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The Snowball stem of one Russian word.

public static string Stem(string word)

Parameters — word is a single Russian word. It is lowercased and NFC-normalised before the rules run, and ё is then folded to е, so a decomposed ё, a composed one and a plain е are the same letter to the algorithm.

Returns — string, the stem, always lowercase, and always spelled with е where the input had ё.

Exceptions — ArgumentNullException when word is null. An empty string is returned empty, and a word with no Russian vowel — в, or a word in another script — comes back whole: its RV is empty and no table can reach it.

Example — a verb reduced to the stem its participle and its finite forms share.

using Lodestar.Text.Stemming;

string infinitive = RussianSnowballStemmer.Stem("читать");  // => чита
string past = RussianSnowballStemmer.Stem("читал");  // => чита
string participle = RussianSnowballStemmer.Stem("читающий");  // => чита

Remarks — возможность → возможн and радость → радост is the pair most likely to look like a bug. Both end in the same derivational ость, and step 3 removes it only when the whole ending lies in R2: in возможность it does, and in the shorter радость it does not. радости reaches the same радост by a different route — its и goes in step 1 — so the two forms still collide, which is what the step is for.

The last step is the other one worth knowing. It does exactly one of three things: undouble a final нн, or remove a superlative ейш/ейше and undouble what that uncovers, or drop a final ь. A word that took the second keeps its ь, and that is why сильнейший and сильный meet at сильн while мысль keeps its own.

using Lodestar.Text.Stemming;

string abstractNoun = RussianSnowballStemmer.Stem("возможности");  // => возможн
string shortNoun = RussianSnowballStemmer.Stem("радости");  // => радост
string superlative = RussianSnowballStemmer.Stem("сильнейший");  // => сильн

Applies to — net10.0, netstandard2.0.

See also — RussianSnowballStemmer, the stemming index.