// @flow strict
import inspect from '../jsutils/inspect';
import isAsyncIterable from '../jsutils/isAsyncIterable';
import { addPath, pathToArray } from '../jsutils/Path';

import { GraphQLError } from '../error/GraphQLError';
import { locatedError } from '../error/locatedError';

import type { DocumentNode } from '../language/ast';

import type { ExecutionResult, ExecutionContext } from '../execution/execute';
import { getArgumentValues } from '../execution/values';
import {
  assertValidExecutionArguments,
  buildExecutionContext,
  buildResolveInfo,
  collectFields,
  execute,
  getFieldDef,
} from '../execution/execute';

import type { GraphQLSchema } from '../type/schema';
import type { GraphQLFieldResolver } from '../type/definition';

import { getOperationRootType } from '../utilities/getOperationRootType';

import mapAsyncIterator from './mapAsyncIterator';

export type SubscriptionArgs = {|
  schema: GraphQLSchema,
  document: DocumentNode,
  rootValue?: mixed,
  contextValue?: mixed,
  variableValues?: ?{ +[variable: string]: mixed, ... },
  operationName?: ?string,
  fieldResolver?: ?GraphQLFieldResolver<any, any>,
  subscribeFieldResolver?: ?GraphQLFieldResolver<any, any>,
|};

/**
 * Implements the "Subscribe" algorithm described in the GraphQL specification.
 *
 * Returns a Promise which resolves to either an AsyncIterator (if successful)
 * or an ExecutionResult (error). The promise will be rejected if the schema or
 * other arguments to this function are invalid, or if the resolved event stream
 * is not an async iterable.
 *
 * If the client-provided arguments to this function do not result in a
 * compliant subscription, a GraphQL Response (ExecutionResult) with
 * descriptive errors and no data will be returned.
 *
 * If the source stream could not be created due to faulty subscription
 * resolver logic or underlying systems, the promise will resolve to a single
 * ExecutionResult containing `errors` and no `data`.
 *
 * If the operation succeeded, the promise resolves to an AsyncIterator, which
 * yields a stream of ExecutionResults representing the response stream.
 *
 * Accepts either an object with named arguments, or individual arguments.
 */
declare function subscribe(
  SubscriptionArgs,
  ..._: []
): Promise<AsyncGenerator<ExecutionResult, void, void> | ExecutionResult>;
/* eslint-disable no-redeclare */
declare function subscribe(
  schema: GraphQLSchema,
  document: DocumentNode,
  rootValue?: mixed,
  contextValue?: mixed,
  variableValues?: ?{ +[variable: string]: mixed, ... },
  operationName?: ?string,
  fieldResolver?: ?GraphQLFieldResolver<any, any>,
  subscribeFieldResolver?: ?GraphQLFieldResolver<any, any>,
): Promise<AsyncIterator<ExecutionResult> | ExecutionResult>;
export function subscribe(
  argsOrSchema,
  document,
  rootValue,
  contextValue,
  variableValues,
  operationName,
  fieldResolver,
  subscribeFieldResolver,
) {
  /* eslint-enable no-redeclare */
  // Extract arguments from object args if provided.
  return arguments.length === 1
    ? subscribeImpl(argsOrSchema)
    : subscribeImpl({
        schema: argsOrSchema,
        document,
        rootValue,
        contextValue,
        variableValues,
        operationName,
        fieldResolver,
        subscribeFieldResolver,
      });
}

/**
 * This function checks if the error is a GraphQLError. If it is, report it as
 * an ExecutionResult, containing only errors and no data. Otherwise treat the
 * error as a system-class error and re-throw it.
 */
function reportGraphQLError(error: mixed): ExecutionResult {
  if (error instanceof GraphQLError) {
    return { errors: [error] };
  }
  throw error;
}

function subscribeImpl(
  args: SubscriptionArgs,
): Promise<AsyncGenerator<ExecutionResult, void, void> | ExecutionResult> {
  const {
    schema,
    document,
    rootValue,
    contextValue,
    variableValues,
    operationName,
    fieldResolver,
    subscribeFieldResolver,
  } = args;

  const sourcePromise = createSourceEventStream(
    schema,
    document,
    rootValue,
    contextValue,
    variableValues,
    operationName,
    subscribeFieldResolver,
  );

  // For each payload yielded from a subscription, map it over the normal
  // GraphQL `execute` function, with `payload` as the rootValue.
  // This implements the "MapSourceToResponseEvent" algorithm described in
  // the GraphQL specification. The `execute` function provides the
  // "ExecuteSubscriptionEvent" algorithm, as it is nearly identical to the
  // "ExecuteQuery" algorithm, for which `execute` is also used.
  const mapSourceToResponse = (payload) =>
    execute({
      schema,
      document,
      rootValue: payload,
      contextValue,
      variableValues,
      operationName,
      fieldResolver,
    });

  // Resolve the Source Stream, then map every source value to a
  // ExecutionResult value as described above.
  return sourcePromise.then((resultOrStream) =>
    // Note: Flow can't refine isAsyncIterable, so explicit casts are used.
    isAsyncIterable(resultOrStream)
      ? mapAsyncIterator(
          resultOrStream,
          mapSourceToResponse,
          reportGraphQLError,
        )
      : ((resultOrStream: any): ExecutionResult),
  );
}

/**
 * Implements the "CreateSourceEventStream" algorithm described in the
 * GraphQL specification, resolving the subscription source event stream.
 *
 * Returns a Promise which resolves to either an AsyncIterable (if successful)
 * or an ExecutionResult (error). The promise will be rejected if the schema or
 * other arguments to this function are invalid, or if the resolved event stream
 * is not an async iterable.
 *
 * If the client-provided arguments to this function do not result in a
 * compliant subscription, a GraphQL Response (ExecutionResult) with
 * descriptive errors and no data will be returned.
 *
 * If the the source stream could not be created due to faulty subscription
 * resolver logic or underlying systems, the promise will resolve to a single
 * ExecutionResult containing `errors` and no `data`.
 *
 * If the operation succeeded, the promise resolves to the AsyncIterable for the
 * event stream returned by the resolver.
 *
 * A Source Event Stream represents a sequence of events, each of which triggers
 * a GraphQL execution for that event.
 *
 * This may be useful when hosting the stateful subscription service in a
 * different process or machine than the stateless GraphQL execution engine,
 * or otherwise separating these two steps. For more on this, see the
 * "Supporting Subscriptions at Scale" information in the GraphQL specification.
 */
export function createSourceEventStream(
  schema: GraphQLSchema,
  document: DocumentNode,
  rootValue?: mixed,
  contextValue?: mixed,
  variableValues?: ?{ +[variable: string]: mixed, ... },
  operationName?: ?string,
  fieldResolver?: ?GraphQLFieldResolver<any, any>,
): Promise<AsyncIterable<mixed> | ExecutionResult> {
  // If arguments are missing or incorrectly typed, this is an internal
  // developer mistake which should throw an early error.
  assertValidExecutionArguments(schema, document, variableValues);

  return new Promise((resolve) => {
    // If a valid context cannot be created due to incorrect arguments,
    // this will throw an error.
    const exeContext = buildExecutionContext(
      schema,
      document,
      rootValue,
      contextValue,
      variableValues,
      operationName,
      fieldResolver,
    );

    resolve(
      // Return early errors if execution context failed.
      Array.isArray(exeContext)
        ? { errors: exeContext }
        : executeSubscription(exeContext),
    );
  }).catch(reportGraphQLError);
}

function executeSubscription(
  exeContext: ExecutionContext,
): Promise<AsyncIterable<mixed>> {
  const { schema, operation, variableValues, rootValue } = exeContext;
  const type = getOperationRootType(schema, operation);
  const fields = collectFields(
    exeContext,
    type,
    operation.selectionSet,
    Object.create(null),
    Object.create(null),
  );
  const responseNames = Object.keys(fields);
  const responseName = responseNames[0];
  const fieldNodes = fields[responseName];
  const fieldNode = fieldNodes[0];
  const fieldName = fieldNode.name.value;
  const fieldDef = getFieldDef(schema, type, fieldName);

  if (!fieldDef) {
    throw new GraphQLError(
      `The subscription field "${fieldName}" is not defined.`,
      fieldNodes,
    );
  }

  const path = addPath(undefined, responseName, type.name);
  const info = buildResolveInfo(exeContext, fieldDef, fieldNodes, type, path);

  // Coerce to Promise for easier error handling and consistent return type.
  return new Promise((resolveResult) => {
    // Implements the "ResolveFieldEventStream" algorithm from GraphQL specification.
    // It differs from "ResolveFieldValue" due to providing a different `resolveFn`.

    // Build a JS object of arguments from the field.arguments AST, using the
    // variables scope to fulfill any variable references.
    const args = getArgumentValues(fieldDef, fieldNodes[0], variableValues);

    // The resolve function's optional third argument is a context value that
    // is provided to every resolve function within an execution. It is commonly
    // used to represent an authenticated user, or request-specific caches.
    const contextValue = exeContext.contextValue;

    // Call the `subscribe()` resolver or the default resolver to produce an
    // AsyncIterable yielding raw payloads.
    const resolveFn = fieldDef.subscribe ?? exeContext.fieldResolver;
    resolveResult(resolveFn(rootValue, args, contextValue, info));
  }).then(
    (eventStream) => {
      if (eventStream instanceof Error) {
        throw locatedError(eventStream, fieldNodes, pathToArray(path));
      }

      // Assert field returned an event stream, otherwise yield an error.
      if (!isAsyncIterable(eventStream)) {
        throw new Error(
          'Subscription field must return Async Iterable. ' +
            `Received: ${inspect(eventStream)}.`,
        );
      }
      return eventStream;
    },
    (error) => {
      throw locatedError(error, fieldNodes, pathToArray(path));
    },
  );
}
