Overflow in division

[sharkdp]

The following program

use num_rational::Rational64;

fn main() {
    let a = Rational64::new(1, 3000000000);
    let b = Rational64::from_integer(3080000000);
    println!("{}", a / b);
}

panics with

thread 'main' panicked at 'attempt to multiply with overflow', /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/ops/arith.rs:352:1
stack backtrace:
   0: rust_begin_unwind
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/std/src/panicking.rs:578:5
   1: core::panicking::panic_fmt
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/panicking.rs:67:14
   2: core::panicking::panic
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/panicking.rs:117:5
   3: <i64 as core::ops::arith::Mul>::mul
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/ops/arith.rs:345:45
   4: <num_rational::Ratio<T> as core::ops::arith::Div>::div
             at /home/shark/.cargo/registry/src/index.crates.io-6f17d22bba15001f/num-rational-0.4.1/src/lib.rs:738:13
   5: t::main
             at ./src/main.rs:6:20
   6: core::ops::function::FnOnce::call_once
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/ops/function.rs:250:5

in this line:

num-rational/src/lib.rs

Line 737 in d89ce12

self.denom / gcd_bd * (rhs.numer / gcd_ac),

Both large integers are far from i64::MAX — but their product overflows the 64-bit boundary. Is the panic expected in this case?

This looks similar to #6 because it also happens in division, but I think it's a separate issue.

[cuviper]

Both large integers are far from i64::MAX — but their product overflows the 64-bit boundary. Is the panic expected in this case?

Yes, this is expected. The correct fully-reduced result would be "1 / 9223372036854775808", and that denominator is too large for i64. You can use CheckedDiv to guard against the panic though.

[sharkdp]

Thank you for the explanation. I didn't think about using .checked_div — that works perfectly!