I try to write a kind of object/record serializer with Delphi 2010 and wonder if there is a way to detect, if a record is a variant record. E.g. the TRect record as defined in Types.pas:
TRect = record
case Integer of
0: (Left, Top, Right, Bottom: Longint);
1: (TopLeft, BottomRight: TPoint);
end;
As my serializer should work recursively on my data structures, it will descent on the TPoint records and generate redundant information in my serialized file. Is there a way to avoid this, by getting detailed information on the record?
One solution could be as follows:
procedure SerializeRecord (RttiRecord : TRttiRecord)
var
AField : TRttiField;
Offset : Integer;
begin
Offset := 0;
for AField in RttiRecord.Fields do
begin
if AField.Offset < Offset then Exit;
Offset := AField.Offset; //store last offset
SerializeField (AField);
end;
end;
But this solution is not a proper solution for all cases. It only works for serialization, if the different variants contain the same information and the same types. If you have something like the following (from wikipedia.org):
type
TVarRec = packed record
case Byte of
0: (FByte: Byte;
FDouble: Double);
1: (FStr: ShortString);
end;
Would you serialize
FByte=6
FDouble=1.81630607010916E-0310
or would it be better to serialize
FStr=Hello!
Yes, for sure, this would also be the same for a computer but not for a file which should be readable or even editable for humans.
So I think, the only way to solve the problem is using an Attribute, to define, which variant should be used for serialization.
Related
Consider the following record:
TMyRecord = record
b: Boolean;
// 3 bytes of padding in here with default record alignment settings
i: Integer;
end;
I wish to implement IEqualityComparer<TMyRecord>. In order to do so I want to call TEqualityComparer<TMyRecord>.Construct. This needs to be supplied with a TEqualityComparison<TMyRecord> which presents no problems to me.
However, Construct also requires a THasher<TMyRecord> and I would like to know the canonical method for implementing that. The function needs to have the following form:
function MyRecordHasher(const Value: TMyRecord): Integer;
begin
Result := ???
end;
I expect that I need to call BobJenkinsHash on both fields of the record value and then combine them some how. Is this the right approach, and how should I combine them?
The reason I don't use TEqualityComparison<TMyRecord>.Default is that it uses CompareMem and so will be incorrect due to the record's padding.
The Effective Java (by Joshua Bloch) section about overriding hashCode could be useful. It shows how the individual parts of the object (or record) can be combined to efficiently construct a hashCode.
A good hash function tends to produce unequal hash codes for unequal
objects. This is exactly what is meant by the third provision of the
hashCode contract. Ideally, a hash function should distribute any
reasonable collection of unequal instances uniformly across all
possible hash values. Achieving this ideal can be extremely difficult.
Luckily it is not too difficult to achieve a fair approximation. Here
is a simple recipe:
Store some constant nonzero value, say 17, in an int variable called result.
For each significant field f in your object (each field taken into account by the equals method, that is), do the following:
a. Compute an int hash code c for the field: ..... details omitted ....
b. Combine the hash code c computed in step a into
result as follows: result = 37*result + c;
Return result.
When you are done writing the hashCode method, ask yourself whether equal instances have equal hash codes. If not, figure out why
and fix the problem.
This can be translated into Delphi code as follows:
{$IFOPT Q+}
{$DEFINE OverflowChecksEnabled}
{$Q-}
{$ENDIF}
function CombinedHash(const Values: array of Integer): Integer;
var
Value: Integer;
begin
Result := 17;
for Value in Values do begin
Result := Result*37 + Value;
end;
end;
{$IFDEF OverflowChecksEnabled}
{$Q+}
{$ENDIF}
This then allows the implementation of MyRecordHasher:
function MyRecordHasher(const Value: TMyRecord): Integer;
begin
Result := CombinedHash([IfThen(Value.b, 0, 1), Value.i]);
end;
Good day,
TValue is a Delphi-2010 and up RTTI feature.
Following on from my earlier question, I had tried to make recurrent function to return a TValue as a n-dimensional. matrix(2D, 3D, 4D...)
for example, this procedure will show a n-dimensional matrix(it will list all elements from a n-dimensional matrix as TValue variable):
Procedure Show(X:TValue);
var i:integer;
begin
if x.IsArray then
begin
for i:=0 to x.GetArrayLength-1 do
show(x.GetArrayElement(i));
writeln;
end else
write(x.ToString,' ');
end;
I don't understand how to create a function to create from a TValue an n-dimensional matrix. For example i need a Function CreateDynArray(Dimensions:array of integer; Kind:TTypeKind):TValue; and the function will return a TValue which is a dynamic array how contain the dimenssions for example:
Return=CreateDynArray([2,3],tkInteger); will return a TValue as tkDynArray
and if i will show(Return) will list
0 0 0
0 0 0
Is not terminated.
From a TValue i try to create a DynArray with n-dimensions
Procedure CreateArray(var Value:TValue; NewDimmension:integer; NewValue2Kind:TTypeKind; NewValue2:TValue; IsLast:Boolean);
var i:integer;
NewValue:TValue;
len:Longint;
begin
If Value.IsArray then// we have components in this dimension
begin
for i:=0 to Value.GetArrayLength-1 do// list all
begin
NewValue:=Value.GetArrayElement[i];
CreateArray(newValue,NewDimension,NewValue2Kind,NewValue2,IsLast);
Value.SetArrayElement(i,NewValue);
end;
end;
end else
begin
if isLast then
begin
len:=NewDimension;
DynArraySetLength(PPointer(Value.GetRefereneToRawData)^,Value.TypeInfo,1,#len); //set length to NewDimension
for i:=0 to NewDimension-1 do //Fill all with 0
Value.SetArrayElement(i,NewValue2);
end else
begin
len:=NewDimension;
DynArraySetLength(PPointer(Value.GetRefereneToRawData)^,Value.TypeInfo,1,#len);//I will create len TValues
end;
end;
var Index:array of integer;
Value:TValue;
ValuKind:TTypeKind;
......
......
....
Case token of
tokInt:
begin
ValueKind:=tkInteger;
Value:=0;
end;
.....
end;
Index:=GetIndexFromSintacticTree;//for example if i have int[20][30] index=[20,30]
for i:=0 to high(index) do
begin
if i = high(index) then CreateArray(Variable.Value,Index[i],ValueKind,Value,True)
else CreateArray(Variable.Value,Index[i],ValueKind,Value,False)
//Variable.Value is TValue
end;
//first TValue have 1 element, after that it will have 20 elements, and after that will have 20*30 elements
Thank you very much, and have a nice day!
To create a dynamic array dynamically, you need a reference to its type info structure (PTypeInfo) to pass to DynArraySetLength; calling DynArraySetLength and passing a reference to a nil pointer is how you can create a new dynamic array. If the specific shape of dynamic array does not already exist in your Delphi program, there will be no specific PTypeInfo pointer that the compiler will generate for you. In this case, you would have to generate the corresponding PTypeInfo data structure yourself. This is possible, though tedious.
Frankly, I'd recommend you use a different structure than built-in Delphi dynamic arrays to represent arrays in your scripting language-like problem. In the long run it will probably be a lot less work than trying to dynamically generate the low-level RTTI data, which is more likely to change from version to version now that it has a much higher level abstraction in the Rtti unit.
I'm implementing a N x M matrix (class) with a record and an internal dynamic array like below.
TMat = record
public
// contents
_Elem: array of array of Double;
//
procedure SetSize(Row, Col: Integer);
procedure Add(const M: TMat);
procedure Subtract(const M: TMat);
function Multiply(const M: TMat): TMat;
//..
class operator Add(A, B: TMat): TMat;
class operator Subtract(A, B: TMat): TMat;
//..
class operator Implicit(A: TMat): TMat; // call assign inside proc.
// <--Self Implicit(which isn't be used in D2007, got compilation error in DelphiXE)
procedure Assign(const M: TMat); // copy _Elem inside proc.
// <-- I don't want to use it explicitly.
end;
I choose a record, because I don't want to Create/Free/Assign to use it.
But with dynamic array, values can't be (deep-)copied with M1 := M2, instead of M1.Assign(M2).
I tried to declare self-Implicit conversion method, but it can't be used for M1:=M2.
(Implicit(const pA: PMat): TMat and M1:=#M2 works, but it's pretty ugly and unreadable..)
Is there any way to hook assignment of record ?
Or is there any suggestion to implement N x M matrix with records ?
Thanks in advance.
Edit:
I implemented like below with Barry's method and confirmed working properly.
type
TDDArray = array of array of Double;
TMat = record
private
procedure CopyElementsIfOthersRefer;
public
_Elem: TDDArray;
_FRefCounter: IInterface;
..
end;
procedure TMat.SetSize(const RowSize, ColSize: Integer);
begin
SetLength(_Elem, RowSize, ColSize);
if not Assigned(_FRefCounter) then
_FRefCounter := TInterfacedObject.Create;
end;
procedure TMat.Assign(const Source: TMat);
var
I: Integer;
SrcElem: TDDArray;
begin
SrcElem := Source._Elem; // Allows self assign
SetLength(Self._Elem, 0, 0);
SetLength(Self._Elem, Length(SrcElem));
for I := 0 to Length(SrcElem) - 1 do
begin
SetLength(Self._Elem[I], Length(SrcElem[I]));
Self._Elem[I] := Copy(SrcElem[I]);
end;
end;
procedure TMat.CopyElementsIfOthersRefer;
begin
if (_FRefCounter as TInterfacedObject).RefCount > 1 then
begin
Self.Assign(Self); // Self Copy
end;
end;
I agree it's not efficient. Just using Assign with pure record is absolutely faster.
But it's pretty handy and more readable.(and interesting. :-)
I think it's useful for light calculation or pre-production prototyping. Isn't it ?
Edit2:
kibab gives function getting reference count of dynamic array itself.
Barry's solution is more independent from internal impl, and perhaps works on upcoming 64bit compilers without any modification, but in this case, I prefer kibab's for it's simplicity & efficiency. Thanks.
TMat = record
private
procedure CopyElementsIfOthersRefer;
public
_Elem: TDDArray;
..
end;
procedure TMat.SetSize(const RowSize, ColSize: Integer);
begin
SetLength(_Elem, RowSize, ColSize);
end;
function GetDynArrayRefCnt(const ADynArray): Longword;
begin
if Pointer(ADynArray) = nil then
Result := 1 {or 0, depending what you need}
else
Result := PLongword(Longword(ADynArray) - 8)^;
end;
procedure TMat.CopyElementsIfOthersRefer;
begin
if GetDynArrayRefCnt(_Elem) > 1 then
Self.Assign(Self);
end;
You can use an interface field reference inside your record to figure out if your array is shared by more than one record: simply check the reference count on the object behind the interface, and you'll know that the data in the arrays is shared. That way, you can lazily copy on modification, but still use data sharing when the matrices aren't being modified.
You can't override record assignment by Implicit or Explicit operators.
The best you can do IMO is not to use direct assignment, using M.Assign method instead:
procedure TMat.Assign(const M: TMat);
begin
// "Copy" currently only copies the first dimension,
// bug report is open - see comment by kibab
// _Elem:= Copy(M._Elem);
..
end;
ex
M1.Assign(M2);
instead of
M1:= M2;
I've just realised a reason why this may not be such a great idea. It's true that the calling code becomes much simpler with operator overloading. But you may have performance problems.
Consider, for example, the simple code A := A+B; and suppose you use the idea in Barry's accepted answer. Using operator overloading this simple operation will result in a new dynamic array being allocated. In reality you would want to perform this operation in place.
Such in-place operations are very common in linear algebra matrix algorithms for the simple reason that you don't want to hit the heap if you can avoid it - it's expensive.
For small value types (e.g. complex numbers, 3x3 matrices etc.) then operator overloading inside records is efficient, but I think, if performance matters, then for large matrices operator overloading is not the best solution.
I implemented language translation in an application by putting all strings at runtime in a TStringList with:
procedure PopulateStringList;
begin
EnglishStringList.Append('CAN_T_FIND_FILE=It is not possible to find the file');
EnglishStringList.Append('DUMMY=Just a dummy record');
// total of 2000 record appended in the same way
EnglishStringList.Sorted := True; // Updated comment: this is USELESS!
end;
Then I get the translation using:
function GetTranslation(ResStr:String):String;
var
iIndex : Integer;
begin
iIndex := -1;
iIndex := EnglishStringList.IndexOfName(ResStr);
if iIndex >= 0 then
Result := EnglishStringList.ValueFromIndex[iIndex] else
Result := ResStr + ' (Translation N/A)';
end;
Anyway with this approach it takes about 30 microseconds to locate a record, is there a better way to achieve the same result?
UPDATE: For future reference I write here the new implementation that uses TDictionary as suggested (works with Delphi 2009 and newer):
procedure PopulateStringList;
begin
EnglishDictionary := TDictionary<String, String>.Create;
EnglishDictionary.Add('CAN_T_FIND_FILE','It is not possible to find the file');
EnglishDictionary.Add('DUMMY','Just a dummy record');
// total of 2000 record appended in the same way
end;
function GetTranslation(ResStr:String):String;
var
ValueFound: Boolean;
begin
ValueFound:= EnglishDictionary.TryGetValue(ResStr, Result);
if not ValueFound then Result := Result + '(Trans N/A)';
end;
The new GetTranslation function performs 1000 times faster (on my 2000 sample records) then the first version.
THashedStringList should be better, I think.
In Delphi 2009 or later I would use TDictionary< string,string > in Generics.Collections.
Also note that there are free tools such as http://dxgettext.po.dk/ for translating applications.
If THashedStringList works for you, that's great. Its biggest weakness is that every time you change the contents of the list, the Hash table is rebuilt. So it will work for you as long as your list remains small or doesn't change very often.
For more info on this, see: THashedStringList weakness, which gives a few alternatives.
If you have a big list that may be updated, you might want to try GpStringHash by gabr, that doesn't have to recompute the whole table at every change.
I think that you don't use the EnglishStringList(TStringList) correctly. This is a sorted list, you add elements (strings), you sort it, but when you search, you do this by a partial string (only the name, with IndexOfName).
If you use IndexOfName in a sorted list, the TStringList can't use Dicotomic search. It use sequential search.
(this is the implementation of IndexOfName)
for Result := 0 to GetCount - 1 do
begin
S := Get(Result);
P := AnsiPos('=', S);
if (P <> 0) and (CompareStrings(Copy(S, 1, P - 1), Name) = 0) then Exit;
end;
I think that this is the reason of poor performance.
The alternative is use 2 TStringList:
* The first (sorted) only containts the "Name" and a pointer to the second list that contain the value; You can implement this pointer to the second list using the "pointer" of Object property.
* The second (not sorted) list containt the values.
When you search, you do it at first list; In this case you can use the Find method. when you find the name, the pointer (implemented with Object property) give you the position on second list with the value.
In this case, Find method on Sorted List is more efficient that HashList (that must execute a funcion to get the position of a value).
Regards.
Pd:Excuse-me for mistakes with english.
You can also use a CLASS HELPER to re-program the "IndexOfName" function:
TYPE
TStringsHelper = CLASS HELPER FOR TStrings
FUNCTION IndexOfName(CONST Name : STRING) : INTEGER;
END;
FUNCTION TStringsHelper.IndexOfName(CONST Name : STRING) : INTEGER;
VAR
SL : TStringList ABSOLUTE Self;
S,T : STRING;
I : INTEGER;
BEGIN
IF (Self IS TStringList) AND SL.Sorted THEN BEGIN
S:=Name+NameValueSeparator;
IF SL.Find(S,I) THEN
Result:=I
ELSE IF (I<0) OR (I>=Count) THEN
Result:=-1
ELSE BEGIN
T:=SL[I];
IF CompareStrings(COPY(T,1,LENGTH(S)),S)=0 THEN Result:=I ELSE Result:=-1
END;
EXIT
END;
Result:=INHERITED IndexOfName(Name)
END;
(or implement it in a descendant TStrings class if you dislike CLASS HELPERs or don't have them in your Delphi version).
This will use a binary search on a sorted TStringList and a sequential search on other TStrings classes.
I have a record that holds data about a file:
TYPE
RFile= record
public
FileName : string;
Resolution : Integer;
FileSize : Cardinal;
Rating : Byte;
end;
PFile= ^RFile;
And I keep a list of these files/records in a TList<>
TFileList= class(TList<PFile>)
procedure SortByFilename;
procedure SortByRating;
procedure SortByResolution;
procedure SortBySize;
end;
I have methods like SortByFilename, SortBySize, etc in which I sort the list.
I do "classic" sorting.
Now I want to upgrade to the new-and-cool System.Generics.Defaults.TComparer.
From what I understand I need to assign a comparer to my TFileList, like
TIntStringComparer = class(TComparer<String>)
public
function Compare(const Left, Right: String): Integer; override;
end;
How do I do this?
How do I deal with one comparer for each data field (filename, filesize, resolution)?
Update:
This code compiles but I have an EIntegerOverflow because the FileSize is a cardinal while I return an integer (diff between two cardinals).
Sort(TComparer<PFile>.Construct(
function(CONST A,B: PFile): integer
begin
Result:= A.FileSize - B.FileSize;
end
));
When you write a comparer for a numeric type, you should never use subtraction, even if your data type is signed.
Indeed, try to compare a = 100 and b = -2147483640 as Integers; clearly a > b, but subtraction will yield the wrong result.
Instead, you should always do something similar to
if a = b then
Result := 0
else if a < b then
Result := -1
else
Result := 1;
But Delphi's RTL already contains functions for this: there are several CompareValue overloads in the Math unit (for different types of integers and floats -- but, unfortunately, not for Cardinals).
Thus, although your snippet will work "most of the time" if you do
Result := Integer(A.FileSize) - Integer(B.FileSize)
this is not good enough: For one thing, not every Cardinal will fit in a Integer. Also, as noted above, subtraction is not the way to go.
In your case, you can simply use the if thing above directly, or you could create a new CompareValue overload for Cardinals. Or, you could do
Result := CompareValue(Int64(A.FileSize), Int64(B.FileSize)).
(Also, as others have stated in comments, you should reconsider if it is wise to use a Cardinal to store a file size in the first place. If you upgrade this to an Int64 or UInt64 you can write simply
Result := CompareValue(A.FileSize, B.FileSize).)