import {
  GraphQLExecutionResult,
  GraphQLRequestContext,
} from 'apollo-server-types';
import { Headers } from 'apollo-server-env';
import {
  execute,
  GraphQLError,
  Kind,
  TypeNameMetaFieldDef,
  GraphQLFieldResolver,
} from 'graphql';
import { Trace, google } from 'apollo-engine-reporting-protobuf';
import { defaultRootOperationNameLookup } from '@apollo/federation';
import { GraphQLDataSource } from './datasources/types';
import {
  FetchNode,
  PlanNode,
  QueryPlan,
  ResponsePath,
  OperationContext,
  QueryPlanSelectionNode,
  QueryPlanFieldNode,
  getResponseName
} from './QueryPlan';
import { deepMerge } from './utilities/deepMerge';

export type ServiceMap = {
  [serviceName: string]: GraphQLDataSource;
};

type ResultMap = Record<string, any>;

interface ExecutionContext<TContext> {
  queryPlan: QueryPlan;
  operationContext: OperationContext;
  serviceMap: ServiceMap;
  requestContext: GraphQLRequestContext<TContext>;
  errors: GraphQLError[];
}

export async function executeQueryPlan<TContext>(
  queryPlan: QueryPlan,
  serviceMap: ServiceMap,
  requestContext: GraphQLRequestContext<TContext>,
  operationContext: OperationContext,
): Promise<GraphQLExecutionResult> {
  const errors: GraphQLError[] = [];

  const context: ExecutionContext<TContext> = {
    queryPlan,
    operationContext,
    serviceMap,
    requestContext,
    errors,
  };

  let data: ResultMap | undefined | null = Object.create(null);

  const captureTraces = !!(
    requestContext.metrics && requestContext.metrics.captureTraces
  );

  if (queryPlan.node) {
    const traceNode = await executeNode(
      context,
      queryPlan.node,
      data!,
      [],
      captureTraces,
    );
    if (captureTraces) {
      requestContext.metrics!.queryPlanTrace = traceNode;
    }
  }

  // FIXME: Re-executing the query is a pretty heavy handed way of making sure
  // only explicitly requested fields are included and field ordering follows
  // the original query.
  // It is also used to allow execution of introspection queries though.
  try {
    const executionResult = await execute({
      schema: operationContext.schema,
      document: {
        kind: Kind.DOCUMENT,
        definitions: [
          operationContext.operation,
          ...Object.values(operationContext.fragments),
        ],
      },
      rootValue: data,
      variableValues: requestContext.request.variables,
      // We have a special field resolver which ensures we support aliases.
      // FIXME: It's _possible_ this will change after `graphql-extensions` is
      // deprecated, though not certain. See here, also: https://git.io/Jf8cS.
      fieldResolver: defaultFieldResolverWithAliasSupport,
    });
    data = executionResult.data;
    if (executionResult.errors?.length) {
      errors.push(...executionResult.errors)
    }
  } catch (error) {
    return { errors: [error] };
  }

  return errors.length === 0 ? { data } : { errors, data };
}

// Note: this function always returns a protobuf QueryPlanNode tree, even if
// we're going to ignore it, because it makes the code much simpler and more
// typesafe. However, it doesn't actually ask for traces from the backend
// service unless we are capturing traces for Engine.
async function executeNode<TContext>(
  context: ExecutionContext<TContext>,
  node: PlanNode,
  results: ResultMap | ResultMap[],
  path: ResponsePath,
  captureTraces: boolean,
): Promise<Trace.QueryPlanNode> {
  if (!results) {
    // XXX I don't understand `results` threading well enough to understand when this happens
    //     and if this corresponds to a real query plan node that should be reported or not.
    //
    // This may be if running something like `query { fooOrNullFromServiceA {
    // somethingFromServiceB } }` and the first field is null, then we don't bother to run the
    // inner field at all.
    return new Trace.QueryPlanNode();
  }

  switch (node.kind) {
    case 'Sequence': {
      const traceNode = new Trace.QueryPlanNode.SequenceNode();
      for (const childNode of node.nodes) {
        const childTraceNode = await executeNode(
          context,
          childNode,
          results,
          path,
          captureTraces,
        );
        traceNode.nodes.push(childTraceNode!);
      }
      return new Trace.QueryPlanNode({ sequence: traceNode });
    }
    case 'Parallel': {
      const childTraceNodes = await Promise.all(
        node.nodes.map(async childNode =>
          executeNode(context, childNode, results, path, captureTraces),
        ),
      );
      return new Trace.QueryPlanNode({
        parallel: new Trace.QueryPlanNode.ParallelNode({
          nodes: childTraceNodes,
        }),
      });
    }
    case 'Flatten': {
      return new Trace.QueryPlanNode({
        flatten: new Trace.QueryPlanNode.FlattenNode({
          responsePath: node.path.map(
            id =>
              new Trace.QueryPlanNode.ResponsePathElement(
                typeof id === 'string' ? { fieldName: id } : { index: id },
              ),
          ),
          node: await executeNode(
            context,
            node.node,
            flattenResultsAtPath(results, node.path),
            [...path, ...node.path],
            captureTraces,
          ),
        }),
      });
    }
    case 'Fetch': {
      const traceNode = new Trace.QueryPlanNode.FetchNode({
        serviceName: node.serviceName,
        // executeFetch will fill in the other fields if desired.
      });
      try {
        await executeFetch(
          context,
          node,
          results,
          path,
          captureTraces ? traceNode : null,
        );
      } catch (error) {
        context.errors.push(error);
      }
      return new Trace.QueryPlanNode({ fetch: traceNode });
    }
  }
}

async function executeFetch<TContext>(
  context: ExecutionContext<TContext>,
  fetch: FetchNode,
  results: ResultMap | ResultMap[],
  _path: ResponsePath,
  traceNode: Trace.QueryPlanNode.FetchNode | null,
): Promise<void> {
  const logger = context.requestContext.logger || console;
  const service = context.serviceMap[fetch.serviceName];
  if (!service) {
    throw new Error(`Couldn't find service with name "${fetch.serviceName}"`);
  }

  const entities = Array.isArray(results) ? results : [results];
  if (entities.length < 1) return;

  let variables = Object.create(null);
  if (fetch.variableUsages) {
    for (const variableName of fetch.variableUsages) {
      const providedVariables = context.requestContext.request.variables;
      if (
        providedVariables &&
        typeof providedVariables[variableName] !== 'undefined'
      ) {
        variables[variableName] = providedVariables[variableName];
      }
    }
  }

  if (!fetch.requires) {
    const dataReceivedFromService = await sendOperation(
      context,
      fetch.operation,
      variables,
    );

    for (const entity of entities) {
      deepMerge(entity, dataReceivedFromService);
    }
  } else {
    const requires = fetch.requires;

    const representations: ResultMap[] = [];
    const representationToEntity: number[] = [];

    entities.forEach((entity, index) => {
      const representation = executeSelectionSet(entity, requires);
      if (representation && representation[TypeNameMetaFieldDef.name]) {
        representations.push(representation);
        representationToEntity.push(index);
      }
    });

    if ('representations' in variables) {
      throw new Error(`Variables cannot contain key "representations"`);
    }

    const dataReceivedFromService = await sendOperation(
      context,
      fetch.operation,
      { ...variables, representations },
    );

    if (!dataReceivedFromService) {
      return;
    }

    if (
      !(
        dataReceivedFromService._entities &&
        Array.isArray(dataReceivedFromService._entities)
      )
    ) {
      throw new Error(`Expected "data._entities" in response to be an array`);
    }

    const receivedEntities = dataReceivedFromService._entities;

    if (receivedEntities.length !== representations.length) {
      throw new Error(
        `Expected "data._entities" to contain ${representations.length} elements`,
      );
    }

    for (let i = 0; i < entities.length; i++) {
      deepMerge(entities[representationToEntity[i]], receivedEntities[i]);
    }
  }

  async function sendOperation(
    context: ExecutionContext<TContext>,
    source: string,
    variables: Record<string, any>,
  ): Promise<ResultMap | void | null> {
    // We declare this as 'any' because it is missing url and method, which
    // GraphQLRequest.http is supposed to have if it exists.
    let http: any;

    // If we're capturing a trace for Engine, then save the operation text to
    // the node we're building and tell the federated service to include a trace
    // in its response.
    if (traceNode) {
      http = {
        headers: new Headers({ 'apollo-federation-include-trace': 'ftv1' }),
      };
      if (
        context.requestContext.metrics &&
        context.requestContext.metrics.startHrTime
      ) {
        traceNode.sentTimeOffset = durationHrTimeToNanos(
          process.hrtime(context.requestContext.metrics.startHrTime),
        );
      }
      traceNode.sentTime = dateToProtoTimestamp(new Date());
    }

    const response = await service.process({
      request: {
        query: source,
        variables,
        http,
      },
      context: context.requestContext.context,
    });

    if (response.errors) {
      const errors = response.errors.map(error =>
        downstreamServiceError(
          error.message,
          fetch.serviceName,
          source,
          variables,
          error.extensions,
          error.path,
        ),
      );
      context.errors.push(...errors);
    }

    // If we're capturing a trace for Engine, save the received trace into the
    // query plan.
    if (traceNode) {
      traceNode.receivedTime = dateToProtoTimestamp(new Date());

      if (response.extensions && response.extensions.ftv1) {
        const traceBase64 = response.extensions.ftv1;

        let traceBuffer: Buffer | undefined;
        let traceParsingFailed = false;
        try {
          // XXX support non-Node implementations by using Uint8Array? protobufjs
          // supports that, but there's not a no-deps base64 implementation.
          traceBuffer = Buffer.from(traceBase64, 'base64');
        } catch (err) {
          logger.error(
            `error decoding base64 for federated trace from ${fetch.serviceName}: ${err}`,
          );
          traceParsingFailed = true;
        }

        if (traceBuffer) {
          try {
            const trace = Trace.decode(traceBuffer);
            traceNode.trace = trace;
          } catch (err) {
            logger.error(
              `error decoding protobuf for federated trace from ${fetch.serviceName}: ${err}`,
            );
            traceParsingFailed = true;
          }
        }
        if (traceNode.trace) {
          // Federation requires the root operations in the composed schema
          // to have the default names (Query, Mutation, Subscription) even
          // if the implementing services choose different names, so we override
          // whatever the implementing service reported here.
          const rootTypeName =
            defaultRootOperationNameLookup[
              context.operationContext.operation.operation
            ];
          traceNode.trace.root?.child?.forEach((child) => {
            child.parentType = rootTypeName;
          });
        }
        traceNode.traceParsingFailed = traceParsingFailed;
      }
    }

    return response.data;
  }
}

/**
 *
 * @param source Result of GraphQL execution.
 * @param selectionSet
 */
function executeSelectionSet(
  source: Record<string, any> | null,
  selections: QueryPlanSelectionNode[],
): Record<string, any> | null {

  // If the underlying service has returned null for the parent (source)
  // then there is no need to iterate through the parent's selection set
  if (source === null) {
    return null;
  }

  const result: Record<string, any> = Object.create(null);

  for (const selection of selections) {
    switch (selection.kind) {
      case Kind.FIELD:
        const responseName = getResponseName(selection as QueryPlanFieldNode);
        const selections = (selection as QueryPlanFieldNode).selections;

        if (typeof source[responseName] === 'undefined') {
          throw new Error(`Field "${responseName}" was not found in response.`);
        }
        if (Array.isArray(source[responseName])) {
          result[responseName] = source[responseName].map((value: any) =>
            selections ? executeSelectionSet(value, selections) : value,
          );
        } else if (selections) {
          result[responseName] = executeSelectionSet(
            source[responseName],
            selections,
          );
        } else {
          result[responseName] = source[responseName];
        }
        break;
      case Kind.INLINE_FRAGMENT:
        if (!selection.typeCondition) continue;

        const typename = source && source['__typename'];
        if (!typename) continue;

        if (typename === selection.typeCondition) {
          deepMerge(
            result,
            executeSelectionSet(source, selection.selections),
          );
        }
        break;
    }
  }

  return result;
}

function flattenResultsAtPath(value: any, path: ResponsePath): any {
  if (path.length === 0) return value;
  if (value === undefined || value === null) return value;

  const [current, ...rest] = path;
  if (current === '@') {
    return value.flatMap((element: any) => flattenResultsAtPath(element, rest));
  } else {
    return flattenResultsAtPath(value[current], rest);
  }
}

function downstreamServiceError(
  message: string | undefined,
  serviceName: string,
  query: string,
  variables?: Record<string, any>,
  extensions?: Record<string, any>,
  path?: ReadonlyArray<string | number> | undefined,
) {
  if (!message) {
    message = `Error while fetching subquery from service "${serviceName}"`;
  }
  extensions = {
    code: 'DOWNSTREAM_SERVICE_ERROR',
    // XXX The presence of a serviceName in extensions is used to
    // determine if this error should be captured for metrics reporting.
    serviceName,
    query,
    variables,
    ...extensions,
  };
  return new GraphQLError(
    message,
    undefined,
    undefined,
    undefined,
    path,
    undefined,
    extensions,
  );
}

export const defaultFieldResolverWithAliasSupport: GraphQLFieldResolver<
  any,
  any
> = function(source, args, contextValue, info) {
  // ensure source is a value for which property access is acceptable.
  if (typeof source === 'object' || typeof source === 'function') {
    // if this is an alias, check it first because a downstream service
    // would have returned the data *already cast* to an alias responseName
    const property = source[info.path.key];
    if (typeof property === 'function') {
      return source[info.fieldName](args, contextValue, info);
    }
    return property;
  }
};

// Converts an hrtime array (as returned from process.hrtime) to nanoseconds.
//
// ONLY CALL THIS ON VALUES REPRESENTING DELTAS, NOT ON THE RAW RETURN VALUE
// FROM process.hrtime() WITH NO ARGUMENTS.
//
// The entire point of the hrtime data structure is that the JavaScript Number
// type can't represent all int64 values without loss of precision:
// Number.MAX_SAFE_INTEGER nanoseconds is about 104 days. Calling this function
// on a duration that represents a value less than 104 days is fine. Calling
// this function on an absolute time (which is generally roughly time since
// system boot) is not a good idea.
//
// XXX We should probably use google.protobuf.Duration on the wire instead of
// ever trying to store durations in a single number.
function durationHrTimeToNanos(hrtime: [number, number]) {
  return hrtime[0] * 1e9 + hrtime[1];
}

// Converts a JS Date into a Timestamp.
function dateToProtoTimestamp(date: Date): google.protobuf.Timestamp {
  const totalMillis = +date;
  const millis = totalMillis % 1000;
  return new google.protobuf.Timestamp({
    seconds: (totalMillis - millis) / 1000,
    nanos: millis * 1e6,
  });
}