Difference between revisions of "Hex"

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Note: Results with eight nybbles begin always with one of the positive signed nybbles 1 2 3 4 5 6 7, never with a zero or unsigned nybble 0 8 9 A B C D E F, except for the boundary test case of the most negative integer "-0x80000000".
Note: Always begins any result of eight nybbles with one of the positive signed nybbles 1 2 3 4 5 6 7, never with a zero or unsigned nybble 0 8 9 A B C D E F, except for the boundary test case of the most negative integer "-0x80000000".
 
Caution: This page was a work in progress as of 2007-10. The specification, the implementations, the demo, the demo results, and the alternatives may not yet be totally consistent. See the [[Talk:Hex|discussion]] tab.
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== Implementations ==
== Code ==
<div style="padding: 0.5em;">
<div style="padding: 0.5em;">


Here you see code that implements the hex function specification, then you see demo code that calls the hex function, and then you see a copy of exactly the results that you running the demo should produce.
The brief, clear, conventional code here implements this specification exactly.
 
You see not one but actually three kinds of code that implement the one specification. The concise & conventional code details and implements the specification. The small code reduces the code size reported by llGetFreeMemory. The fast code reduces the run time reported by llGetTimestamp when called to return the hex for the integer value -0x80000000.
 
=== Concise & Conventional ===
 
You should choose this code when you care more about concise & conventional than about small & fast.


You should feel that this concise & conventional code is easy to review and modify. For example, you should immediately understand the comment that tells you how to substitute the easy-to-read upper case A B C D E F nybbles of IBM style for the easy-to-type lower case a b c d e f nybbles of AT&T style. Also you should immediately see what results to expect this code to return for any input.
The [[Efficient Hex]] article presents an alternative approach: clever, small, and fast code to implement the same specification and related specifications, and also links to instruments that measure small and fast.


<pre>
<source lang="lsl2">
// http://wiki.secondlife.com/wiki/hex
// http://wiki.secondlife.com/wiki/hex


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     }
     }
}
}
</pre>
</source>


347 bytes of code reported by llGetFreeMemory -- more than the small code and more than the fast code
</div>
</div>


FIXME milliseconds of run time reported by llGetTimestamp -- more than the small code and more than the fast code
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=== Small ===
== Demo Results ==
<div style="padding: 0.5em;">


You should choose this code when you care more about small & fast than about concise & conventional.
Running the demo should produce exactly these results:
 
We wrote this small code to show how to reduce the code size reported by [[llGetFreeMemory]]. We think this small code produces the same results as the concise & conventional code. After adequate education & review, you should agree.


<pre>
<pre>
// http://wiki.secondlife.com/wiki/hex
Hello
 
0x0 == 0
string hex(integer value)
0x400 == (0x00FEDC00 & -0x00FEDC00)
{
0x40000000 == (1 << 30)
    string lead = "0x";
-0x80000000 == 0x80000000
    if (value & 0x80000000) // means (integer < 0) but is smaller and faster
-0x123678a == 0xFEDC9876
    {
-0x1 == -1
        lead = "-0x";
-0x1 == 0x123456789
        value = -value; // unnecessary when value == -0x80000000
OK
    }
Hello again
 
0x7fffffff as base
    string nybbles = "";
0x7fffffff by owner
    do
0x0 by group
    {
0x0 by anyone
        integer lsn = value & 0xF; // least significant nybble
0x82000 by next owner
        nybbles = llGetSubString("0123456789abcdef", lsn, lsn) + nybbles;
aka 532480
    } while ((value = (0xfffFFFF & (value >> 4))));
OK
 
    return lead + nybbles;
}
</pre>
</pre>


197 bytes of code reported by llGetFreeMemory -- less than the concise & conventional code and less than the fast code
</div>
</div>


FIXME milliseconds of run time reported by llGetTimestamp -- less than the concise & conventional code, but more than the fast code
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=== Fast ===
== Demo ==
<div style="padding: 0.5em;">


You should choose this code when you care more about fast & small than about concise & conventional.
To reproduce exactly the expected demo results above, run the demo code below.


We wrote this fast code to show how to reduce the run time reported by [[llGetTimestamp]]. We think this fast code produces the same results as the concise & conventional code. After adequate education & review, you should agree. This code leaves as little work inside the loop as possible. This code might not be as fast as code that unrolls the loop completely. This code might not be as fast as code which implements hex as one function rather than as two functions.
We chose test cases that astonish people new to hex and test cases that astonish people new to LSL permission masks.


<pre>
You'll get the permission mask results we show if you create a New Script to run this demo in. If instead you try modifying some old script to run this demo, then you might have to edit its permission masks to get the demo results that we show here.
// http://wiki.secondlife.com/wiki/hex
 
string XDIGITS = "0123456789abcdef"; // could be "0123456789ABCDEF"
 
string hexu(integer bits)
{
    integer lsn = (bits & 0xF); // least significant nybble
    string nybbles = llGetSubString(XDIGITS, lsn, lsn);
    if ((bits = (0xfffFFFF & (bits >> 4))))
    {
        do
        {
            nybbles = llGetSubString(XDIGITS, lsn = (bits & 0xF), lsn) + nybbles;
        } while ((bits = (bits >> 4)));
    }
    return "0x" + nybbles;
}


string hex(integer value)
<source lang="lsl2">
{
    //saves one byte over "value < 0" and is faster.
    if (value & 0x80000000) return "-" + hexu(-value);
    return hexu(value);
}
</pre>
 
337 bytes of code reported by llGetFreeMemory - less than concise & conventional code, but more than the small code
 
FIXME milliseconds of run time reported by llGetTimestamp - less than the concise & conventional code and less than the small code
 
===== Demo =====
 
Run this demo code to confirm that you get exactly the expected demo results below. We chose test cases that astonish people new to hex.
 
We also chose test cases that astonish people new to LSL permission masks. You'll get the permission mask results we show if you create a New Script to run this demo in. If instead you tried modifying some old script to run this demo, then you might have to edit its permission masks to get the demo results that we show here.
 
<pre>
default
default
{
{
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     }
     }
}
}
</pre>
</source>
 
===== Demo Results =====
 
Running the demo above to call your choice of code should produce exactly these results:
 
<pre>
Hello
0x0 == 0
0x400 == (0x00FEDC00 & -0x00FEDC00)
0x40000000 == (1 << 30)
-0x80000000 == 0x80000000
-0x123678a == 0xFEDC9876
-0x1 == -1
-0x1 == 0x123456789
OK
Hello again
0x7fffffff as base
0x7fffffff by owner
0x0 by group
0x0 by anyone
0x82000 by next owner
aka 532480
OK
</pre>


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== Design Rationale ==
== Specification ==
<div style="padding: 0.5em">
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The specification requires exactly the same results as the hex function of the Python scripting language.
We chose requirements that astonish people who usually write clever, small, or fast code by taking as precedent a specification from the far off world of people who usually write brief, clear, and conventional code.


The specification reproduces how hex integer literals often appear in LSL script, conforming to such arbitrary and traditional AT&T C conventions as:
We require exactly the same results as the hex function of the popular Python scripting language. We thus reproduce how hex integer literals often appear in LSL script, conforming to such arbitrary and traditional AT&T C conventions as:


# return lower case a b c d e f rather than upper case A B C D E F,
# return lower case a b c d e f rather than upper case A B C D E F,
# return a signed 31-bit result if negative, rather than an unsigned 32-bit result,
# return a signed 31-bit result if negative, rather than an unsigned 32-bit result,
# omit the leading quads of zeroed bits, except returns "0x0" rather than "0x" when the result is zero,
# omit the leading zeroed nybbles, except return "0x0" rather than "0x" when the result is zero,
# return a meaningless "0" before the "x",  as LSL and C compilers require,
# return a meaningless "0" before the "x",  as LSL and C compilers require,
# return the "x" on the left as in LSL and C, not the "h" on the right as in Assembly code, and
# return the "x" on the left as in LSL and C, not the "h" on the right as in Assembly code, and
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Disputes over the detailed specification of the Python hex function appear buried deep within http://www.python.org/dev/peps/pep-0237/
Disputes over the detailed specification of the Python hex function appear buried deep within http://www.python.org/dev/peps/pep-0237/
As you read this page, you have to wade thru more than one implementation because our LSL wiki serves more than one community of LSL authors. New authors need to see concise & conventional exemplars first, experts find appropriate uses for the small & fast exemplars.
We don't know how to write LSL that implements a hex function that is concise and conventional and small and fast, all at once. We think that design goal is an impossibility in LSL, as in many other programming languages.


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== Alternatives ==
== See Also ==
<div style="padding: 0.5em;">
<div style="padding: 0.5em;">


When you can accept the cost of abandoning the conventional specification, you can make the code still more concise and smaller and faster.
'''Articles'''
 
* [[Efficient Hex]]
=== Different & Concise & Small ===
 
This code sees the most significant bit of the integer as just another data bit, rather than as the sign bit, so results with eight nybbles can begin with any of the nonzero unsigned nybbles 1 2 3 4 5 6 7 8 9 A B C D E F. This code also doesn't bother to calculate the "-0x" or "0x" part of the conventional answer.
 
This code returns "0" for 0, "80000000" for -0x80000000, "FFFFFFFF" for -0x1, etc.
 
The code presented here does return easy-to-read upper case A B C D E F nybbles, but exactly the same small size of code can return easy-to-type lower case a b c d e f nybbles.
 
<pre>
// http://wiki.secondlife.com/wiki/hex
 
string bits2nybbles(integer bits)
{
    string nybbles = "";
    do
    {
        integer lsn = bits & 0xF; // least significant nybble
        nybbles = llGetSubString("0123456789ABCDEF", lsn, lsn) + nybbles;
    } while (bits = (0xfffFFFF & (bits >> 4)));
    return nybbles;
}
</pre>
 
139 bytes of code reported by llGetFreeMemory
 
FIXME milliseconds of run time reported by llGetTimestamp
 
</div>
</div>
 
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== See Also ==
<div style="padding: 0.5em">


'''Functions'''
'''Functions'''
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* [[llGetObjectPermMask]]
* [[llGetObjectPermMask]]
* [[llIntegerToBase64]]
* [[llIntegerToBase64]]
* [[llBase64ToInteger]]
* [[llGetFreeMemory]]
* [[llGetTimestamp]]
* [[llMessageLinked]]
* [[llList2Integer]]
* [[llList2String]]


'''Wikipedia'''
'''Wikipedia'''
* http://en.wikipedia.org/wiki/Exemplar
* {{Wikipedia|Exemplar}}
* http://en.wikipedia.org/wiki/Optimization_(computer_science)#When_to_optimize
* {{Wikipedia|Principle_of_least_astonishment}}
* http://en.wikipedia.org/wiki/Principle_of_least_astonishment


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</div>

Latest revision as of 14:14, 24 January 2015

Function: string hex(integer value);

Returns the hexadecimal nybbles of the signed integer value in order. Specifically returns the nybbles from most to least significant, starting with the first nonzero nybble, folding every nybble to lower case, and beginning with the nonnegative prefix "0x" or the negative prefix "-0x".

Parameters:

• integer value – signed value to be expressed as negative or nonnegative hex

Note: Always begins any result of eight nybbles with one of the positive signed nybbles 1 2 3 4 5 6 7, never with a zero or unsigned nybble 0 8 9 A B C D E F, except for the boundary test case of the most negative integer "-0x80000000".

Code

The brief, clear, conventional code here implements this specification exactly.

The Efficient Hex article presents an alternative approach: clever, small, and fast code to implement the same specification and related specifications, and also links to instruments that measure small and fast. 

// http://wiki.secondlife.com/wiki/hex

string XDIGITS = "0123456789abcdef"; // could be "0123456789ABCDEF"

string hexes(integer bits)
{
    string nybbles = "";
    while (bits)
    {
        integer lsn = bits & 0xF; // least significant nybble
        string nybble = llGetSubString(XDIGITS, lsn, lsn);
        nybbles = nybble + nybbles;
        bits = bits >> 4; // discard the least significant bits at right
        bits = bits & 0xfffFFFF; // discard the sign bits at left
    }
    return nybbles;
}

string hex(integer value)
{
    if (value < 0)
    {
        return "-0x" + hexes(-value);
    }
    else if (value == 0)
    {
        return "0x0"; // hexes(value) == "" when (value == 0)
    }
    else // if (0 < value)
    {
        return "0x" + hexes(value);
    }
}

Demo Results

Running the demo should produce exactly these results: 

Hello
0x0 == 0
0x400 == (0x00FEDC00 & -0x00FEDC00)
0x40000000 == (1 << 30)
-0x80000000 == 0x80000000
-0x123678a == 0xFEDC9876
-0x1 == -1
-0x1 == 0x123456789
OK
Hello again
0x7fffffff as base
0x7fffffff by owner
0x0 by group
0x0 by anyone
0x82000 by next owner
aka 532480
OK

Demo

To reproduce exactly the expected demo results above, run the demo code below.

We chose test cases that astonish people new to hex and test cases that astonish people new to LSL permission masks.

You'll get the permission mask results we show if you create a New Script to run this demo in. If instead you try modifying some old script to run this demo, then you might have to edit its permission masks to get the demo results that we show here. 

default
{
    state_entry()
    {
        llOwnerSay("Hello");
        llOwnerSay(hex(0) + " == 0");
        llOwnerSay(hex(0x00FEDC00 & -0x00FEDC00) + " == (0x00FEDC00 & -0x00FEDC00)");
        llOwnerSay(hex(1 << 30) + " == (1 << 30)");
        llOwnerSay(hex(0x80000000) + " == 0x80000000");
        llOwnerSay(hex(0xFEDC9876) + " == 0xFEDC9876");
        llOwnerSay(hex(-1) + " == -1");
        llOwnerSay(hex(0x123456789) + " == 0x123456789");
        llOwnerSay("OK");
        
        llOwnerSay("Hello again");
        string item = llGetScriptName();
        llOwnerSay(hex(llGetInventoryPermMask(item, MASK_BASE)) + " as base");
        llOwnerSay(hex(llGetInventoryPermMask(item, MASK_OWNER)) + " by owner");
        llOwnerSay(hex(llGetInventoryPermMask(item, MASK_GROUP)) + " by group");
        llOwnerSay(hex(llGetInventoryPermMask(item, MASK_EVERYONE)) + " by anyone");
        llOwnerSay(hex(llGetInventoryPermMask(item, MASK_NEXT)) + " by next owner");
        llOwnerSay("aka " + (string) llGetInventoryPermMask(item, MASK_NEXT));
        llOwnerSay("OK");
    }
}

Specification

We chose requirements that astonish people who usually write clever, small, or fast code by taking as precedent a specification from the far off world of people who usually write brief, clear, and conventional code.

We require exactly the same results as the hex function of the popular Python scripting language. We thus reproduce how hex integer literals often appear in LSL script, conforming to such arbitrary and traditional AT&T C conventions as:

  1. return lower case a b c d e f rather than upper case A B C D E F,
  2. return a signed 31-bit result if negative, rather than an unsigned 32-bit result,
  3. omit the leading zeroed nybbles, except return "0x0" rather than "0x" when the result is zero,
  4. return a meaningless "0" before the "x", as LSL and C compilers require,
  5. return the "x" on the left as in LSL and C, not the "h" on the right as in Assembly code, and
  6. return the nybbles listed from most to least significant as in English, not listed from least to most significant as in Arabic.

Brief doc for the Python hex function appears buried deep within http://docs.python.org/lib/built-in-funcs.html

Disputes over the detailed specification of the Python hex function appear buried deep within http://www.python.org/dev/peps/pep-0237/

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