Module: Philiprehberger::FuzzyMatch::DamerauLevenshtein

Defined in:
lib/philiprehberger/fuzzy_match/damerau_levenshtein.rb

Overview

Damerau-Levenshtein distance (optimal string alignment variant)

Extends Levenshtein by counting adjacent transpositions as a single edit. Operations: insertion, deletion, substitution, and adjacent transposition.

Class Method Summary collapse

Class Method Details

.distance(str_a, str_b) ⇒ Object 



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# File 'lib/philiprehberger/fuzzy_match/damerau_levenshtein.rb', line 10

def self.distance(str_a, str_b)
  a = str_a.to_s.downcase
  b = str_b.to_s.downcase

  return b.length if a.empty?
  return a.length if b.empty?
  return 0 if a == b

  n = a.length
  m = b.length

  # Build full matrix (need 2 previous rows for transpositions)
  d = Array.new(n + 1) { Array.new(m + 1, 0) }

  (0..n).each { |i| d[i][0] = i }
  (0..m).each { |j| d[0][j] = j }

  (1..n).each do |i|
    (1..m).each do |j|
      cost = a[i - 1] == b[j - 1] ? 0 : 1

      d[i][j] = [
        d[i - 1][j] + 1,
        d[i][j - 1] + 1,
        d[i - 1][j - 1] + cost
      ].min

      if i > 1 && j > 1 && a[i - 1] == b[j - 2] && a[i - 2] == b[j - 1]
        d[i][j] = [d[i][j], d[i - 2][j - 2] + 1].min
      end
    end
  end

  d[n][m]
end