8000 Major precision loss when passing 64-bit values back and forth? · Issue #542 · mlua-rs/mlua · GitHub
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Major precision loss when passing 64-bit values back and forth? #542

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ghost opened this issue Mar 3, 2025 · 3 comments
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Major precision loss when passing 64-bit values back and forth? #542

ghost opened this issue Mar 3, 2025 · 3 comments

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@ghost
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ghost commented Mar 3, 2025

Hey, I was testing some Discord API stuff in my project, which involves receiving and sending 64-bit values back and forth. I was under the assumption that Lua handled up to 64-bit numbers, but I ran into a bug when passing the application ID to Discord, from Lua, in Rust: the value received from Lua is off by quite a bit. You can test this yourself with this code:

fn test(_: &Lua, value: u64) -> mlua::Result<u64> {
    println!("rust_max: {}", u64::max_value());
    println!("rust: {value}");

    if value == 0 {
        Ok(u64::max_value())
    } else {
        Ok(value)
    }
}
local test = test(0)
print(test)
test(test)

Output:

rust_max: 18446744073709551615
rust: 0
18446744073709552000

In addition, mlua will crash when passing the test value in Lua back to Rust, with the error error converting Lua number to u64 (out of range).
@khvzak
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khvzak commented Mar 3, 2025 •
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Lua 5.4 API is limited to i64 integers.

mlua actually should reject pushing u64::MAX into Lua (similar to how it rejects vice-versa conversion).

For your case using strings can be more preferable.

@khvzak
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khvzak commented Mar 3, 2025

This is the behaviour of Lua:

lua5.4 -e 'print(18446744073709551615)'
1.844674407371e+19

The same precision loss

@ghost
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ghost commented Mar 3, 2025

Ah, alright. Thanks!

@ghost ghost closed this as completed Mar 3, 2025
This issue was closed.
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