bitwise operators tricks

bitwise-cheatsheet.txt

atoi example

https://fr.wikipedia.org/wiki/Table_de_v%C3%A9rit%C3%A9

Bitwise tricks

All these tricks are also used in cryptography and obfuscation methods.

AND
Take bits in bytes
0xDEADBEEF & 0xFFFF0000 = 0xDEAD0000
0xDEADBEEF & 0x0000FFFF = 0xBEEF

char to int digits

51('3') in ascii in hex is 0x33 & 0xF = we get 3 in integer

Check is odd or even
0x2 & 0x1 == 0 is even

NOT

Increment by one
-~n = -~1 = 2

Decrement by one
~-n = ~-1 = 0

XOR

Xor is very used in cryptography systems

set zero variable or anything
a = 42
a ^= a // a is now at 0

Swap lowercase & Uppercase
a = 'a'
a ^= 32 // a is now Uppercase

a = 'A'
a ^= 32 // A is now lowercase

a = 'A'
a |= 32 // a is now lowercase

Check if two integer have same sign

-2 ^ -2 >= 0 = yes
3 ^ 3 >= 0 = yes
3 ^ -3 >= 0 = No

Swap two elements
Xor swap algorithm

https://en.wikipedia.org/wiki/XOR_swap_algorithm

a = 2
b = 4

a ^= b
b ^= a
a ^= b
a is now 4
and b is now 2

Compare two elements

a = 4
b = 4
a ^ b == 0 // then is equal

Add two binary numbers

4 | 1 = 5
2 | 1 = 3
2 | 2 = 3

Bitshift

Concat two bytes

(0xDE << 8 | 0xAD)

Swap bits

Swap one byte (replace two nibble bytes)
((42 >> 4) | (42 << 4)) & 0xFF

Swap word

((0xBEEF >> 8) | (0xBEEF << 8)) & 0xFFFF

Swap dword

((0xDEADBEEF >> 16) | (0xDEADBEEF << 16)) & 0xFFFFFFFF

Swap qword

((0xDEADBEEFBADCAFFE >> 32) | (0xDEADBEEFBADCAFFE << 32)) & 0xFFFFFFFFFFFFFFFF

Increase power

1 << 1 = 2
1 << 2 = 4

Decrease power

4 >> 1 = 2
4 >> 2 = 1

Mul by 10

(2 << 1) + (2 << 3) = 20

Encode In Hex

97 = 0x61

97 >> 4  = 6
97 & 0xF = 1

97 / 16  = 6
97 % 16  = 1

https://fr.wikipedia.org/wiki/Alg%C3%A8bre_de_Boole_(logique)

Get max of two integer

a = 5
b = 12
b & ((a - b) >> 31) | a & (~(a - b) >> 31)

Get min of two integer

a = 5
b = 12
a & ((a-b) >> 31) | b & (~(a-b) >> 31);

Check if number is a power of two

num > 0 && (num & (~-num)) == 0;

Get max int Example

~-(1 << 31) & 0x7FFFFFFF Signed

Get min int Example

-~(~(1 << 31))

Check if factorial of 2

(n & (~-n)) == 0)

Get ABS value

(-3 ^ (-3 >> 31)) - (-3 >> 31)

if (x==a) x=b; if (x==b) x=a;

x = a ^ b ^ x;

Concat binary number

1 << 2 | 0 << 1 | 1 = 5 (101)

int	ft_atoi(const char *str)
{
	int	to_dec;
	int	neg;

	to_dec = 0;
	neg = 1;
	while (*str && (*str == ' ' || *str == '\t'))
		str++;
	while (*str && (*str == '+' || *str == '-'))
		if (*str++ == '-')
			neg = ~(neg - 1);
	while (*str && *str >= '0' && *str <= '9')
		to_dec = ((to_dec << 1) + (to_dec << 3)) + (*str++ & 0xF);
	return (to_dec * neg);
}

Thanks for this sheecheat, so many tricks as usual.