ICU format parser in typescript types

icu-format-parse.ts

// https://unicode-org.github.io/icu/userguide/format_parse/messages/
type Trim<S extends string> =
    S extends ` ${infer R}` | `${infer R} ` | `\n${infer R}` | `${infer R}\n` ? Trim<R> : S;

type AccumulateBeforeClosing<S extends string, TStack extends unknown[] = [], R extends string = ''> = S extends `${infer TBeforeClosing}}${infer TAfterClosing}`
    ? S extends `${infer TBeforeOpening}{${infer TAfterOpening}`
    ? TBeforeOpening extends `${TBeforeClosing}${string}`
    ? TStack['length'] extends 0
    ? `${R}${TBeforeClosing}`
    : TStack extends [...infer TStart, infer TLast]
    ? AccumulateBeforeClosing<TAfterClosing, TStart, `${R}${TBeforeClosing}}`>
    : never // can't happen
    : AccumulateBeforeClosing<TAfterOpening, [...TStack, undefined], `${R}${TBeforeOpening}{`>
    : TStack['length'] extends 0
    ? `${R}${TBeforeClosing}`
    : TStack extends [...infer TStart, infer TLast]
    ? AccumulateBeforeClosing<TAfterClosing, TStart, `${R}${TBeforeClosing}}`>
    : never // can't happen
    : `${R}${S}`

type ICUTypes = 'number' | 'number, currency' | 'date' | 'time'

type DetectICU<S extends string> = S extends `${infer TIdentifier}, ${ICUTypes}`
    ? TIdentifier
    : S extends `${infer TSelectIdentifier}, select,${infer RestSelect}`
    ? S extends `${infer TPluralIdentifier}, plural,${infer RestPlural}`
    /**
     * In case we got nested select and plural
     * we pick smallest of TSelectIdentifier/TPluralIdentifier, because otherwise we catch something bigger than identifier
     */
    ? (TSelectIdentifier extends `${TPluralIdentifier}${string}` ? [TPluralIdentifier, RestPlural] : [TSelectIdentifier, RestSelect]) extends [infer TIdentifier, infer Rest extends string]
    ? TIdentifier | ParseInterpolation<ParseOneLevelOfInterpolation<Rest>>
    : never // can't happen
    : TSelectIdentifier | ParseInterpolation<ParseOneLevelOfInterpolation<RestSelect>>
    : S extends `${infer TPluralIdentifier}, plural,${infer RestPlural}`
    ? TPluralIdentifier | ParseInterpolation<ParseOneLevelOfInterpolation<RestPlural>>
    : S

type ParseOneLevelOfInterpolation<S extends string> = S extends `${string}{${infer TAfterOpening}`
    ? AccumulateBeforeClosing<TAfterOpening> extends infer TInside extends string
        ? TAfterOpening extends `${TInside}}${infer TAfterClosing}`
            ? Trim<TInside> extends infer TTrimmedInside extends string
                ? TTrimmedInside | ParseOneLevelOfInterpolation<TAfterClosing>
                : never // can't happen
            : never // means no interpolation strings inside S (because no "}")
        : never // can't happen
    : never; // means no interpolation strings inside S (because no "{")

type ParseInterpolation<S extends string> = DetectICU<ParseOneLevelOfInterpolation<S>>

type Test = ParseInterpolation<"I have {apples_count, plural, zero {# {apple_color} apples} one {# {apple_color} apple} other {# many {qualification} {apple_color} apples}} in my {color} backpack">;
//   ^?

type Test2 = ParseInterpolation<'Hello {user}, you have {count, number} messages.'>;

type test_text = `{num_guests, plural, offset:1 
      =0 {{host} does not give a party.}
      =1 {{host} invites {guest} to their party.}
      =2 {{host} invites {guest} and one other person to their party.}
      other {{host} invites {guest} and # other people to their party.}}}`;
type Test99 = ParseInterpolation<test_text>
//   ^?

type large_test_text = `{gender_of_host, select, 
  female {
    {num_guests, plural, offset:1 
      =0 {{host} does not give a party.}
      =1 {{host} invites {guest} to her party.}
      =2 {{host} invites {guest} and one other person to her party.}
      other {{host} invites {guest} and # other people to her party.}}}
  male {
    {num_guests, plural, offset:1 
      =0 {{host} does not give a party.}
      =1 {{host} invites {guest} to his party.}
      =2 {{host} invites {guest} and one other person to his party.}
      other {{host} invites {guest} and # other people to his party.}}}
  other {
    {num_guests, plural, offset:1 
      =0 {{host} does not give a party.}
      =1 {{host} invites {guest} to their party.}
      =2 {{host} invites {guest} and one other person to their party.}
      other {{host} invites {guest} and # other people to their party.}}}}`;

type Test3 = ParseInterpolation<large_test_text>
//   ^?

type Test4 = ParseInterpolation<' {foo} '>
//   ^?
type Test5 = ParseInterpolation<'{foo} '>
//   ^?
type Test6 = ParseInterpolation<' {foo}'>
//   ^?
type Test7 = ParseInterpolation<'{foo}'>
//   ^?
type Test8 = ParseInterpolation<'{foo}' | ' {bar} {kek}'>
//   ^?