I'm writing to report a bug in the porter-stemmer algorithm supplied as part of
the FTS3 implementation.
The stemmer has an inverted logic error that prevents it from properly stemming
words of the following form:
dry -> dri
cry -> cri
This means, for instance, that the following words don't stem the same:
dried -> dri -doesn't match- dry
cried -> cry -doesn't match- cry
The bug seems to have been introduced as a simple logic error by whoever wrote
the stemmer code. The original description of step 1c is here:
http://snowball.tartarus.org/algorithms/english/stemmer.html
Step 1c:
replace suffix y or Y by i if preceded by a non-vowel which is
not the first letter of the word (so cry -> cri, by -> by, say -> say)
But the code in sqlite reads like this:
/* Step 1c */
if( z[0]=='y' && hasVowel(z+1) ){
z[0] = 'i';
}
In other words, sqlite turns the y into an i only if it is preceded by a vowel
(say -> sai), while the algorithm intends this to be done if it is _not_
preceded by a vowel.
But there are two other problems in that same line of code:
(1) hasVowel checks whether a vowel exists anywhere in the string, not
just in the next character, which is incorrect, and goes against the step 1c
directions above. (amplify would not be properly stemmed to amplifi, for
instance)
(2) The check for the first letter is not performed (for words like
"by", etc)
I've fixed both of those errors in the patch below:
/* Step 1c */
- if( z[0]=='y' && hasVowel(z+1) ){
+ if( z[0]=='y' && isConsonant(z+1) && z[2] ){
z[0] = 'i';
}
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