artesanias/node_modules/streamsearch/lib/sbmh.js

/*
  Based heavily on the Streaming Boyer-Moore-Horspool C++ implementation
  by Hongli Lai at: https://github.com/FooBarWidget/boyer-moore-horspool
*/
var EventEmitter = require('events').EventEmitter,
    inherits = require('util').inherits;

function jsmemcmp(buf1, pos1, buf2, pos2, num) {
  for (var i = 0; i < num; ++i, ++pos1, ++pos2)
    if (buf1[pos1] !== buf2[pos2])
      return false;
  return true;
}

function SBMH(needle) {
  if (typeof needle === 'string')
    needle = new Buffer(needle);
  var i, j, needle_len = needle.length;

  this.maxMatches = Infinity;
  this.matches = 0;

  this._occ = new Array(256);
  this._lookbehind_size = 0;
  this._needle = needle;
  this._bufpos = 0;

  this._lookbehind = new Buffer(needle_len);

  // Initialize occurrence table.
  for (j = 0; j < 256; ++j)
    this._occ[j] = needle_len;

  // Populate occurrence table with analysis of the needle,
  // ignoring last letter.
  if (needle_len >= 1) {
    for (i = 0; i < needle_len - 1; ++i)
      this._occ[needle[i]] = needle_len - 1 - i;
  }
}
inherits(SBMH, EventEmitter);

SBMH.prototype.reset = function() {
  this._lookbehind_size = 0;
  this.matches = 0;
  this._bufpos = 0;
};

SBMH.prototype.push = function(chunk, pos) {
  var r, chlen;
  if (!Buffer.isBuffer(chunk))
    chunk = new Buffer(chunk, 'binary');
  chlen = chunk.length;
  this._bufpos = pos || 0;
  while (r !== chlen && this.matches < this.maxMatches)
    r = this._sbmh_feed(chunk);
  return r;
};

SBMH.prototype._sbmh_feed = function(data) {
  var len = data.length, needle = this._needle, needle_len = needle.length;

  // Positive: points to a position in `data`
  //           pos == 3 points to data[3]
  // Negative: points to a position in the lookbehind buffer
  //           pos == -2 points to lookbehind[lookbehind_size - 2]
  var pos = -this._lookbehind_size,
      last_needle_char = needle[needle_len - 1],
      occ = this._occ,
      lookbehind = this._lookbehind;

  if (pos < 0) {
    // Lookbehind buffer is not empty. Perform Boyer-Moore-Horspool
    // search with character lookup code that considers both the
    // lookbehind buffer and the current round's haystack data.
    //
    // Loop until
    //   there is a match.
    // or until
    //   we've moved past the position that requires the
    //   lookbehind buffer. In this case we switch to the
    //   optimized loop.
    // or until
    //   the character to look at lies outside the haystack.
    while (pos < 0 && pos <= len - needle_len) {
       var ch = this._sbmh_lookup_char(data, pos + needle_len - 1);

      if (ch === last_needle_char
          && this._sbmh_memcmp(data, pos, needle_len - 1)) {
        this._lookbehind_size = 0;
        ++this.matches;
        if (pos > -this._lookbehind_size)
          this.emit('info', true, lookbehind, 0, this._lookbehind_size + pos);
        else
          this.emit('info', true);

        this._bufpos = pos + needle_len;
        return pos + needle_len;
      } else
        pos += occ[ch];
    }

    // No match.

    if (pos < 0) {
      // There's too few data for Boyer-Moore-Horspool to run,
      // so let's use a different algorithm to skip as much as
      // we can.
      // Forward pos until
      //   the trailing part of lookbehind + data
      //   looks like the beginning of the needle
      // or until
      //   pos == 0
      while (pos < 0 && !this._sbmh_memcmp(data, pos, len - pos))
        pos++;
    }

    if (pos >= 0) {
      // Discard lookbehind buffer.
      this.emit('info', false, lookbehind, 0, this._lookbehind_size);
      this._lookbehind_size = 0;
    } else {
      // Cut off part of the lookbehind buffer that has
      // been processed and append the entire haystack
      // into it.
      var bytesToCutOff = this._lookbehind_size + pos;

      if (bytesToCutOff > 0) {
        // The cut off data is guaranteed not to contain the needle.
        this.emit('info', false, lookbehind, 0, bytesToCutOff);
      }

      lookbehind.copy(lookbehind, 0, bytesToCutOff,
                      this._lookbehind_size - bytesToCutOff);
      this._lookbehind_size -= bytesToCutOff;

      data.copy(lookbehind, this._lookbehind_size);
      this._lookbehind_size += len;

      this._bufpos = len;
      return len;
    }
  }

  if (pos >= 0)
    pos += this._bufpos;

  // Lookbehind buffer is now empty. Perform Boyer-Moore-Horspool
  // search with optimized character lookup code that only considers
  // the current round's haystack data.
  while (pos <= len - needle_len) {
    var ch = data[pos + needle_len - 1];

    if (ch === last_needle_char
        && data[pos] === needle[0]
        && jsmemcmp(needle, 0, data, pos, needle_len - 1)) {
      ++this.matches;
      if (pos > 0)
        this.emit('info', true, data, this._bufpos, pos);
      else
        this.emit('info', true);

      this._bufpos = pos + needle_len;
      return pos + needle_len;
    } else
      pos += occ[ch];
  }

  // There was no match. If there's trailing haystack data that we cannot
  // match yet using the Boyer-Moore-Horspool algorithm (because the trailing
  // data is less than the needle size) then match using a modified
  // algorithm that starts matching from the beginning instead of the end.
  // Whatever trailing data is left after running this algorithm is added to
  // the lookbehind buffer.
  if (pos < len) {
    while (pos < len && (data[pos] !== needle[0]
                         || !jsmemcmp(data, pos, needle, 0, len - pos))) {
      ++pos;
    }
    if (pos < len) {
      data.copy(lookbehind, 0, pos, pos + (len - pos));
      this._lookbehind_size = len - pos;
    }
  }

  // Everything until pos is guaranteed not to contain needle data.
  if (pos > 0)
    this.emit('info', false, data, this._bufpos, pos < len ? pos : len);

  this._bufpos = len;
  return len;
};

SBMH.prototype._sbmh_lookup_char = function(data, pos) {
  if (pos < 0)
    return this._lookbehind[this._lookbehind_size + pos];
  else
    return data[pos];
}

SBMH.prototype._sbmh_memcmp = function(data, pos, len) {
  var i = 0;

  while (i < len) {
    if (this._sbmh_lookup_char(data, pos + i) === this._needle[i])
      ++i;
    else
      return false;
  }
  return true;
}

module.exports = SBMH;