS: 'translation';

    /**
     * Fallback namespace used if translation not found in given namespace
     * @default false
     */
    fallbackNS: false;

    /**
     * Json Format Version - V4 allows plural suffixes
     */
    jsonFormat: 'v4';

    /**
     * Resources to initialize with
     */
    resources: object;

    /**
     * Flag that allows HTML elements to receive objects. This is only useful for React applications
     * where you pass objects to HTML elements so they can be replaced to their respective interpolation
     * values (mostly with Trans component)
     */
    allowObjectInHTMLChildren: false;

    /**
     * Prefix for interpolation
     */
    interpolationPrefix: '{{';

    /**
     * Suffix for interpolation
     */
    interpolationSuffix: '}}';
  },
  CustomTypeOptions
>;

export type PluginOptions<T> = $MergeBy<
  {
    /**
     * Options for language detection - check documentation of plugin
     * @default undefined
     */
    detection?: object;

    /**
     * Options for backend - check documentation of plugin
     * @default undefined
     */
    backend?: T;

    /**
     * Options for cache layer - check documentation of plugin
     * @default undefined
     */
    cache?: object;

    /**
     * Options for i18n message format - check documentation of plugin
     * @default undefined
     */
    i18nFormat?: object;
  },
  CustomPluginOptions
>;

export type FormatFunction = (
  value: any,
  format?: string,
  lng?: string,
  options?: InterpolationOptions & $Dictionary,
) => string;

export interface InterpolationOptions {
  /**
   * Format function see formatting for details
   * @default noop
   */
  format?: FormatFunction;
  /**
   * Used to separate format from interpolation value
   * @default ','
   */
  formatSeparator?: string;
  /**
   * Escape function
   * @default str => str
   */
  escape?(str: string): string;

  /**
   * Always format interpolated values.
   * @default false
   */
  alwaysFormat?: boolean;
  /**
   * Escape passed in values to avoid xss injection
   * @default true
   */
  escapeValue?: boolean;
  /**
   * If true, then value passed into escape function is not casted to string, use with custom escape function that does its own type check
   * @default false
   */
  useRawValueToEscape?: boolean;
  /**
   * Prefix for interpolation
   * @default '{{'
   */
  prefix?: string;
  /**
   * Suffix for interpolation
   * @default '}}'
   */
  suffix?: string;
  /**
   * Escaped prefix for interpolation (regexSafe)
   * @default undefined
   */
  prefixEscaped?: string;
  /**
   * Escaped suffix for interpolation (regexSafe)
   * @default undefined
   */
  suffixEscaped?: string;
  /**
   * Suffix to unescaped mode
   * @default undefined
   */
  unescapeSuffix?: string;
  /**
   * Prefix to unescaped mode
   * @default '-'
   */
  unescapePrefix?: string;
  /**
   * Prefix for nesting
   * @default '$t('
   */
  nestingPrefix?: string;
  /**
   * Suffix for nesting
   * @default ')'
   */
  nestingSuffix?: string;
  /**
   * Escaped prefix for nesting (regexSafe)
   * @default undefined
   */
  nestingPrefixEscaped?: string;
  /**
   * Escaped suffix for nesting (regexSafe)
   * @default undefined
   */
  nestingSuffixEscaped?: string;
  /**
   * Separates options from key
   * @default ','
   */
  nestingOptionsSeparator?: string;
  /**
   * Global variables to use in interpolation replacements
   * @default undefined
   */

  defaultVariables?: { [index: string]: any };
  /**
   * After how many interpolation runs to break out before throwing a stack overflow
   * @default 1000
   */
  maxReplaces?: number;

  /**
   * If true, it will skip to interpolate the variables
   * @default true
   */
  skipOnVariables?: boolean;
}

export interface FallbackLngObjList {
  [language: string]: readonly string[];
}

export type FallbackLng =
  | string
  | readonly string[]
  | FallbackLngObjList
  | ((code: string) => string | readonly string[] | FallbackLngObjList);

export interface ReactOptions {
  /**
   * Set it to fallback to let passed namespaces to translated hoc act as fallbacks
   * @default 'default'
   */
  nsMode?: 'default' | 'fallback';
  /**
   * Set it to the default parent element created by the Trans component.
   * @default 'div'
   */
  defaultTransParent?: string;
  /**
   * Set which events trigger a re-render, can be set to false or string of events
   * @default 'languageChanged'
   */
  bindI18n?: string | false;
  /**
   * Set which events on store trigger a re-render, can be set to false or string of events
   * @default ''
   */
  bindI18nStore?: string | false;
  /**
   * Set fallback value for Trans components without children
   * @default undefined
   */
  transEmptyNodeValue?: string;
  /**
   * Set it to false if you do not want to use Suspense
   * @default true
   */
  useSuspense?: boolean;
  /**
   * Function to generate an i18nKey from the defaultValue (or Trans children)
   * when no key is provided.
   * By default, the defaultValue (Trans text) itself is used as the key.
   * If you want to require keys for all translations, supply a function
   * that always throws an error.
   * @default undefined
   */
  hashTransKey?(defaultValue: TOptionsBase['defaultValue']): TOptionsBase['defaultValue'];
  /**
   * Convert eg. <br/> found in translations to a react component of type br
   * @default true
   */
  transSupportBasicHtmlNodes?: boolean;
  /**
   * Which nodes not to convert in defaultValue generation in the Trans component.
   * @default ['br', 'strong', 'i', 'p']
   */
  transKeepBasicHtmlNodesFor?: readonly string[];
  /**
   * Wrap text nodes in a user-specified element.
   * @default ''
   */
  transWrapTextNodes?: string;
  /**
   * Optional keyPrefix that will be automatically applied to returned t function in useTranslation for example.
   * @default undefined
   */
  keyPrefix?: string;
  /**
   * Unescape function
   * by default it unescapes some basic html entities
   */
  unescape?(str: string): string;
}

export interface InitOptions<T = object> extends PluginOptions<T> {
  /**
   * Logs info level to console output. Helps finding issues with loading not working.
   * @default false
   */
  debug?: boolean;

  /**
   * Resources to initialize with (if not using loading or not appending using addResourceBundle)
   * @default undefined
   */
  resources?: Resource;

  /**
   * Allow initializing with bundled resources while using a backend to load non bundled ones.
   * @default false
   */
  partialBundledLanguages?: boolean;

  /**
   * Language to use (overrides language detection)
   * @default undefined
   */
  lng?: string;

  /**
   * Language to use if translations in user language are not available.
   * @default 'dev'
   */
  fallbackLng?: false | FallbackLng;

  /**
   * Array of allowed languages
   * @default false
   */
  supportedLngs?: false | readonly string[];

  /**
   * If true will pass eg. en-US if finding en in supportedLngs
   * @default false
   */
  nonExplicitSupportedLngs?: boolean;

  /**
   * Language codes to lookup, given set language is
   * 'en-US': 'all' --> ['en-US', 'en', 'dev'],
   * 'currentOnly' --> 'en-US',
   * 'languageOnly' --> 'en'
   * @default 'all'
   */
  load?: 'all' | 'currentOnly' | 'languageOnly';

  /**
   * Array of languages to preload. Important on server-side to assert translations are loaded before rendering views.
   * @default false
   */
  preload?: false | readonly string[];

  /**
   * Language will be lowercased eg. en-US --> en-us
   * @default false
   */
  lowerCaseLng?: boolean;

  /**
   * Language will be lowercased EN --> en while leaving full locales like en-US
   * @default false
   */
  cleanCode?: boolean;

  /**
   * String or array of namespaces to load
   * @default 'translation'
   */
  ns?: string | readonly string[];

  /**
   * Default namespace used if not passed to translation function
   * @default 'translation'
   */
  defaultNS?: string | false | readonly string[];

  /**
   * String or array of namespaces to lookup key if not found in given namespace.
   * @default false
   */
  fallbackNS?: false | string | readonly string[];

  /**
   * Calls save missing key function on backend if key not found.
   * @default false
   */
  saveMissing?: boolean;

  /**
   * Calls save missing key function on backend if key not found also for plural forms.
   * @default false
   */
  saveMissingPlurals?: boolean;

  /**
   * Experimental: enable to update default values using the saveMissing
   * (Works only if defaultValue different from translated value.
   * Only useful on initial development or when keeping code as source of truth not changing values outside of code.
   * Only supported if backend supports it already)
   * @default false
   */
  updateMissing?: boolean;

  /**
   * @default 'fallback'
   */
  saveMissingTo?: 'current' | 'all' | 'fallback';

  /**
   * Used to not fallback to the key as default value, when using saveMissing functionality.
   * i.e. when using with i18next-http-backend this will result in having a key with an empty string value.
   * @default false
   */
  missingKeyNoValueFallbackToKey?: boolean;

  /**
   * Used for custom missing key handling (needs saveMissing set to true!)
   * @default false
   */
  missingKeyHandler?:
    | false
    | ((
        lngs: readonly string[],
        ns: string,
        key: string,
        fallbackValue: string,
        updateMissing: boolean,
        options: any,
      ) => void);

  /**
   * Receives a key that was not found in `t()` and returns a value, that will be returned by `t()`
   * @default noop
   */
  parseMissingKeyHandler?(key: string, defaultValue?: string): any;

  /**
   * Appends namespace to missing key
   * @default false
   */
  appendNamespaceToMissingKey?: boolean;

  /**
   * Gets called in case a interpolation value is undefined. This method will not be called if the value is empty string or null
   * @default noop
   */
  missingInterpolationHandler?: (text: string, value: any, options: InitOptions) => any;

  /**
   * Will use 'plural' as suffix for languages only having 1 plural form, setting it to false will suffix all with numbers
   * @default true
   */
  simplifyPluralSuffix?: boolean;

  /**
   * String or array of postProcessors to apply per default
   * @default false
   */
  postProcess?: false | string | readonly string[];

  /**
   * passthrough the resolved object including 'usedNS', 'usedLang' etc into options object of postprocessors as 'i18nResolved' property
   * @default false
   */
  postProcessPassResolved?: boolean;

  /**
   * Allows null values as valid translation
   * @default false
   */
  returnNull?: boolean;

  /**
   * Allows empty string as valid translation
   * @default true
   */
  returnEmptyString?: boolean;

  /**
   * Allows objects as valid translation result
   * @default false
   */
  returnObjects?: boolean;

  /**
   * Returns an object that includes information about the used language, namespace, key and value
   * @default false
   */
  returnDetails?: boolean;

  /**
   * Gets called if object was passed in as key but returnObjects was set to false
   * @default noop
   */
  returnedObjectHandler?(key: string, value: string, options: any): void;

  /**
   * Char, eg. '\n' that arrays will be joined by
   * @default false
   */
  joinArrays?: false | string;

  /**
   * Sets defaultValue
   * @default args => ({ defaultValue: args[1] })
   */
  overloadTranslationOptionHandler?(args: string[]): TOptions;

  /**
   * @see https://www.i18next.com/interpolation.html
   */
  interpolation?: InterpolationOptions;

  /**
   * Options for react - check documentation of plugin
   * @default undefined
   */
  react?: ReactOptions;

  /**
   * Triggers resource loading in init function inside a setTimeout (default async behaviour).
   * Set it to false if your backend loads resources sync - that way calling i18next.t after
   * init is possible without relaying on the init callback.
   * @default true
   */
  initImmediate?: boolean;

  /**
   * Char to separate keys
   * @default '.'
   */
  keySeparator?: false | string;

  /**
   * Char to split namespace from key
   * @default ':'
   */
  nsSeparator?: false | string;

  /**
   * Char to split plural from key
   * @default '_'
   */
  pluralSeparator?: string;

  /**
   * Char to split context from key
   * @default '_'
   */
  contextSeparator?: string;

  /**
   * Prefixes the namespace to the returned key when using `cimode`
   * @default false
   */
  appendNamespaceToCIMode?: boolean;

  /**
   * Compatibility JSON version
   * @default 'v4'
   */
  compatibilityJSON?: 'v1' | 'v2' | 'v3' | 'v4';

  /**
   * Options for https://github.com/locize/locize-lastused
   * @default undefined
   */
  locizeLastUsed?: {
    /**
     * The id of your locize project
     */
    projectId: string;

    /**
     * An api key if you want to send missing keys
     */
    apiKey?: string;

    /**
     * The reference language of your project
     * @default 'en'
     */
    referenceLng?: string;

    /**
     * Version
     * @default 'latest'
     */
    version?: string;

    /**
     * Debounce interval to send data in milliseconds
     * @default 90000
     */
    debounceSubmit?: number;

    /**
     * Hostnames that are allowed to send last used data.
     * Please keep those to your local system, staging, test servers (not production)
     * @default ['localhost']
     */
    allowedHosts?: readonly string[];
  };

  /**
   * Automatically lookup for a flat key if a nested key is not found an vice-versa
   * @default true
   */
  ignoreJSONStructure?: boolean;

  /**
   * Limit parallelism of calls to backend
   * This is needed to prevent trying to open thousands of
   * sockets or file descriptors, which can cause failures
   * and actually make the entire process take longer.
   * @default 10
   */
  maxParallelReads?: number;

  /**
   * The maximum number of retries to perform.
   * Note that retries are only performed when a request has no response
   * and throws an error.
   * The default value is used if value is set below 0.
   * @default 5
   */
  maxRetries?: number;

  /**
   * Set how long to wait, in milliseconds, betweeen retries of failed requests.
   * This number is compounded by a factor of 2 for subsequent retry.
   * The default value is used if value is set below 1ms.
   * @default 350
   */
  retryTimeout?: number;
}

export interface TOptionsBase {
  /**
   * Default value to return if a translation was not found
   */
  defaultValue?: any;
  /**
   * Count value used for plurals
   */
  count?: number;
  /**
   * Ordinal flag for ordinal plurals
   */
  ordinal?: boolean;
  /**
   * Used for contexts (eg. male\female)
   */
  context?: any;
  /**
   * Object with vars for interpolation - or put them directly in options
   */
  replace?: any;
  /**
   * Override language to use
   */
  lng?: string;
  /**
   * Override languages to use
   */
  lngs?: readonly string[];
  /**
   * Override language to lookup key if not found see fallbacks for details
   */
  fallbackLng?: FallbackLng;
  /**
   * Override namespaces (string or array)
   */
  ns?: Namespace;
  /**
   * Override char to separate keys
   */
  keySeparator?: false | string;
  /**
   * Override char to split namespace from key
   */
  nsSeparator?: false | string;
  /**
   * Accessing an object not a translation string (can be set globally too)
   */
  returnObjects?: boolean;
  /**
   * Returns an object that includes information about the used language, namespace, key and value
   */
  returnDetails?: boolean;
  /**
   * Char, eg. '\n' that arrays will be joined by (can be set globally too)
   */
  joinArrays?: string;
  /**
   * String or array of postProcessors to apply see interval plurals as a sample
   */
  postProcess?: string | readonly string[];
  /**
   * Override interpolation options
   */
  interpolation?: InterpolationOptions;
}

// Type Options
type _ReturnObjects = TypeOptions['returnObjects'];
type _ReturnNull = TypeOptions['returnNull'];
type _KeySeparator = TypeOptions['keySeparator'];
type _NsSeparator = TypeOptions['nsSeparator'];
type _PluralSeparator = TypeOptions['pluralSeparator'];
type _DefaultNamespace = TypeOptions['defaultNS'];
type _FallbackNamespace = TypeOptions['fallbackNS'];
type _Resources = TypeOptions['resources'];
type _JSONFormat = TypeOptions['jsonFormat'];
type _InterpolationPrefix = TypeOptions['interpolationPrefix'];
type _InterpolationSuffix = TypeOptions['interpolationSuffix'];

type Resources = $ValueIfResourcesDefined<_Resources, $Dictionary<string>>;
export type FlatNamespace = $PreservedValue<keyof _Resources, string>;
export type Namespace<T = FlatNamespace> = T | readonly T[];

export type TOptions<TInterpolationMap extends object = $Dictionary> = TOptionsBase &
  TInterpolationMap;

type PluralSuffix = 'zero' | 'one' | 'two' | 'few' | 'many' | 'other';

type WithOrWithoutPlural<Key> = _JSONFormat extends 'v4'
  ? Key extends `${infer KeyWithoutOrdinalPlural}${_PluralSeparator}ordinal${_PluralSeparator}${PluralSuffix}`
    ? KeyWithoutOrdinalPlural | Key
    : Key extends `${infer KeyWithoutPlural}${_PluralSeparator}${PluralSuffix}`
    ? KeyWithoutPlural | Key
    : Key
  : Key;

type JoinKeys<K1, K2> = `${K1 & string}${_KeySeparator}${K2 & string}`;
type AppendNamespace<Ns, Keys> = `${Ns & string}${_NsSeparator}${Keys & string}`;

/******************************************************
 * Build all keys and key prefixes based on Resources *
 ******************************************************/
type KeysBuilderWithReturnObjects<Res, Key = keyof Res> = Key extends keyof Res
  ? Res[Key] extends $Dictionary
    ?
        | JoinKeys<Key, WithOrWithoutPlural<keyof $OmitArrayKeys<Res[Key]>>>
        | JoinKeys<Key, KeysBuilderWithReturnObjects<Res[Key]>>
    : never
  : never;

type KeysBuilderWithoutReturnObjects<Res, Key = keyof $OmitArrayKeys<Res>> = Key extends keyof Res
  ? Res[Key] extends $Dictionary
    ? JoinKeys<Key, KeysBuilderWithoutReturnObjects<Res[Key]>>
    : Key
  : never;

type KeysBuilder<Res, WithReturnObjects> = $IsResourcesDefined extends true
  ? WithReturnObjects extends true
    ? keyof Res | KeysBuilderWithReturnObjects<Res>
    : KeysBuilderWithoutReturnObjects<Res>
  : string;

type KeysWithReturnObjects = {
  [Ns in FlatNamespace]: WithOrWithoutPlural<KeysBuilder<Resources[Ns], true>>;
};
type KeysWithoutReturnObjects = {
  [Ns in FlatNamespace]: WithOrWithoutPlural<KeysBuilder<Resources[Ns], false>>;
};

type ResourceKeys<WithReturnObjects = _ReturnObjects> = WithReturnObjects extends true
  ? KeysWithReturnObjects
  : KeysWithoutReturnObjects;

/************************************************************************
 * Parse t function keys based on the namespace, options and key prefix *
 ************************************************************************/
type KeysByTOptions<TOpt extends TOptions> = TOpt['returnObjects'] extends true
  ? ResourceKeys<true>
  : ResourceKeys;

type NsByTOptions<Ns extends Namespace, TOpt extends TOptions> = TOpt['ns'] extends Namespace
  ? TOpt['ns']
  : Ns;

type ParseKeysByKeyPrefix<Keys, KPrefix> = KPrefix extends string
  ? Keys extends `${KPrefix}${_KeySeparator}${infer Key}`
    ? Key
    : never
  : Keys;

type ParseKeysByNamespaces<Ns extends Namespace, Keys> = Ns extends readonly (infer UnionNsps)[]
  ? UnionNsps extends keyof Keys
    ? AppendNamespace<UnionNsps, Keys[UnionNsps]>
    : never
  : never;

type ParseKeysByFallbackNs<Keys extends $Dictionary> = _FallbackNamespace extends false
  ? never
  : _FallbackNamespace extends (infer UnionFallbackNs extends string)[]
  ? Keys[UnionFallbackNs]
  : Keys[_FallbackNamespace & string];

export type ParseKeys<
  Ns extends Namespace = _DefaultNamespace,
  TOpt extends TOptions = {},
  KPrefix = undefined,
  Keys extends $Dictionary = KeysByTOptions<TOpt>,
  ActualNS extends Namespace = NsByTOptions<Ns, TOpt>,
> = $IsResourcesDefined extends true
  ?
      | ParseKeysByKeyPrefix<Keys[$FirstNamespace<ActualNS>], KPrefix>
      | ParseKeysByNamespaces<ActualNS, Keys>
      | ParseKeysByFallbackNs<Keys>
  : string;

/*********************************************************
 * Parse t function return type and interpolation values *
 *********************************************************/
type ParseInterpolationValues<Ret> =
  Ret extends `${string}${_InterpolationPrefix}${infer Value}${_InterpolationSuffix}${infer Rest}`
    ?
        | (Value extends `${infer ActualValue},${string}` ? ActualValue : Value)
        | ParseInterpolationValues<Rest>
    : never;
type InterpolationMap<Ret> = Record<$PreservedValue<ParseInterpolationValues<Ret>, string>, any>;

type ParseTReturnPlural<
  Res,
  Key,
  KeyWithPlural = `${Key & string}${_PluralSeparator}${PluralSuffix}`,
  KeyWithOrdinalPlural = `${Key &
    string}${_PluralSeparator}ordinal${_PluralSeparator}${PluralSuffix}`,
> = KeyWithOrdinalPlural extends keyof Res
  ? Res[KeyWithOrdinalPlural]
  : KeyWithPlural extends keyof Res
  ? Res[KeyWithPlural]
  : $Value<Res, Key>;

type ParseTReturn<Key, Res> = Key extends `${infer K1}${_KeySeparator}${infer RestKey}`
  ? ParseTReturn<RestKey, $Value<Res, K1>>
  : ParseTReturnPlural<Res, Key>;

type TReturnOptionalNull = _ReturnNull extends true ? null : never;
type TReturnOptionalObjects<TOpt extends TOptions> = _ReturnObjects extends true
  ? $SpecialObject
  : TOpt['returnObjects'] extends true
  ? $SpecialObject
  : never;
type DefaultTReturn<TOpt extends TOptions> =
  | string
  | TReturnOptionalObjects<TOpt>
  | TReturnOptionalNull;

export type TFunctionReturn<
  Ns extends Namespace,
  Key,
  TOpt extends TOptions,
  ActualNS extends Namespace = NsByTOptions<Ns, TOpt>,
> = $IsResourcesDefined extends true
  ? Key extends `${infer Nsp}${_NsSeparator}${infer RestKey}`
    ? ParseTReturn<RestKey, $Value<Resources, Nsp>>
    : ParseTReturn<Key, Resources[$FirstNamespace<ActualNS>]>
  : DefaultTReturn<TOpt>;

type TFunctionReturnOptionalDetails<Ret, TOpt extends TOptions> = TOpt['returnDetails'] extends true
  ? TFunctionDetailedResult<Ret>
  : Ret;

type AppendKeyPrefix<Key, KPrefix> = KPrefix extends string
  ? `${KPrefix}${_KeySeparator}${Key & string}`
  : Key;

/**************************
 * T function declaration *
 **************************/
export interface TFunction<Ns extends Namespace = _DefaultNamespace, KPrefix = undefined> {
  <
    Key extends ParseKeys<Ns, TOpt, KPrefix> | TemplateStringsArray,
    TOpt extends TOptions,
    Ret extends TFunctionReturn<Ns, AppendKeyPrefix<Key, KPrefix>, TOpt>,
  >(
    ...args:
      | [key: Key | Key[], options?: TOpt & InterpolationMap<Ret>]
      | [key: Key | Key[], defaultValue: string, options?: TOpt & InterpolationMap<Ret>]
  ): TFunctionReturnOptionalDetails<Ret, TOpt>;
}

export type KeyPrefix<Ns extends Namespace> = ResourceKeys<true>[$FirstNamespace<Ns>] | undefined;

export interface WithT<Ns extends Namespace = _DefaultNamespace> {
  // Expose parameterized t in the i18next interface hierarchy
  t: TFunction<Ns>;
}

export type TFunctionDetailedResult<T = string> = {
  /**
   * The plain used key
   */
  usedKey: string;
  /**
   * The translation result.
   */
  res: T;
  /**
   * The key with context / plural
   */
  exactUsedKey: string;
  /**
   * The used language for this translation.
   */
  usedLng: string;
  /**
   * The used namespace for this translation.
   */
  usedNS: string;
};

export interface Resource {
  [language: string]: ResourceLanguage;
}

export interface ResourceLanguage {
  [namespace: string]: ResourceKey;
}

export type ResourceKey =
  | string
  | {
      [key: string]: any;
    };

export interface Interpolator {
  init(options: InterpolationOptions, reset: boolean): undefined;
  reset(): undefined;
  resetRegExp(): undefined;
  interpolate(str: string, data: object, lng: string, options: InterpolationOptions): string;
  nest(str: string, fc: (...args: any[]) => any, options: InterpolationOptions): string;
}

export class ResourceStore {
  constructor(data: Resource, options: InitOptions);

  public data: Resource;
  public options: InitOptions;

  /**
   * Gets fired when resources got added or removed
   */
  on(event: 'added' | 'removed', callback: (lng: string, ns: string) => void): void;
  /**
   * Remove event listener
   * removes all callback when callback not specified
   */
  off(event: 'added' | 'removed', callback?: (lng: string, ns: string) => void): void;
}

export interface Formatter {
  init(services: Services, i18nextOptions: InitOptions): void;
  add(name: string, fc: (value: any, lng: string | undefined, options: any) => string): void;
  addCached(
    name: string,
    fc: (lng: string | undefined, options: any) => (value: any) => string,
  ): void;
  format: FormatFunction;
}

export interface Services {
  backendConnector: any;
  i18nFormat: any;
  interpolator: Interpolator;
  languageDetector: any;
  languageUtils: any;
  logger: any;
  pluralResolver: any;
  resourceStore: ResourceStore;
  formatter?: Formatter;
}

export type ModuleType =
  | 'backend'
  | 'logger'
  | 'languageDetector'
  | 'postProcessor'
  | 'i18nFormat'
  | 'formatter'
  | '3rdParty';

export interface Module {
  type: ModuleType;
}

export type CallbackError = Error | string | null | undefined;
export type ReadCallback = (
  err: CallbackError,
  data: ResourceKey | boolean | null | undefined,
) => void;
export type MultiReadCallback = (err: CallbackError, data: Resource | null | undefined) => void;

/**
 * Used to load data for i18next.
 * Can be provided as a singleton or as a prototype constructor (preferred for supporting multiple instances of i18next).
 * For singleton set property `type` to `'backend'` For a prototype constructor set static property.
 */
export interface BackendModule<TOptions = object> extends Module {
  type: 'backend';
  init(services: Services, backendOptions: TOptions, i18nextOptions: InitOptions): void;
  read(language: string, namespace: string, callback: ReadCallback): void;
  /** Save the missing translation */
  create?(
    languages: readonly string[],
    namespace: string,
    key: string,
    fallbackValue: string,
  ): void;
  /** Load multiple languages and namespaces. For backends supporting multiple resources loading */
  readMulti?(
    languages: readonly string[],
    namespaces: readonly string[],
    callback: MultiReadCallback,
  ): void;
  /** Store the translation. For backends acting as cache layer */
  save?(language: string, namespace: string, data: ResourceLanguage): void;
}

/**
 * Used to detect language in user land.
 * Can be provided as a singleton or as a prototype constructor (preferred for supporting multiple instances of i18next).
 * For singleton set property `type` to `'languageDetector'` For a prototype constructor set static property.
 */
export interface LanguageDetectorModule extends Module {
  type: 'languageDetector';
  init?(services: Services, detectorOptions: object, i18nextOptions: InitOptions): void;
  /** Must return detected language */
  detect(): string | readonly string[] | undefined;
  cacheUserLanguage?(lng: string): void;
}

/**
 * Used to detect language in user land.
 * Can be provided as a singleton or as a prototype constructor (preferred for supporting multiple instances of i18next).
 * For singleton set property `type` to `'languageDetector'` For a prototype constructor set static property.
 */
export interface LanguageDetectorAsyncModule extends Module {
  type: 'languageDetector';
  /** Set to true to enable async detection */
  async: true;
  init?(services: Services, detectorOptions: object, i18nextOptions: InitOptions): void;
  /** Must call callback passing detected language or return a Promise*/
  detect(
    callback: (lng: string | readonly string[] | undefined) => void | undefined,
  ): void | Promise<string | readonly string[] | undefined>;
  cacheUserLanguage?(lng: string): void | Promise<void>;
}

/**
 * Used to extend or manipulate the translated values before returning them in `t` function.
 * Need to be a singleton object.
 */
export interface PostProcessorModule extends Module {
  /** Unique name */
  name: string;
  type: 'postProcessor';
  process(value: string, key: string | string[], options: TOptions, translator: any): string;
}

/**
 * Override the built-in console logger.
 * Do not need to be a prototype function.
 */
export interface LoggerModule extends Module {
  type: 'logger';
  log(...args: any[]): void;
  warn(...args: any[]): void;
  error(...args: any[]): void;
}

export interface I18nFormatModule extends Module {
  type: 'i18nFormat';
}

export interface FormatterModule extends Module, Formatter {
  type: 'formatter';
}

export interface ThirdPartyModule extends Module {
  type: '3rdParty';
  init(i18next: i18n): void;
}

export interface Modules {
  backend?: BackendModule;
  logger?: LoggerModule;
  languageDetector?: LanguageDetectorModule | LanguageDetectorAsyncModule;
  i18nFormat?: I18nFormatModule;
  formatter?: FormatterModule;
  external: ThirdPartyModule[];
}

// helper to identify class https://stackoverflow.com/a/45983481/2363935
export interface Newable<T> {
  new (...args: any[]): T;
}
export interface NewableModule<T extends Module> extends Newable<T> {
  type: T['type'];
}

export type Callback = (error: any, t: TFunction) => void;

/**
 * Uses similar args as the t function and returns true if a key exists.
 */
export interface ExistsFunction<
  TKeys extends string = string,
  TInterpolationMap extends object = $Dictionary,
> {
  (key: TKeys | TKeys[], options?: TOptions<TInterpolationMap>): boolean;
}

export interface CloneOptions extends InitOptions {
  /**
   * Will create a new instance of the resource store and import the existing translation resources.
   * This way it will not shared the resource store instance.
   * @default false
   */
  forkResourceStore?: boolean;
}

export interface i18n {
  // Expose parameterized t in the i18next interface hierarchy
  t: TFunction<[_DefaultNamespace, ...Exclude<FlatNamespace, _DefaultNamespace>[]]>;

  /**
   * The default of the i18next module is an i18next instance ready to be initialized by calling init.
   * You can create additional instances using the createInstance function.
   *
   * @param options - Initial options.
   * @param callback - will be called after all translations were loaded or with an error when failed (in case of using a backend).
   */
  init(callback?: Callback): Promise<TFunction>;
  init<T>(options: InitOptions<T>, callback?: Callback): Promise<TFunction>;

  loadResources(callback?: (err: any) => void): void;

  /**
   * The use function is there to load additional plugins to i18next.
   * For available module see the plugins page and don't forget to read the documentation of the plugin.
   *
   * @param module Accepts a class or object
   */
  use<T extends Module>(module: T | NewableModule<T> | Newable<T>): this;

  /**
   * List of modules used
   */
  modules: Modules;

  /**
   * Internal container for all used plugins and implementation details like languageUtils, pluralResolvers, etc.
   */
  services: Services;

  /**
   * Internal container for translation resources
   */
  store: ResourceStore;

  /**
   * Uses similar args as the t function and returns true if a key exists.
   */
  exists: ExistsFunction;

  /**
   * Returns a resource data by language.
   */
  getDataByLanguage(lng: string): { [key: string]: { [key: string]: string } } | undefined;

  /**
   * Returns a t function that defaults to given language or namespace.
   * Both params could be arrays of languages or namespaces and will be treated as fallbacks in that case.
   * On the returned function you can like in the t function override the languages or namespaces by passing them in options or by prepending namespace.
   *
   * Accepts optional keyPrefix that will be automatically applied to returned t function.
   */
  getFixedT<
    Ns extends Namespace | null = _DefaultNamespace,
    TKPrefix extends KeyPrefix<ActualNs> = undefined,
    ActualNs extends Namespace = Ns extends null ? _DefaultNamespace : Ns,
  >(
    ...args:
      | [lng: string | readonly string[], ns?: Ns, keyPrefix?: TKPrefix]
      | [lng: null, ns: Ns, keyPrefix?: TKPrefix]
  ): TFunction<ActualNs, TKPrefix>;

  /**
   * Changes the language. The callback will be called as soon translations were loaded or an error occurs while loading.
   * HINT: For easy testing - setting lng to 'cimode' will set t function to always return the key.
   */
  changeLanguage(lng?: string, callback?: Callback): Promise<TFunction>;

  /**
   * Is set to the current detected or set language.
   * If you need the primary used language depending on your configuration (supportedLngs, load) you will prefer using i18next.languages[0].
   */
  language: string;

  /**
   * Is set to an array of language-codes that will be used it order to lookup the translation value.
   */
  languages: readonly string[];

  /**
   * Is set to the current resolved language.
   * It can be used as primary used language, for example in a language switcher.
   */
  resolvedLanguage?: string;

  /**
   * Loads additional namespaces not defined in init options.
   */
  loadNamespaces(ns: string | readonly string[], callback?: Callback): Promise<void>;

  /**
   * Loads additional languages not defined in init options (preload).
   */
  loadLanguages(lngs: string | readonly string[], callback?: Callback): Promise<void>;

  /**
   * Reloads resources on given state. Optionally you can pass an array of languages and namespaces as params if you don't want to reload all.
   */
  reloadResources(
    lngs?: string | readonly string[],
    ns?: string | readonly string[],
    callback?: () => void,
  ): Promise<void>;
  reloadResources(lngs: null, ns: string | readonly string[], callback?: () => void): Promise<void>;

  /**
   * Changes the default namespace.
   */
  setDefaultNamespace(ns: string): void;

  /**
   * Checks if a namespace has been loaded.
   */
  hasLoadedNamespace(ns: string, options?: Pick<InitOptions, 'fallbackLng'>): boolean;

  /**
   * Returns rtl or ltr depending on languages read direction.
   */
  dir(lng?: string): 'ltr' | 'rtl';

  /**
   * Exposes interpolation.format function added on init.
   */
  format: FormatFunction;

  /**
   * Will return a new i18next instance.
   * Please read the options page for details on configuration options.
   * Providing a callback will automatically call init.
   * The callback will be called after all translations were loaded or with an error when failed (in case of using a backend).
   */
  createInstance(options?: InitOptions, callback?: Callback): i18n;

  /**
   * Creates a clone of the current instance. Shares store, plugins and initial configuration.
   * Can be used to create an instance sharing storage but being independent on set language or namespaces.
   */
  cloneInstance(options?: CloneOptions, callback?: Callback): i18n;

  /**
   * Gets fired after initialization.
   */
  on(event: 'initialized', callback: (options: InitOptions) => void): void;

  /**
   * Gets fired on loaded resources.
   */
  on(
    event: 'loaded',
    callback: (loaded: { [language: string]: { [namespace: string]: boolean } }) => void,
  ): void;

  /**
   * Gets fired if loading resources failed.
   */
  on(event: 'failedLoading', callback: (lng: string, ns: string, msg: string) => void): void;

  /**
   * Gets fired on accessing a key not existing.
   */
  on(
    event: 'missingKey',
    callback: (lngs: readonly string[], namespace: string, key: string, res: string) => void,
  ): void;

  /**
   * Gets fired when resources got added or removed.
   */
  on(event: 'added' | 'removed', callback: (lng: string, ns: string) => void): void;

  /**
   * Gets fired when changeLanguage got called.
   */
  on(event: 'languageChanged', callback: (lng: string) => void): void;

  /**
   * Event listener
   */
  on(event: string, listener: (...args: any[]) => void): void;

  /**
   * Remove event listener
   * removes all callback when callback not specified
   */
  off(event: string, listener?: (...args: any[]) => void): void;

  /**
   * Gets one value by given key.
   */
  getResource(
    lng: string,
    ns: string,
    key: string,
    options?: Pick<InitOptions, 'keySeparator' | 'ignoreJSONStructure'>,
  ): any;

  /**
   * Adds one key/value.
   */
  addResource(
    lng: string,
    ns: string,
    key: string,
    value: string,
    options?: { keySeparator?: string; silent?: boolean },
  ): i18n;

  /**
   * Adds multiple key/values.
   */
  addResources(lng: string, ns: string, resources: any): i18n;

  /**
   * Adds a complete bundle.
   * Setting deep param to true will extend existing translations in that file.
   * Setting overwrite to true it will overwrite existing translations in that file.
   */
  addResourceBundle(
    lng: string,
    ns: string,
    resources: any,
    deep?: boolean,
    overwrite?: boolean,
  ): i18n;

  /**
   * Checks if a resource bundle exists.
   */
  hasResourceBundle(lng: string, ns: string): boolean;

  /**
   * Returns a resource bundle.
   */
  getResourceBundle(lng: string, ns: string): any;

  /**
   * Removes an existing bundle.
   */
  removeResourceBundle(lng: string, ns: string): i18n;

  /**
   * Current options
   */
  options: InitOptions;

  /**
   * Is initialized
   */
  isInitialized: boolean;

  /**
   * Emit event
   */
  emit(eventName: string): void;
}

declare const i18next: i18n;
export default i18next;

export const createInstance: i18n['createInstance'];

export const dir: i18n['dir'];
export const init: i18n['init'];
export const loadResources: i18n['loadResources'];
export const reloadResources: i18n['reloadResources'];
export const use: i18n['use'];
export const changeLanguage: i18n['changeLanguage'];
export const getFixedT: i18n['getFixedT'];
export const t: i18n['t'];
export const exists: i18n['exists'];
export const setDefaultNamespace: i18n['setDefaultNamespace'];
export const hasLoadedNamespace: i18n['hasLoadedNamespace'];
export const loadNamespaces: i18n['loadNamespaces'];
export const loadLanguages: i18n['loadLanguages'];