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Thread: [FPC] Bug in non-Extended floating types

  1. #1

    [FPC] Bug in non-Extended floating types

    System: Windows XP, Athalon X2 4800+, nVidia 8600GTS 256MB
    Compiler/IDE: FPC 2.2.0
    API: N/A (SysUtils/System?)

    I've been using text files to represent data and read it in through the parser interface, but I've encountered a possible bug that is driving me nutty. Any float value converted from a string is completely bogus. For instance, take my logfile for an example, as I logged the value to convert and its conversion result.

    Code:
    BEGIN LOG -- 10/26/2007 9:12:31
    Float encountered.  S = 0.9; V = 0.8999999762;
    Float encountered.  S = 0.8; V = 0.8000000119;
    END LOG -- 10/26/2007 9:12:35
    I know those round to the values I want, it is a float to single assignment if that does anything, but the deviation makes me wonder. I'm using StrToFloat so if there's any problem with StrToFloat then it will affect any user of FreePascal 2.2.0 ... and yet I've seen no such bug reports.

    Any ideas on what might've gone wrong on my system?

    parser.exe (my experiment)
    [pascal]program parser;

    uses SysUtils, Classes, gDebug;

    var AParser: TParser;
    FS: TFileStream;

    type
    TArmorFlag = (afImpactResist, afSlashResist, afElecVuln);
    TArmorFlags = set of TArmorFlag;
    TBodyPart = (bpTorso, bpHead);

    const
    BodyParts: array[TBodyPart ] of string = ('torso','head');
    ArmorFlags: array[TArmorFlag] of string = ('impact_resist','slash_resist','elec_vuln');

    type
    PArmor = ^TArmor;
    TArmor = record
    // NAME:
    single, plural: string;
    // DESC:
    desc: string;
    // VALUE, WEIGHT (OZ):
    value, weight: word;
    // MITIGATION (REDUCTION):
    mitigation: single;
    // BODYPART:
    part: TBodyPart;
    // FLAGS:
    flags: TArmorFlags;
    end;

    var
    Armory: array of TArmor;
    Pos: Integer = -1;

    Tok: Char;
    tempS: string;
    i: integer;

    iP: TBodyPart;
    aF: TArmorFlag;

    BEGIN
    writeln;
    writeln('GameLib TParser Experiment');
    FS := TFileStream.Create('armor.txt', fmOpenRead);
    AParser := TParser.Create(FS);

    writeln;
    write('Generating objects.');

    Tok := AParser.Token;
    while Tok <> toEOF do begin
    write('.');
    tempS := AParser.TokenString;
    if tempS = 'armor' then begin
    Pos += 1;
    SetLength(Armory, Pos+1);
    end else if tempS = 'name' then begin
    Tok := AParser.NextToken;
    if Tok = toString then begin
    Armory[Pos].single := AParser.TokenString;
    end else
    AParser.CheckToken(toString);
    end else if tempS = 'plural' then begin
    Tok := AParser.NextToken;
    if Tok = toString then begin
    Armory[Pos].plural := AParser.TokenString;
    end else
    AParser.CheckToken(toString);
    end else if tempS = 'desc' then begin
    Tok := AParser.NextToken;
    if Tok = toString then begin
    Armory[Pos].desc := AParser.TokenString;
    end else
    AParser.CheckToken(toString);
    end else if tempS = 'value' then begin
    Tok := AParser.NextToken;
    if Tok = toInteger then begin
    Armory[Pos].value := AParser.TokenInt;
    end else
    AParser.CheckToken(toInteger);
    end else if tempS = 'weight' then begin
    Tok := AParser.NextToken;
    if Tok = toInteger then begin
    Armory[Pos].weight := AParser.TokenInt;
    end else
    AParser.CheckToken(toInteger);
    end else if tempS = 'mitigation' then begin
    Tok := AParser.NextToken;
    if Tok = toFloat then begin
    Armory[Pos].mitigation := StrToFloat(AParser.TokenString);
    Log('Float encountered. S = '+AParser.TokenString+'; V = '+FloatToStr(Armory[Pos].mitigation)+';');
    end else
    AParser.CheckToken(toFloat);
    end else if tempS = 'bodypart' then begin
    Tok := AParser.NextToken;
    if Tok = toSymbol then begin
    tempS := AParser.TokenString;
    for iP := low(TBodyPart) to high(TBodyPart) do
    if tempS = BodyParts[iP] then begin
    Armory[Pos].part := iP;
    break;
    end;
    end else
    AParser.CheckToken(toSymbol);
    end else if tempS = 'flags' then begin
    Tok := AParser.NextToken;
    while Tok = toSymbol do begin
    tempS := AParser.TokenString;
    if tempS = 'done' then break;
    for aF := low(TArmorFlag) to high(TArmorFlag) do
    if ArmorFlags[aF] = tempS then
    include(Armory[pos].flags, aF);
    Tok := AParser.NextToken;
    end;
    end else
    AParser.Error('Unrecognized object/token: '+tempS);
    Tok := AParser.NextToken;
    end;
    writeln(' Completed.');

    for i := low(Armory) to high(Armory) do begin
    writeln('Armor #',i+1,' {');
    writeln('Singular: ', Armory[i].single);
    writeln('Plural: ', Armory[i].plural);
    writeln('Description: ', Armory[i].desc);
    writeln('Value: $', Armory[i].value);
    writeln('Weight: ', Armory[i].weight);
    writeln('Mitigation: ', FloatToStr(Armory[i].mitigation));
    writeln('Bodypart: ', BodyParts[Armory[i].part]);
    write ('Flags: ');
    for aF := low(TArmorFlag) to high(TArmorFlag) do
    if aF in Armory[Pos].flags then
    write(ArmorFlags[aF],' ');
    writeln;
    writeln('}');
    writeln;
    end;

    readln;
    AParser.Free;
    FS.Free;
    END.[/pascal]
    armor.txt (the datafile)
    Code:
    armor
    name		'bronzium plate'
    plural		'bronzium plating'
    desc		'The key behind bronzium is the polymer chain, of bronze itself, that transforms ordinary bronze into a lattice-like structure that is exponentially tougher.  This change is costly and takes great skill, but produces the greatest of armors able to resist even the finest steel.'
    value		25000
    weight		208
    mitigation	0.9
    bodypart	torso
    flags		impact_resist slash_resist elec_vuln
    done
    
    armor
    name		'bronzium helmet'
    plural		'bronzium helmets'
    desc		'The key behind bronzium is the polymer chain, of bronze itself, that transforms ordinary bronze into a lattice-like structure that is exponentially tougher.  This change is costly and takes great skill, but produces the greatest of armors able to resist even the finest steel.'
    value		17500
    weight		80
    mitigation	0.8
    bodypart	head
    flags		slash_resist elec_vuln
    done

  2. #2

    [FPC] Bug in non-Extended floating types

    I wrote myself a custom StrToSingle function using Val, the same thing used for the StrToFloat conversions, and got the exact same result. Honestly I really don't understand how this is happening. As both functions use Val this implicates FPC as having a potentially bad bug, especially where accuracy is required in these routines.

    I hope that somewhere it's my fault; that's so much simpler to fix! :?

    Code:
    function StrToSingle&#40;S&#58; String&#41;&#58; Single;
    var e&#58; Integer;
        f&#58; Single;
    begin
      Val&#40;s,f,e&#41;;
      if e > 0 then
        LogCritical&#40;'Error in StrToSingle!  STR = "'+S+'" Error at char #'+IntToStr&#40;E&#41;&#41;
      else
        StrToSingle &#58;= f;
    end;

  3. #3

    [FPC] Bug in non-Extended floating types

    This is bizarre. I branched into a separate project and created my own function to convert a string into a float without the Val code. And lo and behold, both functions worked! I then glanced and realized that I used an Extended variable here, and not a Single size variable.

    I expanded my test to cover the four floating point types I'm familiar with. Real, Extended, Single, and Double. The results are far less than satisfactory, and now I'm certain there's a bug somewhere in floating-point types that aren't Extended. See it, compile it, and try it for yourself. Even floating point comparisons and expansions appear flawed right now.

    My results:
    Code:
    Float Debugger
    
    Value = 0.9
    Custom converter results&#58; true
    Default converter results&#58;
      Single   -> false&#40;0.8999999762&#41;
      Double   -> false&#40;0.9&#41;
      Real     -> false&#40;0.9&#41;
      Extended -> true &#40;0.9&#41;
    Code:
    program floattest;
    
    uses SysUtils, StrUtils, Math;
    
    procedure CutStr&#40;const s, delim&#58; string; var s1,s2&#58; string&#41;;
    var i&#58; integer;
        flag&#58; boolean;
    begin
      if length&#40;s&#41; = 0 then
        exit;
    
      i &#58;= 1;
      flag &#58;= false;
      s1 &#58;= '';
      s2 &#58;= '';
      while i <= length&#40;s&#41; do begin
        if s&#91;i&#93; = '.' then begin
          flag &#58;= true;
          i += 1;
        end;
        if flag then
          s2 &#58;= s2+s&#91;i&#93;
        else
          s1 &#58;= s1+s&#91;i&#93;;
        i += 1;
      end;
    end;
    
    function CustStrToFloat&#40;S&#58; String&#41;&#58; Extended;
    var middle&#58; integer;
        whole,part&#58; string;
    begin
      middle &#58;= Pos&#40;'.',S&#41;;
      if middle <= 0 then
        Exit&#40;StrToInt&#40;S&#41;&#41;
      else begin
        CutStr&#40;s,'.',whole,part&#41;;
        exit&#40;StrToInt&#40;whole&#41; + StrToInt&#40;part&#41;/power&#40;10,Length&#40;part&#41;&#41;&#41;;
      end;
    end;
    
    var
      f1&#58; single;
      f2&#58; double;
      f3&#58; real;
      f4&#58; extended;
    
    BEGIN
      writeln&#40;'Float Debugger'&#41;;
      writeln;
      writeln&#40;'Value = 0.9'&#41;;
      write&#40;'Custom converter results&#58; '&#41;;
      if CustStrToFloat&#40;'0.9'&#41; = 0.9 then
        writeln&#40;'true'&#41;
      else
        writeln&#40;'false'&#41;;
      writeln&#40;'Default converter results&#58;'&#41;;
      f1 &#58;= StrToFloat&#40;'0.9'&#41;;
      f2 &#58;= StrToFloat&#40;'0.9'&#41;;
      f3 &#58;= StrToFloat&#40;'0.9'&#41;;
      f4 &#58;= StrToFloat&#40;'0.9'&#41;;
      write&#40;'  Single   ->'&#41;;
      if f1 = 0.9 then
        writeln&#40;' true &#40;',FloatToStr&#40;f1&#41;,'&#41;'&#41;
      else
        writeln&#40;' false&#40;',FloatToStr&#40;f1&#41;,'&#41;'&#41;;
      write&#40;'  Double   ->'&#41;;
      if f2 = 0.9 then
        writeln&#40;' true &#40;',FloatToStr&#40;f2&#41;,'&#41;'&#41;
      else
        writeln&#40;' false&#40;',FloatToStr&#40;f2&#41;,'&#41;'&#41;;
      write&#40;'  Real     ->'&#41;;
      if f3 = 0.9 then
        writeln&#40;' true &#40;',FloatToStr&#40;f3&#41;,'&#41;'&#41;
      else
        writeln&#40;' false&#40;',FloatToStr&#40;f3&#41;,'&#41;'&#41;;
      write&#40;'  Extended ->'&#41;;
      if f4 = 0.9 then
        writeln&#40;' true &#40;',FloatToStr&#40;f4&#41;,'&#41;'&#41;
      else
        writeln&#40;' false&#40;',FloatToStr&#40;f4&#41;,'&#41;'&#41;;
      readln;
    END.

  4. #4

    [FPC] Bug in non-Extended floating types

    Submitted to FPC Bugtracker as issue 10024.

    Anyone who can reproduce this problem with FPC 2.2.0, the current public build, please post here and confirm this. Or, if you have a FPC bugtracker account, you can post confirmation over at FPC's bugtracker. Please help track this bug down and kill it!

  5. #5

    [FPC] Bug in non-Extended floating types

    Robert, did you test this in Delphi? I think Delphi has the same "bug".
    Some numbers cannot be represented in floating-point form precisely. So some approximation used.

  6. #6

    [FPC] Bug in non-Extended floating types

    Yes, someone else did actually.

    The problem is improper conversion of numbers. Just force a typecast of any constant numbers into the type they're being compared to--this includes casual constants that aren't named. As soon as you do this the problem vanishes faster than a goblin in a lava pit.

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