Quickly find whether a value is present in a C array?

Question

I have an embedded application with a time-critical ISR that needs to iterate through an array of size 256 (preferably 1024, but 256 is the minimum) and check if a value matches

Answer 1

This is more like an addendum than an answer.

I've had a similar case in the past, but my array was constant over a considerable number of searches.

In half of them, the searched value was NOT present in array. Then I realized I could apply a "filter" before doing any search.

This "filter" is just a simple integer number, calculated ONCE and used in each search.

It's in Java, but it's pretty simple:

binaryfilter = 0;
for (int i = 0; i < array.length; i++)
{
    // just apply "Binary OR Operator" over values.
    binaryfilter = binaryfilter | array[i];
}

So, before do a binary search, I check binaryfilter:

// Check binaryfilter vs value with a "Binary AND Operator"
if ((binaryfilter & valuetosearch) != valuetosearch)
{
    // valuetosearch is not in the array!
    return false;
}
else
{
    // valuetosearch MAYBE in the array, so let's check it out
    // ... do binary search stuff ...

}

You can use a 'better' hash algorithm, but this can be very fast, specially for large numbers. May be this could save you even more cycles.