declare module 'mongoose' {
  type IfAny<IFTYPE, THENTYPE, ELSETYPE = IFTYPE> = 0 extends 1 & IFTYPE
    ? THENTYPE
    : ELSETYPE;
  type IsUnknown<T> = unknown extends T ? true : false;

  type IsNotNever<T> = [T] extends [never] ? false : true;
  type IsAny<T> = 0 extends 1 & T ? true : false;

  type WithLevel1NestedPaths<T, K extends keyof T = keyof T> = IsItRecordAndNotAny<T> extends true ? {
    [P in K | NestedPaths<Required<T>, K>]: P extends K
      // Handle top-level paths
      // First, drill into documents so we don't end up surfacing `$assertPopulated`, etc.
      ? Extract<NonNullable<T[P]>, Document> extends never
        // If not a document, then return the type. Otherwise, get the DocType.
        ? NonNullable<T[P]>
        : Extract<NonNullable<T[P]>, Document> extends Document<any, any, infer DocType, any>
          ? DocType | Exclude<NonNullable<T[P]>, Document>
          : never
      // Handle nested paths
      : P extends `${infer Key}.${infer Rest}`
        ? Key extends keyof T
          ? NonNullable<T[Key]> extends (infer U)[]
            ? NonNullable<T[Key]> extends Types.DocumentArray<infer TRawDocType>
              ? Rest extends keyof NonNullable<TRawDocType>
                ? NonNullable<TRawDocType>[Rest]
                : never
            : Rest extends keyof NonNullable<U>
              ? NonNullable<U>[Rest]
              : never
            : Rest extends keyof NonNullable<T[Key]>
              ? NonNullable<T[Key]>[Rest]
              : never
          : never
        : never;
  } : T;

  type HasStringIndex<T> =
    string extends Extract<keyof T, string> ? true : false;

  type SafeObjectKeys<T> =
    HasStringIndex<T> extends true ? never : Extract<keyof T, string>;

  type NestedPaths<T, K extends keyof T> =
    K extends string
      ? T[K] extends TreatAsPrimitives
        ? never
        : Extract<NonNullable<T[K]>, Document> extends never
          ? NonNullable<T[K]> extends Array<infer U>
            ? NonNullable<T[K]> extends Types.DocumentArray<infer TRawDocType>
              ? SafeObjectKeys<NonNullable<TRawDocType>> extends never
                ? never
                : `${K}.${SafeObjectKeys<NonNullable<TRawDocType>>}`
              : NonNullable<U> extends Record<string, any>
                ? SafeObjectKeys<NonNullable<U>> extends never
                  ? never
                  : `${K}.${SafeObjectKeys<NonNullable<U>>}`
                : never
            : NonNullable<T[K]> extends object
              ? SafeObjectKeys<NonNullable<T[K]>> extends never
                ? never
                : `${K}.${SafeObjectKeys<NonNullable<T[K]>>}`
              : never
          : Extract<NonNullable<T[K]>, Document> extends Document<any, any, infer DocType, any>
            ? DocType extends object
              ? SafeObjectKeys<NonNullable<DocType>> extends never
                ? never
                : `${K}.${SafeObjectKeys<NonNullable<DocType>>}`
              : never
            : never
      : never;

  type WithoutUndefined<T> = T extends undefined ? never : T;

  /**
   * @summary Removes keys from a type
   * @description It helps to exclude keys from a type
   * @param {T} T A generic type to be checked.
   * @param {K} K Keys from T that are to be excluded from the generic type
   * @returns T with the keys in K excluded
   */
  type ExcludeKeys<T, K extends keyof T> = {
    [P in keyof T as P extends K ? never : P]: T[P];
  };

  type Unpacked<T> = T extends (infer U)[]
    ? U
    : T extends ReadonlyArray<infer U>
    ? U
    : T;

  type DeepPartial<T> =
    T extends TreatAsPrimitives ? T :
    T extends Array<infer U> ? DeepPartial<U>[] :
    T extends Record<string, any> ? { [K in keyof T]?: DeepPartial<T[K]> } :
    T;

  type UnpackedIntersection<T, U> = T extends null
    ? null
    : T extends (infer A)[]
    ? (A extends any ? (Omit<A, keyof U> & U) : never)[]
    : keyof U extends never
    ? T
    : T extends any ? (Omit<T, keyof U> & U) : never;

  type MergeType<A, B> = A extends unknown ? Omit<A, keyof B> & B : never;

  /**
   * @summary Converts Unions to one record "object".
   * @description It makes intellisense dialog box easier to read as a single object instead of showing that in multiple object unions.
   * @param {T} T The type to be converted.
   */
  type FlatRecord<T> = { [K in keyof T]: T[K] };

  /** Force an operation like `{ a: 0 } & { b: 1 }` to be computed so that it displays `{ a: 0; b: 1 }`. */
  export type Show<t> = { [k in keyof t]: t[k] } & unknown;

  /**
   * @summary Checks if a type is "Record" or "any".
   * @description It Helps to check if user has provided schema type "EnforcedDocType"
   * @param {T} T A generic type to be checked.
   * @returns true if {@link T} is Record OR false if {@link T} is of any type.
   */
  type IsItRecordAndNotAny<T> = IfEquals<
    T,
    any,
    false,
    T extends Record<any, any> ? true : false
  >;

  /**
   * @summary Checks if two types are identical.
   * @param {T} T The first type to be compared with {@link U}.
   * @param {U} U The second type to be compared with {@link T}.
   * @param {Y} Y A type to be returned if {@link T} &  {@link U} are identical.
   * @param {N} N A type to be returned if {@link T} &  {@link U} are not identical.
   */
  type IfEquals<T, U, Y = true, N = false> = (<G>() => G extends T
    ? 1
    : 0) extends <G>() => G extends U ? 1 : 0
    ? Y
    : N;

  /**
   * @summary Extracts 'this' parameter from a function, if it exists. Otherwise, returns fallback.
   * @param {T} T Function type to extract 'this' parameter from.
   * @param {F} F Fallback type to return if 'this' parameter does not exist.
   */
  type ThisParameter<T, F> = T extends { (this: infer This, ...args: never): void } ? This : F;

  /**
   * @summary Decorates all functions in an object with 'this' parameter.
   * @param {T} T Object with functions as values to add 'D' parameter to as 'this'. {@link D}
   * @param {D} D The type to be added as 'this' parameter to all functions in {@link T}.
   */
  type AddThisParameter<T, D> = {
    [K in keyof T]: T[K] extends (...args: infer A) => infer R
      ? IsUnknown<ThisParameter<T[K], unknown>> extends true
        ? (this: D, ...args: A) => R
        : T[K]
      : T[K];
  };

  /**
   * @summary Adds timestamp fields to a type
   * @description Adds createdAt and updatedAt fields of type Date, or custom timestamp fields if specified
   * @param {T} T The type to add timestamp fields to
   * @param {P} P Optional SchemaTimestampsConfig or boolean to customize timestamp field names
   * @returns T with timestamp fields added
   */
  export type WithTimestamps<
    T,
    P extends SchemaTimestampsConfig | boolean = true
  > = ResolveTimestamps<T, { timestamps: P }>;
}
