Re: Reading and writing a big file in Ada (GNAT) on Windows XP

[Fionn Mac Cumhaill <invisible@hiding.from.spam>]

On Sun, 29 Apr 2007 21:46:29 GMT, Fionn Mac Cumhaill
<invisible@hiding.from.spam> wrote:

>On Sat, 28 Apr 2007 19:12:33 GMT, "Jeffrey R. Carter"
><jrcarter@acm.org> wrote:
>
>>Fionn Mac Cumhaill wrote:
>>> 
>>> All it does is read lines in a loop from a text file with
>>> Ada.Text_IO.Get_Line, does minor modifications on about 80% of the
>>> lines that it reads, and writes the lines to an output file with
>>> Put_Line. 
>>> 
>>> The modifications consist of replacing a slice of text at the end of a
>>> line with another bit of text. The biggest slice is 10 characters, and
>>> the replacement slice is always smaller than the original slice. An
>>> occasional line of text is about 6000 characters long, but most are
>>> about 700 haracters.  Get_Line reads them into a String variable that
>>> is 10,000 characters long.
>>> 
>>> The problem is that the input file has more than 10 million lines of
>>> text in it. The program works perfectly, but takes about 5 hours to
>>> run. The Cygwin version of wc can count the lines in the input file in
>>> less than one minute.
>>> 
>>> Why is this so slow? 
>>> Do I have an Ada problem, a GNAT problem, or a MinGW problem?
>>
>>It's hard to tell. Text_IO is quite heavy-weight compared to C text I/O, 
>>but I'd be surprised if it made that much difference.
>>
>>This sounds as if it would be a pretty simple program; if so, you could 
>>post it (or a reasonable facsimile, if there's a reason you can't post 
>>the actual code) here, and we would be better able to help.
>
>I'm posting to cla from home. The program is at work. I'll post the
>code when I get back to work.

Well, I was too pressed for time to do any one-change-at-a-time
experimentation; I removed the buffer initialization, and changed  the
Index searches to use Backward. I removed some other uses of Index, as
the slice for which I was looking was at a fixed location. Run time
fell to 8 minutes.