Programing Puzzles

[Koroskin Denis]

On Thu, 07 Aug 2008 19:37:27 +0400, Wyverex <wyverex.cypher at gmail.com>  
wrote:

> JAnderson wrote:
>> Wyverex wrote:
>>> just some fun little programming puzzles I found around online...
>>>
>>>
>>> Problem #2 Test if an int is even or odd without looping or if  
>>> statement (Cant use: do, while, for, foreach, if).
>>>
>>> Problem #4 Find if the given number is a power of 2.
>>>
>>> Post Solutions to this root, comments to someones solution in that  
>>> thread.
>>   Some of these are pretty standard interview questions. Although I  
>> don't personally like to ask these sort of questions because they are  
>> often about knowing a "trick" which you an easily lookup.  The can be  
>> fun to figure out though.
>>  Here's another common one:
>>  | Write a bitcount for a 32-bit number.
>>  And a little more challenging:
>>  | Write a bitcount for a 32-bit number that is less then 15 operations  
>> without using a lookup table.
>>  | Can you do that in 12 or less?
>>  -Joel
>
> int count( in int i )
> {
>     int c = (i & 1) + (i & 2 ? 1 : 0) + (i & 4 ? 1 : 0) + (i & 8 ? 1 :  
> 0);
>     i >>= 4;
>     c += (i & 1) + (i & 2 ? 1 : 0) + (i & 4 ? 1 : 0) + (i & 8 ? 1 : 0);
>     i >>= 4;
>     c += (i & 1) + (i & 2 ? 1 : 0) + (i & 4 ? 1 : 0) + (i & 8 ? 1 : 0);
>     i >>= 4;
>     c += (i & 1) + (i & 2 ? 1 : 0) + (i & 4 ? 1 : 0) + (i & 8 ? 1 : 0);
>
>     return c;
> }
>
> int count2( in int i )
> {
>     int c = (i & 1) + ((i >> 1) & 1) + ((i >> 2) & 1) + ((i >> 3) & 1);
>     i >>= 4;
>     c += (i & 1) + ((i >> 1) & 1) + ((i >> 2) & 1) + ((i >> 3) & 1);
>     i >>= 4;
>     c += (i & 1) + ((i >> 1) & 1) + ((i >> 2) & 1) + ((i >> 3) & 1);
>     i >>= 4;
>     c += (i & 1) + ((i >> 1) & 1) + ((i >> 2) & 1) + ((i >> 3) & 1);
>
>     return c;
> }

Much simpler:

int getBitCount32(int i) {
     return getBitCount16(i) + getBitCount16(i >> 16);
}

int getBitCount16(int i) {
     return getBitCount8(i) + getBitCount8(i >> 8);
}

int getBitCount8(int i) {
     return getBitCount4(i) + getBitCount4(i >> 4);
}

int getBitCount4(int i) {
     return getBitCount2(i) + getBitCount2(i >> 2);
}

int getBitCount2(int i) {
     return (i & 2) + (i & 1);
}