Re: [PATCH] avoid double scanning in function byteain

在 2025/3/26 16:37, Kirill Reshke 写道:
> On Wed, 26 Mar 2025 at 12:17, Steven Niu <niushiji(at)gmail(dot)com> wrote:
>>
>> Hi,
>
> Hi!
>
>> This double scanning can be inefficient, especially for large inputs.
>> So I optimized the function to eliminate the need for two scans,
>> while preserving correctness and efficiency.
>
> While the argument that processing input once not twice is fast is
> generally true, may we have some simple bench here just to have an
> idea how valuable this patch is?
>
>
> Patch:
>
>
>> + /* Handle traditional escaped style in a single pass */
>> + input_len = strlen(inputText);
>> + result = palloc(input_len + VARHDRSZ); /* Allocate max possible size */
>> rp = VARDATA(result);
>> + tp = inputText;
>> +
>> while (*tp != '\0')
>
>
> Isn't this `strlen` O(n) + `while` O(n)? Where is the speed up?
>
>
>
> [0] https://github.com/bminor/glibc/blob/master/string/strlen.c#L43-L45
>

Hi, Kirill,

Your deep insight suprised me!

Yes, you are correct that strlen() actually performed a loop operation.
So maybe the performance difference is not so obvious.

However, there are some other reasons that drive me to make this change.

1. The author of original code left comment: "BUGS: The input is scanned
twice." .
You can find this line of code in my patch. This indicates a left work
to be done.

2. If I were the author of this function, I would not be satisfied with
myself that I used two loops to do something which actually can be done
with one loop.
I prefer to choose a way that would not add more burden to readers.

3. The while (*tp != '\0') loop has some unnecessary codes and I made
some change.

Thanks,
Steven