// this entire module is depressing. i should have spent my time learning
// how to patch v8 so that these options would just be available on the
// process object.

const os = require('os');
const fs = require('fs');
const path = require('path');
const execFile = require('child_process').execFile;
const env = process.env;
const user = env.LOGNAME || env.USER || env.LNAME || env.USERNAME;
const configfile = '.v8flags.'+process.versions.v8+'.'+user+'.json';
const exclusions = ['--help'];

const failureMessage = [
  'Unable to cache a config file for v8flags to a your home directory',
  'or a temporary folder. To fix this problem, please correct your',
  'environment by setting HOME=/path/to/home or TEMP=/path/to/temp.',
  'NOTE: the user running this must be able to access provided path.',
  'If all else fails, please open an issue here:',
  'http://github.com/tkellen/js-v8flags'
].join('\n');

function fail (err) {
  err.message += '\n\n' + failureMessage;
  return err;
}

function openConfig (cb) {
  var userHome = require('user-home');
  var configpath = path.join(userHome || os.tmpdir(), configfile);
  var content;
  try {
    // if the config file is valid, it should be json and therefore
    // node should be able to require it directly. if this doesn't
    // throw, we're done!
    content = require(configpath);
    process.nextTick(function () {
      cb(null, content);
    });
  } catch (e) {
    // if requiring the config file failed, maybe it doesn't exist, or
    // perhaps it has become corrupted. instead of calling back with the
    // content of the file, call back with a file descriptor that we can
    // write the cached data to
    fs.open(configpath, 'w+', function (err, fd) {
      if (err) {
        return cb(err);
      }
      return cb(null, fd);
    });
  }
}

// i can't wait for the day this whole module is obsolete because these
// options are available on the process object. this executes node with
// `--v8-options` and parses the result, returning an array of command
// line flags.
function getFlags (cb) {
  execFile(process.execPath, ['--v8-options'], function (execErr, result) {
    if (execErr) {
      return cb(execErr);
    }
    var flags = result.match(/\s\s--(\w+)/gm).map(function (match) {
      return match.substring(2);
    }).filter(function (name) {
      return exclusions.indexOf(name) === -1;
    });
    return cb(null, flags);
  });
}

// write some json to a file descriptor. if this fails, call back
// with both the error and the data that was meant to be written.
function writeConfig (fd, flags, cb) {
  var buf = new Buffer(JSON.stringify(flags));
  return fs.write(fd, buf, 0, buf.length, 0 , function (writeErr) {
    fs.close(fd, function (closeErr) {
      var err = writeErr || closeErr;
      if (err) {
        return cb(fail(err), flags);
      }
      return cb(null, flags);
    });
  });
}

module.exports = function (cb) {
  // attempt to open/read cache file
  openConfig(function (openErr, result) {
    if (!openErr && typeof result !== 'number') {
      return cb(null, result);
    }
    // if the result is not an array, we need to go fetch
    // the flags by invoking node with `--v8-options`
    getFlags(function (flagsErr, flags) {
      // if there was an error fetching the flags, bail immediately
      if (flagsErr) {
        return cb(flagsErr);
      }
      // if there was a problem opening the config file for writing
      // throw an error but include the flags anyway so that users
      // can continue to execute (at the expense of having to fetch
      // flags on every run until they fix the underyling problem).
      if (openErr) {
        return cb(fail(openErr), flags);
      }
      // write the config file to disk so subsequent runs can read
      // flags out of a cache file.
      return writeConfig(result, flags, cb);
    });
  });
};

module.exports.configfile = configfile;
