I am in the process of writing off-line browser Webseizer, use a lot of string processing functions, this is a CPU consumption of a processing process, in order to write an efficient web analytics engine, to optimize the code made a study.
1. High-precision timing function
A precise timer is needed for code optimization. Commonly used with GetTickCount functions, can achieve the accuracy of the millisecond level. But it is not enough, at this time can be used to improve the number of cycles. In addition, there is a more accurate timing-"High resolution performance counter" (high-resolution performance counter), which provides two API functions, Gets the queryperformancefrequency of the counter frequency and gets the QueryPerformanceCounter of the counter value. The implementation principle is realized by using the 8253,8254 programmable time interval timer chip in the computer. There are three separate 16-bit counters inside the computer.
Counters can be counted in binary or two-decimal (BDC). The counter produces 1,193,180 pulses per second, each pulse to reduce the number of counters, the frequency is variable, with queryperformancefrequency can be obtained, under normal circumstances are 1193180. Queryperformance counter can get the current counter value. So as long as your computer
Fast enough, theoretical accuracy can reach 1/1193180 seconds.
2, code optimization examples
The following is an example of a custom string function that describes the code optimization process.
The string function provided by Delphi has a POS function, which is defined as:
function Pos(Substr: string; S: string): Integer;
Its role is to find the string substr in the string s, the return value is the first occurrence of substr in S, if not found, the return value is 0.
In my writing Webseizer software (Sky software station has download) process, POS can not meet the requirements. On the one hand: when dealing with strings in a Web page, it is not sensitive to case, that is, the meaning of < H T > and <HTML> representatives is exactly the same. On the other hand: We also require a function where the return value is the last occurrence of substr in S, not the first occurrence. The following is an optimized code for this function.
function RightPos(const Substr,S: string): Integer;
var
iPos: Integer;
TmpStr:string;
begin
TmpStr:=s;
iPos := Pos(Substr,TmpStr); Result:=0;
//查找Substr第一次出现位置
while iPos<>0 do
begin
Delete(TmpStr,1,iPos+length(Substr)-1);
//删除已经查找过的字符
Result:=Result+iPos;
iPos := Pos(Substr,TmpStr); //查找Substr出现位置
if iPos=0 then break;
Result:=Result+length(Substr)-1;
end;
end;
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