import type {WordSeparators} from '../source/internal';
import type {Split} from './split';

/**
Step by step takes the first item in an array literal, formats it and adds it to a string literal, and then recursively appends the remainder.

Only to be used by `CamelCaseStringArray<>`.

@see CamelCaseStringArray
*/
type InnerCamelCaseStringArray<Parts extends readonly any[], PreviousPart> =
	Parts extends [`${infer FirstPart}`, ...infer RemainingParts]
		? FirstPart extends undefined
			? ''
			: FirstPart extends ''
					? InnerCamelCaseStringArray<RemainingParts, PreviousPart>
					: `${PreviousPart extends '' ? FirstPart : Capitalize<FirstPart>}${InnerCamelCaseStringArray<RemainingParts, FirstPart>}`
		: '';

/**
Starts fusing the output of `Split<>`, an array literal of strings, into a camel-cased string literal.

It's separate from `InnerCamelCaseStringArray<>` to keep a clean API outwards to the rest of the code.

@see Split
*/
type CamelCaseStringArray<Parts extends readonly string[]> =
	Parts extends [`${infer FirstPart}`, ...infer RemainingParts]
		? Uncapitalize<`${FirstPart}${InnerCamelCaseStringArray<RemainingParts, FirstPart>}`>
		: never;

/**
Convert a string literal to camel-case.

This can be useful when, for example, converting some kebab-cased command-line flags or a snake-cased database result.

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

// Simple

const someVariable: CamelCase<'foo-bar'> = 'fooBar';

// Advanced

type CamelCasedProperties<T> = {
	[K in keyof T as CamelCase<K>]: T[K]
};

interface RawOptions {
	'dry-run': boolean;
	'full_family_name': string;
	foo: number;
	BAR: string;
	QUZ_QUX: number;
	'OTHER-FIELD': boolean;
}

const dbResult: CamelCasedProperties<RawOptions> = {
	dryRun: true,
	fullFamilyName: 'bar.js',
	foo: 123,
	bar: 'foo',
	quzQux: 6,
	otherField: false
};
```

@category Change case
@category Template literal
*/
export type CamelCase<K> = K extends string ? CamelCaseStringArray<Split<K extends Uppercase<K> ? Lowercase<K> : K, WordSeparators>> : K;