我如何在lua中模拟带符号的32位整数

Question

In Java, when I do following left shift operation, I get a negative result due to integer / long overflow: 0xAAAAAAAA << 7 gives me -183251938048

But, In Lua since everything is a Lua number which is 52 bit float; I am not able to trigger overflow upon left shift: bit_lshift(0xAAAAAAAA,7) gives me 1431655680

How do I simulate 32bit signed integer in Lua??

Answer 1

You write some C functions that handle this and then export them to Lua.

Though generally, Lua code shouldn't be touching things this low-level.

Answer 2

You are looking for bit manipulating libraries in Lua. One such library is bitop from the author of LuaJIT, which directly contains it without the need for installation. You can also install it in standard Lua.

Another library is the bit32 library, which is contained in Lua 5.2.

Both libraries let you manipulate 32-bit numbers. For example with bitop :

local bit = require 'bit
print(bit.lshift(0xAAAAAAAA, 7)) --> 1431655680

I do not know how you got the negative number, since 1431655680 is what I get by doing (0xAAAAAAAA<<7)&0xFFFFFFFF in C (and also doing that in a "programming calculator").

Answer 3

I hope I'm not seen as trolling for saying this, but the best way to simulate Java from Lua would be to use Java from Lua.

If you need to emulate Java, chances are that your Lua is already embedded in it. Just expose Java's binary operations to the Lua program, so it can use them.