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Reputation: 116850

String searching library's outcome - bug or feature or my coding error?

I am using this python library that implements the Aho-Corasick string searching algorithm that finds a set of patterns in a given string in one pass. The output is not what I am expecting:

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}

The output I was expecting was this:

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

Am I missing something? I am doing this on large strings so post processing is not my first option. Is there a way to get the desired output?

Upvotes: 1

Views: 226

Answers (2)

Lemur
Lemur

Reputation: 462

I don't know about the ahocorasick module, but those results seem suspect. The acora module shows this:

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

Results:

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

Upvotes: 1

Jim Mischel
Jim Mischel

Reputation: 134005

The way I understand the Aho-Corasick algorithm and the way I've implemented it would have me agree with your expected output. It looks like the Python library you're using is in error, or perhaps there's a flag that you can tell it to give you all matches starting at a position rather than just the longest match starting at a particular position.

The examples in the original paper, http://www.win.tue.nl/~watson/2R080/opdracht/p333-aho-corasick.pdf, support your understanding.

Upvotes: 1

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