Change diff variable to custom variable if strings do not match

lib/simetric/levenshtein.ex

@@ -14,33 +14,33 @@ defmodule Simetric.Levenshtein do
       3
 
   """
-  @spec compare(String.t, String.t) :: non_neg_integer
-  def compare(string1, string2)
+  @spec compare(String.t, String.t, integer) :: non_neg_integer
+  def compare(string1, string2, diff_score \\ 1)
 
-  def compare(string, string), do: 0
+  def compare(string, string, _), do: 0
 
-  def compare(string1, ""), do: String.length(string1)
+  def compare(string1, "", _), do: String.length(string1)
 
-  def compare("", string2), do: String.length(string2)
+  def compare("", string2, _), do: String.length(string2)
 
-  def compare(string1, string2) do
+  def compare(string1, string2, diff_score) do
     chars1 = String.graphemes(string1)
     chars2 = String.graphemes(string2)
-    distance(chars1, chars2, length(chars2)..0, 1)
+    distance(chars1, chars2, length(chars2)..0, 1, diff_score)
   end
 
-  defp distance([], _, [result | _], _), do: result
+  defp distance([], _, [result | _], _, _), do: result
 
-  defp distance([char | rest], chars2, distlist, step) do
-    distlist = proceed(char, chars2, Enum.reverse(distlist), [step], step)
-    distance(rest, chars2, distlist, step + 1)
+  defp distance([char | rest], chars2, distlist, step, diff_score) do
+    distlist = proceed(char, chars2, Enum.reverse(distlist), [step], step, diff_score)
+    distance(rest, chars2, distlist, step + 1, diff_score)
   end
 
-  defp proceed(_, [], _, acc, _), do: acc
+  defp proceed(_, [], _, acc, _, _), do: acc
 
-  defp proceed(char1, [char2 | rest], [head | [prev | _] = distlist], acc, score) do
-    diff = if char1 == char2, do: 0, else: 1
+  defp proceed(char1, [char2 | rest], [head | [prev | _] = distlist], acc, score, diff_score) do
+    diff = if char1 == char2, do: 0, else: diff_score
     score = min(min(score + 1, prev + 1), head + diff)
-    proceed(char1, rest, distlist, [score | acc], score)
+    proceed(char1, rest, distlist, [score | acc], score, diff_score)
   end
 end

test/simetric/levenshtein_test.exs

@@ -33,9 +33,33 @@ defmodule Simetric.LevenshteinTest do
     ["distance", "difference"] => 5
   }
 
+  cases_custom = %{
+    # empty strings
+    ["", ""] => 0,
+    ["", "ab"] => 2,
+    ["abc", ""] => 3,
+    # equal strings
+    ["abc", "abc"] => 0,
+    # inserts only
+    ["ac", "abc"] => 1,
+    # deletions only
+    ["xabxcdxxefxgx", "abcdefg"] => 6,
+    # substitutions only
+    ["a", "b"] => 2,
+    ["xabxcdxxefxgx", "1ab2cd34ef5g6"] => 12,
+    # mixed operations
+    ["levenshtein", "frankenstein"] => 9
+  }
+
   for {input, distance} <- cases do
     test "compare #{inspect input}" do
       assert Simetric.Levenshtein.compare(unquote_splicing(input)) == unquote(distance)
     end
   end
+
+  for {input, distance} <- cases_custom do
+    test "compare custom #{inspect input} with diff_distance = 2" do
+      assert Simetric.Levenshtein.compare(unquote_splicing(input), 2) == unquote(distance)
+    end
+  end
 end