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I have come up with this function to return the number of occurrences of a string in a Delphi Stream. However, I suspect there is a more efficient way to achieve this, since I am using "higher level" constructs (char), and not working at the lower byte/pointer level (which I am not that familiar with)
function ReadStream(const S: AnsiString; Stream: TMemoryStream): Integer;
var
Arr: Array of AnsiChar;
Buf: AnsiChar;
ReadCount: Integer;
procedure AddChar(const C: AnsiChar);
var
I: Integer;
begin
for I := 1 to Length(S) - 1 do
Arr[I] := Arr[I+1];
Arr[Length(S)] := C;
end;
function IsEqual: Boolean;
var
I: Integer;
begin
Result := True;
for I := 1 to Length(S) do
if S[I] <> Arr[I] then
begin
Result := False;
Break;;
end;
end;
begin
Stream.Position := 0;
SetLength(Arr, Length(S));
Result := 0;
repeat
ReadCount := Stream.Read(Buf, 1);
AddChar(Buf);
if IsEqual then
Inc(Result);
until ReadCount = 0;
end;
Can someone supply a procedure that is more efficient?
Stream has a method that will let you get into the internal buffer.
You can get a pointer to the internal buffer using the Memory property.
If you are working in 32 bit and you are willing to let go of the deprecated TMemoryStream and use TBytesStream instead you can use abuse the fact that a dynamic array and an AnsiString share the same structure in 32 bit.
Unfortunately Emba broke that compatibility in X64, Which means that for no good reason whatsoever you cannot have strings > 2GB in X64.
Note that this trick will break in 64 bit! (See fix below)
You can use Boyer-Moore string searching.
This allows you to write code like this:
function CountOccurrances(const Needle: AnsiString; const Haystack: TBytesStream): integer;
var
Start: cardinal;
Count: integer;
begin
Start:= 1;
Count:= 0;
repeat
{$ifdef CPUx86}
Start:= _FindStringBoyerAnsiString(string(HayStack.Memory), Needle, false, Start);
{$else}
Start:= __FindStringBoyerAnsiStringIn64BitTArrayByte(TArray<Byte>(HaySAtack.Memory), Needle, false, Start);
{$endif}
if Start >= 1 then begin
Inc(Start, Length(Needle));
Inc(Count);
end;
until Start <= 0;
Result:= Count;
end;
For 32 bit you'll have to rewrite the BoyerMoore code to use AnsiString; a trivial rewrite.
For 64 bit you'll have to rewrite the BoyerMoore code to use a TArray<byte> as a first parameter; a relatively simple task.
If you are looking for efficiency, please try and avoid WinAPI calls that use pchars. c-style strings are a horrible idea, because they do not have a length prefix.
Johan has given you a good answer about Boyer-Moore searching. BM is fine if
your are content to use it as a "black box", but if you want to understand what's going on,
there is a bit of a gulf between the complexity of your own code and a BM implementation.
You might find it helpful to explore searching that's more efficient than your own code
but not so complex as BM. There is one ultra-simple way to do what you want without
getting invoved with pointers, PChars, etc.
Let's leave aside for a moment the fact that you want to work with a TMemoryStream, and
consider finding the number of occurrences of a string SubStr in another string Target.
For efficiency, things you want to avoid are a) repeatedly scanning the same characters
over and over and b) copying one or both strings.
Since D7, Delphi has included a PosEx function:
function PosEx(const SubStr, S: string; Offset: Cardinal = 1): Integer;
Description
PosEx returns the index of SubStr in S, beginning the search at Offset. If Offset is 1 (default), PosEx is equivalent to Pos.
PosEx returns 0 if SubStr is not found, if Offset is greater than the length of S, or if Offset is less than 1.
So what you can do is repeatedly call PosEx, starting with Offset = 1, and each time it
finds SubStr in Target you increment Offset to skip over it, like this (in a console application):
function ContainsCount(const SubStr, Target : String) : Integer;
var
i : Integer;
begin
Result := 0;
i := 1;
repeat
i := PosEx(SubStr, Target, i);
if i > 0 then begin
Inc(Result);
i := i + Length(SubStr);
end;
until i <= 0;
end;
var
Count : Integer;
Target : String;
begin
Target := 'aa b ca';
Count := ContainsCount('a', Target);
writeln(Count);
readln;
end.
The fact that PosEx and ContainsCount both pass SubStr and Target as
consts meants that no string copying is involved, and it should be obvious
that ContainsCount never scans the same characters more that once.
Once you've satisfied yourself that this works, you might care to trace
into PosEx to see how it does its stuff.
You can do something which works in a similar way on PChars using the RTL functions StrPos/AnsiStrPos
To convert your memory stream to a string, you could use this code from
Rob Kennedy's answer to this q Converting TMemoryStream to 'String' in Delphi 2009
function MemoryStreamToString(M: TMemoryStream): string;
begin
SetString(Result, PChar(M.Memory), M.Size div SizeOf(Char));
end;
(Note what he says about the alternative version later in his answer)
Btw, if you look through the VCL + RTL code, you'll see that quite a lot of the string-parsing and processing code (e.g. in TParser, TStringList, TExpressionParser) all does its work with PChars. There's a reason for that of course, because it minimizes character copying and means that most scanning operations can be done by changing pointer (PChar) values.
I'm want to convert a string value to a global memory handle and vice versa, using the following functions I've just written.
But StrToGlobalHandle() causes my testing program hangs. So GlobalHandleToStr() is untest-able yet and I'm also wondering if my code is logical or not.
function StrToGlobalHandle(const aText: string): HGLOBAL;
var
ptr: PChar;
begin
Result := 0;
if aText <> '' then
begin
Result := GlobalAlloc(GMEM_MOVEABLE or GMEM_ZEROINIT, length(aText) + 1);
if Result <> 0 then
begin
ptr := GlobalLock(Result);
if Assigned(ptr) then
begin
StrCopy(ptr, PChar(aText));
GlobalUnlock(Result);
end
end;
end;
end;
function GlobalHandleToStr(const aHandle: HGLOBAL): string;
var
ptrSrc: PChar;
begin
ptrSrc := GlobalLock(aHandle);
if Assigned(ptrSrc) then
begin
SetLength(Result, Length(ptrSrc));
StrCopy(PChar(Result), ptrSrc);
GlobalUnlock(aHandle);
end
end;
Testing code:
procedure TForm3.Button1Click(Sender: TObject);
var
h: HGLOBAL;
s: string;
s2: string;
begin
s := 'this is a test string';
h := StrToGlobalHandle(s);
s2 := GlobalHandleToStr(h);
ShowMessage(s2);
GlobalFree(h);
end;
BTW, I want to use these two functions as helpers to send string values between programs - send a global handle from process A to process B, and process B get the string using GlobalHandleToStr().
BTW 2, I know WM_COPY and other IPC methods, those are not suitable in my case.
The strings in Delphi 2010 are unicode, so you are not allocating the proper buffer size.
replace this line
Result := GlobalAlloc(GMEM_MOVEABLE or GMEM_ZEROINIT, length(aText) + 1);
with this
Result := GlobalAlloc(GMEM_MOVEABLE or GMEM_ZEROINIT, (length(aText) + 1)* SizeOf(Char));
If your program hangs when you call GlobalAlloc, then you probably have heap corruption from earlier in your program. That leads to undefined behavior; the function might detect the problem and return an error, it might crash your program, it might silently corrupt yet more of your memory, it might hang, or it might do any number of other things.
That heap corruption might come from a previous call to StrToGlobalHandle because your StrCopy call writes beyond the end of the allocated memory. You're allocating bytes, but the Length function returns the number of characters in the string. That's only valid when characters are one byte wide, which isn't the case as of Delphi 2009. Multiply by SizeOf(Char) to get a byte count:
Result := GlobalAlloc(GMEM_MOVEABLE or GMEM_ZEROINIT, SizeOf(Char) * (Length(aText) + 1));
You can't send data between programs using GlobalAlloc - it worked only in 16-bit Windows. Use Memory Mapped File instead.
In Delphi you can speed up your code by passing parameters as const, e.g.
function A(const AStr: string): integer;
//or
function B(AStr: string): integer;
Suppose both functions have the same code inside, the speed difference between them is negligible and I doubt it can even be measured with a cycle-counter like:
function RDTSC: comp;
var
TimeStamp: record case byte of
1: (Whole: comp);
2: (Lo, Hi: Longint);
end;
begin
asm
db $0F; db $31;
mov [TimeStamp.Lo], eax
mov [TimeStamp.Hi], edx
end;
Result := TimeStamp.Whole;
end;
The reason for this is that all the const does in function A is to prevent the reference count of AStr to be incremented.
But the increment only takes one cycle of one core of my multicore CPU, so...
Why should I bother with const?
If there is no other reason for the function to contain an implicit try/finally, and the function itself is not doing much work, the use of const can result in a significant speedup (I once got one function that was using >10% of total runtime in a profiling run down to <2% just by adding a const in the right place).
Also, the reference counting takes much much more than one cycle because it has to be performed with the lock prefix for threadsafety reasons, so we are talking more like 50-100 cycles. More if something in the same cache line has been modified by another core in between.
As for not being able to measure it:
program Project;
{$APPTYPE CONSOLE}
uses
Windows,
SysUtils,
Math;
function GetThreadTime: Int64;
var
CreationTime, ExitTime, KernelTime, UserTime: TFileTime;
begin
GetThreadTimes(GetCurrentThread, CreationTime, ExitTime, KernelTime, UserTime);
Result := PInt64(#UserTime)^;
end;
function ConstLength(const s: string): Integer;
begin
Result := Length(s);
end;
function NoConstLength(s: string): Integer;
begin
Result := Length(s);
end;
var
s : string;
i : Integer;
j : Integer;
ConstTime, NoConstTime: Int64;
begin
try
// make sure we got an heap allocated string;
s := 'abc';
s := s + '123';
//make sure we minimize thread context switches during the timing
SetThreadPriority(GetCurrentThread, THREAD_PRIORITY_TIME_CRITICAL);
j := 0;
ConstTime := GetThreadTime;
for i := 0 to 100000000 do
Inc(j, ConstLength(s));
ConstTime := GetThreadTime - ConstTime;
j := 0;
NoConstTime := GetThreadTime;
for i := 0 to 100000000 do
Inc(j, NoConstLength(s));
NoConstTime := GetThreadTime - NoConstTime;
SetThreadPriority(GetCurrentThread, THREAD_PRIORITY_NORMAL);
WriteLn('Const: ', ConstTime);
WriteLn('NoConst: ', NoConstTime);
WriteLn('Const is ', (NoConstTime/ConstTime):2:2, ' times faster.');
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
if DebugHook <> 0 then
ReadLn;
end.
Produces this output on my system:
Const: 6084039
NoConst: 36192232
Const is 5.95 times faster.
EDIT: it gets a bit more interesting if we add some thread contention:
program Project;
{$APPTYPE CONSOLE}
uses
Windows,
SysUtils,
Classes,
Math;
function GetThreadTime: Int64;
var
CreationTime, ExitTime, KernelTime, UserTime: TFileTime;
begin
GetThreadTimes(GetCurrentThread, CreationTime, ExitTime, KernelTime, UserTime);
Result := PInt64(#UserTime)^;
end;
function ConstLength(const s: string): Integer;
begin
Result := Length(s);
end;
function NoConstLength(s: string): Integer;
begin
Result := Length(s);
end;
function LockedAdd(var Target: Integer; Value: Integer): Integer; register;
asm
mov ecx, eax
mov eax, edx
lock xadd [ecx], eax
add eax, edx
end;
var
x : Integer;
s : string;
ConstTime, NoConstTime: Integer;
StartEvent: THandle;
ActiveCount: Integer;
begin
try
// make sure we got an heap allocated string;
s := 'abc';
s := s + '123';
ConstTime := 0;
NoConstTime := 0;
StartEvent := CreateEvent(nil, True, False, '');
ActiveCount := 0;
for x := 0 to 2 do
TThread.CreateAnonymousThread(procedure
var
i : Integer;
j : Integer;
ThreadConstTime: Int64;
begin
//make sure we minimize thread context switches during the timing
SetThreadPriority(GetCurrentThread, THREAD_PRIORITY_HIGHEST);
InterlockedIncrement(ActiveCount);
WaitForSingleObject(StartEvent, INFINITE);
j := 0;
ThreadConstTime := GetThreadTime;
for i := 0 to 100000000 do
Inc(j, ConstLength(s));
ThreadConstTime := GetThreadTime - ThreadConstTime;
SetThreadPriority(GetCurrentThread, THREAD_PRIORITY_NORMAL);
LockedAdd(ConstTime, ThreadConstTime);
InterlockedDecrement(ActiveCount);
end).Start;
while ActiveCount < 3 do
Sleep(100);
SetEvent(StartEvent);
while ActiveCount > 0 do
Sleep(100);
WriteLn('Const: ', ConstTime);
ResetEvent(StartEvent);
for x := 0 to 2 do
TThread.CreateAnonymousThread(procedure
var
i : Integer;
j : Integer;
ThreadNoConstTime: Int64;
begin
//make sure we minimize thread context switches during the timing
SetThreadPriority(GetCurrentThread, THREAD_PRIORITY_HIGHEST);
InterlockedIncrement(ActiveCount);
WaitForSingleObject(StartEvent, INFINITE);
j := 0;
ThreadNoConstTime := GetThreadTime;
for i := 0 to 100000000 do
Inc(j, NoConstLength(s));
ThreadNoConstTime := GetThreadTime - ThreadNoConstTime;
SetThreadPriority(GetCurrentThread, THREAD_PRIORITY_NORMAL);
LockedAdd(NoConstTime, ThreadNoConstTime);
InterlockedDecrement(ActiveCount);
end).Start;
while ActiveCount < 3 do
Sleep(100);
SetEvent(StartEvent);
while ActiveCount > 0 do
Sleep(100);
WriteLn('NoConst: ', NoConstTime);
WriteLn('Const is ', (NoConstTime/ConstTime):2:2, ' times faster.');
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
if DebugHook <> 0 then
ReadLn;
end.
On a 6 core machine, this gives me:
Const: 19968128
NoConst: 1313528420
Const is 65.78 times faster.
EDIT2: replacing the call to Length with a call to Pos (I picked the worst case, search for something not contained in the string):
function ConstLength(const s: string): Integer;
begin
Result := Pos('x', s);
end;
function NoConstLength(s: string): Integer;
begin
Result := Pos('x', s);
end;
results in:
Const: 51792332
NoConst: 1377644831
Const is 26.60 times faster.
for the threaded case, and:
Const: 15912102
NoConst: 44616286
Const is 2.80 times faster.
for the non-threaded case.
Don't forget that const isn't only there to provide those tiny performance improvements.
Using const explains to anybody reading or maintaining the code that the value shouldn't be updated, and allows the compiler to catch any accidental attempts to do so.
So making your code more readable and maintainable can also make it marginally faster. What good reasons are there for not using const?
Using const prevents an implicit try/finally block which on x86 is rather more expensive than reference counting. That's really a separate issue to the semantic meaning of const. It's a shame that performance and semantics are mixed up in this way.
The type String is a special case, because it is managed by Delphi (copy on demand), and therefore not ideal to answer your question.
If you test your function with other types that are bigger than a pointer, records or arrays for example, you should see a bigger time difference, because with const only a pointer is passed, without const the record would be copied before passing to the function.
Using the keyword const, you can leave the decision of optimization to the compiler.
The documentation says:
Using const allows the compiler to optimize code for structured- and string-type parameters.
So, it is better, thus rational, to use const for string parameters, simply because the manual says so. ;)
Now, this may be well enough an answer for the questioner, but it is even more interesting to look at the general question whether to use const parameters or not.
Again, the documentation says at just one click away from the Delphi Language Guide Index:
Value and constant (const) parameters are passed by value or by reference, depending on the type and size of the parameter:
Note the apparent equality of value and constant parameters in this sentence. This concludes that using const for parameters, being not string- or structured-typed, makes no difference in performance nor code-size. (A short test, derived from Thorsten Engler's test code, indeed shows an average indifference between with and without const for parameters of ordinal and real types.)
So it turns out that whether or not using const only makes a difference to the programmer, not the executable.
As follow-up, and as LukeH already asked: What good reasons are there for not using const?
To follow Delphi's own syntax:
function FindDragTarget(const Pos: TPoint; AllowDisabled: Boolean): TControl;
function UpperCase(const S: string): string;
function UpCase(Ch: Char): Char;
function EncodeDate(Year, Month, Day: Word): TDateTime;
To produce more compact are therefore possibly slightly more readable code. For instance: using constant parameters in property setters really is superfluous, which surprisingly often leads to single line declarations instead of double, if you like to honour a line length limit.
To comfortably provide variables to virtual methods and event handlers. Note that none of the VCL event handler types use const parameters (for other than string- or record-typed members). It is just nice service for the users of your code or your components.
Of course, there also may be fine reasons for using const:
As LukeH already answered, if there is really no need at all to change the value of the parameter.
For (personal) protection, like the documentation says:
Using const also provides a safeguard against unintentionally passing a parameter by reference to another routine.
Partial origin of this answer: http://www.nldelphi.com.
Generally, I would avoid any optimizations (in any language) that don't solve real problems that you can measure. Profile your code, and fix the problems that you can actually see. Optimizing for theoretical issues is just a waste of your time.
If you suspect that something is wrong, and this somehow fixes it/speeds it up, then great, but implementing these kinds of micro optimizations by default are rarely worth the time.
One of the most important fact that people omitted. Interlock ... instruction is very costly in Multicore CPUs of x86 instruction. Read Intel manual. The cost is when refcounter var is taken placed and it is not in cpu cache, ALL other CPUs must be stopped for instruction to carried out.
Cheers
This is not exactly a straight-out question because I have just solved it, but more like "am I getting it right" type of question and a reminder for those who might get stuck into that.
Turns out, Delphi does not align variables on stack and there are no directives/options to control this behavior. Default COM marshaller on my XP SP3 seem to require 4-byte alignment when marshaling records though. Worse, when it encounters unaligned pointer, it does not return an error, oh no: it rounds the pointer down to the nearest 4-byte boundary and continues like that.
Therefore, if you pass a record you have allocated on stack into COM-marshaled function by reference, you're screwed and you won't even know.
The problem can be solved by using New/Dispose to allocate records, as memory managers tend to align everything at 8 bytes or better, but god, this is annoying, both the misalignment part and the "trim-down-pointers" part.
Is this really the reason, or am I wrong somewhere?
Update: How to reproduce (Delphi 2007 for Win32).
uses SysUtils;
type
TRec = packed record
a, b, c, d, e: int64;
end;
TDummy = class
protected
procedure Proc(param1: integer);
end;
procedure TDummy.Proc(param1: integer);
var a, b, c: byte;
rec: TRec;
begin
a := 5;
b := 9;
c := 100;
rec.a := param1;
rec.b := a;
rec.c := b;
rec.d := c;
writeln(IntToHex(integer(#rec), 8));
readln;
end;
var Obj: TDummy;
begin
obj := TDummy.Create;
try
obj.Proc(0);
finally
FreeAndNil(obj);
end;
end.
This gives odd result address, clearly not aligned on anything. If it doesn't, try adding more byte variables to "a, b, c: byte" (and don't forget to simulate some work with them at the end of the function).
The part with COM is easier to reproduce but longer to explain. Create a new VCL app called Sample Server, add a COM object SampleObject implementing ISampleObject, with a type library, free-threaded, single instance (make sure to check ISampleObject is marked as Ole Automation in type library). Open the type library, declare a new SampleRecord with five __int64 fields. Add a SampleFunction with a single SampleRecord* out parameter to ISampleObject. Implement SampleFunction in TSampleObject by returning fixed values:
function TSampleObject.SampleFunction(out rec: SampleRecord): HResult;
begin
rec.a := 1291;
rec.b := 742310;
//...
Result := S_OK;
end;
Note how Delphi declares SampleRecord as "packed record" in automatically generated type library header code:
SampleRecord = packed record
a: Int64;
b: Int64;
//...
end;
I have checked, and this, at least, was fixed in Delphi 2010. Automatically generated records are not packed there.
Register the COM server. Run it.
Now modify the source above (sample 1) to call this server instead of just doing writeln:
uses SysUtils, Windows, ActiveX, SampleServer_TLB;
procedure TDummy.Proc(param1: integer);
var a, b, c: byte;
rec: SampleRecord;
Server: ISampleObject;
begin
a := 5;
b := 9;
c := 100;
rec.a := param1;
rec.b := a;
rec.c := b;
rec.d := c;
Server := CoSampleObject.Create;
hr := Server.SampleFunction(rec);
writeln('#: 'IntToHex(integer(#rec), 8)+', rec.a='+IntToStr(rec.a));
readln;
end;
var Obj: TDummy;
begin
CoInitializeEx(nil, COINIT_MULTITHREADED);
obj := TDummy.Create;
try
obj.Proc(0);
finally
FreeAndNil(obj);
CoUninitialize();
end;
end.
Observe that when the address of rec is not aligned, values of rec fields are wrong (specifically, bitshifted to 8, 16 or 24 bits, sometimes wrapped over to the next value).
Do you have an example of the stack being misaligned? Delphi should be aligning everything to 4 byte boundaries. The only case I can think of that would cause misalignment is if someplace along the call-chain is some assembler code that has explicitly done something to the stack to mis-align it.
In my program, I process millions of strings that have a special character, e.g. "|" to separate tokens within each string. I have a function to return the n'th token, and this is it:
function GetTok(const Line: string; const Delim: string; const TokenNum: Byte): string;
{ LK Feb 12, 2007 - This function has been optimized as best as possible }
var
I, P, P2: integer;
begin
P2 := Pos(Delim, Line);
if TokenNum = 1 then begin
if P2 = 0 then
Result := Line
else
Result := copy(Line, 1, P2-1);
end
else begin
P := 0; { To prevent warnings }
for I := 2 to TokenNum do begin
P := P2;
if P = 0 then break;
P2 := PosEx(Delim, Line, P+1);
end;
if P = 0 then
Result := ''
else if P2 = 0 then
Result := copy(Line, P+1, MaxInt)
else
Result := copy(Line, P+1, P2-P-1);
end;
end; { GetTok }
I developed this function back when I was using Delphi 4. It calls the very efficient PosEx routine that was originally developed by Fastcode and is now included in the StrUtils library of Delphi.
I recently upgraded to Delphi 2009 and my strings are all Unicode. This GetTok function still works and still works well.
I have gone through the new libraries in Delphi 2009 and there are many new functions and additions to it.
But I have not seen a GetToken function like I need in any of the new Delphi libraries, in the various fastcode projects, and I can't find anything with a Google search other than Zarko Gajic's: Delphi Split / Tokenizer Functions, which is not as optimized as what I already have.
Any improvement, even 10% would be noticeable in my program. I know an alternative is StringLists and to always keep the tokens separate, but this has a big overhead memory-wise and I'm not sure if I did all that work to convert whether it would be any faster.
Whew. So after all this long winded talk, my question really is:
Do you know of any very fast implementations of a GetToken routine? An assembler optimized version would be ideal?
If not, are there any optimizations that you can see to my code above that might make an improvement?
Followup: Barry Kelly mentioned a question I asked a year ago about optimizing the parsing of the lines in a file. At that time I hadn't even thought of my GetTok routine which was not used for the that read or parsing. It is only now that I saw the overhead of my GetTok routine which led me to ask this question. Until Carl Smotricz and Barry's answers, I had never thought of connecting the two. So obvious, but it just didn't register. Thanks for pointing that out.
Yes, my Delim is a single character, so obviously I have some major optimization I can do. My use of Pos and PosEx in the GetTok routine (above) blinded me to the idea that I can do it faster with a character by character search instead, with bits of code like:
while (cp^ > #0) and (cp^ <= Delim) do
Inc(cp);
I'm going to go through everyone's answers and try the various suggestions and compare them. Then I'll post the results.
Confusion: Okay, now I'm really perplexed.
I took Carl and Barry's recommendation to go with PChars, and here is my implementation:
function GetTok(const Line: string; const Delim: string; const TokenNum: Byte): string;
{ LK Feb 12, 2007 - This function has been optimized as best as possible }
{ LK Nov 7, 2009 - Reoptimized using PChars instead of calls to Pos and PosEx }
{ See; https://stackoverflow.com/questions/1694001/is-there-a-fast-gettoken-routine-for-delphi }
var
I: integer;
PLine, PStart: PChar;
begin
PLine := PChar(Line);
PStart := PLine;
inc(PLine);
for I := 1 to TokenNum do begin
while (PLine^ <> #0) and (PLine^ <> Delim) do
inc(PLine);
if I = TokenNum then begin
SetString(Result, PStart, PLine - PStart);
break;
end;
if PLine^ = #0 then begin
Result := '';
break;
end;
inc(PLine);
PStart := PLine;
end;
end; { GetTok }
On paper, I don't think you can do much better than this.
So I put both routines to the task and used AQTime to see what's happening. The run I had included 1,108,514 calls to GetTok.
AQTime timed the original routine at 0.40 seconds. The million calls to Pos took 0.10 seconds. A half a million of the TokenNum = 1 copies took 0.10 seconds. The 600,000 PosEx calls only took 0.03 seconds.
Then I timed my new routine with AQTime for the same run and exactly the same calls. AQTime reports that my new "fast" routine took 3.65 seconds, which is 9 times as long. The culprit according to AQTime was the first loop:
while (PLine^ <> #0) and (PLine^ <> Delim) do
inc(PLine);
The while line, which was executed 18 million times, was reported at 2.66 seconds. The inc line, executed 16 million times, was said to take 0.47 seconds.
Now I thought I knew what was happening here. I had a similar problem with AQTime in a question I posed last year: Why is CharInSet faster than Case statement?
Again it was Barry Kelly who clued me in. Basically, an instrumenting profiler like AQTime does not necessarily do the job for microoptimization. It adds an overhead to each line which may swamp the results which is shown clearly in these numbers. The 34 million lines executed in my new "optimized code" overwhelm the several million lines of my original code, with apparently little or no overhead from the Pos and PosEx routines.
Barry gave me a sample of code using QueryPerformanceCounter to check that he was correct, and in that case he was.
Okay, so let's do the same now with QueryPerformanceCounter to prove that my new routine is faster and not 9 times slower as AQTime says it is. So here I go:
function TimeIt(const Title: string): double;
var i: Integer;
start, finish, freq: Int64;
Seconds: double;
begin
QueryPerformanceCounter(start);
for i := 1 to 250000 do
GetTokOld('This is a string|that needs|parsing', '|', 1);
for i := 1 to 250000 do
GetTokOld('This is a string|that needs|parsing', '|', 2);
for i := 1 to 250000 do
GetTokOld('This is a string|that needs|parsing', '|', 3);
for i := 1 to 250000 do
GetTokOld('This is a string|that needs|parsing', '|', 4);
QueryPerformanceCounter(finish);
QueryPerformanceFrequency(freq);
Seconds := (finish - start) / freq;
Result := Seconds;
end;
So this will test 1,000,000 calls to GetTok.
My old procedure with the Pos and PosEx calls took 0.29 seconds.
The new one with PChars took 2.07 seconds.
Now I am completely befuddled! Can anyone tell me why the PChar procedure is not only slower, but is 8 to 9 times slower!?
Mystery solved! Andreas said in his answer to change the Delim parameter from a string to a Char. I'll always be using just a Char, so at least for my implementation this is very possible. I was amazed at what happened.
The time for the 1 million calls went down from 1.88 seconds to .22 seconds.
And surprisingly, the time for my original Pos/PosEx routine went UP from .29 to .44 seconds when I changed it's Delim parameter to a Char.
Frankly, I'm disappointed by Delphi's optimizer. That Delim is a constant parameter. The optimizer should have noticed that the same conversion is happening within the loop and should have moved it out so that it would only be done once.
Double checking my Code generation parameters, yes I do have Optimization True and String format checking Off.
Bottom line is that the new PChar routine with Andrea's fix is about 25% faster than my original (.22 versus .29).
I still want to follow up on the other comments here and test them out.
Turning off optimization and turning on String format checking only increases the time from .22 to .30. It adds about the same to the original.
The advantage to using assembler code, or calling routines written in assembler like Pos or PosEx is that they are NOT subject to what code generation options you have set. They will always run the same way, a pre-optimized and non-bloated way.
I have reaffirmed in the last couple of days, that the best way to compare code for microoptimization is to look at and compare the Assembler code in the CPU window. It would be nice if Embarcadero could make that window a bit more convenient, and allow us to copy portions to the clipboard or to print sections of it.
Also, I unfairly slammed AQTime earlier in this post, thinking that the extra time added for my new routine was solely because of the instrumentation it added. Now that I go back and check with the Char parameter instead of String, the while loop is down to .30 seconds (from 2.66) and the inc line is down to .14 seconds (from .47). Strange that the inc line would go down as well. But I'm getting worn out from all this testing already.
I took Carl's idea of looping by characters, and rewrote that code with that idea. It makes another improvement, down to .19 seconds from .22. So here is now the best so far:
function GetTok(const Line: string; const Delim: Char; const TokenNum: Byte): string;
{ LK Nov 8, 2009 - Reoptimized using PChars instead of calls to Pos and PosEx }
{ See; https://stackoverflow.com/questions/1694001/is-there-a-fast-gettoken-routine-for-delphi }
var
I, CurToken: Integer;
PLine, PStart: PChar;
begin
CurToken := 1;
PLine := PChar(Line);
PStart := PLine;
for I := 1 to length(Line) do begin
if PLine^ = Delim then begin
if CurToken = TokenNum then
break
else begin
CurToken := CurToken + 1;
inc(PLine);
PStart := PLine;
end;
end
else
inc(PLine);
end;
if CurToken = TokenNum then
SetString(Result, PStart, PLine - PStart)
else
Result := '';
end;
There still may be some minor optimizations to this, such as the CurToken = Tokennum comparison, which should be the same type, Integer or Byte, whichever is faster.
But let's say, I'm happy now.
Thanks again to the StackOverflow Delphi community.
It makes a big difference what "Delim" is expected to be. If it's expected to be a single character, you're far better off stepping through the string character by character, ideally through a PChar, and testing specifically.
If it's a long string, Boyer-Moore and similar searches have a set-up phase for skip tables, and the best way would be to build the tables once, and reuse them for each subsequent find. That means you need state between calls, and this function would be better off as a method on an object instead.
You might be interested in this answer I gave to a question some time before, about the fastest way to parse a line in Delphi. (But I see that it is you that asked the question! Nevertheless, in solving your problem, I would hew to how I described parsing, not using PosEx like you are using, depending on what Delim normally looks like.)
UPDATE: OK, I spent about 40 minutes looking at this. If you know the delimiter is going to be a character, you're pretty much always better off with the second version (i.e. PChar scanning), but you have to pass Delim as a character. At the time of writing, you're converting the PLine^ expression - of type Char - to a string for comparison with Delim. That will be very slow; even indexing into the string, with Delim[1] will also be somewhat slow.
However, depending on how large your lines are, and how many delimited pieces you want to pull out, you may be better off with a resumable approach, rather than skipping unwanted delimited pieces inside the tokenizing routine. If you call GetTok with successively increasing indexes, like you are currently doing in your mini benchmark, you'll end up with O(n*n) performance, where n is the number of delimited sections. That can be turned into O(n) if you save the state of the scan and restore it for the next iteration, or pack all extracted items into an array.
Here's a version that does all tokenization once, and returns an array. It needs to tokenize twice though, in order to know how large to make the array. On the other hand, only the second tokenization needs to extract the strings:
// Do all tokenization up front.
function GetTok4(const Line: string; const Delim: Char): TArray<string>;
var
cp, start: PChar;
count: Integer;
begin
// Count sections
count := 1;
cp := PChar(Line);
start := cp;
while True do
begin
if cp^ <> #0 then
begin
if cp^ <> Delim then
Inc(cp)
else
begin
Inc(cp);
Inc(count);
end;
end
else
begin
Inc(count);
Break;
end;
end;
SetLength(Result, count);
cp := start;
count := 0;
while True do
begin
if cp^ <> #0 then
begin
if cp^ <> Delim then
Inc(cp)
else
begin
SetString(Result[count], start, cp - start);
Inc(cp);
Inc(count);
end;
end
else
begin
SetString(Result[count], start, cp - start);
Break;
end;
end;
end;
Here's the resumable approach. The loads and stores of the current position and delimiter character do have a cost, though:
type
TTokenizer = record
private
FSource: string;
FCurrPos: PChar;
FDelim: Char;
public
procedure Reset(const ASource: string; ADelim: Char); inline;
function GetToken(out AResult: string): Boolean; inline;
end;
procedure TTokenizer.Reset(const ASource: string; ADelim: Char);
begin
FSource := ASource; // keep reference alive
FCurrPos := PChar(FSource);
FDelim := ADelim;
end;
function TTokenizer.GetToken(out AResult: string): Boolean;
var
cp, start: PChar;
delim: Char;
begin
// copy members to locals for better optimization
cp := FCurrPos;
delim := FDelim;
if cp^ = #0 then
begin
AResult := '';
Exit(False);
end;
start := cp;
while (cp^ <> #0) and (cp^ <> Delim) do
Inc(cp);
SetString(AResult, start, cp - start);
if cp^ = Delim then
Inc(cp);
FCurrPos := cp;
Result := True;
end;
Here's the full program I used for benchmarking.
Here are the results:
*** count=3, Length(src)=200
GetTok1: 595 ms
GetTok2: 547 ms
GetTok3: 2366 ms
GetTok4: 407 ms
GetTokBK: 226 ms
*** count=6, Length(src)=350
GetTok1: 1587 ms
GetTok2: 1502 ms
GetTok3: 6890 ms
GetTok4: 679 ms
GetTokBK: 334 ms
*** count=9, Length(src)=500
GetTok1: 3055 ms
GetTok2: 2912 ms
GetTok3: 13766 ms
GetTok4: 947 ms
GetTokBK: 446 ms
*** count=12, Length(src)=650
GetTok1: 4997 ms
GetTok2: 4803 ms
GetTok3: 23021 ms
GetTok4: 1213 ms
GetTokBK: 543 ms
*** count=15, Length(src)=800
GetTok1: 7417 ms
GetTok2: 7173 ms
GetTok3: 34644 ms
GetTok4: 1480 ms
GetTokBK: 653 ms
Depending on the characteristics of your data, whether the delimiter is likely to be a character or not, and how you work with it, different approaches may be faster.
(I made a mistake in my earlier program, I wasn't measuring the same operations for each style of routine. I updated the pastebin link and benchmark results.)
Your new function (the one with PChar) should declare "Delim" as Char and not as String. In your current implementation the compiler has to convert the PLine^ char into a string to compare it with "Delim". And that happens in a tight loop resulting is an enormous performance hit.
function GetTok(const Line: string; const Delim: Char{<<==}; const TokenNum: Byte): string;
{ LK Feb 12, 2007 - This function has been optimized as best as possible }
{ LK Nov 7, 2009 - Reoptimized using PChars instead of calls to Pos and PosEx }
{ See; http://stackoverflow.com/questions/1694001/is-there-a-fast-gettoken-routine-for-delphi }
var
I: integer;
PLine, PStart: PChar;
begin
PLine := PChar(Line);
PStart := PLine;
inc(PLine);
for I := 1 to TokenNum do begin
while (PLine^ <> #0) and (PLine^ <> Delim) do
inc(PLine);
if I = TokenNum then begin
SetString(Result, PStart, PLine - PStart);
break;
end;
if PLine^ = #0 then begin
Result := '';
break;
end;
inc(PLine);
PStart := PLine;
end;
end; { GetTok }
Delphi compiles to VERY efficient code; in my experience, it was very difficult to do better in assembler.
I think you should just point a PChar (they still exist, don't they? I parted ways with Delphi around 4.0) at the beginning of the string and increment it while counting "|"s, until you've found n-1 of them. I suspect that will be faster than calling PosEx repeatedly.
Take note of that position, then increment the pointer some more until you hit the next pipe. Pull out your substring. Done.
I'm only guessing, but I wouldn't be surprised if this was close to the quickest this problem can be solved.
EDIT: Here's what I had in mind. This code is, alas, uncompiled and untested, but it should demonstrate what I meant.
In particular, Delim is treated as a single char, which I believe makes a world of difference if that will fulfill the requirements, and the character at PLine is tested only once. Finally, there's no more comparison against TokenNum; I believe it's faster to decrement a counter to 0 for counting delimiters.
function GetTok(const Line: string; const Delim: string; const TokenNum: Byte): string;
var
Del: Char;
PLine, PStart: PChar;
Nth, I, P0, P9: Integer;
begin
Del := Delim[1];
Nth := TokenNum + 1;
P0 := 1;
P9 := Line.length + 1;
PLine := PChar(line);
for I := 1 to P9 do begin
if PLine^ = Del then begin
if Nth = 0 then begin
P9 := I;
break;
end;
Dec(Nth);
if Nth = 0 then P0 := I + 1
end;
Inc(PLine);
end;
if (Nth <= 1) or (TokenNum = 1) then
Result := Copy(Line, P0, P9 - P0);
else
Result := ''
end;
Using assembler would be a micro-optimization. There are much greater gains to be had by optimizing the algorithm. Not doing work beats doing work in the fastest possible way, every time.
One example would be if you have places in your program where you need several tokens of the same line. Another procedure that returns an array of tokens which you can then index into should be faster than calling your function more than once, especially if you let the procedure not return all tokens, but only as many as you need.
But in general I agree with Carl's answer (+1), using a PChar for scanning would probably be faster than your current code.
This is a function that I've had in my personal library for quite some time that I use extensively. I believe this is the most current version of it. I've had multiple versions in the past being optimized for a variety of different reasons. This one tries to take into account Quoted strings, but if that code is removed it makes the function a slight bit faster.
I actually have a number of other routines, CountSections and ParseSectionPOS being a couple of examples.
Unfortuately this routine is ansi/pchar based only. Although I don't think it would be difficult to move it to unicode. Maybe I've already done that...I'll have to check on that.
Note: This routine is 1 based in the ParseNum indexing.
function ParseSection(ParseLine: string; ParseNum: Integer; ParseSep: Char; QuotedStrChar:char = #0) : string;
var
wStart, wEnd : integer;
wIndex : integer;
wLen : integer;
wQuotedString : boolean;
begin
result := '';
wQuotedString := false;
if not (ParseLine = '') then
begin
wIndex := 1;
wStart := 1;
wEnd := 1;
wLen := Length(ParseLine);
while wEnd <= wLen do
begin
if (QuotedStrChar <> #0) and (ParseLine[wEnd] = QuotedStrChar) then
wQuotedString := not wQuotedString;
if not wQuotedString and (ParseLine[wEnd] = ParseSep) then
begin
if wIndex=ParseNum then
break
else
begin
inc(wIndex);
wStart := wEnd+1;
end;
end;
inc(wEnd);
end;
result := copy(ParseLine, wStart, wEnd-wStart);
if (length(result) > 0) and (QuotedStrChar <> #0) and (result[1] = QuotedStrChar) then
result := AnsiDequotedStr(result, QuotedStrChar);
end;
end; { ParseSection }
In your code, I think this is the only line that can be optimized:
Result := copy(Line, P+1, MaxInt)
If you calculate the new Length there, it might get a bit faster, but not the 10% you are looking for.
Your tokenizing algorithm seems pretty OK.
For optimizing it, I would run it through a profiler (like AQTime from AutomatedQA) with a representative subset of your production data. That will point you to the weakest spot.
The only RTL function that comes close is this one in the Classes unit:
procedure TStrings.SetDelimitedText(const Value: string);
It tokenizes, but uses both QuoteChar and Delimiter, but you only use a Delimiter.
It uses the SetString function in the System unit which is a pretty fast way to set the content of a string based on a PChar/PAnsiChar/PUnicodeChar and a length.
That might get you some improvement as well; on the other hand, Copy is really fast too.
I'm not the person always blaming the algorithm, but if I look at the first piece of source,
the problem is that for string N, you do the POS/posexes for string 1..n-1 again too.
This means for N items, you do sum (n, n-1,n-2...1) POSes (=+/- 0.5*N^2) , while only N are needed.
If you simply cache the position of the last found result, e.g. in a record that is passed by VAR parameter, you can gain a lot.
type
TLastPosition = record
elementnr : integer; // last tokennumber
elementpos: integer; // character index of last match
end;
and then something
if tokennum=(lastposition.elementnr+1) then
begin
newpos:=posex(delim,line,lastposition.elementpos);
end;
Unfortunately, I don't have the time now to write it out, but I hope you get the idea