bit-manipulation

问题 I would like to manipulate the bitwise representation of floating-point numbers in C#. BinaryWriter and BinaryReader do it this way: public virtual unsafe void Write(double value) { ulong num = *((ulong*) &value); ... } public virtual unsafe double ReadDouble() { ... ulong num3 = ...; return *((double*) &num3); } Is there a way to do this without unsafe code, and without the overhead of actually using BinaryWriter and BinaryReader? 回答1: Are you trying to avoid unsafe code altogether, or do

问题 I'm implementing a quantization algorithm from a textbook. I'm at a point where things pretty much work, except I get off-by-one errors when rounding. This is what the textbook has to say about that: Rounded division by 2^p may be carried out by adding an offset and right-shifting by p bit positions Now, I get the bit about the right shift, but what offset are they talking about? Here's my sample code: def scale(x, power2=16): if x < 0: return -((-x) >> power2) else: return x >> power2 def

问题 I can already convert 32bit integers into their rgba values like this: pixelData[i] = { red: pixelValue >> 24 & 0xFF, green: pixelValue >> 16 & 0xFF, blue: pixelValue >> 8 & 0xFF, alpha: pixelValue & 0xFF }; But I don't really know how to reverse it. 回答1: To reverse it, you just have to combine the bytes into an integer. Simply use left-shift and add them, and it will work. var rgb = (red << 24) + (green << 16) + (blue << 8) + (alpha); Alternatively, to make it safer, you could first AND each

问题 I am trying to understand how to use Bitwise AND to extract the values of individual bytes. What I have is a 4-byte array and am casting the last 2 bytes into a single 2 byte value. Then I am trying to extract the original single byte values from that 2 byte value. See the attachment for a screen shot of my code and values. The problem I am having is I am not able to get the value of the last byte in the 2 byte value. How would I go about doing this with Bitwise AND? Thanks for your help,

问题 I was looking at the below bit reversal code and just wondering how does one come up with these kind of things. (source : http://www.cl.cam.ac.uk/~am21/hakmemc.html) /* reverse 8 bits (Schroeppel) */ unsigned reverse_8bits(unsigned41 a) { return ((a * 0x000202020202) /* 5 copies in 40 bits */ & 0x010884422010) /* where bits coincide with reverse repeated base 2^10 */ /* PDP-10: 041(6 bits):020420420020(35 bits) */ % 1023; /* casting out 2^10 - 1's */ } Can someone explain what does comment

问题 How to swap the nybble bit positions of a number? For example: 534, convert it into binary, the rightmost 4 bits has to be interchanged with the leftmost 4 bits and then make a new number with that. Anyone know how to do this? 回答1: Start from the fact that hexadecimal 0xf covers exactly four bits. There are four nibbles in a 16-bit number. The masks for the nibbles are 0xf000 , 0xf00 , 0xf0 , and 0xf . Then start masking, shifting and bitwise OR-ing. 回答2: Sean Anderson's bit twiddling guide

问题 Abstract Hi, suppose you have two different independent 64-bit binary matrices A and T ( T is a transposed version of itself, using the transposed version of matrix allows during multiplication to operate on T 's rows rather than columns which is super cool for binary arithmetic) and you want to multiply these matrices the only thing is that matrix multiplication result is truncated to 64-bits and if you yield to a value greater that 1 in some specific matrix cell the resulting matrix cell

问题 The problem is to find all the subarrays of the given array with xor of all its elements equal to zero. For example, if array contains elements [13,8,5,3,3] , the solution should give the indices of all subarrays like 0-2 , 3-4 , 0-4 , etc. The question is similar to the one asked here The only difference is that I want the indices of all the subarrays that satisfies the equation A0 xor A1 xor...xor An = 0 回答1: This is a fairly straightforward extension of the linked question. In Python, #

问题 Let's say I have this int variable v1 : 1100 1010 And this variable int v2 : 1001 1110 I need to copy the last four bits from v2 to the last four bits of v1 so that the result is: 1100 1110 ^ ^ last four bits of v2 | | first four bits of v1 How would I got about doing this in C or C++? I read a few articles about bitwise operations but I couldn't find any information specifically about this. 回答1: Bitwise operations were the right things to look for. v1 = (v1 & ~0xf) | (v2 & 0xf); Is there

问题 I've been going through the K&R C Programming Language book and I'm stuck on Exercise 2-6 which reads: Write a function setbits(x,p,n,y) that returns x with the n bits that begin at position p set to the rightmost n bits of y, leaving the other bits unchanged. I'm having trouble understanding the exact thing they're looking for me to do. I looked at a possible answer here, but I still don't really understand. I think it's the wording that's throwing me off. Can anyone maybe explain what they