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350 lines
12 KiB
Python
350 lines
12 KiB
Python
# Natural Language Toolkit: Stemmers
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#
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# Copyright (C) 2001-2020 NLTK Project
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# Author: Steven Tomcavage <stomcava@law.upenn.edu>
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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"""
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A word stemmer based on the Lancaster (Paice/Husk) stemming algorithm.
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Paice, Chris D. "Another Stemmer." ACM SIGIR Forum 24.3 (1990): 56-61.
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"""
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import re
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from nltk.stem.api import StemmerI
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class LancasterStemmer(StemmerI):
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"""
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Lancaster Stemmer
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>>> from nltk.stem.lancaster import LancasterStemmer
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>>> st = LancasterStemmer()
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>>> st.stem('maximum') # Remove "-um" when word is intact
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'maxim'
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>>> st.stem('presumably') # Don't remove "-um" when word is not intact
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'presum'
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>>> st.stem('multiply') # No action taken if word ends with "-ply"
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'multiply'
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>>> st.stem('provision') # Replace "-sion" with "-j" to trigger "j" set of rules
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'provid'
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>>> st.stem('owed') # Word starting with vowel must contain at least 2 letters
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'ow'
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>>> st.stem('ear') # ditto
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'ear'
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>>> st.stem('saying') # Words starting with consonant must contain at least 3
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'say'
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>>> st.stem('crying') # letters and one of those letters must be a vowel
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'cry'
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>>> st.stem('string') # ditto
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'string'
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>>> st.stem('meant') # ditto
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'meant'
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>>> st.stem('cement') # ditto
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'cem'
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>>> st_pre = LancasterStemmer(strip_prefix_flag=True)
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>>> st_pre.stem('kilometer') # Test Prefix
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'met'
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>>> st_custom = LancasterStemmer(rule_tuple=("ssen4>", "s1t."))
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>>> st_custom.stem("ness") # Change s to t
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'nest'
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"""
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# The rule list is static since it doesn't change between instances
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default_rule_tuple = (
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"ai*2.", # -ia > - if intact
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"a*1.", # -a > - if intact
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"bb1.", # -bb > -b
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"city3s.", # -ytic > -ys
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"ci2>", # -ic > -
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"cn1t>", # -nc > -nt
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"dd1.", # -dd > -d
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"dei3y>", # -ied > -y
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"deec2ss.", # -ceed >", -cess
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"dee1.", # -eed > -ee
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"de2>", # -ed > -
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"dooh4>", # -hood > -
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"e1>", # -e > -
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"feil1v.", # -lief > -liev
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"fi2>", # -if > -
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"gni3>", # -ing > -
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"gai3y.", # -iag > -y
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"ga2>", # -ag > -
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"gg1.", # -gg > -g
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"ht*2.", # -th > - if intact
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"hsiug5ct.", # -guish > -ct
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"hsi3>", # -ish > -
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"i*1.", # -i > - if intact
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"i1y>", # -i > -y
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"ji1d.", # -ij > -id -- see nois4j> & vis3j>
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"juf1s.", # -fuj > -fus
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"ju1d.", # -uj > -ud
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"jo1d.", # -oj > -od
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"jeh1r.", # -hej > -her
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"jrev1t.", # -verj > -vert
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"jsim2t.", # -misj > -mit
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"jn1d.", # -nj > -nd
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"j1s.", # -j > -s
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"lbaifi6.", # -ifiabl > -
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"lbai4y.", # -iabl > -y
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"lba3>", # -abl > -
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"lbi3.", # -ibl > -
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"lib2l>", # -bil > -bl
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"lc1.", # -cl > c
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"lufi4y.", # -iful > -y
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"luf3>", # -ful > -
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"lu2.", # -ul > -
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"lai3>", # -ial > -
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"lau3>", # -ual > -
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"la2>", # -al > -
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"ll1.", # -ll > -l
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"mui3.", # -ium > -
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"mu*2.", # -um > - if intact
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"msi3>", # -ism > -
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"mm1.", # -mm > -m
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"nois4j>", # -sion > -j
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"noix4ct.", # -xion > -ct
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"noi3>", # -ion > -
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"nai3>", # -ian > -
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"na2>", # -an > -
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"nee0.", # protect -een
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"ne2>", # -en > -
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"nn1.", # -nn > -n
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"pihs4>", # -ship > -
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"pp1.", # -pp > -p
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"re2>", # -er > -
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"rae0.", # protect -ear
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"ra2.", # -ar > -
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"ro2>", # -or > -
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"ru2>", # -ur > -
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"rr1.", # -rr > -r
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"rt1>", # -tr > -t
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"rei3y>", # -ier > -y
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"sei3y>", # -ies > -y
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"sis2.", # -sis > -s
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"si2>", # -is > -
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"ssen4>", # -ness > -
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"ss0.", # protect -ss
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"suo3>", # -ous > -
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"su*2.", # -us > - if intact
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"s*1>", # -s > - if intact
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"s0.", # -s > -s
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"tacilp4y.", # -plicat > -ply
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"ta2>", # -at > -
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"tnem4>", # -ment > -
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"tne3>", # -ent > -
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"tna3>", # -ant > -
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"tpir2b.", # -ript > -rib
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"tpro2b.", # -orpt > -orb
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"tcud1.", # -duct > -duc
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"tpmus2.", # -sumpt > -sum
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"tpec2iv.", # -cept > -ceiv
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"tulo2v.", # -olut > -olv
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"tsis0.", # protect -sist
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"tsi3>", # -ist > -
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"tt1.", # -tt > -t
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"uqi3.", # -iqu > -
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"ugo1.", # -ogu > -og
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"vis3j>", # -siv > -j
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"vie0.", # protect -eiv
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"vi2>", # -iv > -
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"ylb1>", # -bly > -bl
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"yli3y>", # -ily > -y
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"ylp0.", # protect -ply
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"yl2>", # -ly > -
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"ygo1.", # -ogy > -og
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"yhp1.", # -phy > -ph
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"ymo1.", # -omy > -om
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"ypo1.", # -opy > -op
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"yti3>", # -ity > -
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"yte3>", # -ety > -
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"ytl2.", # -lty > -l
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"yrtsi5.", # -istry > -
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"yra3>", # -ary > -
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"yro3>", # -ory > -
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"yfi3.", # -ify > -
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"ycn2t>", # -ncy > -nt
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"yca3>", # -acy > -
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"zi2>", # -iz > -
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"zy1s.", # -yz > -ys
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)
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def __init__(self, rule_tuple=None, strip_prefix_flag=False):
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"""Create an instance of the Lancaster stemmer.
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"""
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# Setup an empty rule dictionary - this will be filled in later
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self.rule_dictionary = {}
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# Check if a user wants to strip prefix
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self._strip_prefix = strip_prefix_flag
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# Check if a user wants to use his/her own rule tuples.
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self._rule_tuple = rule_tuple if rule_tuple else self.default_rule_tuple
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def parseRules(self, rule_tuple=None):
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"""Validate the set of rules used in this stemmer.
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If this function is called as an individual method, without using stem
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method, rule_tuple argument will be compiled into self.rule_dictionary.
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If this function is called within stem, self._rule_tuple will be used.
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"""
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# If there is no argument for the function, use class' own rule tuple.
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rule_tuple = rule_tuple if rule_tuple else self._rule_tuple
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valid_rule = re.compile("^[a-z]+\*?\d[a-z]*[>\.]?$")
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# Empty any old rules from the rule set before adding new ones
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self.rule_dictionary = {}
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for rule in rule_tuple:
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if not valid_rule.match(rule):
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raise ValueError("The rule {0} is invalid".format(rule))
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first_letter = rule[0:1]
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if first_letter in self.rule_dictionary:
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self.rule_dictionary[first_letter].append(rule)
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else:
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self.rule_dictionary[first_letter] = [rule]
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def stem(self, word):
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"""Stem a word using the Lancaster stemmer.
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"""
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# Lower-case the word, since all the rules are lower-cased
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word = word.lower()
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word = self.__stripPrefix(word) if self._strip_prefix else word
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# Save a copy of the original word
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intact_word = word
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# If rule dictionary is empty, parse rule tuple.
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if not self.rule_dictionary:
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self.parseRules()
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return self.__doStemming(word, intact_word)
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def __doStemming(self, word, intact_word):
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"""Perform the actual word stemming
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"""
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valid_rule = re.compile("^([a-z]+)(\*?)(\d)([a-z]*)([>\.]?)$")
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proceed = True
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while proceed:
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# Find the position of the last letter of the word to be stemmed
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last_letter_position = self.__getLastLetter(word)
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# Only stem the word if it has a last letter and a rule matching that last letter
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if (
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last_letter_position < 0
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or word[last_letter_position] not in self.rule_dictionary
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):
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proceed = False
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else:
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rule_was_applied = False
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# Go through each rule that matches the word's final letter
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for rule in self.rule_dictionary[word[last_letter_position]]:
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rule_match = valid_rule.match(rule)
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if rule_match:
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(
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ending_string,
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intact_flag,
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remove_total,
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append_string,
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cont_flag,
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) = rule_match.groups()
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# Convert the number of chars to remove when stemming
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# from a string to an integer
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remove_total = int(remove_total)
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# Proceed if word's ending matches rule's word ending
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if word.endswith(ending_string[::-1]):
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if intact_flag:
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if word == intact_word and self.__isAcceptable(
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word, remove_total
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):
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word = self.__applyRule(
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word, remove_total, append_string
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)
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rule_was_applied = True
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if cont_flag == ".":
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proceed = False
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break
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elif self.__isAcceptable(word, remove_total):
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word = self.__applyRule(
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word, remove_total, append_string
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)
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rule_was_applied = True
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if cont_flag == ".":
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proceed = False
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break
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# If no rules apply, the word doesn't need any more stemming
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if rule_was_applied == False:
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proceed = False
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return word
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def __getLastLetter(self, word):
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"""Get the zero-based index of the last alphabetic character in this string
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"""
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last_letter = -1
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for position in range(len(word)):
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if word[position].isalpha():
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last_letter = position
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else:
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break
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return last_letter
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def __isAcceptable(self, word, remove_total):
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"""Determine if the word is acceptable for stemming.
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"""
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word_is_acceptable = False
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# If the word starts with a vowel, it must be at least 2
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# characters long to be stemmed
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if word[0] in "aeiouy":
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if len(word) - remove_total >= 2:
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word_is_acceptable = True
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# If the word starts with a consonant, it must be at least 3
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# characters long (including one vowel) to be stemmed
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elif len(word) - remove_total >= 3:
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if word[1] in "aeiouy":
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word_is_acceptable = True
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elif word[2] in "aeiouy":
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word_is_acceptable = True
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return word_is_acceptable
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def __applyRule(self, word, remove_total, append_string):
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"""Apply the stemming rule to the word
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"""
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# Remove letters from the end of the word
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new_word_length = len(word) - remove_total
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word = word[0:new_word_length]
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# And add new letters to the end of the truncated word
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if append_string:
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word += append_string
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return word
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def __stripPrefix(self, word):
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"""Remove prefix from a word.
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This function originally taken from Whoosh.
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"""
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for prefix in (
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"kilo",
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"micro",
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"milli",
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"intra",
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"ultra",
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"mega",
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"nano",
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"pico",
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"pseudo",
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):
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if word.startswith(prefix):
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return word[len(prefix) :]
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return word
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def __repr__(self):
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return "<LancasterStemmer>"
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