Module: LevenshteinLite

Defined in:
lib/levenshtein_lite.rb,
lib/levenshtein_lite/version.rb

Overview

Pure-Ruby implementation of the Levenshtein algorithm, tuned for speed and minimal allocations.

Constant Summary collapse

VERSION = 
"1.0.2"

Class Method Summary collapse

Class Method Details

.distance(str1, str2) ⇒ Object

rubocop:disable Metrics/AbcSize rubocop:disable Metrics/MethodLength



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# File 'lib/levenshtein_lite.rb', line 16

def self.distance(str1, str2)
  return str1.length if str2.empty?
  return str2.length if str1.empty?
  return 0 if str1 == str2

  # Distance from a prefix of str1 to each prefix in str2.
  #
  # On the first iteration, the prefix of str1 is just '',
  # and so the distance measurement for any prefix of str2
  # is the prefix's length. For example, the distance from
  # 'abc' to '' is 3.
  #
  # So, we initialize the array to [0, 1, 2,.. str2.length].
  #
  distances = Array.new(str2.length + 1) { _1 }

  # Workspace for calculating the next set of prefix distances.
  workspace = Array.new(str2.length + 1)

  i = 0 # avoid allocations from each_with_index
  str1.each_codepoint do |c1|
    workspace[0] = i + 1

    j = 0 # avoid allocation from each_with_index
    str2.each_codepoint do |c2|
      deletion_cost = distances[j + 1] + 1
      insertion_cost = workspace[j] + 1
      substitution_cost = distances[j] + ((c1 == c2) ? 0 : 1)

      min = deletion_cost
      min = insertion_cost if insertion_cost < min
      min = substitution_cost if substitution_cost < min

      workspace[j + 1] = min

      j += 1
    end

    # The newly completed workspace becomes the set of distances
    # for the next iteration, and the old distances become our
    # next workspace.
    #
    # Swap them the old-fashioned way to avoid an Array allocation.
    #
    # rubocop:disable Style/SwapValues
    tmp = distances
    distances = workspace
    workspace = tmp
    # rubocop:enable Style/SwapValues

    i += 1
  end

  distances[-1]
end