String hash function in Lua

gistfile1.txt

function hash(str)
    h = 5381;

    for c in str:gmatch"." do
        h = ((h << 5) + h) + string.byte(c)
    end
    return h
end

Note that this is djb2 hash function (DJB is Daniel J. Bernstein)

http://www.cse.yorku.ca/~oz/hash.html

https://stackoverflow.com/questions/10696223/reason-for-the-number-5381-in-the-djb-hash-function

https://theartincode.stanis.me/008-djb2/

@karelbilek thanks

Also, this will work with older lua versions without << and will not use gmatch

function hash(str)
    local h = 5381

    for i = 1, #str do
       h = h*32 + h + str:byte(i)
    end
    return h
end

(I am not sure if the h*32 + h or h<<5 + h is really faster than just h*33 )

You'll want to mod by 2^31 to keep it within 32 bits or you'll get a double floating point number eventually.

function hash(str)
    h = 5381;

    for c in str:gmatch"." do
        h = math.fmod(((h << 5) + h) + string.byte(c), 2147483648)
    end
    return h
end