How fast can you make linear search?

Question

I\'m looking to optimize this linear search:

static int
linear (const int *arr, int n, int key)
{
        int i = 0;
        while (i < n) {
                      


        

Answer 1

In one of the comments you said the array length is 64.

Well if you must do it linearly, you can do:

int i = -1;
do {
  if (arr[0] >= key){i = 0; break;}
  if (arr[1] >= key){i = 1; break;}
  ...
  if (arr[62] >= key){i = 62; break;}
  if (arr[63] >= key){i = 63; break;}
} while(0);

However, I seriously doubt if that is faster than this binary search: *

int i = 0;
if (key >= arr[i+32]) i += 32;
if (key >= arr[i+16]) i += 16;
if (key >= arr[i+ 8]) i +=  8;
if (key >= arr[i+ 4]) i +=  4;
if (key >= arr[i+ 2]) i +=  2;
if (key >= arr[i+ 1]) i +=  1;

*Thanks to Jon Bentley for that one.

Added: since you said this table is prepared once for a large number of searches, and you want fast, you could allocate some space somewhere and generate machine code with the values hard-wired into it. It could either be linear or binary search. If binary, the machine code would look like what the compiler would generate from this:

if (key < value32){
  if (key < value16){
    ...
  }
  else {
    ...
  }
}
else {
  if (key < value48){
    ...
  }
  else {
    ...
  }
}

Then you just copy that into a place where you can call it.

OR you could print the code above, compile and link it on the fly into a dll, and load the dll.