OpenEuphoria

Re: sequence storage space

• Posted by MAP <ddhinc at ALA.NET> Apr 17, 1998
• 609 views

Hello,
I wrote a couple of functions based on the info provided in
Robert Craig's function that achieve a fair amount of compression
for strings. By fair I mean not quite 2:1 for short strings,
and approaching 3:1 for longer ones. If saving space is really
critical though you'd be better off using memory allocation
with peek and poke or some other method.

The two functions are quite simple, str_pack() compresses the
string by putting 3 characters into each element of the sequence
and str_unpack() decompresses it. You might wonder why I disn't use
4 characters instead of 3... 4 seemed to prove less compact in
every case I tried, possibly I just didn't try enough cases or
long enough strings. Of course, don't attempt to use this on
non-string sequences (i.e. if they contain values greater than
255 in any of the elements).

-----------------------------------------------------------------------
function str_pack(sequence str)
    integer norm_strlen,
            packed_strlen,
            padding
    atom this_atom
    sequence packed_str

    norm_strlen = length(str)
    padding = remainder(norm_strlen, 3)
    packed_strlen = floor(norm_strlen / 3)
    packed_str = {}
    for cnt = 1 to packed_strlen do
        this_atom = 0
        for cnt2 = (3 * (cnt - 1)) + 1 to 3 * cnt do
            this_atom = this_atom * 256 + str[cnt2]
        end for
        packed_str = append(packed_str, this_atom)
    end for
    if padding then
        this_atom = 0
        for cnt = (3 * packed_strlen) + 1 to (3 * packed_strlen) + 3 do
            if padding then
                this_atom = this_atom * 256 + str[cnt]
                padding = padding - 1
            else
                this_atom = this_atom * 256
            end if
        end for
        packed_str = append(packed_str, this_atom)
    end if
    return packed_str
end function

function str_unpack(sequence pstr)
    sequence str,
             sub_str
    integer pstr_len
    atom this_atom

    pstr_len = length(pstr)
    str = {}
    sub_str = {0, 0, 0}
    for cnt = 1 to pstr_len do
        this_atom = pstr[cnt]
        for cnt2 = 1 to 3 do
            sub_str[4 - cnt2] = and_bits(this_atom, 255)
            this_atom = floor(this_atom / 256)
        end for
        str = str & sub_str
    end for
    while not compare(str[length(str)], 0) do
        str = str[1..length(str) - 1]
    end while
    return str
end function
-----------------------------------------------------------------------

Christopher D. Hickman