changeCase, C++ (vc++)

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/******** Untested. ************/

// ASCII characters are flipped to the opposite case (upper <-> lower)
// non-ASCII characters are left unchanged
void changeCase (char char_array[], int array_size ) {

__asm{
            // BEGIN YOUR CODE HERE

mov   esi, char_array;      // MSVC inline asm requires these potentially-redundant copies :(
        mov   ecx, array_size;

test  ecx,ecx;       // return if(size <= 0)
        jle  early_out;

next_char:
        mov   al, [esi];     // load the current character
        mov   dl, al;

// check if the character is alphabetic or not
        // there are two equal-size ranges of characters: one with 0x20 set, and one without
        or    al, 0x20;      // set 0x20 and then just check that lowercase range

// unsigned compare trick: 0 <= n < high  can be done with one unsigned compare instead of two signed compares
        // low < n < high  can be done by shifting the range first
        sub   al, 'a';       // if al is less than 'a', it will become a large unsigned number
        cmp   al, 'z'-'a';
        ja  non_alpha;      // conditionally skip the flip & store

xor   dl, 0x20;      // toggle the ASCII case bit
        mov   [esi], dl;
                             // xor [esi], 0x20   // saves the mov earlier, but is otherwise slower
    non_alpha:

inc   esi;
        dec   ecx;
        jz next_char;

early_out:
            // END YOUR CODE HERE
    }
}

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Compilation time: 0.89 sec, absolute service time: 0,95 sec