/**
 * @license
 * Copyright Google LLC All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */

import {APP_BOOTSTRAP_LISTENER, ApplicationRef, inject, InjectionToken, makeStateKey, Provider, StateKey, TransferState, ɵENABLED_SSR_FEATURES as ENABLED_SSR_FEATURES, ɵwhenStable as whenStable} from '@angular/core';
import {Observable, of} from 'rxjs';
import {tap} from 'rxjs/operators';

import {HttpHeaders} from './headers';
import {HTTP_ROOT_INTERCEPTOR_FNS, HttpHandlerFn} from './interceptor';
import {HttpRequest} from './request';
import {HttpEvent, HttpResponse} from './response';

interface TransferHttpResponse {
  body: any;
  headers: Record<string, string[]>;
  status?: number;
  statusText?: string;
  url?: string;
  responseType?: HttpRequest<unknown>['responseType'];
}

const CACHE_STATE = new InjectionToken<{isCacheActive: boolean}>(
    ngDevMode ? 'HTTP_TRANSFER_STATE_CACHE_STATE' : '');

/**
 * A list of allowed HTTP methods to cache.
 */
const ALLOWED_METHODS = ['GET', 'HEAD'];

export function transferCacheInterceptorFn(
    req: HttpRequest<unknown>, next: HttpHandlerFn): Observable<HttpEvent<unknown>> {
  const {isCacheActive} = inject(CACHE_STATE);

  // Stop using the cache if the application has stabilized, indicating initial rendering
  // is complete.
  if (!isCacheActive || !ALLOWED_METHODS.includes(req.method)) {
    // Cache is no longer active or method is not HEAD or GET.
    // Pass the request through.
    return next(req);
  }

  const transferState = inject(TransferState);
  const storeKey = makeCacheKey(req);
  const response = transferState.get(storeKey, null);

  if (response) {
    // Request found in cache. Respond using it.
    let body: ArrayBuffer|Blob|string|undefined = response.body;

    switch (response.responseType) {
      case 'arraybuffer':
        body = new TextEncoder().encode(response.body).buffer;
        break;
      case 'blob':
        body = new Blob([response.body]);
        break;
    }

    return of(
        new HttpResponse({
          body,
          headers: new HttpHeaders(response.headers),
          status: response.status,
          statusText: response.statusText,
          url: response.url,
        }),
    );
  }

  // Request not found in cache. Make the request and cache it.
  return next(req).pipe(
      tap((event: HttpEvent<unknown>) => {
        if (event instanceof HttpResponse) {
          transferState.set<TransferHttpResponse>(storeKey, {
            body: event.body,
            headers: getHeadersMap(event.headers),
            status: event.status,
            statusText: event.statusText,
            url: event.url || '',
            responseType: req.responseType,
          });
        }
      }),
  );
}

function getHeadersMap(headers: HttpHeaders): Record<string, string[]> {
  const headersMap: Record<string, string[]> = {};

  for (const key of headers.keys()) {
    const values = headers.getAll(key);
    if (values !== null) {
      headersMap[key] = values;
    }
  }

  return headersMap;
}

function makeCacheKey(request: HttpRequest<any>): StateKey<TransferHttpResponse> {
  // make the params encoded same as a url so it's easy to identify
  const {params, method, responseType, url} = request;
  const encodedParams = params.keys().sort().map((k) => `${k}=${params.getAll(k)}`).join('&');
  const key = method + '.' + responseType + '.' + url + '?' + encodedParams;

  const hash = generateHash(key);

  return makeStateKey(hash);
}

/**
 * A method that returns a hash representation of a string using a variant of DJB2 hash
 * algorithm.
 *
 * This is the same hashing logic that is used to generate component ids.
 */
function generateHash(value: string): string {
  let hash = 0;

  for (const char of value) {
    hash = Math.imul(31, hash) + char.charCodeAt(0) << 0;
  }

  // Force positive number hash.
  // 2147483647 = equivalent of Integer.MAX_VALUE.
  hash += 2147483647 + 1;

  return hash.toString();
}

/**
 * Returns the DI providers needed to enable HTTP transfer cache.
 *
 * By default, when using server rendering, requests are performed twice: once on the server and
 * other one on the browser.
 *
 * When these providers are added, requests performed on the server are cached and reused during the
 * bootstrapping of the application in the browser thus avoiding duplicate requests and reducing
 * load time.
 *
 */
export function withHttpTransferCache(): Provider[] {
  return [
    {
      provide: CACHE_STATE,
      useFactory: () => {
        inject(ENABLED_SSR_FEATURES).add('httpcache');
        return {isCacheActive: true};
      }
    },
    {
      provide: HTTP_ROOT_INTERCEPTOR_FNS,
      useValue: transferCacheInterceptorFn,
      multi: true,
      deps: [TransferState, CACHE_STATE]
    },
    {
      provide: APP_BOOTSTRAP_LISTENER,
      multi: true,
      useFactory: () => {
        const appRef = inject(ApplicationRef);
        const cacheState = inject(CACHE_STATE);

        return () => {
          whenStable(appRef).then(() => {
            cacheState.isCacheActive = false;
          });
        };
      }
    }
  ];
}