similar_text算相似性时归一化时的疑问
我在算两个字符串的长度时，发现归一化时好像此函数采取的方式不一样。
第一次，我试了两个不一样长的字符串，算其编辑距离：
echo "levenshtein计算：\n";echo levenshtein("seller_id","selr_id");echo "\n";
得到的结果是：2
再用同样的两个字符串，用PHP的similar_text函数来求其相似性
echo "similar_text计算：\n";similar_text("seller_id","selr_id",$percent);
echo $percent;
出现在相似性是：87.5
把2这个距离归一化时，正好符合公式：1-（编辑距离/(两个字符串的长度之和)）
第二次，我试了两个一样长度的字符串，分别算其编辑距离和相似性
similar_text("abcd","1234",$percent);echo $percent;echo "\n";
echo levenshtein("abcd","1234");
得到的值分别为：4和0
正好符合公式：1-（编辑距离/(任一个字符串的长度)）
我的问题是：为什么对两个不一样长的字符串求相似性时，分母是两个字符串的长度之和呢？
我在网上找了些pdf文档看，对编辑距离归一化时，其分母是最长的那个字符串的长度呢。
------解决思路----------------------
应该说 similar_text 函数的设计者，考虑的还是蛮周到的
当传入的两个串长度相同时，计算的相似度与理论上并无差异
当传入的两个串长度不同时，得到的相似度不像理论上的那么陡峭。也就是说被匹配的概率变大
当然如果你不希望这样的话可以自行计算，串都是你的，他也返回了已匹配的数量。计算一下并不困难
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