7 ms·

I've tested this with both LuaJIT and Lua 5.5. It appears to only affect LuaJIT. Worth mentioning in the article I'd say.

• purplesyringa 2mo ago
  That's interesting! On my PC it reproduces under Lua 5.5 (`log_a x > log_b x`), but not under LuaJIT (`log_a x = log_b x`). I took a look at LuaJIT's implementation (https://github.com/LuaJIT/LuaJIT/blob/faaf663340347a78b22ed94c63c24fe090bd9784/src/lib_math.c#L58 https://github.com/LuaJIT/LuaJIT/blob/faaf663340347a78b22ed9...) and noticed that it always uses the `ln x / ln a` formula -- or, rather, `log_2 x / log_2 a`, which is just as correct I guess. Have you perhaps misinterpreted the results? (I do think it's valuable to add that this doesn't apply to LuaJIT though.)
  • Ennea 2mo ago
    Hm, perhaps, but I am confused now. In the article, you're comparing log_a x < log_b x, but now you're comparing log_a x > log_b x in your comment. To make it clear, I am running this code: https://bpa.st/ZCKA https://bpa.st/ZCKA Which prints, for LuaJIT: true true false And for Lua 5.5: true false false
    • purplesyringa 2mo ago
      `log_a x < log_b x` is the mathematically correct result. Due to inherent rounding from floats being a finite representation of real numbers, `log_a x <= log_b x` would be expected from any correct implementation using floats. So either `true true false` or `true false true` would be reasonable. (I made a mistake in the previous comment, LuaJIT returns `<` for me, just like in your comment, not `=`.) The topic of the post is that in PHP and Lua (without LuaJIT), sometimes this inequality doesn't hold, and instead we get `log_a x > log_b x`, which is very incorrect and cannot be explained away by rounding. Does that make more sense?
      • Ennea 2mo ago
        Ahem. Clearly my math knowledge failing me here. Apologies, and thanks for clearing it up :)