'use strict';

var BaseService = require('../../service');
var inherits = require('util').inherits;
var Encoding = require('./encoding');
var index = require('../../');
var log = index.log;
var utils = require('../../utils');
var async = require('async');
var BN = require('bn.js');
var consensus = require('bcoin').consensus;

var HeaderService = function(options) {

  BaseService.call(this, options);

  this._tip = null;
  this._p2p = this.node.services.p2p;
  this._db = this.node.services.db;
  this._headers = [];
};

inherits(HeaderService, BaseService);

HeaderService.dependencies = [ 'p2p', 'db' ];

HeaderService.MAX_CHAINWORK = new BN(1).ushln(256);
HeaderService.STARTING_CHAINWORK = '0000000000000000000000000000000000000000000000000000000100010001';

// --- public prototype functions
HeaderService.prototype.getAPIMethods = function() {

  var methods = [
    ['getAllHeaders', this, this.getAllHeaders, 0],
    ['getBestHeight', this, this.getBestHeight, 0]
  ];

  return methods;

};

HeaderService.prototype.getBestHeight = function() {
  return this._tip.height;
};

HeaderService.prototype.start = function(callback) {

  var self = this;

  async.waterfall([
    function(next) {
      self._db.getPrefix(self.name, next);
    },
    function(prefix, next) {
      self._encoding = new Encoding(prefix);
      self._db.getServiceTip(self.name, next);
    },
    function(tip, next) {
      self._tip = tip;
      self._getPersistedHeaders(next);
    }
  ], function(err, headers) {

    if (err) {
      return callback(err);
    }

    this._headers = headers;
    self._setListeners();
    self._startSubscriptions();
    callback();

  });

};

HeaderService.prototype.stop = function(callback) {
  callback();
};

HeaderService.prototype._startSubscriptions = function() {

  if (this._subscribed) {
    return;
  }

  this._subscribed = true;
  if (!this._bus) {
    this._bus = this.node.openBus({remoteAddress: 'localhost'});
  }

  this._bus.on('p2p/headers', this._onHeaders.bind(this));
  this._bus.on('p2p/block', this._onHeaders.bind(this));
  this._bus.subscribe('p2p/headers');
  this._bus.subscribe('p2p/block');

};

HeaderService.prototype._onBlock = function(block) {
  // we just want the header to keep a running list
  this._onHeaders([block.header]);
};

HeaderService.prototype._onHeaders = function(headers) {

  if (!headers || headers.length < 1) {
    return;
  }

  var operations = this._getHeaderOperations(headers);

  this._tip.hash = headers[headers.length - 1].hash;
  this._tip.height = this._tip.height + headers.length;

  var tipOps = utils.encodeTip(this._tip, this.name);

  operations.push({
    type: 'put',
    key: tipOps.key,
    value: tipOps.value
  });

  this._db.batch(operations);
  this._headers.concat(headers);

  if (this._tip.height >= this._bestHeight) {
    log.info('Header download complete.');
    this.emit('headers', this._headers);
    return;
  }

  this._sync();

};

HeaderService.prototype._getHeaderOperations = function(headers) {

  var self = this;
  var runningHeight = this._tip.height;
  var prevHeader = this._headers[this._headers.length - 1];
  return headers.map(function(header) {
    header.height = ++runningHeight;
    header.chainwork = self._getChainwork(header, prevHeader).toString(16, 32);
    prevHeader = header;
    return {
      type: 'put',
      key: self._encoding.encodeHeaderKey(header.height, header.hash),
      value: self._encoding.encodeHeaderValue(header)
    };
  });

};

HeaderService.prototype._setListeners = function() {

  this._p2p.once('bestHeight', this._onBestHeight.bind(this));

};

HeaderService.prototype._onBestHeight = function(height) {
  this._bestHeight = height;
  this._startSync();
};

HeaderService.prototype._startSync = function() {

  this._numNeeded = this._bestHeight - this._tip.height;
  if (this._numNeeded <= 0) {
    return;
  }

  log.info('Gathering: ' + this._numNeeded + ' ' + 'header(s) from the peer-to-peer network.');

  this._p2pHeaderCallsNeeded = Math.ceil(this._numNeeded / 500);
  this._sync();

};

HeaderService.prototype._sync = function() {

  if (--this._p2pHeaderCallsNeeded > 0) {

    log.info('Headers download progress: ' + this._tip.height + '/' +
      this._numNeeded + '  (' + (this._tip.height / this._numNeeded*100).toFixed(2) + '%)');

    this._p2p.getHeaders({ startHash: this._tip.hash });

    return;

  }

};

HeaderService.prototype.getAllHeaders = function() {
  return this._headers;
};

HeaderService.prototype._getPersistedHeaders = function(callback) {

  var self = this;
  var results = [];
  var start = self._encoding.encodeHeaderKey(0);
  var end = self._encoding.encodeHeaderKey(0xffffffff);
  var criteria = {
    gte: start,
    lte: end
  };

  var stream = self._db.createReadStream(criteria);

  var streamErr;
  stream.on('error', function(error) {
    streamErr = error;
  });

  stream.on('data', function(data) {
    var res = {};
    res[self._encoding.decodeHeaderKey(data.key).hash] = self._encoding.decodeHeaderValue(data.value);
    results.push(res);
  });

  stream.on('end', function() {
    if (streamErr) {
      return streamErr;
    }
    callback(null, results);
  });

};

HeaderService.prototype._getChainwork = function(header, prevHeader) {

  var lastChainwork = prevHeader ? prevHeader.chainwork : HeaderService.STARTING_CHAINWORK;
  var prevChainwork = new BN(new Buffer(lastChainwork, 'hex'));

  return this._computeChainwork(header.bits, prevChainwork);
};

HeaderService.prototype._computeChainwork = function(bits, prev) {

  var target = consensus.fromCompact(bits);

  if (target.isNeg() || target.cmpn(0) === 0) {
    return new BN(0);
  }

  var proof =  HeaderService.MAX_CHAINWORK.div(target.iaddn(1));

  if (!prev) {
    return proof;
  }

  return proof.iadd(prev);

};

module.exports = HeaderService;