How much function call overhead is this incurring?
Would it run faster if you took the digit function and just embedded
it directly into the itoa function?
On 9/21/07, Charles Curley <[EMAIL PROTECTED]> wrote:
> On Fri, Sep 21, 2007 at 08:37:11PM -0600, Levi Pearson wrote:
> > Charles Curley <[EMAIL PROTECTED]> writes:
> > >
> > > Have we beaten this thing into the ground yet?
> >
> > Not quite yet. Try this one:
>
> I like it. A few points.
>
> * It is base ten only. My solution is much more general. Need base 36?
> Recall that base 10 was not specified in the original problem. But
> your method can be adapted at the cost of:
>
> * expanding the lookup table,
>
> * dividing by a variable instead of an inline constant, usually
> slower.
>
> Also, your lookup table is probably faster than my calculation.
>
> * You have a call to abs inside your main loop. I think I see why, but
> would you safely speed things up by moving it outside?
>
> * Your reverse function is nice, but probably slower than strcat
> because it uses an intermediate variable. But I think you can speed
> it up considerably by only running for (strlen (s))/2 iterations. If
> so, it probably would be better than my temporary buffer.
>
> If the compiler will optimize the /2 into a right shift, you get rid
> of a divide instruction. (Or, on most 16 bit processors, a divide
> function.) Or do it yourself with (strlen (s))>>.
>
> * I replaced my while loop with a do ... while loop, and found (to no
> great surprise) that it is slower. I didn't check, but I doubt it
> saved any space because of the way do ... while loops usually
> compile. That did let me get rid of the 0 special case, making
> things more elegant. The same might apply here.
>
> I made a few other optimizations in my version.
>
> * I now initialize i to SIZE and decrement it. This gets rid of a
> subtraction instruction inside the main loop.
>
> * I declared some variables to be register variables. That helps on
> Intel architecture, which is so pathetically register poor. It might
> or might not help on register rich processors because the compiler
> might do it for you.
>
> * I declared the function digit inline. That helped. But it would make
> for a smaller code size only if digit isn't used elsewhere. It could
> be a useful routine for special cases like a hex dump.
>
> --------------------------------------------------
>
> /* We depend on a pure ASCII environment here. */
> inline char digit (int d)
> {
> register char ret = '0' + d;
>
> /* For bases greater than 10, skip from '9' to 'A' in the ASCII
> * table. This may have been a trick your examiner was looking
> * for. */
> if (ret > '9') {
> ret += 7;
> }
> return (ret);
> }
>
> char *itoa (int value, char *buffer)
> {
> register int i = SIZE;
>
> /* I have no way of knowing the size of the incoming buffer, so I
> * have to build in a buffer, then copy into the given one. strcat
> * is likely more efficient and faster than strncat, and probably
> * coded in assembler. */
> char tmp[SIZE+1];
>
> tmp[SIZE] = 0;
>
> /* For negative numbers, set the sign. Having done that, do
> * the conversion as a positive number. */
> if (value < 0) {
> strcat (buffer, "-");
> value = 0 - value;
> }
>
> do {
> /* Since we are starting at the right end of the string, we
> * fill our temporary buffer from the right. This is likely
> * another trick the examiner wanted. */
> tmp[--i] = digit (value%base);
> value = value/base;
> } while (value);
>
> strcat (buffer, &tmp[i]);
> }
> --------------------------------------------------
>
> --
>
> Charles Curley /"\ ASCII Ribbon Campaign
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