core: add Text::levenshtein_distance

[nit.git] / lib / core / text / abstract_text.nit

diff --git a/lib/core/text/abstract_text.nit b/lib/core/text/abstract_text.nit
index ef8a24b..d4873ad 100644 (file)
--- a/lib/core/text/abstract_text.nit
+++ b/lib/core/text/abstract_text.nit
@@ -68,7 +68,7 @@ abstract class Text
        fun substring(from: Int, count: Int): SELFTYPE is abstract
 
        # Iterates on the substrings of self if any
-       fun substrings: Iterator[FlatText] is abstract
+       private fun substrings: Iterator[FlatText] is abstract
 
        # Is the current Text empty (== "")
        #
@@ -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
@@ -1709,6 +1768,16 @@ redef class Char
                return (self >= 'a' and self <= 'z') or (self >= 'A' and self <= 'Z')
        end
 
+       # Is `self` an hexadecimal digit ?
+       #
+       #     assert 'A'.is_hexdigit
+       #     assert not 'G'.is_hexdigit
+       #     assert 'a'.is_hexdigit
+       #     assert not 'g'.is_hexdigit
+       #     assert '5'.is_hexdigit
+       fun is_hexdigit: Bool do return (self >= '0' and self <= '9') or (self >= 'A' and self <= 'F') or
+                                       (self >= 'a' and self <= 'f')
+
        # Returns true if the char is an alpha or a numeric digit
        #
        #     assert 'a'.is_alphanumeric