java-string-similarity
A library implementing different string similarity and distance measures. A dozen of algorithms (including Levenshtein edit distance and sibblings, Jaro-Winkler, Longest Common Subsequence, cosine similarity etc.) are currently implemented. Check the summary table below for the complete list...
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- Overview
- Normalized, metric, similarity and distance
- Shingles (n-gram) based similarity and distance
- Levenshtein
- Normalized Levenshtein
- Weighted Levenshtein
- Damerau-Levenshtein
- Optimal String Alignment
- Jaro-Winkler
- Longest Common Subsequence
- Metric Longest Common Subsequence
- N-Gram
- Shingle (n-gram) based algorithms
- Ratcliff-Obershelp
- Experimental
- Users
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Using maven:
<dependency>
<groupId>info.debatty</groupId>
<artifactId>java-string-similarity</artifactId>
<version>RELEASE</version>
</dependency>
Or check the releases.
This library requires Java 8 or more recent.
Overview
The main characteristics of each implemented algorithm are presented below. The "cost" column gives an estimation of the computational cost to compute the similarity between two strings of length m and n respectively.
| Normalized? | Metric? | Type | Cost | Typical usage | ||
|---|---|---|---|---|---|---|
| Levenshtein | distance | No | Yes | O(m*n) 1 | ||
| Normalized Levenshtein | distance similarity |
Yes | No | O(m*n) 1 | ||
| Weighted Levenshtein | distance | No | No | O(m*n) 1 | OCR | |
| Damerau-Levenshtein 3 | distance | No | Yes | O(m*n) 1 | ||
| Optimal String Alignment 3 | distance | No | No | O(m*n) 1 | ||
| Jaro-Winkler | similarity distance |
Yes | No | O(m*n) | typo correction | |
| Longest Common Subsequence | distance | No | No | O(m*n) 1,2 | diff utility, GIT reconciliation | |
| Metric Longest Common Subsequence | distance | Yes | Yes | O(m*n) 1,2 | ||
| N-Gram | distance | Yes | No | O(m*n) | ||
| Q-Gram | distance | No | No | Profile | O(m+n) | |
| Cosine similarity | similarity distance |
Yes | No | Profile | O(m+n) | |
| Jaccard index | similarity distance |
Yes | Yes | Set | O(m+n) | |
| Sorensen-Dice coefficient | similarity distance |
Yes | No | Set | O(m+n) | |
| Ratcliff-Obershelp | similarity distance |
Yes | No | ? |
[1] In this library, Levenshtein edit distance, LCS distance and their sibblings are computed using the dynamic programming method, which has a cost O(m.n). For Levenshtein distance, the algorithm is sometimes called Wagner-Fischer algorithm ("The string-to-string correction problem", 1974). The original algorithm uses a matrix of size m x n to store the Levenshtein distance between string prefixes.
If the alphabet is finite, it is possible to use the method of four russians (Arlazarov et al. "On economic construction of the transitive closure of a directed graph", 1970) to speedup computation. This was published by Masek in 1980 ("A Faster Algorithm Computing String Edit Distances"). This method splits the matrix in blocks of size t x t. Each possible block is precomputed to produce a lookup table. This lookup table can then be used to compute the string similarity (or distance) in O(nm/t). Usually, t is choosen as log(m) if m > n. The resulting computation cost is thus O(mn/log(m)). This method has not been implemented (yet).
[2] In "Length of Maximal Common Subsequences", K.S. Larsen proposed an algorithm that computes the length of LCS in time O(log(m).log(n)). But the algorithm has a memory requirement O(m.n²) and was thus not implemented here.
[3] There are two variants of Damerau-Levenshtein string distance: Damerau-Levenshtein with adjacent transpositions (also sometimes called unrestricted Damerau–Levenshtein distance) and Optimal String Alignment (also sometimes called restricted edit distance). For Optimal String Alignment, no substring can be edited more than once.
Normalized, metric, similarity and distance
Although the topic might seem simple, a lot of different algorithms exist to measure text similarity or distance. Therefore the library defines some interfaces to categorize them.
(Normalized) similarity and distance
- StringSimilarity : Implementing algorithms define a similarity between strings (0 means strings are completely different).
- NormalizedStringSimilarity : Implementing algorithms define a similarity between 0.0 and 1.0, like Jaro-Winkler for example.
- StringDistance : Implementing algorithms define a distance between strings (0 means strings are identical), like Levenshtein for example. The maximum distance value depends on the algorithm.
- NormalizedStringDistance : This interface extends StringDistance. For implementing classes, the computed distance value is between 0.0 and 1.0. NormalizedLevenshtein is an example of NormalizedStringDistance.
Generally, algorithms that implement NormalizedStringSimilarity also implement NormalizedStringDistance, and similarity = 1 - distance. But there are a few exceptions, like N-Gram similarity and distance (Kondrak)...
Metric distances
The MetricStringDistance interface : A few of the distances are actually metric distances, which means that verify the triangle inequality d(x, y) <= d(x,z) + d(z,y). For example, Levenshtein is a metric distance, but NormalizedLevenshtein is not.
A lot of nearest-neighbor search algorithms and indexing structures rely on the triangle inequality. You can check "Similarity Search, The Metric Space Approach" by Zezula et al. for a survey. These cannot be used with non metric similarity measures.
Read Javadoc for a detailed description
Shingles (n-gram) based similarity and distance
A few algorithms work by converting strings into sets of n-grams (sequences of n characters, also sometime
