import type { Flow } from '@equinor/fusion-observable';
import { actions, type Actions } from './actions';
import type { QueryClientState, QueryFn, RetryOptions } from './types';
import {
  filter,
  mergeMap,
  from,
  map,
  catchError,
  of,
  takeUntil,
  withLatestFrom,
  timer,
  type Observable,
  tap,
} from 'rxjs';
import { QueryClientError } from './QueryClientError';

/**
 * Handles incoming request actions by transforming them into execute actions.
 *
 * @param action$ - The stream of actions being dispatched in the system.
 * @returns An Observable that emits execute actions for each request action.
 */
export const handleRequests: Flow<Actions, QueryClientState> = (action$) =>
  action$.pipe(
    filter(actions.request.match),
    map((action) => actions.execute(action.meta.transaction)),
  );

/**
 * Handles the execution of a query.
 *
 * @param fetch - The function used to execute the query.
 * @returns A flow that handles the execution of the query.
 */
export const handleExecution =
  <TType, TArgs>(fetch: QueryFn<TType, TArgs>): Flow<Actions, QueryClientState<TArgs>> =>
  (action$, state$) =>
    action$.pipe(
      filter(actions.execute.match),
      withLatestFrom(state$),
      mergeMap(([action, state]) => {
        const transaction = action.payload;
        const request = state[transaction];

        // Create an AbortController instance to manage cancellation.
        const controller = new AbortController();

        // Listen for cancel actions specifically targeting this transaction.
        const cancel$ = action$.pipe(
          filter(actions.cancel.match),
          filter((next) => next.payload.transaction === transaction),
          tap(() => {
            // If the request hasn't been aborted yet, abort it.
            if (!controller.signal.aborted) {
              controller.abort();
            }
          }),
        );

        try {
          return from(fetch(request.args, controller.signal)).pipe(
            map((value) =>
              actions.execute.success({
                ...request,
                status: 'complete',
                completed: Date.now(),
                value,
              }),
            ),
            catchError((err) => of(actions.execute.failure(err, transaction))),
            takeUntil(cancel$), // Complete the observable chain if a cancel action is received.
          );
        } catch (err) {
          // Normally errors thrown during the execution of the fetch function are caught by the `catchError` operator.
          // However, if the fetch function itself throws an error, it will be caught here.
          // This can happen if the fetch function is not a function or if it throws synchronously.
          return of(actions.execute.failure(err as Error, transaction));
        }
      }),
    );

/**
 * Handles execution failure by scheduling retries according to the provided retry options.
 * If the maximum number of retries is exceeded or no retry options are provided, it emits an error action.
 * Otherwise, it schedules a retry after a delay determined by the retry options.
 *
 * @param config - Optional configuration for retry behavior, including the maximum number of retries and delay between retries.
 */
export const handleFailure = (config?: Partial<RetryOptions>): Flow<Actions, QueryClientState> => {
  return (action$, state$) => {
    return action$.pipe(
      // Filter for actions that indicate an execution failure.
      filter(actions.execute.failure.match),
      // Combine each failure action with the latest state.
      withLatestFrom(state$),
      // Handle the retry logic for each failure.
      mergeMap(([action, state]) => {
        // Extract the transaction identifier from the action metadata.
        const { transaction } = action.meta;
        // Retrieve the corresponding request from the state.
        const request = state[transaction];

        // If the request no longer exists in the state, it cannot be retried.
        if (!request) {
          return of(
            actions.error(
              transaction,
              new QueryClientError('error', {
                message:
                  'request not found, cannot retry request, most likely removed while scheduled for retry!',
              }),
            ),
          );
        }

        // Count the number of execution attempts made for this request.
        const executions = request.execution.length;
        // Merge the provided configuration with the request-specific retry options.
        const retryOptions = Object.assign({}, config, request.options?.retry) as RetryOptions;

        // Identify the cause of the failure, either the last known error or a generic message.
        const cause =
          request.errors?.slice(-1)[0] ??
          new QueryClientError('error', {
            message: 'no errors registered for request!',
            request,
          });

        // If the maximum number of retries has been reached, report an error.
        if (executions > retryOptions.count) {
          const error =
            // if retry is disabled return the last error, else return all registered errors
            retryOptions.count === 0
              ? cause
              : new QueryClientError('error', {
                  message: 'maximum retries executed!',
                  cause: request.errors,
                  request,
                });
          return of(actions.error(transaction, error));
        }

        // Create an Observable that emits after the specified delay for the retry.
        const delay$ =
          typeof retryOptions.delay === 'function'
            ? from(retryOptions.delay(cause)).pipe(
                // Catch errors that occur within the delay calculation.
                catchError((cause) =>
                  of(
                    actions.error(
                      transaction,
                      new QueryClientError('error', {
                        message: 'retry delay callback failed!',
                        cause: [...(request.errors ?? []), cause],
                        request,
                      }),
                    ),
                  ),
                ),
              )
            : (timer(retryOptions.delay) as unknown as Observable<void>);

        // After the delay, check if the request is still present and initiate a retry.
        return delay$.pipe(
          withLatestFrom(state$),
          // Ensure the request has not been removed from the state during the delay.
          filter(([_, state]) => !!state[transaction]),
          // If the request has been removed, throw an error.
          tap(([, state]) => {
            if (!state[transaction]) {
              throw new QueryClientError('error', {
                message: 'request not found, most likely removed while scheduled for retry',
                cause: request.errors,
                request,
              });
            }
          }),
          // Emit an action to re-execute the request.
          map(() => actions.execute(transaction)),
        );
      }),
    );
  };
};