type-fest/source/delimiter-case.d.ts

import type {UpperCaseCharacters, WordSeparators} from './internal';

// Transforms a string that is fully uppercase into a fully lowercase version. Needed to add support for SCREAMING_SNAKE_CASE, see https://github.com/sindresorhus/type-fest/issues/385
type UpperCaseToLowerCase<T extends string> = T extends Uppercase<T> ? Lowercase<T> : T;

// This implementation does not support SCREAMING_SNAKE_CASE, it is used internally by `SplitIncludingDelimiters`.
type SplitIncludingDelimiters_<Source extends string, Delimiter extends string> =
	Source extends '' ? [] :
		Source extends `${infer FirstPart}${Delimiter}${infer SecondPart}` ?
			(
				Source extends `${FirstPart}${infer UsedDelimiter}${SecondPart}`
					? UsedDelimiter extends Delimiter
						? Source extends `${infer FirstPart}${UsedDelimiter}${infer SecondPart}`
							? [...SplitIncludingDelimiters<FirstPart, Delimiter>, UsedDelimiter, ...SplitIncludingDelimiters<SecondPart, Delimiter>]
							: never
						: never
					: never
			) :
			[Source];

/**
Unlike a simpler split, this one includes the delimiter splitted on in the resulting array literal. This is to enable splitting on, for example, upper-case characters.

@category Template literal
*/
export type SplitIncludingDelimiters<Source extends string, Delimiter extends string> = SplitIncludingDelimiters_<UpperCaseToLowerCase<Source>, Delimiter>;

/**
Format a specific part of the splitted string literal that `StringArrayToDelimiterCase<>` fuses together, ensuring desired casing.

@see StringArrayToDelimiterCase
*/
type StringPartToDelimiterCase<StringPart extends string, Start extends boolean, UsedWordSeparators extends string, UsedUpperCaseCharacters extends string, Delimiter extends string> =
	StringPart extends UsedWordSeparators ? Delimiter :
		Start extends true ? Lowercase<StringPart> :
			StringPart extends UsedUpperCaseCharacters ? `${Delimiter}${Lowercase<StringPart>}` :
				StringPart;

/**
Takes the result of a splitted string literal and recursively concatenates it together into the desired casing.

It receives `UsedWordSeparators` and `UsedUpperCaseCharacters` as input to ensure it's fully encapsulated.

@see SplitIncludingDelimiters
*/
type StringArrayToDelimiterCase<Parts extends readonly any[], Start extends boolean, UsedWordSeparators extends string, UsedUpperCaseCharacters extends string, Delimiter extends string> =
	Parts extends [`${infer FirstPart}`, ...infer RemainingParts]
		? `${StringPartToDelimiterCase<FirstPart, Start, UsedWordSeparators, UsedUpperCaseCharacters, Delimiter>}${StringArrayToDelimiterCase<RemainingParts, false, UsedWordSeparators, UsedUpperCaseCharacters, Delimiter>}`
		: Parts extends [string]
			? string
			: '';

/**
Convert a string literal to a custom string delimiter casing.

This can be useful when, for example, converting a camel-cased object property to an oddly cased one.

@see KebabCase
@see SnakeCase

@example
```
import type {DelimiterCase} from 'type-fest';

// Simple

const someVariable: DelimiterCase<'fooBar', '#'> = 'foo#bar';

// Advanced

type OddlyCasedProperties<T> = {
	[K in keyof T as DelimiterCase<K, '#'>]: T[K]
};

interface SomeOptions {
	dryRun: boolean;
	includeFile: string;
	foo: number;
}

const rawCliOptions: OddlyCasedProperties<SomeOptions> = {
	'dry#run': true,
	'include#file': 'bar.js',
	foo: 123
};
```

@category Change case
@category Template literal
*/
export type DelimiterCase<Value, Delimiter extends string> = string extends Value ? Value : Value extends string
	? StringArrayToDelimiterCase<
	SplitIncludingDelimiters<Value, WordSeparators | UpperCaseCharacters>,
	true,
	WordSeparators,
	UpperCaseCharacters,
	Delimiter
	>
	: Value;