An implementation of the Levenshtein distance algorithm in LUA.

levenshtein_algorithm.lua

-- Returns the Levenshtein distance between the two given strings
function string.levenshtein(str1, str2)
	local len1 = string.len(str1)
	local len2 = string.len(str2)
	local matrix = {}
	local cost = 0
	
        -- quick cut-offs to save time
	if (len1 == 0) then
		return len2
	elseif (len2 == 0) then
		return len1
	elseif (str1 == str2) then
		return 0
	end
	
        -- initialise the base matrix values
	for i = 0, len1, 1 do
		matrix[i] = {}
		matrix[i][0] = i
	end
	for j = 0, len2, 1 do
		matrix[0][j] = j
	end
	
        -- actual Levenshtein algorithm
	for i = 1, len1, 1 do
		for j = 1, len2, 1 do
			if (str1:byte(i) == str2:byte(j)) then
				cost = 0
			else
				cost = 1
			end
			
			matrix[i][j] = math.min(matrix[i-1][j] + 1, matrix[i][j-1] + 1, matrix[i-1][j-1] + cost)
		end
	end
	
        -- return the last value - this is the Levenshtein distance
	return matrix[len1][len2]
end

Thanks for this!

Oustanding!

This is great, thanks. :)

who came here from that one devforum

quick note: because in line 35 you only ever refer to matrix[i] and matrix[i-1], you can get away with just storing the two most recent lines of the matrix, rather than computing the whole thing. This gets you down to O(2n) space (while still taking O(n^2) time)

You can use a modified version of this algorithm to search for substrings that closely match another string.

Thanks!

Thanks!
matrix declaration in line 5 could appear after the quick cut-offs to prevent Lua from creating the table and later collecting it