core: add Text::levenshtein_distance

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/lib/core/text/abstract_text.nit b/lib/core/text/abstract_text.nit
index cacee99..d4873ad 100644 (file)
--- a/lib/core/text/abstract_text.nit
+++ b/lib/core/text/abstract_text.nit
@@ -974,6 +974,65 @@ abstract class Text
                return s.plain_to_s
        end
 
+       # Return the Levenshtein distance between two strings
+       #
+       # ~~~
+       # assert "abcd".levenshtein_distance("abcd") == 0
+       # assert "".levenshtein_distance("abcd")     == 4
+       # assert "abcd".levenshtein_distance("")     == 4
+       # assert "abcd".levenshtein_distance("xyz")  == 4
+       # assert "abcd".levenshtein_distance("xbdy") == 3
+       # ~~~
+       fun levenshtein_distance(other: String): Int
+       do
+               var slen = self.length
+               var olen = other.length
+
+               # fast cases
+               if slen == 0 then return olen
+               if olen == 0 then return slen
+               if self == other then return 0
+
+               # previous row of distances
+               var v0 = new Array[Int].with_capacity(olen+1)
+
+               # current row of distances
+               var v1 = new Array[Int].with_capacity(olen+1)
+
+               for j in [0..olen] do
+                       # prefix insert cost
+                       v0[j] = j
+               end
+
+               for i in [0..slen[ do
+
+                       # prefix delete cost
+                       v1[0] = i + 1
+
+                       for j in [0..olen[ do
+                               # delete cost
+                               var cost1 = v1[j] + 1
+                               # insert cost
+                               var cost2 = v0[j + 1] + 1
+                               # same char cost (+0)
+                               var cost3 = v0[j]
+                               # change cost
+                               if self[i] != other[j] then cost3 += 1
+                               # keep the min
+                               v1[j+1] = cost1.min(cost2).min(cost3)
+                       end
+
+                       # Switch columns:
+                       # * v1 become v0 in the next iteration
+                       # * old v0 is reused as the new v1
+                       var tmp = v1
+                       v1 = v0
+                       v0 = tmp
+               end
+
+               return v0[olen]
+       end
+
        # Copies `n` bytes from `self` at `src_offset` into `dest` starting at `dest_offset`
        #
        # Basically a high-level synonym of NativeString::copy_to