levenshtein_distance.rb

# This code is based directly on the Text gem implementation.
# Copyright (c) 2006-2013 Paul Battley, Michael Neumann, Tim Fletcher.
#
# Returns a value representing the "cost" of transforming str1 into str2.
# https://github.com/rails/rails/blob/c994a893c18c0456fd2a30efe4debfc2b18e2508/railties/lib/rails/command/behavior.rb

def levenshtein_distance(str1, str2)
	s = str1
	t = str2
	n = s.length
	m = t.length

	return m if (0 == n)
	return n if (0 == m)

	d = (0..m).to_a
	x = nil

	# avoid duplicating an enumerable object in the loop
	str2_codepoint_enumerable = str2.each_codepoint

	str1.each_codepoint.with_index do |char1, i|
		e = i+1

		str2_codepoint_enumerable.with_index do |char2, j|
			cost = (char1 == char2) ? 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

	x
end