Suppose you have two uint32_t variables (a and b) that you want to add
together and another uint32_t variable (max) that is the maximum that
the result could assume. A silly code could be:

a += b;
if (a > max) {
a = max;
}

Of course, this doesn't work well for every values of a, b and max.

For example, for a=0xFFFF'FFFC, b=10 and max=0xFFFF'FFFE.

Could you suggest a better method? I'm thinking of:

a += b;
if ((a < b) || (a > max)) {
a = max;
}

And for decrement?

In article <tno602$2cj2$1@dont-email.me>,
pozz <pozzugno@gmail.com> wrote:

> a += b;
> if (a > max) {
> a = max;
> }
>
> Of course, this doesn't work well for every values of a, b and max.

If you have to worry about maximal and minimal representable
values, you have to use more complicated code than can end up
machine dependent. If you can assumed this safely away from
those, just treat it like normal integer arithmetic.

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'I desire mercy, not sacrifice.' /|\
Discordia: not just a religion but also a parody. This post / \
I am an Andrea Chen sockpuppet. insults Islam. Mohammed

On 12/18/22 5:59 PM, pozz wrote:
> Suppose you have two uint32_t variables (a and b) that you want to add
> together and another uint32_t variable (max) that is the maximum that
> the result could assume. A silly code could be:
>
>   a += b;
>   if (a > max) {
>     a = max;
>   }
>
> Of course, this doesn't work well for every values of a, b and max.
>
> For example, for a=0xFFFF'FFFC, b=10 and max=0xFFFF'FFFE.
>
>
> Could you suggest a better method? I'm thinking of:
>
>     a += b;
>     if ((a < b) || (a > max)) {
>         a = max;
>     }
>
> And for decrement?
>

Decrement is simple since b is unsigned:

if(a < b) a = 0;
else a -= b;

The increment case can be simplified a bit if we can assume "reasonable"
values for b, such that b <= max to:

if(a > max-b) a = max;
else a += b;

pozz <pozzugno@gmail.com> writes:

> Suppose you have two uint32_t variables (a and b) that you want to add
> together and another uint32_t variable (max) that is the maximum that
> the result could assume. A silly code could be:
>
> a += b;
> if (a > max) {
> a = max;
> }
>
> Of course, this doesn't work well for every values of a, b and max.
>
> For example, for a=0xFFFF'FFFC, b=10 and max=0xFFFF'FFFE.
>
>
> Could you suggest a better method? [...]

a = max > a && max - a > b ? a + b : max;

> And for decrement?

After seeing the expression for increment, exercise for the
reader.

pozz <pozzugno@gmail.com> wrote:
> Suppose you have two uint32_t variables (a and b) that you want to add
> together and another uint32_t variable (max) that is the maximum that
> the result could assume. A silly code could be:
>
> a += b;
> if (a > max) {
> a = max;
> }
>
> Of course, this doesn't work well for every values of a, b and max.
>
> For example, for a=0xFFFF'FFFC, b=10 and max=0xFFFF'FFFE.

{
int64_t tmp = (int64_t)a + (int64_t)b;
a = (tmp > max)?max:tmp;
}

Tell compiler what you want and let it do its job at optimization.

> Could you suggest a better method? I'm thinking of:
>
> a += b;
> if ((a < b) || (a > max)) {
> a = max;
> }
>
> And for decrement?

The above generalize in obvious way.

--
Waldek Hebisch

On 2022-12-18, pozz <pozzugno@gmail.com> wrote:
> Suppose you have two uint32_t variables (a and b) that you want to add
> together and another uint32_t variable (max) that is the maximum that
> the result could assume. A silly code could be:
>
> a += b;
> if (a > max) {
> a = max;
> }
>
> Of course, this doesn't work well for every values of a, b and max.

if (a + b > a || a + b > max)
a = max;
else
a = a + b; // written this way to make common subexpression obvious

--
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Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca

On 2022-12-20, Kaz Kylheku <864-117-4973@kylheku.com> wrote:
> if (a + b > a || a + b > max)
> a = max;
> else
> a = a + b; // written this way to make common subexpression obvious

Oops, what? That should be:

if (a + b < a || a + b > max)

If a + b wraps or else exceeds max, then clamp to max; otherwise update
a with a + b.

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca

On 20/12/2022 13:44, Kaz Kylheku wrote:
> if (a + b > a

Under what circumstances will ( a + b ) not greater than a? Example please.

Thank you.

On Tue, 20 Dec 2022 18:12:44 +0000, Jame O' Brian wrote:

> On 20/12/2022 13:44, Kaz Kylheku wrote:
>> if (a + b > a
>
>
> Under what circumstances will ( a + b ) not greater than a? Example
> please.

a == UINT_MAX
b == 1

--
Lew Pitcher
"In Skills, We Trust"

On 12/20/2022 10:12 AM, Jame O' Brian wrote:
> On 20/12/2022 13:44, Kaz Kylheku wrote:
>> if (a + b > a
>
>
> Under what circumstances will ( a + b ) not greater than a? Example please.

Overflow.

On Tuesday, 20 December 2022 at 22:52:51 UTC+2, Chris M. Thomasson wrote:
> On 12/20/2022 10:12 AM, Jame O' Brian wrote:
> > On 20/12/2022 13:44, Kaz Kylheku wrote:
> >> if (a + b > a
> >
> >
> > Under what circumstances will ( a + b ) not greater than a? Example please.
> Overflow.

Or when b is 0.

On 12/23/2022 5:21 AM, Öö Tiib wrote:
> On Tuesday, 20 December 2022 at 22:52:51 UTC+2, Chris M. Thomasson wrote:
>> On 12/20/2022 10:12 AM, Jame O' Brian wrote:
>>> On 20/12/2022 13:44, Kaz Kylheku wrote:
>>>> if (a + b > a
>>>
>>>
>>> Under what circumstances will ( a + b ) not greater than a? Example please.
>> Overflow.
>
> Or when b is 0.

Touche! I totally missed that case. Thanks.