fred.l.kleinschmidt@[EMAIL PROTECTED]
wrote:
> On May 12, 3:58 pm, Bart <b...@[EMAIL PROTECTED]
> wrote:
>> On May 12, 11:18 pm, fred.l.kleinschm...@[EMAIL PROTECTED]
wrote:
>>
>>> If one knows that x and y are unsigned integers (of unknown size -
>>> they may be short, int, long, long long), what is the most ****table
>>> way to determine their difference?
>>> If x is smaller than y, then x-y is negative.
>>> A naive approach is
>>> if ( x > y ) {
>>> ...
>>> }
>>> but a compiler may code (x>y) as (x-y > 0).
>> What do you mean by difference?
>>
>> If you mean x-y, then this will give strange results when x<y (it
>> can't be negative).
>>
>> If you mean max(x,y)-min(x,y) then your 'naive' approach will likely
>> work well.
>>
>
> The problem came up on a Unix platform, where some code was trying
> to check for a double-click. Each time button1 is clicked, it checks
> the time with the previous time it was clicked; if the difference is
> small enough, a double-click is assumed.
>
> The times are stored in a variable of type Time, defined as
> an unsigned int on some platforms, unsigned long on others, storing
> clock time in milliseconds.
>
> For an unsigned int, the value will wrap every 47 days. The original
> code was
>
> if ( (newtime - oldtime) < doubleclicktime ) {
> /* perform double-click action */
> }
>
> That code is unsafe: as oldtime approaches UINT_MAX, the risk
> is that newtime might occur after time wraps, thus might be
> a small integer, and (newtime - oldtime) is mathematically
> negative.
Doesn't matter. You've said the times are either
`unsigned int' or `unsigned long' depending on the platform,
and neither of these is subject to promotion. Hence, the
arithmetic will be carried out according to the unsigned
rules, and `tiny - huge' will yield `moderate', as desired.
> Since others have said that the compiler is obliged to
> perform the if-test properly, my solution is
>
> if ( t2 > t1 ) {
> delta = t2 - t1;
> }
> else {
> delta = (UINT_MAX - t1) + t2 + 1;
> }
The `else' clause is just an obfuscated version of
the `then' clause, producing the same result.
--
Eric.Sosman@[EMAIL PROTECTED]
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