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/// Returns value with `n` digits after floating point where `n` is `precision`.
/// Standard rounding rules apply (if `n+1`th digit >= 5, round up).
///
/// If rounding the `value` will have no effect (e.g., it's infinite or NaN),
/// returns `value` unchanged.
///
/// # Examples
///
/// ```
/// # use typst_utils::round_with_precision;
/// let rounded = round_with_precision(-0.56553, 2);
/// assert_eq!(-0.57, rounded);
/// ```
pub fn round_with_precision(value: f64, precision: u8) -> f64 {
// Don't attempt to round the float if that wouldn't have any effect.
// This includes infinite or NaN values, as well as integer values
// with a filled mantissa (which can't have a fractional part).
// Rounding with a precision larger than the amount of digits that can be
// effectively represented would also be a no-op. Given that, the check
// below ensures we won't proceed if `|value| >= 2^53` or if
// `precision >= 15`, which also ensures the multiplication by `offset`
// won't return `inf`, since `2^53 * 10^15` (larger than any possible
// `value * offset` multiplication) does not.
if value.is_infinite()
|| value.is_nan()
|| value.abs() >= (1_i64 << f64::MANTISSA_DIGITS) as f64
|| precision as u32 >= f64::DIGITS
{
return value;
}
let offset = 10_f64.powi(precision.into());
assert!((value * offset).is_finite(), "{value} * {offset} is not finite!");
(value * offset).round() / offset
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_round_with_precision_0() {
let round = |value| round_with_precision(value, 0);
assert_eq!(0.0, round(0.0));
assert_eq!(-0.0, round(-0.0));
assert_eq!(0.0, round(0.4));
assert_eq!(-0.0, round(-0.4));
assert_eq!(1.0, round(0.56453));
assert_eq!(-1.0, round(-0.56453));
}
#[test]
fn test_round_with_precision_1() {
let round = |value| round_with_precision(value, 1);
assert_eq!(0.0, round(0.0));
assert_eq!(-0.0, round(-0.0));
assert_eq!(0.4, round(0.4));
assert_eq!(-0.4, round(-0.4));
assert_eq!(0.4, round(0.44));
assert_eq!(-0.4, round(-0.44));
assert_eq!(0.6, round(0.56453));
assert_eq!(-0.6, round(-0.56453));
assert_eq!(1.0, round(0.96453));
assert_eq!(-1.0, round(-0.96453));
}
#[test]
fn test_round_with_precision_2() {
let round = |value| round_with_precision(value, 2);
assert_eq!(0.0, round(0.0));
assert_eq!(-0.0, round(-0.0));
assert_eq!(0.4, round(0.4));
assert_eq!(-0.4, round(-0.4));
assert_eq!(0.44, round(0.44));
assert_eq!(-0.44, round(-0.44));
assert_eq!(0.44, round(0.444));
assert_eq!(-0.44, round(-0.444));
assert_eq!(0.57, round(0.56553));
assert_eq!(-0.57, round(-0.56553));
assert_eq!(1.0, round(0.99553));
assert_eq!(-1.0, round(-0.99553));
}
#[test]
fn test_round_with_precision_fuzzy() {
let round = |value| round_with_precision(value, 0);
assert_eq!(f64::INFINITY, round(f64::INFINITY));
assert_eq!(f64::NEG_INFINITY, round(f64::NEG_INFINITY));
assert!(round(f64::NAN).is_nan());
let max_int = (1_i64 << f64::MANTISSA_DIGITS) as f64;
let f64_digits = f64::DIGITS as u8;
// max
assert_eq!(max_int, round(max_int));
assert_eq!(0.123456, round_with_precision(0.123456, f64_digits));
assert_eq!(max_int, round_with_precision(max_int, f64_digits));
// max - 1
assert_eq!(max_int - 1f64, round(max_int - 1f64));
assert_eq!(0.123456, round_with_precision(0.123456, f64_digits - 1));
assert_eq!(max_int - 1f64, round_with_precision(max_int - 1f64, f64_digits));
assert_eq!(max_int, round_with_precision(max_int, f64_digits - 1));
assert_eq!(max_int - 1f64, round_with_precision(max_int - 1f64, f64_digits - 1));
}
}
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