Lair Of The Multimedia Guru

July 21, 2006

Division by zero

Have you ever been curious why you cant divide by zero?

The intuitive explanation with division of positve integers

To divide a by b means to remove b as often as possible from a and assign the number of times which you could do that to a/b, and whats left as the remainder of the division
if b is 0 you can remove it as often or as rare as you want, nothing will ever change

The intuitive explanation with division as the inverse of multiplication

If you have a number a and multiply that by b then divide
by b, so a*b/b then you should get a again if division reverses what multiplication did, but if b is 0 then a*b is a*0 which is 0 no matter what a was, so theres no information left about the value a had and consequently no way to reverse it

The Analytical explanation

if a is non zero constant, then a/b diverges to infinity as b approaches 0, and it converges to x if a=xb
note its important to remember here that the limit of a/b as b approaches 0 is not the same as a/0

The Algebraic explanation

All the above really just deals with the common numbers like integers, reals, complex numbers, …
so you might ask yourself if its not maybe possible to invent some other algebra (set of elements with some operations like addition and multiplication) in which we can divide by zero, well the awnser is of course you can, the problem is just that you have to give up some of the well known properties of common numbers
Lets see which set of properties is incompatible with division by zero
to speak about division by zero we need at least

  1. a set of elements S for our algebra A
  2. an additative identity element (x + 0 = 0 + x = x for all x)
    we use “0” here for simplicity even though the element doesnt need to be related to our well known zero
  3. of course a + and * operation, which again of course doesnt have to be the same as common addition and multiplication
  4. 0 to have a inverse ((x*0)*0-1 = x for all x)

with that set of rules one can easily find many algebras in which you can divide by zero, for example just define the * operation to be identical to + and that identical to the common addition, division by zero is just subtraction by zero, totally useless but it works :)
anoter property we likely want is a multiplicative identity element (x * 1 = 1 * x = x for all x) and the distributive law (a+b)*c= a*c + b*c and c*(a+b)= c*a + c*b sadly that already starts causing problems:

0*0 + 0= 0*0 definition of the identity element of +
0*0 + 1*0 = 0*0 definition of the identity element of *
(0 + 1)*0 = 0*0 distributive law
1*0 = 0*0 definition of the identity element of +
1 = 0 inverting the multiplication by 0

so our algebra would need the identity element of + and the identity element of * to be identical, hmm …

and another odd result:

x*1 = x definition of the identity element of *
x*(1+1) = x definition of the identity element of + and 1=0
x+x = x distributive law

now the question is, is there actualy a non trivial (more then 1 element) algebra left at all?
it seems there is: assume our algebra has 2 elements: “0” and “#”
and we define addition and multiplication like:

+/* 0 #
0 0 #
# # #

its immedeatly obvious that the rules for the 2 identity elements hold
does the distributive law hold too? it does which can be proofen by simply looking at the 8 possible cases, and an inverse element for multiply by 0 well 0 is its own inverse obviously

next, we drop the multiplicative identity element again and try to add a unique multiplicative inverse element x for every element instead of just for zero (a*x=b for all a,b), without that we would either just change the division by zero in a division by foobar problem or we wouldnt be able to reach some elements, sadly only the trivial 1 element algebra is left then:

a*b = a*(b+0) definition of the identity element of +
a*b = a*b+a*0 distributive law
0 = 0+a*0 choose b so that a*b=0
0 = a*0 definition of the identity element of +
0*0-1 = a invert multiplication by zero

the last line means: every algebra with the properties above will have exactly one element and be useless
alternatively if we replace the property that every element must have a unique multiplicative inverse by requiring addition to be invertable ((a+b) + (-b) = a) then we too end up with the useless 1 element algebra:

a=(a*0)*o-1 definition of the inverse of 0
a=(a*(0+0))*0-1 definition of the identity element of +
a=(a*0 + a*0)*0-1 distributve law
a=(a*0)*0-1+(a*0)*0-1 distributve law
a=a + a substituting from row 1
0=a inverting the addition of a

i guess thats enough math for today, and enough chances to embarrass myself with a trivial typo somewhere …

Filed under: Uncategorized — Michael @ 11:25

July 8, 2006

UTF-8

UTF-8 is the most common encoding to store unicode, the following table summarizes how it works

number of bytes byte1 byte2 byte3 byte4 range of unicode symbols
1 0XXX XXXX 0-127
2 110X XXXX 10XX XXXX 128-2047
3 1110 XXXX 10XX XXXX 10XX XXXX 2048-65535
4 1111 0XXX 10XX XXXX 10XX XXXX 10XX XXXX 65536-2097151

Features

  • 0-127 ASCII is stored as is in UTF-8
  • bytewise substring matching works as is, or in other words one UTF-8 symbol will never match anything but one and the same UTF-8 symbol, not maybe just the last byte of a symbol
  • UTF-8 is easy to distinguish from a random byte sequence
  • when starting at a random byte, resynchronization is trivial and always happens at the next symbol

S1D-7

The biggest problem of UTF-8 is that its not very compact, it needs more bytes then other (non-unicode) encodings, can we do better without loosing its advantages?, well apparently yes, its sad because youd expect the standard comittees to do better …
Why “S1D-7”, well 1 “stop” bit to determine if the byte is the last one and 7 data bits for each byte

number of bytes byte1 byte2 byte3 byte4 range of unicode symbols
1 0XXX XXXX 0-127
2 1XXX XXXX 0XXX XXXX 128-16383
3 1XXX XXXX 1XXX XXXX 0XXX XXXX 16384-2097151

Compactness differences

  • In all cases is S1D-7 able to store as many or more symbols per number of bytes
  • where UTF-8 needs up to 4 bytes to store all symbols, S1D-7 never needs more then 3
  • Many languages which need 3 bytes per symbol in UTF-8 can be encoded with 2 bytes per symbol, just look at the Basic Multilingual Plane at wikipedia, all the pages which are in the range of 0x07FF to 0x3FFF need 3 bytes per symbol in UTF-8 and 2 bytes per symbol in S1D-7

Features

Well, the argumentation at the wikipedia page to justify UTF-8 bloatedness is that its “advantages outweigh this concern”, well so lets see if our simpler encoding which requires fewer bytes per symbol and is much more compact looses any features

0-127 ASCII is stored as is in UTF-8

same, no change here

Substring matching

in UTF-8 we can simply use bytewise substring matching while in S1D-7 we need to write a slightly differnt substring matching function, though both are very simple, see below

int match_utf8_or_ascii(uint8_t *heap, uint8_t *needle){
    int i,j;

    for(i=0; heap[i]; i++){
        for(j=0; needle[j]; j++){
            if(heap[i+j] != needle[j])
                break;
        }
        if(!needle[j])
            return i;
    }
    return -1;
}

int match_S1D8(uint8_t *heap, uint8_t *needle){
    int i,j;

    for(i=0; heap[i]; i++){
        for(j=0; needle[j]; j++){
            if(heap[i+j] != needle[j])
                break;
        }
        if(!needle[j] && !(i && heap[i-1]>127))
            return i;
    }
    return -1;
}
Distinguishing S1D-7 from random bytes

Due to the lower overhead there is less to check and it consequently needs more text to detect reliably, but the check is very simple, just check if there are any consecutive 3 bytes which are each larger then 127 and you know its not S1D-7, to distingissh it from UTF-8 just run your UTF-8 detection over it, its very unlikely that S1D-7 will be parsed correctly by a UTF-8 parser
not to mention, the encoding should be specified somehow anyway and not guessed by such encoding analysis

resynchronization

is even more trivial then in UTF-8, in UTF-8 you search for a byte <128 or a byte > 192 when you find it you know its the first byte of a symbol
in S1D-7 you search for a byte < 128 when you find it you know that the next byte is the first byte of a symbol

parsing

Parsing S1D-7 is much easier then UTF-8, see:

read_utf8_symbol(uin8_t **b){
    int val= *((*b)++);
    int ones=0;

    while(val&(128>>ones))
        ones++;

    if(ones==1 || ones>4)
        return -1;
    val&= 127>>ones;
    while(--ones > 0){
        int tmp= *((*b)++) - 128;
         if(tmp>>6)
            return -1;
        val= (val<<6) + tmp;
    }
    return val;
}

read_s1d7_symbol(uin8_t **b){
    int i, val=0;

    for(i=0; i<3; i++){
        int tmp= *((*b)++) - 128;
        val= (val<<7) + tmp;
        if(tmp < 0)
            return val + 128;
    }
    return -1;
}
zero byte occurances (new 2006-07-21)

This issue has been raised by a comment …
While UTF-8 gurantees that a zero byte in the encoding will occur only in a zero symbol, S1D-7 does not gurantee that, for example the value 256 is encoded as 0x82 0x00, UTF-16 also has this problem
zero bytes are problematic if the encoded strings are passed through code which has been written for one byte per symbol zero terminated strings, of course you should never pass something through code which is designed for a different encoding but if for whatever odd reason you still do that then one possible solution is to simply change the value before encoding and after decoding to avoid zero bytes, this is actually very easy:

int avoid_zero(int x){
    x+= x<<7;
    return (x + (x>>14))>>7;
}
int reverse_avoid_zero(int x){
    return x - (x>>7);
}

these transforms have almost no effect on the space efficiency of S1D-7

occurance of specific bytes (new 2006-07-21)

This issue has been raised by a comment …
While UTF-8 gurantees that no ascii bytes below 128 will occur in the encoding of symbols larger then 128, S1D-7 does not gurantee that, UTF-16 also has this “problem”
again this “problem” is limited to the case of passing encoded strings through code which expects some other encoding, and thats something you should never ever do, not only will the code not be able to make sense of at least some symbols its likely going to lead to bugs and exploits, if strings have to be passed through code which expects another encoding then you should convert your strings to that encoding and escape the problematic symbols somehow
S1D-7 can also easily be changed to avoid specific low <128 bytes in its encoded representation, allthough thats just for demonstration and shouldnt be done in practice as its very bad to pass strings though code which expects another encoding, if your filesystem for example expects filenames in ascii without specific letters then thats what the filename should be encoded with, and letters which cannot be represented should be encode with some sort of escape sequence

int avoid_some_bytes(int x){
    if(x>127) x= (x<<1) ^ table1[x&63];
    return x;
}
int reverse_avoid_some_bytes(int x){
    if(x>127) x= (x ^ table2[x&127])>>1;
    return x;
}

with these transforms any 64 low ascii bytes can be avoided at the cost of 1 bit per symbol, that makes S1D-7 somewhat worse then before but still much better then UTF-8 in space efficiency

Filed under: Uncategorized — Michael @ 12:40

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