Module: English::Levenshtein

Extended by:
Levenshtein
Included in:
Levenshtein
Defined in:
lib/gems/english-0.3.1/lib/english/levenshtein.rb

Overview

Levenshtein distance algorithm implementation for Ruby, with UTF-8 support.

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’.

Instance Method Summary collapse

Instance Method Details

#distance(str1, str2) ⇒ Object

Calculate the Levenshtein distance between two strings str1 and str2. str1 and str2 should be ASCII, UTF-8, or a one-byte-per character encoding such as ISO-8859-*.

The strings will be treated as UTF-8 if $KCODE is set appropriately (i.e. ‘u’). Otherwise, the comparison will be performed byte-by-byte. There is no specific support for Shift-JIS or EUC strings.

When using Unicode text, be aware that this algorithm does not perform normalisation. If there is a possibility of different normalised forms being used, normalisation should be performed beforehand.



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# File 'lib/gems/english-0.3.1/lib/english/levenshtein.rb', line 42

def distance(str1, str2)
  if $KCODE =~ /^U/i
    unpack_rule = 'U*'
  else
    unpack_rule = 'C*'
  end
  s = str1.unpack(unpack_rule)
  t = str2.unpack(unpack_rule)
  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