Daniel J. Bernstein's hash algorithm written in Fortran

djb_hash.f90

integer function djb_hash(str) result(hash)
    implicit none
    
    character(len=*),intent(in) :: str
    integer :: hash
    integer :: i

    hash = 5381

    do i=1,len(str)
        hash = (ishft(hash,5) + hash) + ichar(str(i:i))
    end do

end function DJB_hash

This code has a bug. Putting the "=5381" in the variable declaration makes it an "implicit save" variable. This means that the next time djb_hash is called, hash will retain the value it had from the last time it was called. The correct version is below (with a few other modifications):

function djb_hash(str) result(hash)

    implicit none
    character(len=*),intent(in) :: str
    integer :: hash
    integer :: i

    hash = 5381

    do i=1,len(str)
        hash = (ishft(hash,5) + hash) + ichar(str(i:i))
    end do

end function DJB_hash

You are correct. Thank you!

I also find that you can get identical hash values for multiple strings:

B3O2            -814373157
ATO2            -814373157

Maybe this is rare enough but this can throw a wrench into searches using the hashed values.

The low tech solution: search the hashes a-postieriori, and then add another integer to one of the duplicate values.

The hash codes are similar because ishft is used instead of ishftc. Indeed, ishft is not circular, and left bits are discarded as the shift is performed. My guess is that ishftc should be used instead. Just a letter more :

function djb_hash(str) result(hash)

implicit none
character(len=*),intent(in) :: str
integer :: hash
integer :: i

hash = 5381

do i=1,len(str)
    hash = (ishftc(hash,5) + hash) + ichar(str(i:i))
end do

end function DJB_hash

I actually tried this and you still get hash collisions under certain circumstances:

 ### hash of text "IO" :    615763273
 ### hash of text "INA":    615763273

I think that by the nature of hashing, there will always be cases where collisions occur. You will probably always need some kind of "tiebreaker" algorithm to resolve collisions.