Dephi: faster way to convert a “dirty” string to a number - delphi

Question

How can I make this code faster? the string can contain characters such as ", .?#" and possibly others.
Const Nums = ['0'..'9'];
function CleanNumber(s: String): Int64;
Var z: Cardinal;
begin
for z := length(s) downto 1 do
if not (s[z] in Nums) then Delete(s,z,1);
if s = '' then
Result := 0 else
Result := StrToInt64(s);
end;
Results (long loop):
CL2,CL3 = HeartWare's
32-bit, "dirty number" / "clean number"
Mine: 270ms, 165ms
CL2: 220ms, 210ms
CL3: 100ms, 110ms
DirtyStrToNum: 215ms, 90ms
64-bit, "dirty number" / "clean number"
Mine: 2280ms, 75ms
CL2: 1320ms, 130ms
CL3: 280ms, 25ms
DirtyStrToNum: 1390ms, 125ms

Answer 1

Here are two examples that for sure are faster than the one you have (deleting a character from a string is relatively slow):
This one works by pre-allocating a string of the maximum possible length and then filling it out with the digits as I come across them in the source string. No delete for every unsupported character, and no expansion of the target string for every supported character.
FUNCTION CleanNumber(CONST S : STRING) : Int64;
VAR
I,J : Cardinal;
C : CHAR;
T : STRING;
BEGIN
SetLength(T,LENGTH(S));
J:=LOW(T);
FOR I:=LOW(S) TO HIGH(S) DO BEGIN
C:=S[I];
IF (C>='0') AND (C<='9') THEN BEGIN
T[J]:=C;
INC(J)
END
END;
IF J=LOW(T) THEN
Result:=0
ELSE BEGIN
SetLength(T,J-LOW(T)); // or T[J]:=#0 [implementation-specific]
Result:=StrToInt64(T)
END
END;
This one works by simple multiplication of the end result by 10 and adding the corresponding digit value.
{$IFOPT Q+}
{$DEFINE OverflowEnabled }
{$ELSE }
{$Q+ If you want overflow checking }
{$ENDIF }
FUNCTION CleanNumber(CONST S : STRING) : Int64;
VAR
I : Cardinal;
C : CHAR;
BEGIN
Result:=0;
FOR I:=LOW(S) TO HIGH(S) DO BEGIN
C:=S[I];
IF (C>='0') AND (C<='9') THEN Result:=Result*10+(ORD(C)-ORD('0'))
END
END;
{$IFNDEF OverflowEnabled } {$Q-} {$ENDIF }
{$UNDEF OverflowEnabled }
Also note that I don't use IN or CharInSet as these are much slower than a simple inline >= and <= comparison.
Another comment I could make is the use of LOW and HIGH on the string variable. This makes it compatible with both 0-based strings (mobile compilers) and 1-based strings (desktop compilers).

Answer 2

Your function is slow mainly because of the Delete approach. Each call to Delete needs to move a lot of characters around.
A faster approach would be like this:
function DirtyStrToNum(const S: string): Int64;
var
tmp: string;
i, j: Integer;
const
DIGITS = ['0'..'9'];
begin
SetLength(tmp, S.Length);
j := 0;
for i := 1 to S.Length do
if CharInSet(S[i], DIGITS) then
begin
Inc(j);
tmp[j] := S[i];
end;
SetLength(tmp, j);
if tmp.IsEmpty then
Result := 0
else
Result := StrToInt64(tmp);
// Or, but not equivalent: Result := StrToInt64Def(tmp, 0);
end;
Notice I make a single allocation for a new string, and then only copy the minimum number of characters to it.