Why there is a need to Convert the unique filename to a unique number? How is this gonna help?

Why are you asking me the same question as I asked you?

The hash is a mathematical algorithm of producing a string based on a contents file

I showed the hash operating on the file path, not its content.

the .html file can be: 1. renamed 2. moved 3. contents altered

Thus, you want to be able to consider two files with different names, different locations and different contents as "the same file".

Without you arranging to place some piece of information within those files that can be searched for, to uniquely identify them; those criteria mean that any two files could be considered the same, which is a nonsense; which is why I ignored the possibility that you actually meant that; and assumed your description was lacking precision.

However, inserting a piece of information -- say a custom html-like tag or html comment -- into each html file -- regardless of whether the are 100's 1000's or 100s of 1000s would be the works of a few minutes. At least it would be for Perl.