MySQL Simhash 是一種基于文本內容生成 Hash 值的算法，可以用來實現文本去重、相似查詢等功能。

CREATE FUNCTION simhash(text TEXT)
RETURNS BIGINT DETERMINISTIC
BEGIN
DECLARE words TEXT;
DECLARE word TEXT;
DECLARE stopwords TEXT;
DECLARE hash BIGINT DEFAULT 0;
DECLARE weight INT DEFAULT 1;
DECLARE bits INT DEFAULT 64;
DECLARE i INT;
SET words = REPLACE(text, '[^\w\x80-\xff]+', ' ');
SET words = LOWER(words);
SET stopwords = 'a an and are as at be by for from had he i in is it'
+ ' of on or that the there this to was with';
SET i = 1;
wordloop: WHILE i<= LENGTH(words) DO
SET word = SUBSTRING_INDEX(SUBSTRING(words, i), ' ', 1);
SET i = i + LENGTH(word) + 1;
IF FIND_IN_SET(word, stopwords) THEN
SET weight = -1;
ELSE
SET weight = 1;
END IF;
SET hash = hash + weight * CRC32(word);
END WHILE;
SET i = 1;
SET bits = 64;
SET hash = 0;
bitloop: WHILE i<= bits DO
SET hash = hash | ((BIT_COUNT(hash >>i) MOD 2)<< i-1);
SET i = i + 1;
END WHILE;
RETURN hash;
END;

Simhash 函數的實現過程分為兩部分：分詞和 Hash 計算。輸入的文本通過正則表達式替換成空格，并轉成小寫后作為參數傳入函數。函數會對字符串進行遍歷，用空格分隔，判斷詞語是否為停用詞，然后計算出每個詞語的 CRC32 值，并根據權重累加起來。最后將累加結果按位重新排序生成 simhash 值。