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Levenshtein function for MySQL
-- Levenshtein function
-- Source: https://openquery.com.au/blog/levenshtein-mysql-stored-function
-- Levenshtein reference: http://en.wikipedia.org/wiki/Levenshtein_distance
-- Arjen note: because the levenshtein value is encoded in a byte array, distance cannot exceed 255;
-- thus the maximum string length this implementation can handle is also limited to 255 characters.
DELIMITER $$
DROP FUNCTION IF EXISTS LEVENSHTEIN $$
CREATE FUNCTION LEVENSHTEIN(s1 VARCHAR(255) CHARACTER SET utf8, s2 VARCHAR(255) CHARACTER SET utf8)
RETURNS INT
DETERMINISTIC
BEGIN
DECLARE s1_len, s2_len, i, j, c, c_temp, cost INT;
DECLARE s1_char CHAR CHARACTER SET utf8;
-- max strlen=255 for this function
DECLARE cv0, cv1 VARBINARY(256);
SET s1_len = CHAR_LENGTH(s1),
s2_len = CHAR_LENGTH(s2),
cv1 = 0x00,
j = 1,
i = 1,
c = 0;
IF (s1 = s2) THEN
RETURN (0);
ELSEIF (s1_len = 0) THEN
RETURN (s2_len);
ELSEIF (s2_len = 0) THEN
RETURN (s1_len);
END IF;
WHILE (j <= s2_len) DO
SET cv1 = CONCAT(cv1, CHAR(j)),
j = j + 1;
END WHILE;
WHILE (i <= s1_len) DO
SET s1_char = SUBSTRING(s1, i, 1),
c = i,
cv0 = CHAR(i),
j = 1;
WHILE (j <= s2_len) DO
SET c = c + 1,
cost = IF(s1_char = SUBSTRING(s2, j, 1), 0, 1);
SET c_temp = ORD(SUBSTRING(cv1, j, 1)) + cost;
IF (c > c_temp) THEN
SET c = c_temp;
END IF;
SET c_temp = ORD(SUBSTRING(cv1, j+1, 1)) + 1;
IF (c > c_temp) THEN
SET c = c_temp;
END IF;
SET cv0 = CONCAT(cv0, CHAR(c)),
j = j + 1;
END WHILE;
SET cv1 = cv0,
i = i + 1;
END WHILE;
RETURN (c);
END $$
DELIMITER ;
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