typia/src/SnakeCase.ts

Superfast runtime validators with only one line

import { Equal } from "./typings/Equal";
import { NativeClass } from "./typings/NativeClass";
import { ValueOf } from "./typings/ValueOf";

/**
 * Snake case type.
 *
 * `SnakeCase` type is a type that all keys of an object are converted to snake
 * case.
 *
 * It also erases every method property like {@link Resolved} type.
 *
 * @author Jeongho Nam - https://github.com/samchon
 * @template T Target type to be snake cased
 */
export type SnakeCase<T> =
  Equal<T, SnakageMain<T>> extends true ? T : SnakageMain<T>;

/* -----------------------------------------------------------
    OBJECT CONVERSION
----------------------------------------------------------- */

type SnakageMain<T> = T extends [never]
  ? never // special trick for (jsonable | null) type
  : T extends { valueOf(): boolean | bigint | number | string }
    ? ValueOf<T>
    : T extends Function
      ? never
      : T extends object
        ? SnakageObject<T>
        : T;

type SnakageObject<T extends object> =
  T extends Array<infer U>
    ? IsTuple<T> extends true
      ? SnakageTuple<T>
      : SnakageMain<U>[]
    : T extends Set<infer U>
      ? Set<SnakageMain<U>>
      : T extends Map<infer K, infer V>
        ? Map<SnakageMain<K>, SnakageMain<V>>
        : T extends WeakSet<any> | WeakMap<any, any>
          ? never
          : T extends NativeClass
            ? T
            : {
                [Key in keyof T as SnakageString<Key & string>]: SnakageMain<
                  T[Key]
                >;
              };

/* -----------------------------------------------------------
    SPECIAL CASES
----------------------------------------------------------- */
type IsTuple<T extends readonly any[] | { length: number }> = [T] extends [
  never,
]
  ? false
  : T extends readonly any[]
    ? number extends T["length"]
      ? false
      : true
    : false;
type SnakageTuple<T extends readonly any[]> = T extends []
  ? []
  : T extends [infer F]
    ? [SnakageMain<F>]
    : T extends [infer F, ...infer Rest extends readonly any[]]
      ? [SnakageMain<F>, ...SnakageTuple<Rest>]
      : T extends [(infer F)?]
        ? [SnakageMain<F>?]
        : T extends [(infer F)?, ...infer Rest extends readonly any[]]
          ? [SnakageMain<F>?, ...SnakageTuple<Rest>]
          : [];

/* -----------------------------------------------------------
    STRING CONVERTER
----------------------------------------------------------- */
type SnakageString<Key extends string> = Key extends `${infer _}`
  ? SnakageStringRepeatedly<Key, "">
  : Key;
type SnakageStringRepeatedly<
  S extends string,
  Previous extends string,
> = S extends `${infer First}${infer Second}${infer Rest}`
  ? `${Underscore<Previous, First>}${Lowercase<First>}${Underscore<
      First,
      Second
    >}${Lowercase<Second>}${SnakageStringRepeatedly<Rest, Second>}`
  : S extends `${infer First}`
    ? `${Underscore<Previous, First>}${Lowercase<First>}`
    : "";
type Underscore<First extends string, Second extends string> = First extends
  | UpperAlphabetic
  | ""
  | "_"
  ? ""
  : Second extends UpperAlphabetic
    ? "_"
    : "";
type UpperAlphabetic =
  | "A"
  | "B"
  | "C"
  | "D"
  | "E"
  | "F"
  | "G"
  | "H"
  | "I"
  | "J"
  | "K"
  | "L"
  | "M"
  | "N"
  | "O"
  | "P"
  | "Q"
  | "R"
  | "S"
  | "T"
  | "U"
  | "V"
  | "W"
  | "X"
  | "Y"
  | "Z";