/**
* GNU LibreJS - A browser add-on to block nonfree nontrivial JavaScript.
*
* Copyright (C) 2018 Giorgio Maone <giorgio@maone.net>
*
* This file is part of GNU LibreJS.
*
* GNU LibreJS is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* GNU LibreJS is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with GNU LibreJS.  If not, see <http://www.gnu.org/licenses/>.
*/

/**
  An abstraction layer over the StreamFilter API, allowing its clients to process
  only the "interesting" HTML and script requests and leaving the other alone
*/

let {ResponseMetaData} = require("./ResponseMetaData");

let listeners = new WeakMap();
let webRequestEvent = browser.webRequest.onHeadersReceived;

class ResponseProcessor {

  static install(handler, types = ["main_frame", "sub_frame", "script"]) {
    if (listeners.has(handler)) return false;
    let listener =
      async request =>  await new ResponseTextFilter(request).process(handler);
    listeners.set(handler, listener);
    webRequestEvent.addListener(
  		listener,
  		{urls: ["<all_urls>"], types},
  		["blocking", "responseHeaders"]
  	);
    return true;
  }

  static uninstall(handler) {
    let listener = listeners.get(handler);
    if (listener) {
      webRequestEvent.removeListener(listener);
    }
  }
}

Object.assign(ResponseProcessor, {
  // control flow values to be returned by handler.pre() callbacks
	ACCEPT: {},
	REJECT: {cancel: true},
	CONTINUE: null
});

class ResponseTextFilter {
  constructor(request) {
    this.request = request;
    let {type, statusCode} = request;
    let md = this.metaData = new ResponseMetaData(request);
    this.canProcess = // we want to process html documents and scripts only
      (statusCode < 300 || statusCode >= 400) && // skip redirections
      !md.disposition && // skip forced downloads
      (type === "script" || /\bhtml\b/i.test(md.contentType));
  }

  async process(handler) {
    if (!this.canProcess) return ResponseProcessor.ACCEPT;
    let {metaData, request} = this;
    let response = {request, metaData}; // we keep it around allowing callbacks to store state
    if (typeof handler.pre === "function") {
      let res = await handler.pre(response);
      if (res) return res;
      if (handler.post) handler = handler.post;
      if (typeof handler !== "function") return ResponseProcessor.ACCEPT;
    }

    let {requestId, responseHeaders} = request;
    let filter = browser.webRequest.filterResponseData(requestId);
    let buffer = [];

    filter.ondata = event => {
      buffer.push(event.data);
    };

    filter.onstop = async event => {
      // concatenate chunks
      let size = buffer.reduce((sum, chunk, n) => sum + chunk.byteLength, 0)
      let allBytes = new Uint8Array(size);
      let pos = 0;
      for (let chunk of buffer) {
        allBytes.set(new Uint8Array(chunk), pos);
        pos += chunk.byteLength;
      }
      buffer = null; // allow garbage collection
      if (allBytes.indexOf(0) !== -1) {
        console.debug("Warning: zeroes in bytestream, probable cached encoding mismatch.", request);
        if (request.type === "script") {
          console.debug("It's a script, trying to refetch it.");
          response.text = await (await fetch(request.url, {cache: "reload", credentials: "include"})).text();
        } else {
          console.debug("It's a %s, trying to decode it as UTF-16.", request.type);
          response.text = new TextDecoder("utf-16be").decode(allBytes, {stream: true});
        }
      } else {
        response.text = metaData.decode(allBytes);
      }
      let editedText = null;
      try {
        editedText = await handler(response);
      } catch(e) {
        console.error(e);
      }
      if (editedText !== null) {
        // we changed the content, let's re-encode
        let encoded = new TextEncoder().encode(editedText);
        // pre-pending the UTF-8 BOM will force the charset per HTML 5 specs
        allBytes = new Uint8Array(encoded.byteLength + 3);
        allBytes.set(ResponseMetaData.UTF8BOM, 0); // UTF-8 BOM
        allBytes.set(encoded, 3);
      }
      filter.write(allBytes);
      filter.close();
    }

    return ResponseProcessor.ACCEPT;
  }
}

module.exports = { ResponseProcessor };