meow

meow: {
    <Flags extends AnyFlags>(
        helpMessage: string,
        options?: Options<Flags>
    ): Result<Flags>;
    <Flags extends AnyFlags>(options?: Options<Flags>): Result<Flags>;
};

type AnyFlag = StringFlag | BooleanFlag | NumberFlag;

type AnyFlags = Record<string, AnyFlag>;

type BooleanFlag = Flag<'boolean', boolean> | Flag<'boolean', boolean[], true>;

type Flag<Type extends FlagType, Default, IsMultiple = false> = {
    readonly type?: Type;
    readonly alias?: string;
    readonly default?: Default;
    readonly isRequired?: boolean | IsRequiredPredicate;
    readonly isMultiple?: IsMultiple;
};

type FlagType = 'string' | 'boolean' | 'number';

type IsRequiredPredicate = (
    flags: Readonly<AnyFlags>,
    input: readonly string[]
) => boolean;

type NumberFlag = Flag<'number', number> | Flag<'number', number[], true>;

type Options<Flags extends AnyFlags> = {
    /**
	Pass in [`import.meta`](https://nodejs.org/dist/latest/docs/api/esm.html#esm_import_meta). This is used to find the correct package.json file.
	*/
    readonly importMeta: ImportMeta;

    /**
	Define argument flags.

	The key is the flag name in camel-case and the value is an object with any of:

	- `type`: Type of value. (Possible values: `string` `boolean` `number`)
	- `alias`: Usually used to define a short flag alias.
	- `default`: Default value when the flag is not specified.
	- `isRequired`: Determine if the flag is required.
		If it's only known at runtime whether the flag is required or not you can pass a Function instead of a boolean, which based on the given flags and other non-flag arguments should decide if the flag is required.
	- `isMultiple`: Indicates a flag can be set multiple times. Values are turned into an array. (Default: false)
		Multiple values are provided by specifying the flag multiple times, for example, `$ foo -u rainbow -u cat`. Space- or comma-separated values are *not* supported.

	Note that flags are always defined using a camel-case key (`myKey`), but will match arguments in kebab-case (`--my-key`).

	@example
	```
	flags: {
		unicorn: {
			type: 'string',
			alias: 'u',
			default: ['rainbow', 'cat'],
			isMultiple: true,
			isRequired: (flags, input) => {
				if (flags.otherFlag) {
					return true;
				}

				return false;
			}
		}
	}
	```
	*/
    readonly flags?: Flags;

    /**
	Description to show above the help text. Default: The package.json `"description"` property.

	Set it to `false` to disable it altogether.
	*/
    readonly description?: string | false;

    /**
	The help text you want shown.

	The input is reindented and starting/ending newlines are trimmed which means you can use a [template literal](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/template_strings) without having to care about using the correct amount of indent.

	The description will be shown above your help text automatically.

	Set it to `false` to disable it altogether.
	*/
    readonly help?: string | false;

    /**
	Set a custom version output. Default: The package.json `"version"` property.

	Set it to `false` to disable it altogether.
	*/
    readonly version?: string | false;

    /**
	Automatically show the help text when the `--help` flag is present. Useful to set this value to `false` when a CLI manages child CLIs with their own help text.

	This option is only considered when there is only one argument in `process.argv`.
	*/
    readonly autoHelp?: boolean;

    /**
	Automatically show the version text when the `--version` flag is present. Useful to set this value to `false` when a CLI manages child CLIs with their own version text.

	This option is only considered when there is only one argument in `process.argv`.
	*/
    readonly autoVersion?: boolean;

    /**
	`package.json` as an `Object`. Default: Closest `package.json` upwards.

	_You most likely don't need this option._
	*/
    readonly pkg?: Record<string, unknown>;

    /**
	Custom arguments object.

	@default process.argv.slice(2)
	*/
    readonly argv?: readonly string[];

    /**
	Infer the argument type.

	By default, the argument `5` in `$ foo 5` becomes a string. Enabling this would infer it as a number.

	@default false
	*/
    readonly inferType?: boolean;

    /**
	Value of `boolean` flags not defined in `argv`.

	If set to `undefined`, the flags not defined in `argv` will be excluded from the result. The `default` value set in `boolean` flags take precedence over `booleanDefault`.

	_Note: If used in conjunction with `isMultiple`, the default flag value is set to `[]`._

	__Caution: Explicitly specifying `undefined` for `booleanDefault` has different meaning from omitting key itself.__

	@example
	```
	import meow from 'meow';

	const cli = meow(`
		Usage
			$ foo

		Options
			--rainbow, -r  Include a rainbow
			--unicorn, -u  Include a unicorn
			--no-sparkles  Exclude sparkles

		Examples
			$ foo
			🌈 unicorns✨🌈
	`, {
		importMeta: import.meta,
		booleanDefault: undefined,
		flags: {
			rainbow: {
				type: 'boolean',
				default: true,
				alias: 'r'
			},
				unicorn: {
				type: 'boolean',
				default: false,
				alias: 'u'
			},
			cake: {
				type: 'boolean',
				alias: 'c'
			},
			sparkles: {
				type: 'boolean',
				default: true
			}
		}
	});

	//{
	//	flags: {
	//		rainbow: true,
	//		unicorn: false,
	//		sparkles: true
	//	},
	//	unnormalizedFlags: {
	//		rainbow: true,
	//		r: true,
	//		unicorn: false,
	//		u: false,
	//		sparkles: true
	//	},
	//	…
	//}
	```
	*/
    // eslint-disable-next-line @typescript-eslint/ban-types
    readonly booleanDefault?: boolean | null | undefined;

    /**
	Whether to use [hard-rejection](https://github.com/sindresorhus/hard-rejection) or not. Disabling this can be useful if you need to handle `process.on('unhandledRejection')` yourself.

	@default true
	*/
    readonly hardRejection?: boolean;

    /**
	Whether to allow unknown flags or not.

	@default true
	*/
    readonly allowUnknownFlags?: boolean;
};

type PossiblyOptionalFlag<Flag extends AnyFlag, FlagType> = Flag extends {
    isRequired: true;
}
    ? FlagType
    : Flag extends { default: any }
    ? FlagType
    : FlagType | undefined;

type Result<Flags extends AnyFlags> = {
    /**
	Non-flag arguments.
	*/
    input: string[];

    /**
	Flags converted to camelCase excluding aliases.
	*/
    flags: CamelCasedProperties<TypedFlags<Flags>> & Record<string, unknown>;

    /**
	Flags converted camelCase including aliases.
	*/
    unnormalizedFlags: TypedFlags<Flags> & Record<string, unknown>;

    /**
	The `package.json` object.
	*/
    pkg: PackageJson;

    /**
	The help text used with `--help`.
	*/
    help: string;

    /**
	Show the help text and exit with code.

	@param exitCode - The exit code to use. Default: `2`.
	*/
    showHelp: (exitCode?: number) => never;

    /**
	Show the version text and exit.
	*/
    showVersion: () => void;
};

type StringFlag = Flag<'string', string> | Flag<'string', string[], true>;

type TypedFlag<Flag extends AnyFlag> = Flag extends { type: 'number' }
    ? number
    : Flag extends { type: 'string' }
    ? string
    : Flag extends { type: 'boolean' }
    ? boolean
    : unknown;

type TypedFlags<Flags extends AnyFlags> = {
    [F in keyof Flags]: Flags[F] extends { isMultiple: true }
        ? PossiblyOptionalFlag<Flags[F], Array<TypedFlag<Flags[F]>>>
        : PossiblyOptionalFlag<Flags[F], TypedFlag<Flags[F]>>;
};