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C Library Functions  - djb2_hash (3)

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NAME

djb2_hash(3f) - [M_hashkeys:bucket_hash] djb2 string hash (algorithm by Daniel J. Bernstein) (LICENSE:PD)

CONTENTS

Synopsis
Description
Options
Returns
Example

SYNOPSIS

function djb2_hash_arr(anything,continue) result(hash_128)

    class(*),intent(in)          :: anything(:)
    logical,intent(in),optional  :: continue
    !! use,intrinsic : ISO_FORTRAN_ENV, only : int64
    integer(kind=int128)         :: hash_128

DESCRIPTION

djb2_hash(3f) is based on the string hash routine commonly known as djb2(3c). This algorithm was first described by Dan J. Bernstein many years ago in comp.lang.c. This version returns a value calculated using a 64-bit hash, which is returned as a 128bit value (not always available in Fortran) to allow the value to always be a positive value; as Fortran does not (currently) support a standard unsigned integer. If the value is changed to be a 64-bit value on platforms that do not support 128-bit INTEGER values the value may be negative, but is otherwise usable.

Such non-reversible hashes may be used for data or file fingerprints, to confirm unchanging results during regression testing, ...

More information is widely available on string hashes (including the well-known djb2(3c) algorithm) on such sources as Wikipedia. Consult such resources to confirm the suitability of this algorithm for your use. This algorithm was probably first proposed as a bucket hash.

The algorithm does not consider the Endian of the programming environment.

OPTIONS

STR	May be a CHARACTER string or an array of common intrinsic types. Currently, the types defined in the procedure are character(len=*); complex; integer(kind=int8); integer(kind=int16); integer(kind=int32); integer(kind=int64); integer(kind=int128); real(kind=real32); real(kind=real64); real(kind=real128).
CONTINUE
	indicate whether to continue accumulating the hash value from the last call. This is not threadsafe. This allows for continued hashes so that a hash can be calculated for a series of calls.

RETURNS

djb2_hash
	A 128-bit INTEGER hash value for the (possibly accumulated) data.

EXAMPLE

Sample program:

    program demo_djb2_hash
    use M_hashkeys, only : djb2_hash, int128
    implicit none
    integer(kind=int128)         :: hash
    character(len=:),allocatable :: string
    integer                      :: i
    ! string
    string=’test djb2_hash’
    hash=djb2_hash(string)
    write(*,*)’string=’,string,’ hash=’,hash
    ! array of characters
    hash=djb2_hash([’t’,’e’,’s’,’t’,’ ’,’d’,’j’,’b’,’2’,’_’,’h’,’a’,’s’,’h’])
    write(*,*)’string=’,string,’ hash=’,hash
    ! continued hash
    hash=djb2_hash([’t’,’e’,’s’,’t’])
    hash=djb2_hash([’ ’,’d’,’j’,’b’,’2’],continue=.true.)
    hash=djb2_hash([’_’,’h’,’a’,’s’,’h’],continue=.true.)
    write(*,*)’string=’,string,’ hash=’,hash
    ! array of integers
    hash=djb2_hash([(i,i=0,100)])
    write(*,*)’hash for values 0 to 100 is ’,hash
    !
    end program demo_djb2_hash

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Nemo Release 3.1

djb2_hash (3)

February 23, 2025

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