tcl with "unsigned int"

[aotto1968]

Down is a hash code from the TCL source,
I want to use the "unsigned int" as my own hash for internal purpose so I tried to translate this in tcl

proc pHash { str } {
set result 0
if {[string length $str] > 0} {
scan [string index $str 0] %c result
Print str@30 result
for {set idx 1} {$idx < [string length $str]} {incr idx} {
scan [string index $str $idx] %c value
set result [expr {$result + ($result << 3) + $value}]
Print idx result
}
}
format %08X $result
}

>> the result is frustrating <<

pHash SrvMkBufferCreateTLS

pHash -> str<SrvMkBufferCreateTLS>, | result<83>,
pHash -> idx<1>, | result<861>,
pHash -> idx<2>, | result<7867>,
pHash -> idx<3>, | result<70880>,
pHash -> idx<4>, | result<638027>,
pHash -> idx<5>, | result<5742309>,
pHash -> idx<6>, | result<51680898>,
pHash -> idx<7>, | result<465128184>,
pHash -> idx<8>, | result<4186153758>,
pHash -> idx<9>, | result<37675383923>,
pHash -> idx<10>, | result<339078455421>,
pHash -> idx<11>, | result<3051706098856>,
pHash -> idx<12>, | result<27465354889818>,
pHash -> idx<13>, | result<247188194008463>,
pHash -> idx<14>, | result<2224693746076264>,
pHash -> idx<15>, | result<20022243714686492>,
pHash -> idx<16>, | result<180200193432178529>,
pHash -> idx<17>, | result<1621801740889606845>,
pHash -> idx<18>, | result<14596215668006461681>,
pHash -> idx<19>, | result<131365941012058155212>,
1F11935808A528CC

The CORE problem is that in "C" the result is limited to 32bit (unsigned in) and an ADD to a 32bit integer
with overrun of the upper border will result in start at "0". the number will ALWAYS be restricted to 32 bit.

→ *but* in TCL the result is always added and *not* restricted to 32 bit.

Question how I add REAL 32bit unsinged integer in TCL

mfg



unsigned int
TclHashObjKey(
Tcl_HashTable *tablePtr, /* Hash table. */
void *keyPtr) /* Key from which to compute hash value. */
{
Tcl_Obj *objPtr = keyPtr;
int length;
const char *string = TclGetStringFromObj(objPtr, &length);
unsigned int result = 0;

/*
* I tried a zillion different hash functions and asked many other people
* for advice. Many people had their own favorite functions, all
* different, but no-one had much idea why they were good ones. I chose
* the one below (multiply by 9 and add new character) because of the
* following reasons:
*
* 1. Multiplying by 10 is perfect for keys that are decimal strings, and
* multiplying by 9 is just about as good.
* 2. Times-9 is (shift-left-3) plus (old). This means that each
* character's bits hang around in the low-order bits of the hash value
* for ever, plus they spread fairly rapidly up to the high-order bits
* to fill out the hash value. This seems works well both for decimal
* and non-decimal strings.
*
* Note that this function is very weak against malicious strings; it's
* very easy to generate multiple keys that have the same hashcode. On the
* other hand, that hardly ever actually occurs and this function *is*
* very cheap, even by comparison with industry-standard hashes like FNV.
* If real strength of hash is required though, use a custom hash based on
* Bob Jenkins's lookup3(), but be aware that it's significantly slower.
* Tcl does not use that level of strength because it typically does not
* need it (and some of the aspects of that strength are genuinely
* unnecessary given the rest of Tcl's hash machinery, and the fact that
* we do not either transfer hashes to another machine, use them as a true
* substitute for equality, or attempt to minimize work in rebuilding the
* hash table).
*
* See also HashStringKey in tclHash.c.
* See also HashString in tclLiteral.c.
*
* See [tcl-Feature Request #2958832]
*/

if (length > 0) {
result = UCHAR(*string);
while (--length) {
result += (result << 3) + UCHAR(*++string);
}
}
return result;
}

[Ralf Fassel]

* aotto1968 <aott...@t-online.de>

| The CORE problem is that in "C" the result is limited to 32bit (unsigned in) and an ADD to a 32bit integer
| with overrun of the upper border will result in start at "0". the number will ALWAYS be restricted to 32 bit.
>
| → *but* in TCL the result is always added and *not* restricted to 32 bit.
>
| Question how I add REAL 32bit unsinged integer in TCL

I'd say you need an "& 0xffffffff" somewhere in your TCL expr code to
limit to 32bits. Finding the correct position for that is left as an
exercise ;-)

R'

[aotto1968]

I choose

> set result [expr {($result + ($result << 3) + $value) & 0xffffffff}]

and hope it will be fine.

[Ralf Fassel]

* aotto1968 <aott...@t-online.de>

| > I'd say you need an "& 0xffffffff" somewhere in your TCL expr code to
| > limit to 32bits. Finding the correct position for that is left as an
| > exercise ;-)
| >

| I choose
>
| > set result [expr {($result + ($result << 3) + $value) & 0xffffffff}]
>
| and hope it will be fine.

Note that TCLs "scan %c" does something different for chars > 127
(notably Unicode) than the C function, since in TCL you get _one_ value
(which is also probably larger than 256), while in C you get two or more
bytes, depending on the Unicode representation. If you need an exact
replica of the C function, you probably need to get at the individual
bytes on TCL level.

Also I'm not sure if you need another "&..." after the left-shift, since
in C "$result << 3" will probably also overflow. Don't know if this
matters for the final result or the result as a hash value (been a while
since I did bitshifting).

R'