字符串搜索库的结果 - 错误或功能或我的编码错误？

Question

我正在使用this python 库，该库实现了 Aho-Corasick 字符串搜索算法，该算法可以一次性在给定字符串中找到一组模式。输出不是我所期望的：

In [4]: import ahocorasick
In [5]: import collections

In [6]: tree = ahocorasick.KeywordTree()

In [7]: ss = "this is the first sentence in this book the first sentence is really the most interesting the first sentence is always first"

In [8]: words = ["first sentence is", "first sentence", "the first sentence", "the first sentence is"]

In [9]: for w in words:
   ...:     tree.add(w)
   ...:

In [10]: tree.make()

In [13]: final = collections.defaultdict(int)

In [15]: for match in tree.findall(ss, allow_overlaps=True):
   ....:     final[ss[match[0]:match[1]]] += 1
   ....:

In [16]: final
{   'the first sentence': 3, 'the first sentence is': 2}

我期待的输出是这样的：

{ 
  'the first sentence': 3,
  'the first sentence is': 2,
  'first sentence': 3,
  'first sentence is': 2
}

我错过了什么吗？我在大字符串上这样做，所以后处理不是我的第一选择。有没有办法获得所需的输出？

Answer 1

我不知道ahocorasick 模块，但这些结果似乎令人怀疑。 acora 模块显示：

import acora
import collections

ss = "this is the first sentence in this book "
     "the first sentence is really the most interesting "
     "the first sentence is always first"

words = ["first sentence is", 
         "first sentence",
         "the first sentence",
         "the first sentence is"]

tree = acora.AcoraBuilder(*words).build()

for match in tree.findall(ss):
    result[match] += 1

结果：

>>> result
defaultdict(<type 'int'>, 
            {'the first sentence'   : 3,
             'first sentence'       : 3,
             'first sentence is'    : 2,
             'the first sentence is': 2})

Answer 2

我理解 Aho-Corasick 算法的方式以及我实现它的方式会让我同意您的预期输出。看起来您使用的 Python 库有误，或者可能有一个标志，您可以告诉它从某个位置开始为您提供所有匹配项，而不仅仅是从特定位置开始的最长匹配项。

原论文http://www.win.tue.nl/~watson/2R080/opdracht/p333-aho-corasick.pdf中的例子支持你的理解。