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fix(runtime): js_dynamic_mod preserves sign of zero (fmod semantics) #5999

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Merged
proggeramlug merged 1 commit into from
Jul 4, 2026
Merged

fix(runtime): js_dynamic_mod preserves sign of zero (fmod semantics) #5999

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9 changes: 7 additions & 2 deletions crates/perry-runtime/src/value/dynamic_arith.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -498,10 +498,15 @@ pub unsafe extern "C" fn js_dynamic_mod(a: f64, b: f64) -> f64 {
}
let a = dynamic_number_operand(a);
let b = dynamic_number_operand(b);
// Float modulo: a - trunc(a / b) * b
let a = numify_arith_operand(a);
let b = numify_arith_operand(b);
a - (a / b).trunc() * b
// JS `%` is C `fmod`: the result takes the *sign of the dividend*, so
// `-1 % -1` is `-0`, not `+0`. The old `a - (a / b).trunc() * b` closed-form
// lost that (`-1.0 - 1.0 * -1.0 == +0.0`) and also returned `NaN` for
// `x % Infinity` (should be `x`). Rust's `f64 % f64` *is* `fmod`, matching
// the spec exactly on the sign of zero, `x % ±Inf`, and `±Inf % y` / `x % 0`
// → `NaN` (test262 compound-assignment `mod-whitespace`: `-0` expected).
a % b
}

/// Dynamic negate: -BigInt if operand is BigInt, else -f64.
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