Re: Numeric key collision related bug in Lua 5.3

[Tim Hill <drtimhill@...>]

On Apr 21, 2015, at 9:36 AM, Dirk Laurie <dirk.laurie@gmail.com> wrote:

The indexing of tables follows the definition of raw equality in the
language. The expressions a[i] and a[j] denote the same table element
if and only if i and j are raw equal (that is, equal without
metamethods). In particular, floats with integral values are equal to
their respective integers (e.g., 1.0 == 1). To avoid ambiguities, any
float with integral value used as a key is converted to its respective
integer. For instance, if you write a[2.0] = true, the actual key
inserted into the table will be the integer 2. (On the other hand, 2
and "2" are different Lua values and therefore denote different table
entries.)

nkey = (1<<63)-333

fkey = nkey + 0.0

Now, of course, nkey == fkey by the Lua rules of converting both to floats before doing the comparison. But is fkey a “float with integral value”? I dont think it is, and math.tointeger() doesn’t either (it returns nil).

The reality is Lua has three number ranges:

[a] Integers with a magnitude less than 2^52 (and can be represented exactly either as float or integer)

[b] Integers with magnitude greater than 2^52 but less than 2^63 (and can only be represented exactly as integers)

[c] Floats with magnitudes greater than 2^63

It’s the [b] range that is the problem here, and I don’t see any clear guidelines in the Lua docs to indicate how this range is handled when used as table keys.

—Tim