import { share } from 'wonka';
import type { ExchangeIO, Exchange, ExchangeInput } from '../types';

/** Composes an array of Exchanges into a single one.
 *
 * @param exchanges - An array of {@link Exchange | Exchanges}.
 * @returns - A composed {@link Exchange}.
 *
 * @remarks
 * `composeExchanges` returns an {@link Exchange} that when instantiated
 * composes the array of passed `Exchange`s into one, calling them from
 * right to left, with the prior `Exchange`’s {@link ExchangeIO} function
 * as the {@link ExchangeInput.forward} input.
 *
 * This simply merges all exchanges into one and is used by the {@link Client}
 * to merge the `exchanges` option it receives.
 *
 * @throws
 * In development, if {@link ExchangeInput.forward} is called repeatedly
 * by an {@link Exchange} an error is thrown, since `forward()` must only
 * be called once per `Exchange`.
 */
export const composeExchanges =
  (exchanges: Exchange[]): Exchange =>
  ({ client, forward, dispatchDebug }: ExchangeInput): ExchangeIO =>
    exchanges.reduceRight((forward, exchange) => {
      let forwarded = false;
      return exchange({
        client,
        forward(operations$) {
          if (process.env.NODE_ENV !== 'production') {
            if (forwarded)
              throw new Error(
                'forward() must only be called once in each Exchange.'
              );
            forwarded = true;
          }
          return share(forward(share(operations$)));
        },
        dispatchDebug(event) {
          dispatchDebug({
            timestamp: Date.now(),
            source: exchange.name,
            ...event,
          });
        },
      });
    }, forward);