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use bigdecimal::{BigDecimal, FromPrimitive, One, ToPrimitive, Zero};
mod error;
use error::{BigDecimalMathError, BigDecimalMathResult};
pub fn root(n: isize, x: BigDecimal) -> BigDecimalMathResult {
if x < BigDecimal::zero() {
let msg = format!("negative argument {:?} of root", x);
return Err(BigDecimalMathError::ArithmeticError(msg));
}
if n <= 0 {
let msg = format!("negative power {:?} of root", x);
return Err(BigDecimalMathError::ArithmeticError(msg));
}
if n == 1 {
return Ok(x);
}
let x_as_f64 = f64::powf(x.to_f64().unwrap(), 1.0 / (n as f64));
let mut s: BigDecimal = BigDecimal::from_f64(x_as_f64).unwrap();
let nth = BigDecimal::from_isize(n).unwrap();
let xhighpr: BigDecimal = scale_prec(&x, 2);
let mc_precision_only = 2 + get_prec(&x);
let eps_numerator: f64 = ulp(&x).to_f64().unwrap();
let eps_denominator: f64 = 2.0 * n as f64 * x.to_f64().unwrap();
let eps = eps_numerator / eps_denominator;
loop {
let mut c = &xhighpr / pow(&s, (n - 1) as i32)?;
c = c.with_prec(mc_precision_only as u64);
c = &s - &c;
let locmc = get_prec(&c);
c = &c / &nth;
c = c.with_prec(locmc as u64);
s = &s - &c;
if (c.to_f64().unwrap() / s.to_f64().unwrap()) < eps {
break;
}
}
Ok(s.round(err2prec(eps) as i64))
}
fn err2prec(xerr: f64) -> i32 {
1 + ((0.5 / xerr).abs().log10()) as i32
}
fn scale_prec(x: &BigDecimal, d: i64) -> BigDecimal {
let (_, scale) = x.as_bigint_and_exponent();
x.with_scale(d + scale)
}
fn get_prec(x: &BigDecimal) -> usize {
let (bigint, _scale) = x.as_bigint_and_exponent();
bigint.to_string().chars().count()
}
fn ulp(x: &BigDecimal) -> BigDecimal {
let (_, scale) = x.as_bigint_and_exponent();
BigDecimal::new(One::one(), scale)
}
fn pow(x: &BigDecimal, n: i32) -> BigDecimalMathResult {
if !(0..=999999999).contains(&n) {
return Err(BigDecimalMathError::ArithmeticError(
"Invalid power operation".to_owned()
));
}
let (bigint, scale) = x.as_bigint_and_exponent();
let new_scale = scale * n as i64;
Ok(BigDecimal::new(bigint.pow(n as u32), new_scale))
}
#[cfg(test)]
mod tests {
use crate::*;
use bigdecimal::BigDecimal;
use std::str::FromStr;
#[test]
fn root_from_str_test() {
let vals: Vec<(&str, isize, &str)> = vec![
("1.79", 1, "1.79"),
("1.73803", 4, "9.125"),
("1.562880129", 5, "9.3245600"),
("1.453573513976", 13, "129.32456087"),
("1.09280916443673520", 135, "159765.989751345"),
];
vals.iter().for_each(|(expected_result, n, x)| {
assert_eq!(
Ok(BigDecimal::from_str(expected_result).unwrap()),
root(*n, BigDecimal::from_str(x).unwrap())
);
});
}
}