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✏️ Editing: sbmh.js Writable

'use strict'

/**
 * Copyright Brian White. All rights reserved.
 *
 * @see https://github.com/mscdex/streamsearch
 *
 * 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
 */
const EventEmitter = require('events').EventEmitter
const inherits = require('util').inherits

function SBMH (needle) {
  if (typeof needle === 'string') {
    needle = Buffer.from(needle)
  }

if (!Buffer.isBuffer(needle)) {
    throw new TypeError('The needle has to be a String or a Buffer.')
  }

const needleLength = needle.length

if (needleLength === 0) {
    throw new Error('The needle cannot be an empty String/Buffer.')
  }

if (needleLength > 256) {
    throw new Error('The needle cannot have a length bigger than 256.')
  }

this.maxMatches = Infinity
  this.matches = 0

this._occ = new Array(256)
    .fill(needleLength) // Initialize occurrence table.
  this._lookbehind_size = 0
  this._needle = needle
  this._bufpos = 0

this._lookbehind = Buffer.alloc(needleLength)

// Populate occurrence table with analysis of the needle,
  // ignoring last letter.
  for (var i = 0; i < needleLength - 1; ++i) { // eslint-disable-line no-var
    this._occ[needle[i]] = needleLength - 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) {
  if (!Buffer.isBuffer(chunk)) {
    chunk = Buffer.from(chunk, 'binary')
  }
  const chlen = chunk.length
  this._bufpos = pos || 0
  let r
  while (r !== chlen && this.matches < this.maxMatches) { r = this._sbmh_feed(chunk) }
  return r
}

SBMH.prototype._sbmh_feed = function (data) {
  const len = data.length
  const needle = this._needle
  const needleLength = needle.length
  const lastNeedleChar = needle[needleLength - 1]

// 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]
  let pos = -this._lookbehind_size
  let 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 - needleLength) {
      ch = this._sbmh_lookup_char(data, pos + needleLength - 1)

if (
        ch === lastNeedleChar &&
        this._sbmh_memcmp(data, pos, needleLength - 1)
      ) {
        this._lookbehind_size = 0
        ++this.matches
        this.emit('info', true)

return (this._bufpos = pos + needleLength)
      }
      pos += this._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, this._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.
      const bytesToCutOff = this._lookbehind_size + pos
      if (bytesToCutOff > 0) {
        // The cut off data is guaranteed not to contain the needle.
        this.emit('info', false, this._lookbehind, 0, bytesToCutOff)
      }

this._lookbehind.copy(this._lookbehind, 0, bytesToCutOff,
        this._lookbehind_size - bytesToCutOff)
      this._lookbehind_size -= bytesToCutOff

data.copy(this._lookbehind, this._lookbehind_size)
      this._lookbehind_size += len

this._bufpos = len
      return len
    }
  }

pos += (pos >= 0) * this._bufpos

// Lookbehind buffer is now empty. We only need to check if the
  // needle is in the haystack.
  if (data.indexOf(needle, pos) !== -1) {
    pos = data.indexOf(needle, pos)
    ++this.matches
    if (pos > 0) { this.emit('info', true, data, this._bufpos, pos) } else { this.emit('info', true) }

return (this._bufpos = pos + needleLength)
  } else {
    pos = len - needleLength
  }

// 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.
  while (
    pos < len &&
    (
      data[pos] !== needle[0] ||
      (
        (Buffer.compare(
          data.subarray(pos, pos + len - pos),
          needle.subarray(0, len - pos)
        ) !== 0)
      )
    )
  ) {
    ++pos
  }
  if (pos < len) {
    data.copy(this._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) {
  return (pos < 0)
    ? this._lookbehind[this._lookbehind_size + pos]
    : data[pos]
}

SBMH.prototype._sbmh_memcmp = function (data, pos, len) {
  for (var i = 0; i < len; ++i) { // eslint-disable-line no-var
    if (this._sbmh_lookup_char(data, pos + i) !== this._needle[i]) { return false }
  }
  return true
}

module.exports = SBMH

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