it hacks are ingenious little programming tricks that manipulate integers in a smart and efficient manner. Instead of performing some operation (such as counting the 1 bits in an integer) by looping over individual bits, these programming hacks do the same but in a tricky way.
This post assumes that you know about the following bitwise operations and some C/C++ language.
- & : Bitwise AND
- || : Bitwise OR
- ^ : Bitwise XOR
- ~ : Bitwise NOT
- <<: Bitwise Shift Left
- >> : Bitwise Shift Right
I’m taking the size of each variable as 8 bit, where the first bit is b0 and last bit (MSB) is b7.
Let’s go ahead with some bit hacks.
Check if the integer is even or odd
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| if ((a & 1) == 0) {a is even}else {a is odd} |
The righmost or zeroth bit of odd number is always set. Checking the rightmost bit only will do if we want to find out that a is even or odd.
Turn off the rightmost 1-bit
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| b = a & (a - 1) |
If x is even, then x – 1 will be odd and vice versa too. This is gonna unset the zeroth bit. Why? The rightmost bit of odd number is always set.
Test if the n-th bit is set
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| if (a & (1 << n)) {n-th bit is set}else {n-th bit is not set} |
1 is left shifted n times and then anded with a, do a example paper run, and you will get it better.
Set the n-th bit
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| b = a | (1 << n) |
Shift left 1 n times or it with a and then put it into a new variable b.
Unset the nth bit
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| b = a | ~(1 << n) |
Shift 1 n times left, complement it or it with a and then put it into a new variable b.
Toggle the n-th bit
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| b = a ^ (1 << n) |
Shift left 1 n times, xor it with a and assign it to a new variable.
Uppercase to lower case letter
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| "CAPITAL LETTER" XOR " " = small letter |
Xoring with space i.e., ” “, will give you lowercase letter.
If a integer is power of 2
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| b = a && !(a & (a - 1)); |
Check a and a – 1 is 0, and AND the result with a, to check that a is not zero and some power of 2.
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