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# | ||
# Levenshtein distance algorithm implementation for Ruby, with UTF-8 support | ||
# | ||
# Author:: Paul BATTLEY (pbattley @ gmail.com) | ||
# Version:: 1.3 | ||
# Date:: 2005-04-19 | ||
# | ||
# == About | ||
# | ||
# The Levenshtein distance is a measure of how similar two strings s and t are, | ||
# calculated as the number of deletions/insertions/substitutions needed to | ||
# transform s into t. The greater the distance, the more the strings differ. | ||
# | ||
# The Levenshtein distance is also sometimes referred to as the | ||
# easier-to-pronounce-and-spell 'edit distance'. | ||
# | ||
# == Revision history | ||
# | ||
# * 2005-05-19 1.3 Repairing an oversight, distance can now be called via | ||
# Levenshtein.distance(s, t) | ||
# * 2005-05-04 1.2 Now uses just one 1-dimensional array. I think this is as | ||
# far as optimisation can go. | ||
# * 2005-05-04 1.1 Now storing only the current and previous rows of the matrix | ||
# instead of the whole lot. | ||
# | ||
# == Licence | ||
# | ||
# Copyright (c) 2005 Paul Battley | ||
# | ||
# Usage of the works is permitted provided that this instrument is retained | ||
# with the works, so that any entity that uses the works is notified of this | ||
# instrument. | ||
# | ||
# DISCLAIMER: THE WORKS ARE WITHOUT WARRANTY. | ||
# | ||
|
||
module Levenshtein | ||
|
||
# | ||
# Calculate the Levenshtein distance between two strings +str1+ and +str2+. | ||
# +str1+ and +str2+ should be ASCII or UTF-8. | ||
# | ||
def distance(str1, str2) | ||
s = str1.unpack('U*') | ||
t = str2.unpack('U*') | ||
n = s.length | ||
m = t.length | ||
return m if (0 == n) | ||
return n if (0 == m) | ||
|
||
d = (0..m).to_a | ||
x = nil | ||
|
||
(0...n).each do |i| | ||
e = i+1 | ||
(0...m).each do |j| | ||
cost = (s[i] == t[j]) ? 0 : 1 | ||
x = [ | ||
d[j+1] + 1, # insertion | ||
e + 1, # deletion | ||
d[j] + cost # substitution | ||
].min | ||
d[j] = e | ||
e = x | ||
end | ||
d[m] = x | ||
end | ||
|
||
return x | ||
end | ||
|
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extend self | ||
end | ||
|
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if (__FILE__ == $0) | ||
require 'test/unit' | ||
|
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class LevenshteinTest < Test::Unit::TestCase | ||
include Levenshtein | ||
|
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EXPECTED = [ | ||
# Easy ones | ||
['test', 'test', 0], | ||
['test', 'tent', 1], | ||
['gumbo', 'gambol', 2], | ||
['kitten', 'sitting', 3], | ||
# Empty strings | ||
['foo', '', 3], | ||
['', '', 0], | ||
['a', '', 1], | ||
# UTF-8 | ||
["f\303\266o", 'foo', 1], | ||
["fran\303\247ais", 'francais', 1], | ||
["fran\303\247ais", "fran\303\246ais", 1], | ||
["\347\247\201\343\201\256\345\220\215\345\211\215\343\201\257"<< | ||
"\343\203\235\343\203\274\343\203\253\343\201\247\343\201\231", | ||
"\343\201\274\343\201\217\343\201\256\345\220\215\345\211\215\343\201"<< | ||
"\257\343\203\235\343\203\274\343\203\253\343\201\247\343\201\231", | ||
2], # Japanese | ||
# Edge cases | ||
['a', 'a', 0], | ||
['0123456789', 'abcdefghijklmnopqrstuvwxyz', 26] | ||
] | ||
|
||
def test_known_distances | ||
EXPECTED.each do |a,b,x| | ||
assert_equal(x, distance(a, b)) | ||
assert_equal(x, distance(b, a)) | ||
end | ||
end | ||
end | ||
end |
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