Question

I want to use lua (that internally uses only doubles) to represent a integer that can t have rounding errors between 0 and 2^64-1 or terrible things will happen.

Is it possible to do so?

Answer 1

No

At least some bits of a 64-bit double must be used to represent the exponent (position of the binary point), and hence there are fewer than 64-bits available for the actual number. So no, a 64-bit double can t represent all the values a 64-bit integer can (and vice-versa).

Answer 2

Even though you ve gotten some good answers to your question about 64-bit types, you may still want a practical solution to your specific problem. The most reliable solution I know of is to build Lua 5.1 with the LNUM patch (also known as the Lua integer patch) which can be downloaded from LuaForge. If you aren t planning on building Lua from the C source, there is at least one pure Lua library that handles 64-bit signed integers -- see the Lua-users wiki.

Answer 3

The double is a 64bit type itself. However you lose 1 bit for the sign and 11 for the exponent.

So the answer is no: it can t be done.

Answer 4

From memory, a double can represent a 53-bit signed integer exactly.

Answer 5

No, you cannot use Double to store 64-bit integers without losing precision.

However, you can apply a Lua patch that adds support for true 64-bit integers to the Lua interpreter. Apply the LNUM patch to your Lua source and recompile.

Answer 6

On 64 bits you can only store 2^64 different codes. This means that a 64-bit type which can represent 2^64 integers doesn t have any place for representing something else, such as floating point numbers.

Obviously double can represent a lot of non-integers numbers, so it can t fit your requirements.

Answer 7

IEEE 754 double cannot represent 64-bit integers exactly. It can, however, represent exactly every 32-bit integer value.

Answer 8

i know nothing about lua
but if you could figure out how to perform bitwise manipulation of the float in this language, you could in theory make a wrapper class that took your number in the form of a string and set the bits of the float in the order which represents the number you gave it
a more practical solution would be to use some bignum library

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