/**
 * Represents the failure case of an Either
 * @typeParam E - The type of the error
 */
export type Left<E> = {readonly _tag: 'Left', readonly left: E}

/**
 * Represents the success case of an Either
 * @typeParam A - The type of the value
 */
export type Right<A> = {readonly _tag: 'Right', readonly right: A}

/**
 * A type that represents either a success (Right) or failure (Left)
 * @typeParam A - The success value type
 * @typeParam E - The error type
 */
export type Either<A, E> = Right<A> | Left<E>

/**
 * Constructs a Left value
 * @typeParam E - The error type
 * @param e - The error value
 * @returns A Left containing the error
 */
export const left = <E>(e: E): Either<never, E> => ({
  _tag: 'Left',
  left: e,
})

/**
 * Constructs a Right value
 * @typeParam A - The value type
 * @param a - The value
 * @returns A Right containing the value
 */
export const right = <A>(a: A): Either<A, never> => ({
  _tag: 'Right',
  right: a,
})

/**
 * Maps a function over the Right side of an Either
 * @typeParam A - The input value type
 * @typeParam B - The output value type
 * @typeParam E - The error type
 * @param f - The mapping function
 * @returns A function that takes an Either and returns a new Either
 */
export const eitherMap =
  <A, B, E>(f: (a: A) => B) => (fa: Either<A, E>): Either<B, E> =>
    fa._tag === 'Right' ? right(f(fa.right)) : fa

/**
 * Chains computations that may return Either values
 * @typeParam A - The input value type
 * @typeParam B - The output value type
 * @typeParam E - The error type
 * @param f - The function that returns an Either
 * @returns A function that takes an Either and returns a new Either
 */
export const eitherChain =
  <A, B, E>(f: (a: A) => Either<B, E>) => (fa: Either<A, E>): Either<B, E> =>
    fa._tag === 'Right' ? f(fa.right) : fa

/**
 * Folds an Either into a single value by providing handlers for both cases
 * @typeParam A - The success value type
 * @typeParam E - The error type
 * @typeParam B - The output type
 * @param onLeft - Function to handle Left case
 * @param onRight - Function to handle Right case
 * @returns A function that takes an Either and returns the folded value
 */
export const fold =
  <A, E, B>(onLeft: (e: E) => B, onRight: (a: A) => B) =>
  (fa: Either<A, E>): B =>
    fa._tag === 'Left' ? onLeft(fa.left) : onRight(fa.right)

/**
 * A class for working with Either in a do-notation style
 * @typeParam T - The type of the accumulated scope
 * @typeParam E - The error type
 */
export class DoEither<T extends Record<string, unknown>, E> {
  /**
   * @param either - The current Either value
   */
  constructor(private readonly either: Either<T, E>) {}

  /**
   * Binds a new value to the scope
   * @typeParam K - The key to bind to
   * @typeParam A - The type of the value to bind
   * @param key - The property name to bind to
   * @param fa - The Either value to bind
   * @returns A new DoEither instance with the extended scope
   */
  bind<K extends string, A>(
    key: K,
    fa: Either<A, E>,
  ): DoEither<T & Record<K, A>, E> {
    const newEither = eitherChain<T, T & Record<K, A>, E>((scope: T) =>
      eitherMap<A, T & Record<K, A>, E>((a: A) => ({...scope, [key]: a}))(fa)
    )(this.either)
    return new DoEither(newEither)
  }

  /**
   * Binds a new value to the scope using a lazily evaluated Either
   * @typeParam K - The key to bind to
   * @typeParam A - The type of the value to bind
   * @param key - The property name to bind to
   * @param f - Function returning the Either value to bind (receives current scope)
   * @returns A new DoEither instance with the extended scope
   */
  bindL<K extends string, A>(
    key: K,
    f: (scope: T) => Either<A, E>,
  ): DoEither<T & Record<K, A>, E> {
    const newEither = eitherChain<T, T & Record<K, A>, E>((scope: T) =>
      eitherMap<A, T & Record<K, A>, E>((a: A) => ({...scope, [key]: a}))(
        f(scope),
      )
    )(this.either)
    return new DoEither(newEither)
  }

  /**
   * Returns the final result from the scope
   * @typeParam B - The return type
   * @param f - The function to transform the scope
   * @returns An Either containing the result
   */
  return<B>(f: (scope: T) => B): Either<B, E> {
    return eitherMap<T, B, E>(f)(this.either)
  }

  /**
   * Returns the current scope
   * @returns The current Either scope
   */
  done(): Either<T, E> {
    return this.either
  }

  /**
   * Executes an effect without modifying the scope
   * @param fa - The Either effect to execute
   * @returns A new DoEither instance with the same scope
   */
  do(fa: Either<unknown, E>): DoEither<T, E> {
    const newEither = eitherChain<T, T, E>((scope: T) =>
      eitherMap<unknown, T, E>(() => scope)(fa)
    )(this.either)
    return new DoEither(newEither)
  }

  /**
   * Executes an effect that depends on the current scope
   * @param f - The function that returns an Either effect
   * @returns A new DoEither instance with the same scope
   */
  doL(f: (scope: T) => Either<unknown, E>): DoEither<T, E> {
    const newEither = eitherChain<T, T, E>((scope: T) =>
      eitherMap<unknown, T, E>(() => scope)(f(scope))
    )(this.either)
    return new DoEither(newEither)
  }

  /**
   * Starts a new DoEither chain with an empty scope
   * @returns A new DoEither instance with an empty scope
   */
  static start<E>(): DoEither<{}, E> {
    return new DoEither(right({}))
  }
}

/**
 * A class for working with Either in a do-notation style with asynchronous computations
 * @typeParam T - The type of the accumulated scope
 * @typeParam E - The error type
 */
export class AsyncDoEither<T extends Record<string, unknown>, E> {
  /**
   * @param promise - The current Promise of an Either value
   */
  constructor(private readonly promise: Promise<Either<T, E>>) {}

  /**
   * Binds a new value to the scope synchronously
   * @typeParam K - The key to bind to
   * @typeParam A - The type of the value to bind
   * @param key - The property name to bind to
   * @param fa - The Either value to bind
   * @returns A new AsyncDoEither instance with the extended scope
   */
  bind<K extends string, A>(
    key: K,
    fa: Either<A, E>,
  ): AsyncDoEither<T & Record<K, A>, E> {
    return this.bindL(key, () => fa)
  }

  /**
   * Binds a new value to the scope asynchronously
   * @typeParam K - The key to bind to
   * @typeParam A - The type of the value to bind
   * @param key - The property name to bind to
   * @param f - Function returning Either or Promise of Either
   * @returns A new AsyncDoEither instance with the extended scope
   */
  bindL<K extends string, A>(
    key: K,
    f: (scope: T) => Either<A, E> | Promise<Either<A, E>>,
  ): AsyncDoEither<T & Record<K, A>, E> {
    const newPromise = this.promise.then(async (currentEither) => {
      if (isLeft(currentEither)) {
        return currentEither as unknown as Either<T & Record<K, A>, E>
      }

      const scope = currentEither.right
      try {
        const nextEither = await f(scope)

        if (isLeft(nextEither)) {
          return nextEither as unknown as Either<T & Record<K, A>, E>
        }

        return right({
          ...scope,
          [key]: nextEither.right,
        }) as Either<T & Record<K, A>, E>
      } catch (err) {
        return left(err as E) as Either<T & Record<K, A>, E>
      }
    })

    return new AsyncDoEither(newPromise)
  }

  /**
   * Returns the final result from the scope
   * @typeParam B - The return type
   * @param f - The function to transform the scope
   * @returns A Promise of Either containing the result
   */
  return<B>(f: (scope: T) => B): Promise<Either<B, E>> {
    return this.promise.then(either => {
      if (isLeft(either)) {
        return either as unknown as Either<B, E>
      }
      return right(f(either.right))
    })
  }

  /**
   * Returns the current scope
   * @returns A Promise of the current Either scope
   */
  done(): Promise<Either<T, E>> {
    return this.promise
  }

  /**
   * Executes an effect without modifying the scope
   * @param fa - The Either effect to execute
   * @returns A new AsyncDoEither instance with the same scope
   */
  do(fa: Either<unknown, E>): AsyncDoEither<T, E> {
    return this.doL(() => fa)
  }

  /**
   * Executes an effect that depends on the current scope
   * @param f - Function returning Either or Promise of Either
   * @returns A new AsyncDoEither instance with the same scope
   */
  doL(f:
    (scope: T) =>
      | Either<unknown, E>
      | Promise<Either<unknown, E>>): AsyncDoEither<T, E>
  {
    const newPromise = this.promise.then(async (currentEither) => {
      if (isLeft(currentEither)) {
        return currentEither
      }

      const scope = currentEither.right
      try {
        const effectEither = await f(scope)
        if (isLeft(effectEither)) {
          return effectEither as unknown as Either<T, E>
        }
        return currentEither
      } catch (err) {
        return left(err as E)
      }
    })

    return new AsyncDoEither(newPromise)
  }

  /**
   * Starts a new AsyncDoEither chain with an empty scope
   * @returns A new AsyncDoEither instance with an empty scope
   */
  static start<E>(): AsyncDoEither<{}, E> {
    return new AsyncDoEither(Promise.resolve(right({})))
  }
}