Simulate Bitwise Shift Operators in Lua

[BlackBulletIV]

I've found a way to simulate the bitwise shift operators in Lua, by just using some simple mathematics. I've posted a blog post about it, so I'll direct you to there for details. But for those who don't want to read a few paragraphs, here's a couple lines of code that should help:

Code: Select all

number * 2 ^ leftShiftValue -- same as number << leftShiftValue
math.floor(number / 2 ^ rightShiftValue) -- same as number >> rightShiftValue

[schme16]

Whilst it's really interesting to know that it can be done manually like that, I've always used http://lua-users.org/wiki/BitwiseOperators for my projects.

Once again, really neat!

[leiradel]

I think BitOp would be a great addition to LÖVE. It is cross-platform in that it ensures consistent behavior across all supported platforms which includes Linux, MacOS X and Windows.

And it's free software under the same license as Lua!

[slime]

Indeed, although Lua 5.2 will include a bitwise library AFAIK, and LuaJIT has one as well.

[TechnoCat]

Lua 5.2 will include a bitwise library AFAIK, and LuaJIT has one as well.

*Drools*

[leiradel]

Indeed, although Lua 5.2 will include a bitwise library AFAIK, and LuaJIT has one as well.

Mike Pall, author of both LuaJIT and BitOp, has publicly criticized the bitwise ops library that will ship with Lua 5.2 because it does not guarantee consistent results across platforms, because bitwise ops has undefined semantics in C.

One of the problems, if I remember it correctly, is with shift operators. Assuming a 32-bit wide word, 1 << 32 could result in:

• 0 if the right operand is maintained as 32
• 1 if the right operand is truncated to stay in the 0-31 range (32 & 31 = 0, 1 << 0 = 1)

Sometimes the shift implementation in the processor will use only the required bits (5 in this example) to perform the shift, so the second operand is truncated.

Mike's BitOp library does a number of checks and ensures consistent behavior. As an example, it explicitly truncates the second operand so all platforms will evaluate 1 << 32 as 1.

Cheers,

Andre

[BlackBulletIV]

Whilst it's really interesting to know that it can be done manually like that, I've always used http://lua-users.org/wiki/BitwiseOperators for my projects.

Once again, really neat!

As stated, I've found pure Lua implementations to be slow (because they use tables I think). Therefore the advantage to this is that it would a lot faster.

Thanks!

I think BitOp would be a great addition to LÖVE. It is cross-platform in that it ensures consistent behavior across all supported platforms which includes Linux, MacOS X and Windows.

And it's free software under the same license as Lua!

I second that! You do need bitwise operators from time to time.

EDIT: I created an issue for it.

[Lap]

For now there's http://luaforge.net/projects/bit/ if you want to do it in pure lua.

[BlackBulletIV]

I've used that. Its performance is terrible; because it uses tables I believe.

[Lap]

I've used that. Its performance is terrible; because it uses tables I believe.

No argument there. It's damn slow.