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EDIT :: reader.js

var assert = require('assert');
var Buffer = require('safer-buffer').Buffer;

var ASN1 = require('./types');
var errors = require('./errors');

// --- Globals

var newInvalidAsn1Error = errors.newInvalidAsn1Error;

// --- API

function Reader(data) {
  if (!data || !Buffer.isBuffer(data))
    throw new TypeError('data must be a node Buffer');

this._buf = data;
  this._size = data.length;

// These hold the "current" state
  this._len = 0;
  this._offset = 0;
}

Object.defineProperty(Reader.prototype, 'length', {
  enumerable: true,
  get: function () { return (this._len); }
});

Object.defineProperty(Reader.prototype, 'offset', {
  enumerable: true,
  get: function () { return (this._offset); }
});

Object.defineProperty(Reader.prototype, 'remain', {
  get: function () { return (this._size - this._offset); }
});

Object.defineProperty(Reader.prototype, 'buffer', {
  get: function () { return (this._buf.slice(this._offset)); }
});

/**
 * Reads a single byte and advances offset; you can pass in `true` to make this
 * a "peek" operation (i.e., get the byte, but don't advance the offset).
 *
 * @param {Boolean} peek true means don't move offset.
 * @return {Number} the next byte, null if not enough data.
 */
Reader.prototype.readByte = function (peek) {
  if (this._size - this._offset < 1)
    return null;

var b = this._buf[this._offset] & 0xff;

if (!peek)
    this._offset += 1;

return b;
};

Reader.prototype.peek = function () {
  return this.readByte(true);
};

/**
 * Reads a (potentially) variable length off the BER buffer.  This call is
 * not really meant to be called directly, as callers have to manipulate
 * the internal buffer afterwards.
 *
 * As a result of this call, you can call `Reader.length`, until the
 * next thing called that does a readLength.
 *
 * @return {Number} the amount of offset to advance the buffer.
 * @throws {InvalidAsn1Error} on bad ASN.1
 */
Reader.prototype.readLength = function (offset) {
  if (offset === undefined)
    offset = this._offset;

if (offset >= this._size)
    return null;

var lenB = this._buf[offset++] & 0xff;
  if (lenB === null)
    return null;

if ((lenB & 0x80) === 0x80) {
    lenB &= 0x7f;

if (lenB === 0)
      throw newInvalidAsn1Error('Indefinite length not supported');

if (lenB > 4)
      throw newInvalidAsn1Error('encoding too long');

if (this._size - offset < lenB)
      return null;

this._len = 0;
    for (var i = 0; i < lenB; i++)
      this._len = (this._len << 8) + (this._buf[offset++] & 0xff);

} else {
    // Wasn't a variable length
    this._len = lenB;
  }

return offset;
};

/**
 * Parses the next sequence in this BER buffer.
 *
 * To get the length of the sequence, call `Reader.length`.
 *
 * @return {Number} the sequence's tag.
 */
Reader.prototype.readSequence = function (tag) {
  var seq = this.peek();
  if (seq === null)
    return null;
  if (tag !== undefined && tag !== seq)
    throw newInvalidAsn1Error('Expected 0x' + tag.toString(16) +
                              ': got 0x' + seq.toString(16));

var o = this.readLength(this._offset + 1); // stored in `length`
  if (o === null)
    return null;

this._offset = o;
  return seq;
};

Reader.prototype.readInt = function () {
  return this._readTag(ASN1.Integer);
};

Reader.prototype.readBoolean = function () {
  return (this._readTag(ASN1.Boolean) === 0 ? false : true);
};

Reader.prototype.readEnumeration = function () {
  return this._readTag(ASN1.Enumeration);
};

Reader.prototype.readString = function (tag, retbuf) {
  if (!tag)
    tag = ASN1.OctetString;

var b = this.peek();
  if (b === null)
    return null;

if (b !== tag)
    throw newInvalidAsn1Error('Expected 0x' + tag.toString(16) +
                              ': got 0x' + b.toString(16));

var o = this.readLength(this._offset + 1); // stored in `length`

if (o === null)
    return null;

if (this.length > this._size - o)
    return null;

this._offset = o;

if (this.length === 0)
    return retbuf ? Buffer.alloc(0) : '';

var str = this._buf.slice(this._offset, this._offset + this.length);
  this._offset += this.length;

return retbuf ? str : str.toString('utf8');
};

Reader.prototype.readOID = function (tag) {
  if (!tag)
    tag = ASN1.OID;

var b = this.readString(tag, true);
  if (b === null)
    return null;

var values = [];
  var value = 0;

for (var i = 0; i < b.length; i++) {
    var byte = b[i] & 0xff;

value <<= 7;
    value += byte & 0x7f;
    if ((byte & 0x80) === 0) {
      values.push(value);
      value = 0;
    }
  }

value = values.shift();
  values.unshift(value % 40);
  values.unshift((value / 40) >> 0);

return values.join('.');
};

Reader.prototype._readTag = function (tag) {
  assert.ok(tag !== undefined);

var b = this.peek();

if (b === null)
    return null;

if (b !== tag)
    throw newInvalidAsn1Error('Expected 0x' + tag.toString(16) +
                              ': got 0x' + b.toString(16));

var o = this.readLength(this._offset + 1); // stored in `length`
  if (o === null)
    return null;

if (this.length > 4)
    throw newInvalidAsn1Error('Integer too long: ' + this.length);

if (this.length > this._size - o)
    return null;
  this._offset = o;

var fb = this._buf[this._offset];
  var value = 0;

for (var i = 0; i < this.length; i++) {
    value <<= 8;
    value |= (this._buf[this._offset++] & 0xff);
  }

if ((fb & 0x80) === 0x80 && i !== 4)
    value -= (1 << (i * 8));

return value >> 0;
};

// --- Exported API

module.exports = Reader;

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// SYSTEM PANEL //

EDIT :: reader.js