sbmh.js

/*
Copyright Brian White. All rights reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to
deal in the Software without restriction, including without limitation the
rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
sell copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
IN THE SOFTWARE.
*/
/*
  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,
      ch;

  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) {
      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) {
    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;