compressing names

2.12: mis.adda/compressing names:
. not sure where I'm going with this;
may want get through a book of algorithms first .
...
. a sha1 converts a file into a practically unique number
wondering what the conditions of that are
and how it could be of use for creating name id's
. if any user name could be mapped to something like an 8byte number,
then that would make storage of names much more efficient .
. the lookup would not be as quick as a hash;
it would use a binary search ?
2.13:
. compilers use hashing to help them place a found string into a symbol table;
hash converts the string into an byte,
and then instead of putting it in a bucket,
it can add on some more bits to make it unique .
. when that string is found again,
the hash quickly finds the approximate location .
. hashing would also be used for huge symbol tables
like the one converting all words to wordcodes .
. hashing is a much less expensive version of sha1
(sha = Secure Hash Algorithm)
http://en.wikipedia.org/wiki/SHA_hash_functions