Difference between revisions of "Float2Sci"

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(Code: updated to colorized code display)
 
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==Code==
 
==Code==
<lsl>
+
<source lang="lsl2">
 
string Float2Sci(float input)
 
string Float2Sci(float input)
 
{// LSLEditor Unsafe, LSO Safe, Mono Safe
 
{// LSLEditor Unsafe, LSO Safe, Mono Safe
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     return mantissa;
 
     return mantissa;
 
}
 
}
</lsl>
+
</source>
  
 
===LSLEditor===
 
===LSLEditor===
 
Some people will want to run this in LSLEditor where the execution order is reversed and where booleans aren't integers.
 
Some people will want to run this in LSLEditor where the execution order is reversed and where booleans aren't integers.
 
Do keep in mind this will only work support float precision, not double precision. I have no intention of adding double precision.
 
Do keep in mind this will only work support float precision, not double precision. I have no intention of adding double precision.
 
+
<source lang="lsl2">string Float2Sci(float input)
<lsl>string Float2Sci(float input)
+
 
{// LSLEditor Safe, LSO Safe, Mono Safe
 
{// LSLEditor Safe, LSO Safe, Mono Safe
 
     if(input == 0.0)//handles negative zero
 
     if(input == 0.0)//handles negative zero
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         return "-" + mantissa;
 
         return "-" + mantissa;
 
     return mantissa;
 
     return mantissa;
}</lsl>
+
}</source>
  
 
=== Testbed ===
 
=== Testbed ===
<lsl>
+
<source lang="lsl2">
 
test(float a){
 
test(float a){
 
     string b = Float2Sci(a);
 
     string b = Float2Sci(a);
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     }
 
     }
 
}
 
}
</lsl>
+
</source>

Latest revision as of 09:16, 16 June 2016

A script for passing floats through strings without losing precision. Wrote this ages ago, it isn't very fast but it is accurate. If you want fast and accurate but don't care so much about human readable try Float2Hex.

Works flawlessly and LSL can parse directly to floats again without any special code :-) just use a (float) typecast. In Mono the (float) typecast has a bug where anything that would be in the denormalized range gets mapped to zero. Nothing I can do to fix this or work around it.

This code, and it's comments, predate Mono by several years, what the comments have to say is true for LSO, not so much for Mono.

Changes:

  • Fixed: Encoding not working in Mono for denormalized range. I have to use llPow instead of (float) typecast.
  • Fixed: Cannot assume that optical will work for numbers with large magnitudes. This was necessitated by SCR-397 which was an unnoticed breaking change made during the switch over to Mono.
  • Prior versions can be found here: http://forums-archive.secondlife.com/15/28/28006/1.html

Code

string Float2Sci(float input)
{// LSLEditor Unsafe, LSO Safe, Mono Safe
    if(input == 0.0)//handles negative zero
        return llDeleteSubString((string)input, -5, -1);//trim off the trailing zero's, don't need them.
 
    float frac = llFabs(input);//we put the negative back at the end.
    string mantissa = (string)frac;//this may be a string of about 47 characters long.
    if(!~llSubStringIndex(mantissa, ".")) return (string)input; //NaN and Infinities, it's quick, it's easy.
    integer exponent = -6;//default exponent for optical method
    if(frac == (float)mantissa){
        mantissa = llDeleteSubString(mantissa, -7, -7);//remove the decimal point
        jump optical;
    }
    //optical failed
    //Ugly Math version; ugly in the sense that it is slow and not as elegant as working with it as a string.
    //A) calculate the exponent via approximation of C log2()
    //B) use kludge to avert fatal error in approximation of log2 result (only a problem with values >= 0x1.FFFFF8p127)
    //       the exponent is sometimes reported as 128, which will bork float math, so we subtract the test for 128.
    //       max_float btw is 0x1.FFFFFEp127, so we are only talking a very small number of numbers.
    //C) normalize the float with questionable exponent
    //D) calculate rounding error left from log2 approximation and add to normalization value.
    //       the '|' acts like a '+' in this instance but saves us one byte.
    integer position = (24 | (3 <= frac)) - (integer)( //D
            frac /= llPow(2.0, //C
            exponent = (exponent - (( //B
                exponent = llFloor(llLog(frac) / 0.69314718055994530941723212145818) //A
            ) == 128))
            )
            );
 
    //this pushes the float into the integer buffer exactly.
    //since the shift is within integer range, we don't need to make a float.
    integer int = (integer)(frac * (1 << position));
    integer target = (integer)(frac = 0.0);//since the float is in the integer buffer, we need to clear the float buffer.
 
    //we don't use a traditional while loop, and instead opt for a do-while, because it's faster
    //since we may have to do about 128 iteration, this savings is important,
    //the exponent needs one final adjustment because of the shift, we do it here to save memory & it's faster.
 
    //The two loops try to make exponent == position by shifting and multiplying.
    //when they are equal, then this should be true ((int * llPow(10, exponent)) == llFabs(input))
    //That is of course assuming that the llPow(10, exponent) result has enough precision.
 
    //We recycle position for these loops as a temporary buffer. This is so we can save a few operations.
    //If we didn't, then we could actually optimize the variable out of the code; though it would be slower.
    if(target > (exponent -= position))
    {//apply the rest of the bit shift if |input| < 1
        do
        {
            if(int < 0x19999999)//(0x80000000 / 5)
            {//won't overflow, multiply in 5
                int = int * 5 + (position = (integer)(frac *= 5.0));
                frac -= (float)position;
                target = ~-target;
            }
            else
            {//overflow predicted, divide by 2
                frac = (frac + (int & 1))/2;
                int = int >> 1;
                exponent = -~exponent;
            }
        }while(target ^ exponent);
    }
    else if(target ^ exponent)//target < exponent
    {//apply the rest of the bit shift if |input| > 1
        do
        {
            if(int < 0x40000000) //(0x80000000 / 2)
            {//won't overflow, multiply in 2
                int = (int << 1) + (position = (integer)(frac *= 2.0));
                frac -= (float)position;
                exponent = ~-exponent;
            }
            else
            {//overflow predicted, divide by 5
                frac = (frac + int%5) / 5.0;
                int /= 5;
                target = -~target;
            }
        }while(target ^ exponent);
    }
    //int is now properly calculated, it holds enough data to accurately describe the input in conjunction with exponent.
    //we feed this through optical to clean up the answer.
    mantissa = (string)int;
 
    @optical;
    //it's not an issue that we may be jumping over the initialization of some of the variables,
    //we initialize everything we use here.
 
    //to accurately describe a float you only need 9 decimal places; so we throw the extras away
    if(9 < (target = position = llStringLength(mantissa)))
        position = 9;
    //chop off the tailing zero's; we don't need them.
    do; while(llGetSubString(mantissa, position, position) == "0" && (position = ~-position));//faster then a while loop
 
    //we do a bad thing, we recycle 'target' here, position is one less then target,
    //"target + ~position" is the same as "target - (position + 1)" saves 6 bytes.
    //this block of code actually does the cutting.
    if(target + ~position) mantissa = llGetSubString(mantissa, 0, position);
 
    //insert the decimal point (not strictly needed). We add the extra zero for aesthetics.
    //by adding in the decimal point, it simplifies some of the code.
    mantissa = llInsertString(mantissa, 1, llGetSubString(".0", 0, !position));
 
    //adjust exponent from having added the decimal place
    if((exponent += ~-target))
        mantissa += "e" + (string)exponent;
    //return with the correct sign.
    if(input < 0)
        return "-" + mantissa;
    return mantissa;
}

LSLEditor

Some people will want to run this in LSLEditor where the execution order is reversed and where booleans aren't integers. Do keep in mind this will only work support float precision, not double precision. I have no intention of adding double precision.

string Float2Sci(float input)
{// LSLEditor Safe, LSO Safe, Mono Safe
    if(input == 0.0)//handles negative zero
        return llDeleteSubString((string)input, -5, -1);//trim off the trailing zero's, don't need them.
 
    float frac = llFabs(input);//we put the negative back at the end.
    string mantissa = (string)frac;//this may be a string of about 47 characters long.
    integer exponent = -6;//default exponent for optical method
    if(frac == (float)mantissa){
        mantissa = llDeleteSubString(mantissa, -7, -7);//remove the decimal point
        jump optical;
    }
 
    //optical failed
    //Ugly Math version; ugly in the sense that it is slow and not as elegant as working with it as a string.
    //A) calculate the exponent via approximation of C log2()
    //B) use kludge to avert fatal error in approximation of log2 result (only a problem with values >= 0x1.FFFFF8p127)
    //       the exponent is sometimes reported as 128, which will bork float math, so we subtract the test for 128.
    //       max_float btw is 0x1.FFFFFEp127, so we are only talking a very small number of numbers.
    //       but max_double is much larger but we aren't supporting those, we don't have the precision.
    //C) normalize the float with questionable exponent
    //D) calculate rounding error left from log2 approximation and add to normalization value.
    //       the '|' acts like a '+' in this instance but saves us one byte.
 
    if((exponent = llFloor(llLog(frac) / 0.69314718055994530941723212145818)) >= 128)//A
        exponent = ~-exponent;//B
    frac /= llPow(2.0, exponent);//C
    integer position = (24 | (integer)(3 <= frac)) - (integer)(frac); //D
 
    //this pushes the float into the integer buffer exactly.
    //since the shift is within integer range, we don't need to make a float.
    integer int = (integer)(frac * (1 << position));
    integer target = (integer)(frac = 0.0);//since the float is in the integer buffer, we need to clear the float buffer.
 
    //we don't use a traditional while loop, and instead opt for a do-while, because it's faster
    //since we may have to do about 128 iteration, this savings is important,
    //the exponent needs one final adjustment because of the shift, we do it here to save memory & it's faster.
 
    //The two loops try to make exponent == position by shifting and multiplying.
    //when they are equal, then this should be true ((int * llPow(10, exponent)) == llFabs(input))
    //That is of course assuming that the llPow(10, exponent) result has enough precision.
 
    //We recycle position for these loops as a temporary buffer. This is so we can save a few operations.
    //If we didn't, then we could actually optimize the variable out of the code; though it would be slower.
    if(target > (exponent -= position))
    {//apply the rest of the bit shift if |input| < 1
        do
        {
            if(int < 0x19999999)//(0x80000000 / 5)
            {//won't overflow, multiply in 5
                int = int * 5 + (position = (integer)(frac *= 5.0));
                frac -= (float)position;
                target = ~-target;
            }
            else
            {//overflow predicted, divide by 2
                frac = (frac + (int & 1))/2;
                int = int >> 1;
                exponent = -~exponent;
            }
        }while(target ^ exponent);
    }
    else if(target ^ exponent)//target < exponent
    {//apply the rest of the bit shift if |input| > 1
        do
        {
            if(int < 0x40000000) //(0x80000000 / 2)
            {//won't overflow, multiply in 2
                int = (int << 1) + (position = (integer)(frac *= 2.0));
                frac -= (float)position;
                exponent = ~-exponent;
            }
            else
            {//overflow predicted, divide by 5
                frac = (frac + int%5) / 5.0;
                int /= 5;
                target = -~target;
            }
        }while(target ^ exponent);
    }
    //int is now properly calculated, it holds enough data to accurately describe the input in conjunction with exponent.
    //we feed this through optical to clean up the answer.
    mantissa = (string)int;
 
    @optical;
    //it's not an issue that we may be jumping over the initialization of some of the variables,
    //we initialize everything we use here.
 
    //to accurately describe a float you only need 9 decimal places; so we throw the extras away
 
//    if(9 < (target = position = llStringLength(mantissa)))
//        position = 9;
    target = position = llStringLength(mantissa);
 
    //chop off the tailing zero's; we don't need them.
    do
        position = ~-position;
    while((llGetSubString(mantissa, position, position) == "0") && !!position);//faster then a while loop
 
    //we do a bad thing, we recycle 'target' here, position is one less then target,
    //"target + ~position" is the same as "target - (position + 1)" saves 6 bytes.
    //this block of code actually does the cutting.
    if(target + ~position) mantissa = llGetSubString(mantissa, 0, position);
 
    //insert the decimal point (not strictly needed). We add the extra zero for aesthetics.
    //by adding in the decimal point, it simplifies some of the code.
    mantissa = llInsertString(mantissa, 1, llGetSubString(".0", 0, !position));
 
    //adjust exponent from having added the decimal place
    if((exponent += ~-target))
        mantissa += "e" + (string)exponent;
    //return with the correct sign.
    if(input < 0)
        return "-" + mantissa;
    return mantissa;
}

Testbed

test(float a){
    string b = Float2Sci(a);
    llSay(0, llList2CSV([a, (float)b, b, a - (float)b, a == (float)b]));
}
 
default
{
    state_entry()
    {
        llListen(1, "", llGetOwner(), "");
        test(84932901.0);
        test(1.0);
        test(10.0);
    }
    listen(integer channel, string name, key id, string message)
    {
        test((float)message);
    }
}