I know you can get the first byte by using
int x = number & ((1<<8)-1);
or
int x = number & 0xFF;
But I don't know how to get the nth byte of an integer.
For example, 1234 is 00000000 00000000 00000100 11010010 as 32bit integer
How can I get all of those bytes? first one would be 210, second would be 4 and the last two would be 0.
int x = (number >> (8*n)) & 0xff;
where n is 0 for the first byte, 1 for the second byte, etc.
For the (n+1)th byte in whatever order they appear in memory (which is also least- to most- significant on little-endian machines like x86):
int x = ((unsigned char *)(&number))[n];
For the (n+1)th byte from least to most significant on big-endian machines:
int x = ((unsigned char *)(&number))[sizeof(int) - 1 - n];
For the (n+1)th byte from least to most significant (any endian):
int x = ((unsigned int)number >> (n << 3)) & 0xff;
Of course, these all assume that n < sizeof(int), and that number is an int.
int nth = (number >> (n * 8)) & 0xFF;
Carry it into the lowest byte and take it in the "familiar" manner.
If you are wanting a byte, wouldn't the better solution be:
byte x = (byte)(number >> (8 * n));
This way, you are returning and dealing with a byte instead of an int, so we are using less memory, and we don't have to do the binary and operation & 0xff just to mask the result down to a byte. I also saw that the person asking the question used an int in their example, but that doesn't make it right.
I know this question was asked a long time ago, but I just ran into this problem, and I think that this is a better solution regardless.
//was trying to do inplace, would have been better if I had swapped higher and lower bytes somehow
uint32_t reverseBytes(uint32_t value) {
uint32_t temp;
size_t size=sizeof(uint32_t);
for(int i=0; i<size/2; i++){
//get byte i
temp = (value >> (8*i)) & 0xff;
//put higher in lower byte
value = ((value & (~(0xff << (8*i)))) | (value & ((0xff << (8*(size-i-1)))))>>(8*(size-2*i-1))) ;
//move lower byte which was stored in temp to higher byte
value=((value & (~(0xff << (8*(size-i-1)))))|(temp << (8*(size-i-1))));
}
return value;
}
Related
I want to read binary file byte at the time and then store bits of that byte into integer array. And similarly I want to write integer array of 1s and 0s (8 of them ) into binary file as bytes?
If you have an array of bytes:
unsigned char bytes[10];
And want to change it into an array of bits:
unsigned char bits[80];
And assuming you have 8 bits per byte, try this:
int i;
for (i=0; i<sizeof(bytes)*8; i++) {
bits[i] = ((1 << (i % 8)) & (bytes[i/8])) >> (i % 8);
}
In this loop, i loops through the total number of bits. The byte that a given bit lives at is i/8, which as integer division rounds down. The position of the bit within a byte is i%8.
First we create a mask for the desired bit:
1 << (i % 8)
Then the desired byte:
bytes[i/8]
Then we perform a logical AND to clear all bits except the one we want.
(1 << (i % 8)) & (bytes[i/8])
Then we shift the result right by the bit position to put the desired bit at the least significant bit. This gives us a value of 1 or 0.
Note also that the arrays in question are unsigned. That is required for the bit shifting to work properly.
To switch back:
int i;
memset(bytes, 0, sizeof(bytes));
for (i=0; i<sizeof(bytes)*8; i++) {
bytes[i/8] |= bits[i] << (i % 8);
}
We start by clearing out the byte array, since we'll be setting each byte one bit at a time.
Then we take the bit in question:
bits[i]
Shift it into its position:
bits[i] << (i % 8)
Then use a logical OR to set the appropriate byte;
A simple C program to do the job on a byte array 'input' of size 'sz' would be:
int i=0,j=0;
unsigned char mask = 0x01u;
for (i=0;i<sz;i++)
for (j=0;j<8;j++)
output[8*i+j]=((unsigned char)input[i] >> j) & (unsigned char)(mask);
I have a long sequence of bits stored in an array of unsigned long integers, like this
struct bit_array
{
int size; /* nr of bits */
unsigned long *array; /* the container that stores bits */
}
I am trying to design an algorithm to reverse the order of bits in *array. Problems:
size can be anything, i.e. not necessarily a multiple of 8 or 32 etc, so the first bit in the input array can end up at any position within the unsigned long in the output array;
the algorithm should be platform-independent, i.e. work for any sizeof(unsigned long).
Code, pseudocode, algo description etc. -- anything better than bruteforce ("bit by bit") approach is welcome.
My favorite solution is to fill a lookup-table that does bit-reversal on a single byte (hence 256 byte entries).
You apply the table to 1 to 4 bytes of the input operand, with a swap. If the size isn't a multiple of 8, you will need to adjust by a final right shift.
This scales well to larger integers.
Example:
11 10010011 00001010 -> 01010000 11001001 11000000 -> 01 01000011 00100111
To split the number into bytes portably, you need to use bitwise masking/shifts; mapping of a struct or array of bytes onto the integer can make it more efficient.
For brute performance, you can think of mapping up to 16 bits at a time, but this doesn't look quite reasonable.
I like the idea of lookup table. Still it's also a typical task for log(n) group bit tricks that may be very fast. Like:
unsigned long reverseOne(unsigned long x) {
x = ((x & 0xFFFFFFFF00000000) >> 32) | ((x & 0x00000000FFFFFFFF) << 32);
x = ((x & 0xFFFF0000FFFF0000) >> 16) | ((x & 0x0000FFFF0000FFFF) << 16);
x = ((x & 0xFF00FF00FF00FF00) >> 8) | ((x & 0x00FF00FF00FF00FF) << 8);
x = ((x & 0xF0F0F0F0F0F0F0F0) >> 4) | ((x & 0x0F0F0F0F0F0F0F0F) << 4);
x = ((x & 0xCCCCCCCCCCCCCCCC) >> 2) | ((x & 0x3333333333333333) << 2);
x = ((x & 0xAAAAAAAAAAAAAAAA) >> 1) | ((x & 0x5555555555555555) << 1);
return x;
}
The underlying idea is that when we aim to reverse the order of some sequence we may swap the head and tail halves of this sequence and then separately reverse each of halves (which is done here by applying the same procedure recursively to each half).
Here is a more portable version supporting unsigned long widths of 4,8,16 or 32 bytes.
#include <limits.h>
#define ones32 0xFFFFFFFFUL
#if (ULONG_MAX >> 128)
#define fill32(x) (x|(x<<32)|(x<<64)|(x<<96)|(x<<128)|(x<<160)|(x<<192)|(x<<224))
#define patt128 (ones32|(ones32<<32)|(ones32<<64) |(ones32<<96))
#define patt64 (ones32|(ones32<<32)|(ones32<<128)|(ones32<<160))
#define patt32 (ones32|(ones32<<64)|(ones32<<128)|(ones32<<192))
#else
#if (ULONG_MAX >> 64)
#define fill32(x) (x|(x<<32)|(x<<64)|(x<<96))
#define patt64 (ones32|(ones32<<32))
#define patt32 (ones32|(ones32<<64))
#else
#if (ULONG_MAX >> 32)
#define fill32(x) (x|(x<<32))
#define patt32 (ones32)
#else
#define fill32(x) (x)
#endif
#endif
#endif
unsigned long reverseOne(unsigned long x) {
#if (ULONG_MAX >> 32)
#if (ULONG_MAX >> 64)
#if (ULONG_MAX >> 128)
x = ((x & ~patt128) >> 128) | ((x & patt128) << 128);
#endif
x = ((x & ~patt64) >> 64) | ((x & patt64) << 64);
#endif
x = ((x & ~patt32) >> 32) | ((x & patt32) << 32);
#endif
x = ((x & fill32(0xffff0000UL)) >> 16) | ((x & fill32(0x0000ffffUL)) << 16);
x = ((x & fill32(0xff00ff00UL)) >> 8) | ((x & fill32(0x00ff00ffUL)) << 8);
x = ((x & fill32(0xf0f0f0f0UL)) >> 4) | ((x & fill32(0x0f0f0f0fUL)) << 4);
x = ((x & fill32(0xccccccccUL)) >> 2) | ((x & fill32(0x33333333UL)) << 2);
x = ((x & fill32(0xaaaaaaaaUL)) >> 1) | ((x & fill32(0x55555555UL)) << 1);
return x;
}
In a collection of related topics which can be found here, the bits of an individual array entry could be reversed as follows.
unsigned int v; // input bits to be reversed
unsigned int r = v; // r will be reversed bits of v; first get LSB of v
int s = sizeof(v) * CHAR_BIT - 1; // extra shift needed at end
for (v >>= 1; v; v >>= 1)
{
r <<= 1;
r |= v & 1;
s--;
}
r <<= s; // shift when v's highest bits are zero
The reversal of the entire array could be done afterwards by rearranging the individual positions.
You must define what is the order of bits in an unsigned long. You might assume that bit n is corresponds to array[x] & (1 << n) but this needs to be specified. If so, you need to handle the byte ordering (little or big endian) if you are going to use access the array as bytes instead of unsigned long.
I would definitely implement brute force first and measure whether the speed is an issue. No need to waste time trying to optimize this if it is not used a lot on large arrays. An optimized version can be tricky to implement correctly. If you end up trying anyway, the brute force version can be used to verify correctness on test values and benchmark the speed of the optimized version.
The fact that the size is not multiple of sizeof(long) is the hardest part of the problem. This can result in a lot of bit shifting.
But, you don't have to do that if you can introduce new struct member:
struct bit_array
{
int size; /* nr of bits */
int offset; /* First bit position */
unsigned long *array; /* the container that stores bits */
}
Offset would tell you how many bits to ignore at the beginning of the array.
Then you only only have to do following steps:
Reverse array elements.
Swap bits of each element. There are many hacks for in the other answers, but your compiler might also provide intrisic functions to do it in fewer instructions (like RBIT instruction on some ARM cores).
Calculate new starting offset. This is equal to unused bits the last element had.
I would split the problem into two parts.
First, I would ignore the fact that the number of used bits is not a multiple of 32. I would use one of the given methods to swap around the whole array like that.
pseudocode:
for half the longs in the array:
take the first longword;
take the last longword;
swap the bits in the first longword
swap the bits in the last longword;
store the swapped first longword into the last location;
store the swapped last longword into the first location;
and then fix up the fact that the first few bits (call than number n) are actually garbage bits from the end of the longs:
for all of the longs in the array:
split the value in the leftmost n bits and the rest;
store the leftmost n bits into the righthand part of the previous word;
shift the rest bits to the left over n positions (making the rightmost n bits zero);
store them back;
You could try to fold that into one pass over the whole array of course. Something like this:
for half the longs in the array:
take the first longword;
take the last longword;
swap the bits in the first longword
swap the bits in the last longword;
split both value in the leftmost n bits and the rest;
for the new first longword:
store the leftmost n bits into the righthand side of the previous word;
store the remaining bits into the first longword, shifted left;
for the new last longword:
remember the leftmost n bits for the next iteration;
store the remembered leftmost n bits, combined with the remaining bits, into the last longword;
store the swapped first longword into the last location;
store the swapped last longword into the first location;
I'm abstracting from the edge cases here (first and last longword), and you may need to reverse the shifting direction depending on how the bits are ordered inside each longword.
I am new to bit manipulation.
My friend recently asked me this in an interview.
Given an array of bytes
Eg: 1000100101010101 | 001010011100
We need to flip it two bits at a time horizontally inplace.
So the new array should be:
1000 | 0101 and so on.
and so on.
I think we start from the middle (marked by | here) and continue our way outwards taking two bits at a time.
I know how to reverse single bits in a number at a time like this:
unsigned int reverse(unsigned int num)
{
unsigned int x = sizeof(num) * 8;
unsigned int reverse_num = 0, i, temp;
for (i = 0; i < x; i++)
{
temp = (num & (1 << i));
if(temp)
reverse_num |= (1 << ((x - 1) - i));
}
return reverse_num;
}
But I wonder how can we reverse two bits efficiently inplace.
Thanks in advance.
I'd just do a whole byte (or more) at once:
output = (input & 0x55) << 1;
output |= (input & 0xAA) >> 1;
The "trick" way to do this is to precompute a table of bytes with the bits flipped. Then you can just index the table using a byte from the array, and write it back. As others have said - how many bits are in your bytes here?
I know you can get the first byte by using
int x = number & ((1<<8)-1);
or
int x = number & 0xFF;
But I don't know how to get the nth byte of an integer.
For example, 1234 is 00000000 00000000 00000100 11010010 as 32bit integer
How can I get all of those bytes? first one would be 210, second would be 4 and the last two would be 0.
int x = (number >> (8*n)) & 0xff;
where n is 0 for the first byte, 1 for the second byte, etc.
For the (n+1)th byte in whatever order they appear in memory (which is also least- to most- significant on little-endian machines like x86):
int x = ((unsigned char *)(&number))[n];
For the (n+1)th byte from least to most significant on big-endian machines:
int x = ((unsigned char *)(&number))[sizeof(int) - 1 - n];
For the (n+1)th byte from least to most significant (any endian):
int x = ((unsigned int)number >> (n << 3)) & 0xff;
Of course, these all assume that n < sizeof(int), and that number is an int.
int nth = (number >> (n * 8)) & 0xFF;
Carry it into the lowest byte and take it in the "familiar" manner.
If you are wanting a byte, wouldn't the better solution be:
byte x = (byte)(number >> (8 * n));
This way, you are returning and dealing with a byte instead of an int, so we are using less memory, and we don't have to do the binary and operation & 0xff just to mask the result down to a byte. I also saw that the person asking the question used an int in their example, but that doesn't make it right.
I know this question was asked a long time ago, but I just ran into this problem, and I think that this is a better solution regardless.
//was trying to do inplace, would have been better if I had swapped higher and lower bytes somehow
uint32_t reverseBytes(uint32_t value) {
uint32_t temp;
size_t size=sizeof(uint32_t);
for(int i=0; i<size/2; i++){
//get byte i
temp = (value >> (8*i)) & 0xff;
//put higher in lower byte
value = ((value & (~(0xff << (8*i)))) | (value & ((0xff << (8*(size-i-1)))))>>(8*(size-2*i-1))) ;
//move lower byte which was stored in temp to higher byte
value=((value & (~(0xff << (8*(size-i-1)))))|(temp << (8*(size-i-1))));
}
return value;
}
I am constructing a message to send a 24-bit number over the network.
For little endian machines, the code is (ptr is the pointer to the message buffer):
*ptr++ = (num >> 16) & 0xFF;
*ptr++ = (num >> 8) & 0xFF;
*ptr++ = (num) & 0xFF;
(So if num0, num1, num2 and num3 are the individual bytes making up num, the message would be encoded as num2|num1|num0.)
What should be the code for encoding num2|num1|num0 on a big endian machine?
The question here is, in what byte order shall the message be sent/constructed ? Because whether you are on a little or big endian machine doesn't matter with respect to num, as you're already dividing num into individual bytes in an endian-agnostic way.
The code you've posted stores 24 bits of num in big endian (aka network byte order). So if that's what you want you're already done. If you want to store it in big little instead, just reverse the order:
*ptr++ = (num) & 0xFF;
*ptr++ = (num >> 8) & 0xFF;
*ptr++ = (num >> 16) & 0xFF;
Your code is portable regardless of endianess. The shift operators >> << work with the values, not with the representation.
In the receiving machine, regardless of endian-ness, if you receive them in same order as they are stored in ptr, assemble them like this:
num = (ptr[0] << 16) + (ptr[1] << 8) + (ptr[2]);
int main(int argc, char** argv) {
int a, b;
a = 0x0f000000; // Contain 32 bit value
printf("before = %d\n", a);
b = a & (~0xff000000); // convert the last 8 bits to zero so we got only 24 bit value in b
printf("After = %d\n", b);
return (EXIT_SUCCESS);
}
There is a number containing a 32-bit value but number b contains only 24 bits, starting from least significant digit. And that doesn't depend on endianness because bitwise operators don't work with memory representation.
So you can use
num = num & (~0xff000000);
to get the last 24-bit value.