bit-manipulation

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're looking for me to do in a different way? I'm hoping that different wording will help me understand

I'm making a class PackedUnsigned1616 which stores two unsigned shorts in one int, and a class PackedSigned1616 which stores two signed shorts in one int. I've read up on bitwise operations, but I'm still confused on how to deal with signed and unsigned and values that are larger or smaller that a short's range (they are passed in as two ints). Here's what I've got so far: public final class PackedUnsigned1616 { public final int field; private static final int RIGHT = (2 << 15) - 1; private static final int LEFT = ((2 << 31) - 1) ^ RIGHT; public PackedUnsigned1616(int left, int right) { field

I'd like to write a function, in C, which takes the MSB of uint8_t , and if it's set, returns 0xFF and if not 0x00 . In short, which returns an integer where all the bits are set to the same value as the MSB. But I'd like to do it in a completely constant time way, no branches, no array offsets, just mathematical operations which are guaranteed to always touch the same number of bits. And ideally, without any undefined behavior. How can this be done? Ilmari Karonen How about: #define uint8_msb_to_all_bits(x) (0xFF * ((x) >> 7)) or even better: #define uint8_msb_to_all_bits(x) (-((x) >> 7)) The

I need to implement as fast as possible left bit shift of a 16-byte array in JavaCard . I tried this code: private static final void rotateLeft(final byte[] output, final byte[] input) { short carry = 0; short i = (short) 16; do { --i; carry = (short)((input[i] << 1) | carry); output[i] = (byte)carry; carry = (short)((carry >> 8) & 1); } while (i > 0); } Any ideas how to improve the performace? I was thinking about some Util.getShort(...) and Util.setShort(...) magic, but I did not manage to make it work faster then the implementation above. This is one part of CMAC subkeys computation and it

I understand bitwise operations and how they might be useful for different purposes, e.g. permissions. However, I don't seem to understand what use the bit shift operators are. I understand how they work, but I can't think of any scenarios where I might want to use them unless I want to do some really quick multiplication or division. Are there any other reasons to use bit-shifting? Guy Sirton There are many reasons, here are some: Let's say you represent a black and white image as a sequence of bits and you want to set a single pixel in this image generically. For example your byte offset may

In order to utilize a byte to its fullest potential, I'm attempting to store two unique values into a byte: one in the first four bits and another in the second four bits. However, I've found that, while this practice allows for optimized memory allocation, it makes changing the individual values stored in the byte difficult. In my code, I want to change the first set of four bits in a byte while maintaining the value of the second four bits in the same byte. While bitwise operations allow me to easily retrieve and manipulate the first four bit values, I'm finding it difficult to concatenate

I'm just beginning to learn about file compression and I've run into a bit of a roadblock. I have an application that will encode a string such as "program" as a compressed binary representation "010100111111011000" (note this is still stored as a String). Encoding g 111 r 10 a 110 p 010 o 011 m 00 Now I need to write this to the file system using a FileOutputStream , the problem I'm having is, how can I convert the string "010100111111011000" to a byte[] / byte s to be written to the file system with FileOutputStream ? I've never worked with bits/bytes before so I'm kind of at a dead end here