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math.round returns 1 for 0.49999999999999994 #14755

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kromka-chleba opened this issue Jun 15, 2024 · 6 comments · Fixed by #14757
Closed

math.round returns 1 for 0.49999999999999994 #14755

kromka-chleba opened this issue Jun 15, 2024 · 6 comments · Fixed by #14757
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Bug Issues that were confirmed to be a bug @ Script API

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@kromka-chleba
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kromka-chleba commented Jun 15, 2024 •
edited by appgurueu
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Problem

As pointed out below the most popular answer on stackoverflow, Minetest's implementation of math.round suffers from a bug: https://stackoverflow.com/questions/18313171/lua-rounding-numbers-and-then-truncate

> math.round(0.49999999999999994)
1

Solution

I rewrote math random this way:

function math.round(x)
    local int, frac = math.modf(x)
    if x >= 0 then
        if frac >= 0.5 then
            return int + 1
        else
            return int
        end
    else
        if frac <= -0.5 then
            return int - 1
        else
            return int
        end
    end
end

Old:

> math.round(0.49999999999999994)
1
> math.round(-0.49999999999999994)
-1
> math.round(0)
0
> math.round(10.3)
10
> math.round(10.5)
11
> math.round(-10.3)
-10
> math.round(-10.5)
-11

New:

> math.round(0.49999999999999994)
0
> math.round(-0.49999999999999994)
0
> math.round(0)
0
> math.round(10.3)
10
> math.round(10.5)
11
> math.round(-10.3)
-10
> math.round(-10.5)
-11

Steps to reproduce

Do this:

> math.round(0.49999999999999994)
1
@kromka-chleba kromka-chleba added the Unconfirmed bug Bug report that has not been confirmed to exist/be reproducible label Jun 15, 2024
@kromka-chleba
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I guess I can make the function shorter a little:

function math.round(x)
    local int, frac = math.modf(x)
    if x >= 0 and frac >= 0.5 then
        return int + 1
    elseif frac <= -0.5 then
        return int - 1
    end
    return int
end

@SmallJoker SmallJoker added the Upstream issue This bug is the fault of a library, the OS or an external service we use. label Jun 15, 2024
@kromka-chleba

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@SmallJoker SmallJoker removed the Upstream issue This bug is the fault of a library, the OS or an external service we use. label Jun 15, 2024
@SmallJoker

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@appgurueu
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I guess I can make the function shorter a little [...]

x >= 0 should be unnecessary, just frac >= 0.5 should suffice, no? I'd also probably use just early returns, no need for the elseif there.

This could even be written as a single line with and and or, but I don't think that is worthwhile.

It might be interesting to see how the performance compares (and whether there is anyone relying on rounding being particularly fast to begin with).

kromka-chleba added a commit to kromka-chleba/minetest that referenced this issue Jun 16, 2024
@kromka-chleba
Fix math.round floating point bug
eb1654b 
fixes minetest#14755
@kromka-chleba kromka-chleba mentioned this issue Jun 16, 2024
Merged
@kromka-chleba
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Made a PR that fixes it.

@appgurueu appgurueu added Bug Issues that were confirmed to be a bug and removed Unconfirmed bug Bug report that has not been confirmed to exist/be reproducible labels Jun 16, 2024
kromka-chleba added a commit to kromka-chleba/minetest that referenced this issue Jun 16, 2024
@kromka-chleba
Fix math.round floating point bug
390185b 
fixes minetest#14755
kromka-chleba added a commit to kromka-chleba/minetest that referenced this issue Jun 19, 2024
@kromka-chleba
Fix math.round floating point bug
7d833d3 
fixes minetest#14755
@The4codeblocks
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this..... is painful
the number written in scientific notation for base 2 is practically 1.1111111111111111111111111111111111111111111111111111*10^-2
or in straight base 2
0.011111111111111111111111111111111111111111111111111111
IT'S PAINFUL
ROUND CAN BE IMPLEMENTED WITHIN PROCESSOR ARCHITECTURE (or its emulate) VERY EASILY, BUT THEY CHOSE NOT TO.
just a floor and a shift and an add.

E

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Fix math.round floating point bug kromka-chleba/minetest
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@SmallJoker @wsor4035 @appgurueu @kromka-chleba @The4codeblocks