type Whitespace = ' ' | '\n' | '\r' | '\f' | '\t'
type Trim<S extends string> = S extends `${infer T}${Whitespace}`
  ? Trim<T>
  : S extends `${Whitespace}${infer T}`
  ? Trim<T>
  : S

type Combinators = ' ' | '>' | '~' | '+'
type GetLastTag<I> = I extends `${string}${Combinators}${infer Right}`
  ? Right extends '' // right arm can't be empty
    ? unknown
    : GetLastTag<Right>
  : I

type PseudoPrefix =
  | 'a'
  | 'b'
  | 'c'
  | 'd'
  | 'e'
  | 'f'
  | 'h'
  | 'i'
  | 'l'
  | 'n'
  | 'o'
  | 'p'
  | 'r'
  | 's'
  | 't'
  | 'u'
  | 'v'
  | 'w'
  | ':-'

type Split<S> = S extends `${string},` // invalid selector
  ? unknown
  : S extends ''
  ? ''
  : SplitRec<S>
type SplitRec<S, Acc = never> = S extends `${infer Left},${infer Right}`
  ? SplitRec<Right, Acc | Left>
  : S extends ''
  ? Acc
  : SplitRec<'', Acc | S>

type Quotes = '"' | "'"

// DO NOT use union type like `${infer L},${Whitespace}${infer R}` here,
// or it may cause OOM when running tsc in downstream projects.
type PreprocessGrouping<I> = I extends `${infer L}, ${infer R}`
  ? PreprocessGrouping<`${L},${R}`>
  : I extends `${infer L},\n${infer R}`
  ? PreprocessGrouping<`${L},${R}`>
  : I extends `${infer L},\r${infer R}`
  ? PreprocessGrouping<`${L},${R}`>
  : I extends `${infer L},\f${infer R}`
  ? PreprocessGrouping<`${L},${R}`>
  : I extends `${infer L},\t${infer R}`
  ? PreprocessGrouping<`${L},${R}`>
  : I

type Preprocess<I> = I extends `${string}[]${string}` // invalid selector
  ? unknown
  : PreprocessUnchecked<I>
type PreprocessUnchecked<I> = I extends `${infer L}\\${Quotes}${infer R}` // remove escaped quotes
  ? PreprocessUnchecked<`${L}${R}`>
  : I extends `${infer L}"${string}"${infer R}` // remove quoted content in attribute
  ? PreprocessUnchecked<`${L}${R}`>
  : I extends `${infer L}'${string}'${infer R}` // remove quoted content in attribute
  ? PreprocessUnchecked<`${L}${R}`>
  : I extends `${infer L}[${string}]${infer R}` // process attribute
  ? PreprocessUnchecked<`${L}#x${R}`> // replace it with a fake ID selector
  : I extends `${infer L}[${string}${infer R}` // process unclosed attribute
  ? PreprocessUnchecked<`${L}#x${R}`> // replace it with a fake ID selector
  : I

/** Parse `:is()` and `:where()` */
type ExpandFunctions<I> = I extends `${infer L}:is(${infer Args})${infer R}`
  ? Split<Trim<Args>> extends string
    ? ExpandFunctions<`${L}&${Split<Trim<Args>>}${R}`>
    : unknown
  : I extends `${infer L}:where(${infer Args})${infer R}`
  ? Split<Trim<Args>> extends string
    ? ExpandFunctions<`${L}&${Split<Trim<Args>>}${R}`>
    : unknown
  : I extends `${infer L}:${infer Pseudo}(${string})${infer R}`
  ? IsIdentifier<Pseudo> extends true
    ? ExpandFunctions<`${L}${R}`>
    : I
  : I

/** Check whether each tag is valid or not. */
type Postprocess<Tags> = PostprocessEach<GetLastTag<Tags>> extends string
  ? PostprocessEach<GetLastTag<Tags>>
  : unknown
/** Postprocess each tag with simple validation. */
type PostprocessEach<I> = I extends `${string}.` // invalid class selector
  ? unknown
  : I extends `${string}#` // invalid ID selector
  ? unknown
  : PostprocessEachUnchecked<I>
type PostprocessEachUnchecked<I> =
  I extends `${infer Tag}.${string}&${infer Rest}`
    ? PostprocessEachUnchecked<`${Tag}&${Rest}`>
    : I extends `${infer Tag}.${string}`
    ? PostprocessEachUnchecked<Tag>
    : I extends `${infer Tag}#${string}&${infer Rest}`
    ? PostprocessEachUnchecked<`${Tag}&${Rest}`>
    : I extends `${infer Tag}#${string}`
    ? PostprocessEachUnchecked<Tag>
    : I extends `${infer Tag}:${PseudoPrefix}${string}&${infer Rest}`
    ? PostprocessEachUnchecked<`${Tag}&${Rest}`>
    : I extends `${infer Tag}:${PseudoPrefix}${string}`
    ? PostprocessEachUnchecked<Tag>
    : I extends `${string}|${infer Tag}` // namespace prefix
    ? PostprocessEachUnchecked<Tag>
    : I

type ParseSelectorToTagNames<I extends string> = Trim<I> extends ''
  ? unknown
  : Postprocess<Split<ExpandFunctions<Preprocess<PreprocessGrouping<Trim<I>>>>>>

export type ParseSelector<
  I extends string,
  Fallback extends Element = Element,
> = ParseSelectorToTagNames<I> extends string
  ? ExpandAnd<ParseSelectorToTagNames<I>, Fallback> extends Fallback
    ? ExpandAnd<ParseSelectorToTagNames<I>, Fallback>
    : Fallback
  : Fallback

/**
 * Wrapper for `...&...` syntax expander.
 *
 * `&` is valid, but the expander will return the default result which is `unknown`,
 * so we must check the result and if it's `unknown` we will turn it into `Fallback`.
 */
type ExpandAnd<
  I extends string,
  Fallback extends Element,
> = unknown extends ExpandAndInner<I, Fallback>
  ? Fallback
  : ExpandAndInner<I, Fallback>

/**
 * Actually expand the `...&...` syntax.
 *
 * The reason why we choose `unknown` as initial type is
 * that `unknown & T` equals to `T`.
 */
type ExpandAndInner<
  I extends string,
  Fallback extends Element,
  Result extends Element | unknown = unknown,
> = I extends `${'' | '*'}&${infer Rest}`
  ? ExpandAndInner<Rest, Fallback, Result>
  : I extends `${infer Tag}&${infer Rest}`
  ? ExpandAndInner<Rest, Fallback, Result & TagNameToElement<Tag, Fallback>>
  : I extends '' | '*'
  ? Result
  : ExpandAndInner<'', Fallback, Result & TagNameToElement<I, Fallback>>

export type TagNameToElement<
  Tag extends string,
  Fallback extends Element = Element,
> = Tag extends keyof HTMLElementTagNameMap
  ? HTMLElementTagNameMap[Tag]
  : Tag extends keyof SVGElementTagNameMap
  ? SVGElementTagNameMap[Tag]
  : Fallback

// --------------------------------------------------------
// Strict Parser
// --------------------------------------------------------

// Specification is here: https://drafts.csswg.org/css-syntax-3/#ident-token-diagram
// but we don't plan to comply that fully,
// otherwise it will increase type-checking time and the complexity of parser.

type LowerCaseLetter =
  | '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'

type Digit = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'

type IdentifierFirstChar =
  | LowerCaseLetter
  | Uppercase<LowerCaseLetter>
  | '-'
  | '_'
type IdentifierChar = IdentifierFirstChar | Digit

type IsIdentifier<S> = S extends `${infer FirstChar}${infer Rest}`
  ? FirstChar extends IdentifierFirstChar
    ? IsValidRestChars<Rest>
    : false
  : false

type IsValidTags<S> = S extends `${infer Head}&${infer Rest}`
  ? IsValidTagName<Head> extends true
    ? IsValidTags<Rest>
    : false
  : IsValidTagName<S>
type IsValidTagName<S> = S extends '' | '*'
  ? true
  : S extends `${infer H}${infer Rest}`
  ? H extends IdentifierFirstChar
    ? IsValidRestChars<Rest>
    : false
  : string extends S
  ? true
  : false
type IsValidRestChars<S extends string> = S extends `${infer H}${infer Rest}`
  ? H extends IdentifierChar
    ? IsValidRestChars<Rest>
    : false
  : true // no characters left, so it's OK

export type StrictlyParseSelector<
  S extends string,
  Fallback extends Element = Element,
> = string extends S
  ? Fallback
  : ParseSelectorToTagNames<S> extends infer Tags
  ? IsValidTags<Tags> extends true
    ? Tags extends string
      ? ExpandAnd<Tags, Fallback>
      : never
    : never
  : never