import { ExecutionResult } from 'graphql';
import { CacheControlFormat } from 'apollo-cache-control';

export interface PersistedQueryCache {
  set(key: string, data: string): Promise<any>;
  get(key: string): Promise<string | null>;
}

export type HttpHeaderCalculation = (
  responses: Array<ExecutionResult & { extensions?: Record<string, any> }>,
) => Record<string, string>;

export function calculateCacheControlHeaders(
  responses: Array<ExecutionResult & { extensions?: Record<string, any> }>,
): Record<string, string> {
  let lowestMaxAge = Number.MAX_VALUE;
  let publicOrPrivate = 'public';

  // Because of the early exit, we are unable to use forEach. While a reduce
  // loop might be possible, a for loop is more readable
  for (let i = 0; i < responses.length; i++) {
    const response = responses[i];

    const cacheControl: CacheControlFormat =
      response.extensions && response.extensions.cacheControl;

    // If there are no extensions or hints, then the headers should not be present
    if (
      !cacheControl ||
      !cacheControl.hints ||
      cacheControl.hints.length === 0 ||
      cacheControl.version !== 1
    ) {
      if (cacheControl && cacheControl.version !== 1) {
        console.warn('Invalid cacheControl version.');
      }
      return {};
    }

    const rootHints = new Set<string>();
    for (let j = 0; j < cacheControl.hints.length; j++) {
      const hint = cacheControl.hints[j];
      if (hint.scope && hint.scope.toLowerCase() === 'private') {
        publicOrPrivate = 'private';
      }

      // If no maxAge is present, then we ignore the hint
      if (hint.maxAge === undefined) {
        continue;
      }

      // if there is a hint with max age of 0, we don't need to process more
      if (hint.maxAge === 0) {
        return {};
      }

      if (hint.maxAge < lowestMaxAge) {
        lowestMaxAge = hint.maxAge;
      }

      // If this is a root path, store that the root is cacheable:
      if (hint.path.length === 1) {
        rootHints.add(hint.path[0] as string);
      }
    }

    // If a root field inside of data does not have a cache hint, then we do not
    // cache the response
    if (Object.keys(response.data).find(rootKey => !rootHints.has(rootKey))) {
      return {};
    }
  }

  return {
    'Cache-Control': `max-age=${lowestMaxAge}, ${publicOrPrivate}`,
  };
}