Why is stdin.byLine.writeln so slow?

[Jyxent via Digitalmars-d-learn]

On Friday, 13 June 2014 at 21:08:08 UTC, monarch_dodra wrote:
> On Friday, 13 June 2014 at 20:48:16 UTC, Jyxent wrote:
>> I've been playing around with D and noticed that:
>>
>> stdin.byLine.writeln
>>
>> takes ~20 times as long as:
>>
>> foreach(line; stdin.byLine) writeln(line);
>>
>> I asked on IRC and this was suggested:
>>
>> stdin.byLine(KeepTerminator.yes).copy(stdout.lockingTextWriter)
>>
>> which is slightly faster than the foreach case.
>>
>> It was suggested that there is something slow about writeln 
>> taking the input range, but I'm not sure I see why.  If I 
>> follow the code correctly, formatRange in std.format will 
>> eventually be called and iterate over the range.
>
> Because:
> stdin.byLine.writeln
> and
> foreach(line; stdin.byLine) writeln(line);
> Don't produce the same output. One prints a range that contains 
> strings, whereas the second repeatedly prints strings.
>
> Given this input:
> line 1
> line	2
> Yo!
>
> Then "stdin.byLine.writeln" will produce this string:
> ["line 1", "line\t2", "Yo!"]
>
> So that's the extra overhead which is slowing you down, because 
> *each* character needs to be individually parsed, and 
> potentially escaped (eg: "\t").
>
> The "copy" option is the same as the foreach one, since each 
> string is individually passed to the writeln, which doesn't 
> parse your string. The "lockingTextWriter" is just sugar to 
> squeeze out extra speed.

Hah.  You're right.  I had seen writeln being used this way and 
just assumed that it printed every line, without looking at the 
output too closely.

Thanks for clearing that up.