rivo/Str/ToSnakeCase.d.ts

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import type { Args, Fn } from "../HKT";

export type ToSnakeCase<S extends string> =
  _ToSnakeCase<S> extends `_${infer T}` ? T : _ToSnakeCase<S>;
type _ToSnakeCase<S extends string, Kebab extends boolean = false> =
  S extends `${infer T}-${infer U}` ? `${Lowercase<T>}_${_ToSnakeCase<U, true>}`
  : Kebab extends true ? S
  : S extends `${infer T}${infer U}` ?
    `${T extends Capitalize<T> ? "_" : ""}${Lowercase<T>}${_ToSnakeCase<U>}`
  : S;

export default interface ToSnakeCaseFn extends Fn<[string], string> {
  def: ([s]: Args<this>) => ToSnakeCase<typeof s>;
}