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| From | Bartc <bc@freeuk.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: How to encode/decode string of decimal digits |
| Date | 2015-10-19 14:07 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <n02pr6$sqa$1@dont-email.me> (permalink) |
| References | <n02ni7$k3q$1@dont-email.me> |
On 19/10/2015 13:28, pozz wrote:
> I have to manage string of decimal digits 0-9. They aren't integer
> numbers, because the string "01234" is completely different than the
> number 1234.
>
> I have a very limited embedded plaftorm, so I'm searching for a simple
> compression/decompression method of a single string.
>
> I thought to use a uint64_t to store a 16-digits string. One byte (the
> LSB) of uint64_t is used to save the number of leading zeros.
>
> uint64_t
> compress(const char *s)
> {
> uint64_t ret;
> unsigned char nz;
>
> ret = strtoull(s, NULL, 10);
> for (nz = 0; *s++ == '0'; nz++)
> ;
> ret = (ret << 8) + nz;
> return ret;
> }
>
> void
> decompress(char *dst, uint64_t n)
> {
> unsigned char nz;
>
> nz = n & 0xFF;
> n >>= 8;
> memset(dst, '0', nz);
> if (n > 0) {
> sprintf(&dst[nz], "%lld", n);
> } else {
> dst[nz] = '\0';
> }
> }
>
> In this way I can save one 16-digits string in 8 bytes.
>
> Do you think of a better method?
How many leading zeros are there likely to be? And how important that
they be recovered?
Also, is performance important? Because above you are converting strings
into binary, and converting from binary into strings, all of which can
require quite a bit of arithmetic to do.
Assuming you can have up to 255 leading zeros, and up to 16 significant
digits, your basic data is 17 bytes (16 chars + 1 byte count, no
terminator). Your method above reduces that to 8 bytes. Not a bad
reduction, but a lot of work, and we don't know whether space is more
important or speed.
You can reduce that to 9 bytes by using a 4-bit representation for each
digit (BCD, 2 digits per byte), an easier conversion. Or 8 bytes if your
forget the leading zero count.
There are far more elaborate methods to reduce below 8 bytes per string,
but they can depend on how random the strings are.
--
Bartc
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How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-19 14:28 +0200
Re: How to encode/decode string of decimal digits Bartc <bc@freeuk.com> - 2015-10-19 14:07 +0100
Re: How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-19 15:58 +0200
Re: How to encode/decode string of decimal digits Bartc <bc@freeuk.com> - 2015-10-19 15:22 +0100
Re: How to encode/decode string of decimal digits Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-19 15:58 +0100
Re: How to encode/decode string of decimal digits Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-19 22:28 +0100
Re: How to encode/decode string of decimal digits Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-19 13:30 -0400
Re: How to encode/decode string of decimal digits Robert Wessel <robertwessel2@yahoo.com> - 2015-10-19 16:18 -0500
Re: How to encode/decode string of decimal digits Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-19 21:38 -0400
Re: How to encode/decode string of decimal digits Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-20 08:00 +0000
Re: How to encode/decode string of decimal digits Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-19 15:50 +0100
Re: How to encode/decode string of decimal digits Keith Thompson <kst-u@mib.org> - 2015-10-19 09:01 -0700
Re: How to encode/decode string of decimal digits Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-19 12:11 -0700
Re: How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-20 13:51 +0200
Re: How to encode/decode string of decimal digits Bartc <bc@freeuk.com> - 2015-10-20 13:38 +0100
Re: How to encode/decode string of decimal digits glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-20 16:05 +0000
Re: How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-21 09:29 +0200
Re: How to encode/decode string of decimal digits Bartc <bc@freeuk.com> - 2015-10-21 11:27 +0100
Re: How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-21 17:55 +0200
Re: How to encode/decode string of decimal digits Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-21 08:03 -0400
Re: How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-21 17:53 +0200
Re: How to encode/decode string of decimal digits supercat@casperkitty.com - 2015-10-21 09:37 -0700
Re: How to encode/decode string of decimal digits David Brown <david.brown@hesbynett.no> - 2015-10-22 11:47 +0200
Re: How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-22 13:44 +0200
Re: How to encode/decode string of decimal digits Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-22 13:45 +0100
Re: How to encode/decode string of decimal digits Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-22 15:51 +0100
Re: How to encode/decode string of decimal digits supercat@casperkitty.com - 2015-10-22 09:31 -0700
Re: How to encode/decode string of decimal digits Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-22 20:48 +0100
Re: How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-23 10:14 +0200
Re: How to encode/decode string of decimal digits Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-23 11:14 +0100
Re: How to encode/decode string of decimal digits Bartc <bc@freeuk.com> - 2015-10-23 11:47 +0100
Re: How to encode/decode string of decimal digits Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-23 13:22 +0100
Re: How to encode/decode string of decimal digits David Brown <david.brown@hesbynett.no> - 2015-10-22 15:07 +0200
Re: How to encode/decode string of decimal digits glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-22 22:05 +0000
Re: How to encode/decode string of decimal digits pozz <pozzugno@gmail.com> - 2015-10-23 10:15 +0200
Re: How to encode/decode string of decimal digits glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-19 22:02 +0000
Re: How to encode/decode string of decimal digits supercat@casperkitty.com - 2015-10-20 10:09 -0700
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