5 ms·

This is stated to be a puzzle. But, when does this actually get used? Are there use cases of bit manipulation in JS? I've never understood when using bit operat

This is stated to be a puzzle. But, when does this actually get used? Are there use cases of bit manipulation in JS? I've never understood when using bit operators could be necessary or even useful. Of course, I'm speaking from my experience building web apps; ruby, python, js, php, etc. I don't have much other programming experience

gamegoblin 13y ago
The only time I find myself using bit manipulations in production code is when I am writing a library that uses an int to represent a set of option flags (where each bit is a boolean flag). It's pretty common in C libs to define some constants like int FLAG_1 = 1; int FLAG_2 = 2; int FLAG_3 = 4; //These would actually probably be formatted like (1 << N) ... And then pass in your options to a function like f(... FLAG_1+FLAG_2) so you can have an optional number of flags in a single argument.
aguynamedrich 13y ago
The first time I used bit manipulation in web apps was for declaring user roles and permissions. A single integer field in your database can be used to declare any combination of up to 32 different capabilities/permissions for users by setting the individual bits in the value. Also, when dealing with colors as an integer value, you can isolate your ARGB values using bit shifting.
wellingtons 13y ago
To do this exercise in javascript is pretty silly. But as a systems developer, I can tell you that bit manipulation happens all over the place in my line of work.
- amenghra 13y ago
  There is nothing inherently javascript about this, besides providing a UI to validate the answer (that's when the validator works...)
blt 13y ago
- Packing lots of small pieces of information into a single memory word - for example, 3-3-2 bit RGB [1] - Interacting with hardware that uses certain bits in a register to control behavior - for example, Arduino digital ports [2] - Doing arithmetic extra-fast with numbers that are powers of two - modulo becomes bitwise-and, division and multiplication become shifts [1] http://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_palettes#3-3-2_bit_RGB http://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_pale... [2] http://arduino.cc/en/Reference/PortManipulation http://arduino.cc/en/Reference/PortManipulation
10098 13y ago
This is mostly done in lower level code, so no wonder you have never bumped into it. One easy example, though is passing a set of binary options around, which is useful, for example, when you want to say "enable those features, and disable those other features". You can pack up to 32 options into a 4-byte integer.
amenghra 13y ago
real world application: https://github.com/facebook/hhvm/blob/master/hphp/compiler/analysis/type.cpp#L401 https://github.com/facebook/hhvm/blob/master/hphp/compiler/a...
joshvm 13y ago
Embedded applications use bitwise ops all the time. Particularly for performing changes to registers. Example, a microcontroller has an 8 bit status register. Each bit controls some hardware function. You wish to change bit 5, but you can't change anything else because it would cause unwanted side effects. So this rules out simply saying myregister = 0b00001000. Common solution is to make a macro like: #define setbit(BYTE,BIT) BYTE |= (1 << BIT) Thus you can say I want to set bit 5 in my register by calling setbit(foo, 5) while keeping the rest of the byte intact. There are analogous macros for clearing and toggling bits. Or you might want to perform an operation based on a particular bit in a byte, e.g. byte 5 is a status flag that tells you when a timer has overflowed. #define getbit(BYTE, BIT) !!(BYTE & ~(1 << BIT)) Then you can say: if( getbit(myregister, 5)){ // do something } Fun fun :)

gamegoblin 13y ago

The only time I find myself using bit manipulations in production code is when I am writing a library that uses an int to represent a set of option flags (where each bit is a boolean flag). It's pretty common in C libs to define some constants like int FLAG_1 = 1; int FLAG_2 = 2; int FLAG_3 = 4; //These would actually probably be formatted like (1 << N) ... And then pass in your options to a function like f(... FLAG_1+FLAG_2) so you can have an optional number of flags in a single argument.

aguynamedrich 13y ago

The first time I used bit manipulation in web apps was for declaring user roles and permissions. A single integer field in your database can be used to declare any combination of up to 32 different capabilities/permissions for users by setting the individual bits in the value. Also, when dealing with colors as an integer value, you can isolate your ARGB values using bit shifting.

wellingtons 13y ago

To do this exercise in javascript is pretty silly. But as a systems developer, I can tell you that bit manipulation happens all over the place in my line of work.

amenghra 13y ago
There is nothing inherently javascript about this, besides providing a UI to validate the answer (that's when the validator works...)

blt 13y ago

- Packing lots of small pieces of information into a single memory word - for example, 3-3-2 bit RGB [1] - Interacting with hardware that uses certain bits in a register to control behavior - for example, Arduino digital ports [2] - Doing arithmetic extra-fast with numbers that are powers of two - modulo becomes bitwise-and, division and multiplication become shifts [1] http://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_palettes#3-3-2_bit_RGB http://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_pale... [2] http://arduino.cc/en/Reference/PortManipulation http://arduino.cc/en/Reference/PortManipulation

10098 13y ago

This is mostly done in lower level code, so no wonder you have never bumped into it. One easy example, though is passing a set of binary options around, which is useful, for example, when you want to say "enable those features, and disable those other features". You can pack up to 32 options into a 4-byte integer.

amenghra 13y ago

real world application: https://github.com/facebook/hhvm/blob/master/hphp/compiler/analysis/type.cpp#L401 https://github.com/facebook/hhvm/blob/master/hphp/compiler/a...

joshvm 13y ago

Embedded applications use bitwise ops all the time. Particularly for performing changes to registers. Example, a microcontroller has an 8 bit status register. Each bit controls some hardware function. You wish to change bit 5, but you can't change anything else because it would cause unwanted side effects. So this rules out simply saying myregister = 0b00001000. Common solution is to make a macro like: #define setbit(BYTE,BIT) BYTE |= (1 << BIT) Thus you can say I want to set bit 5 in my register by calling setbit(foo, 5) while keeping the rest of the byte intact. There are analogous macros for clearing and toggling bits. Or you might want to perform an operation based on a particular bit in a byte, e.g. byte 5 is a status flag that tells you when a timer has overflowed. #define getbit(BYTE, BIT) !!(BYTE & ~(1 << BIT)) Then you can say: if( getbit(myregister, 5)){ // do something } Fun fun :)